Competing Magnetic Structures and the Evolution of Copper Ion

Article pubs.acs.org/cm

Competing Magnetic Structures and the Evolution of Copper Ion/ Vacancy Ordering with Composition in the Manganite Oxide Chalcogenides Sr2MnO2Cu1.5(S1−xSex)2

Paul Adamson,† Joke Hadermann,b Catherine F. Smura,† Oliver J. Rutt,† Geoffrey Hyett,† David G. Free,† and Simon J. Clarke†,* †

Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, U.K. Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium

b

S Supporting Information *

ABSTRACT: The series Sr2MnO2Cu1.5(S1−xSex)2 (0 ≤ x ≤ 1) contains mixed-valent Mn ions (Mn2+/Mn3+) in MnO2 sheets which are separated by copper-deficient antifluorite-type Cu2−δCh2 layers with δ ∼ 0.5. The compounds crystallize in the structure type first described for Sr2Mn3Sb2O2 and are described in the I4/mmm space group at ambient temperatures. Below about 250 K, ordering between Cu+ ions and tetrahedral vacancies occurs which is long-range and close to complete in the sulfide-containing end member of the series Sr2MnO2Cu1.5S2 but which occurs over shorter length scales as the selenide content increases. The superstructure is an orthorhombic 2√2a × √2a × c expansion in Ibam of the room temperature cell. For x > 0.3 there are no superstructure reflections evident in the X-ray or neutron diffraction patterns, and the I4/mmm description is valid for the average structure at all temperatures. However, in the pure selenide end member, Sr2MnO2Cu1.5Se2, diffuse scattering in electron diffractograms and modulation in high resolution lattice image profiles may arise from short-range Cu/vacancy order. All members of the series exhibit long-range magnetic order. In the sulfide-rich end member and in compounds with x < 0.1 in the formula Sr2MnO2Cu1.5(S1−xSex)2, which show well developed superstructures due to long-range Cu/vacancy order, the magnetic structure has a (1/4 1/4 0) propagation vector in which ferromagnetic zigzag chains of Mn moments in the MnO2 sheets are coupled antiferromagnetically in an arrangement described as the CE-type magnetic structure and found in many mixed-valent perovskite and Ruddlesden−Popper type oxide manganites. In these cases the magnetic cell is an a × 2b × c expansion of the low temperature Ibam structural cell. For x ≥ 0.2 in the formula Sr2MnO2Cu1.5(S1−xSex)2 the magnetic structure has a (0 0 0) propagation vector and is similar to the A-type structure, also commonly adopted by some perovskite-related manganites, in which the Mn moments in the MnO2 sheets are coupled ferromagnetically and long-range antiferromagnetic order results from antiferromagnetic coupling between planes. In the region of the transition between the two different structural and magnetic long-range ordering schemes (0.1 < x < 0.2) the two magnetic structures coexist in the same sample. The evolution of the competition between magnetic ordering schemes and the length scale of the structural order with composition in Sr2MnO2Cu1.5(S1−xSex)2 suggest that the changes in magnetic and structural order are related consequences of the introduction of chemical disorder. KEYWORDS: oxychalcogenide, manganite, magnetic order, vacancy order

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INTRODUCTION Oxide chalcogenides are a relatively under-investigated class of solid state compound compared with oxides and other solids containing only one type of anion. Due to the different sizes and coordination requirements of the oxide and the heavier chalcogenide anions, oxide chalcogenides tend to adopt layered structures.1 In the oxide sulfides A2MO2Cu2S2 (A = electropositive metal; M = transition metal, known for A = Sr: M = Mn,2 Co,3 Ni,4 Cu,4 and Zn;2 A = Ba: M = Co,3 which were first described, along with a series of related compounds, by Zhu and Hor and co-workers,2 the more polarizable sulfide anion bonds to Cu+ ions in (Cu2S2)2− antifluorite-type layers and the less polarizable oxide anion bonds to the divalent M2+ ion in square planar (MO2)2− layers which are two-dimensional © 2012 American Chemical Society

fragments of the perovskite structure. The two layer types stack alternately with A2+ cations in between (Figure 1). The structure type was first reported for Sr2Mn3Sb2O2.5 Oxide chalcogenides and oxide pnictides with these layered structures provide a counterpoint to important perovskite-related oxide phases such as the three-dimensional cubic perovskites AMO3−δ and the layered Ruddlesden−Popper (An+1MnO3n+1) phases. It is well-established6 that the copper chalcogenide layers present in the oxide sulfides can readily accept holes in the antibonding states at the top of a valence band that is composed of wellReceived: May 15, 2012 Revised: June 15, 2012 Published: June 19, 2012 2802

dx.doi.org/10.1021/cm301486v | Chem. Mater. 2012, 24, 2802−2816

Chemistry of Materials

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mm diameter pellets which were sealed under vacuum in alumina-lined silica ampules. The silica tubes and alumina crucibles had been dried under dynamic vacuum at 1000 °C for at least 2.5 h to remove adsorbed water. The sealed ampules were heated to 900 at 10 °C min−1 and held at this temperature for 24 h before being cooled at the natural rate of the chamber furnace. Purity of the samples was assessed using laboratory X-ray powder diffraction. The samples contained, as crystalline impurities, very small amounts of the impurity phases SrS1−xSex and MnO; multiple grinding and heating cycles resulted in a loss of crystallinity and greater amounts of the impurity phases. During synthesis all materials were treated as being air sensitive, but once cooled, the products could be handled and stored outside the drybox as they were all air and moisture stable as assessed by the invariance of laboratory powder diffraction patterns and magnetic susceptibilities over several months. X-ray Powder Diffraction (XRPD). XRPD was used to assess the phase purity of reactants and products prepared as described above. Measurements were made using a PANalytical X’Pert PRO diffractometer operating in Bragg−Brentano geometry with monochromatic Cu Kα1 radiation and a multiangle X’Celerator detector. High resolution measurements on several samples were performed on Station ID31 at the ESRF, Grenoble using X-rays with Si-calibrated wavelengths of about 0.4 Å and with the samples contained in 0.7 mm diameter glass capillary tubes. For low temperature measurements the capillaries were filled with helium exchange gas to ensure good thermal contact between the powder and the He flow cryostat and to avoid heating of the sample by the intense X-ray beam.11 Such measurements were made on warming after cooling the sample to 5 K. Neutron Powder Diffraction (NPD). Time of flight NPD data was collected at the ISIS Facility, Rutherford Appleton Laboratory, U.K., using the medium resolution diffractometer POLARIS, the high resolution powder diffractometer HRPD, and the diffractometer OSIRIS which is optimized for high-resolution studies at long dspacings. Data were also collected using monochromatic radiation (1.59 Å) on the diffractometer D2B at the Institut Laue-Langevin (ILL), Grenoble. Samples of 2−5 g in mass were sealed inside cylindrical thin-walled vanadium cans using indium gaskets and measured at temperatures between 2 and 298 K. Further details of the diffractometers and the measurements are given in the Supporting Information. All structural refinements against NPD and XRPD data were carried out using the Rietveld profile refinement suite, GSAS,13 via the EXPGUI interface.14 Refinements against POLARIS or HRPD data were generally carried out against all data banks simultaneously. Magnetic Susceptibility Measurements. Measurements were carried out using either a Quantum Design MPMS5 or MPMS-XL SQUID magnetometer in the temperature range 5−330 K. Measurements were made on warming in an applied magnetic field of 0.1 T, first after cooling in zero field (zero-field cooled: ZFC) and then again after cooling in the measuring field (field cooled: FC). Magnetization isotherms were measured at applied fields in the range ±5 T. Samples of about 50 mg in mass were contained in gelatin capsules mounted inside drinking straws. Transmission Electron Microscopy (TEM). Electron diffraction patterns were taken on a Philips CM20 transmission electron microscope, using a GATAN cooling holder to reach temperatures as low as 100 K. High resolution transmission electron microscopy images were taken on a JEOL 4000EX. The samples were made by crushing the powders in ethanol and depositing a drop of this solution on a holey carbon grid.

Figure 1. Crystal structure of Sr2MnO2Cu1.5(S1−xSex)2 (0 ≤ x ≤ 1). The Cu sites are 3/4 occupied in a disordered fashion at room temperature.

mixed Cu 3d/S 3p orbitals, and that these holes are very mobile. So the band gap insulator Sr2ZnO2Cu2S2 may be doped to produce a p-type metal NaxSr2−xZnO2Cu2S2 with x ∼ 0.1,7 and there is evidence for very high mobility in the related Sr3Sc2O5Cu2S2.8,9 When the valence band is not doped with holes and when the metal M in the oxide layer is a mid-to-late transition metal, the compounds are semiconducting,3,10 suggesting that the oxide layer behaves as a Mott-Hubbard insulator. The interactions between the two electronically very different layers is via weak bonding interactions between M and S which may be tuned by the size of the A cation.11 It is expected that investigation of compounds with this structure type could lead to unusual magnetic and electronic behaviors complementary to those of the better-studied oxides. The oxide sulfides and oxide selenides, by virtue of containing anions of less electronegative Group 16 elements, also tend to exhibit lower oxidation states for the transition metals than are found in perovskite-type oxides. For example, the homologous series of compounds Sr2MnO2Cu2m−δSm+1 (m = 1, 2, 3; δ = 0.5) which we have described10,12 contains Mn in the +2.5 oxidation state10 instead of the oxidation states between +3 and +4 normally encountered in manganite perovskites and Ruddlesden−Popper phases which show complex structural electronic and magnetic phenomena dependent on the eg electron concentration in mixed valent systems. The oxide chalcogenides offer high symmetry structures with transition metal ions in strictly planar oxide layers irrespective of the nature of the alkaline earth cation or the chalcogenide anion and may be good model compounds which will contribute to the development of theories of low-dimensional solids. Here we show that systems with mixed-valent Mn2+/3+ ions show behavior comparable with that of the better-studied mixedvalent Mn3+/4+ oxide manganites.

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RESULTS AND DISCUSSION Compositions and Room Temperature Crystal Structures. NPD measurements were carried out at room temperature (298 K) on 2−5 g samples of Sr2MnO2Cu2−δ(S1−xSex)2. Initially we synthesized compositions Sr2MnO2Cu1.5+0.06x(S1−xSex)2 with x = 0.125, 0.25, 0.5, 0.75, and 0.875 suspecting that increasing the lattice parameter would lower the Mn valence and result in a Cu content which increased with selenide content. These compounds were

EXPERIMENTAL SECTION

Synthesis. Samples Sr2MnO2Cu1.5(S1−xSex)2 (0 ≤ x ≤ 1) were prepared on the 2−5 g scale by reacting together SrS, SrO, MnO2 (99.9%, Sigma-Aldrich), CuO (99.995%, Alfa Aesar), Mn (99.99%, Sigma-Aldrich) Cu (99.999%, Alfa Aesar) and Se (99.999%, Alfa Aesar) in the ratio 2−2x : 2x : 1−2x : 2x : 2x : 1.5−2x : 2x. Due to the air-sensitive nature of SrS and SrO, all manipulations were performed in an argon-filled drybox. The reaction mixtures were ground manually for about one hour in an agate mortar before being pressed into 13 2803



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measured on POLARIS and Rietveld refinements were carried out using the published structure of Sr2MnO2Cu1.5S2 in space group I4/mmm as the starting model.10 The mixed occupancy of the chalcogenide (Ch) site was fixed to reflect the S:Se ratio used in the synthesis and the positional and displacement parameters were constrained to be equal for sulfide and selenide ions. This assumption of a random distribution of sulfide and selenide is reasonable on the length scale probed by diffraction methods, but the refinements were improved by the inclusion of anisotropic peak broadening terms in the profile function which likely reflect the effect on the lattice parameters of slight inhomogeneities in the composition. In addition to the oxychalcogenide phase, elemental vanadium (from the sample can), SrS1−xSex (∼0.5% by mass), MnO15 (∼0.5% by mass), and elemental copper were added to the refinements to account for all of the Bragg peaks. We have generally been unable to make materials in this series which are entirely devoid of small impurity phases (see Supporting Information). Repeated regrinding and reheating did not reduce the amounts of these impurities, and often the impurity level increased following such treatment. Refinement of the site occupancy of copper in the oxysulfide phase and of the amount of excess elemental Cu in the samples suggested that within the experimental uncertainty of about 1− 2% (based on the variation between independent measurements of refined Cu contents for a given sample at different temperatures rather than on the esd produced by a single refinement) the Cu content was almost invariant with x in the range 0 ≤ x ≤ 1 (Supporting Information, Table S1). One sample with a refined composition of Sr2MnO2Cu1.48(2)S2 used for the analysis of the magnetic structure of this phase1 was prepared with 2 mol of Cu per mol of Mn in the synthesis, and this sample contained a corresponding amount of elemental Cu.10 Subsequently, further samples of the solid solution were prepared as described above with Mn and Cu in the ratio 1:1.5 (5 g scale for 0 < x < 0.25 in steps of 0.025 and on the 2 g scale (in steps of 0.2) for compositions with 0.3 < x < 1). The evolution of the structures and magnetic ordering for the series were obtained from this set of samples. Rietveld refinements are shown in Figure 2 and Figure S1 (Supporting Information), and the results are summarized in Table 1. Figure 3 shows the refined lattice parameters and unit cell volumes obtained from laboratory XRPD measurements and selected bond distances and angles obtained from NPD measurements on D2B and POLARIS. These values are tabulated in the Supporting Information (Tables S2 and S3). Figure 3a shows that the substitution of S by Se results in an anisotropic expansion of the unit cell. The increase in the basal lattice parameter a is restricted by the maximum Mn−O bond length that can be maintained. There is no such restriction on the lattice parameter c so the expansion on substituting S by Se is highly anisotropic. The Cu−Ch distance increases linearly with increasing Se content, but the restriction on the increase in the basal lattice parameter inevitably means that the CuCh4 tetrahedra become compressed in the basal direction as x increases becoming regular at x ∼ 0.65 (Figure 3c). Low Temperature Crystal Structures of Sr2MnO2Cu1.5S2 and Sr2MnO2Cu1.5Se2. Sr2MnO2Cu1.5S2. Low temperature NPD measurements of Sr2MnO2Cu1.5S2 on D2B (a sample prepared containing excess Cu as described above) revealed two sets of additional reflections at low temperatures. One set of these which appears below 29 K is the result of magnetic ordering1 and is discussed further below. But

Figure 2. Rietveld refinements against room temperature POLARIS NPD data (145° detector bank). (a) Sr2MnO2Cu1.5(S0.775Se0.225)2 and (b) Sr2MnO2Cu1.5Se2. The data (red), calculated pattern (green line), and difference (purple line) are shown. Reflections markers are, in (a), Sr2MnO2Cu1.5(S0.775Se0.225)2 (black), vanadium sample holder (red), MnO 0.5 wt % (green), and SrS0.775Se0.225 0.3 wt % (orange). In (b) the markers are for Sr2MnO2Cu2Se2 (black) and MnO 0.7 wt % (red). The insets show the short d-spacing regions. See also Figure S1 (Supporting Information).

another set measured at 100 K, above the magnetic ordering transition, was evidently due to a change in the crystal structure. These were not readily indexed using the powder diffraction data, so electron diffraction measurements were performed at ∼100 K after cooling the sample from room temperature over the course of one hour (to simulate fairly closely the rate of cooling in the NPD measurements). These revealed intense superstructure reflections (Figure 4a) indicating a 2√2aT × √2aT × cT expansion of the nuclear unit cell where aT and cT are the cell parameters of the tetragonal room temperature cell. That is, the matrix giving the relation between the reciprocal lattice vectors of the subcell and the supercell is ⎛ 1 ⎜ ⎜ 4 ⎜ 1 ⎜− ⎜ 2 ⎝ 0

1 ⎞ 0 ⎟⎛ * ⎞ ⎛ * ⎞ 4 ⎟⎜ a T ⎟ ⎜ a O ⎟ 1 ⎟⎜ bT* ⎟ = ⎜ bO* ⎟ 0 ⎟⎜ ⎟ ⎜ ⎟ 2 ⎟⎜ * ⎟ ⎜ * ⎟ ⎝ c T ⎠ ⎝ cO ⎠ 0 1⎠

where aO, bO, and cO are the lattice parameters of the orthorhombic cell. Commonly the diffraction patterns exhibited twinning (Figure 4b), and it was generally difficult to obtain diffraction patterns from a single domain as are shown in Figure 4a. In Figure 4, all patterns are indexed in the orthorhombic cell, and the correspondence of the patterns to the tetragonal subcell is [001] = [001]T, [120] = [100]T, and [010] = [1̅10]T as shown in Figure S2 (Supporting Information). The intense superstructure reflections in the electron diffractograms accounted for all the additional intensities evident in the low temperature powder diffractograms gathered from bulk samples, and the reflection conditions derived from the electron diffraction patterns of Sr2MnO2Cu1.5S2 at 100 K are hkl: h + k + l = 2n, hk0: h + k = 2n, h0l: h, l = 2n, and 0kl: k, l = 2n, giving the extinction symbol Iba− and therefore Ibam 2804



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Table 1. Results of Room Temperature Refinements of Sr2MnO2Cu1.5(S1−xSex)2 against NPD Dataa x b

0.00 0.025c 0.05c 0.10c 0.125b 0.15c 0.20c 0.225b 0.25c 0.5b 0.75b 0.875b 1.00b 1.00c a

z(Ch)

z(Sr)

a (Å)

c (Å)

V (Å3)

χ2

Rwp

0.16985(5) 0.1702(2) 0.1698(2) 0.1701(2) 0.17012(4) 0.1702(2) 0.1699(2) 0.17023(4) 0.1699(1) 0.16932(3) 0.16852(2) 0.16809(2) 0.16777(2) 0.16788(8)

0.09941(2) 0.09948(7) 0.09909(7) 0.09882(8) 0.09875(2) 0.09870(8) 0.09839(8) 0.09834(2) 0.09835(8) 0.09713(2) 0.09601(2) 0.09517(2) 0.09486(2) 0.0947(1)

4.01216(3) 4.01356(6) 4.01525(5) 4.01810(6) 4.02240(3) 4.02118(5) 4.02406(5) 4.02451(3) 4.02713(5) 4.04207(3) 4.05702(2) 4.06815(2) 4.06655(3) 4.06955(7)

17.1915(2) 17.2133(3) 17.2229(3) 17.2536(3) 17.3008(1) 17.2901(3) 17.3231(3) 17.3411(1) 17.3592(2) 17.5402(2) 17.7057(1) 17.8316(1) 17.8830(1) 17.8999(3)

276.739(6) 277.28(1) 277.67(1) 278.56(1) 279.921(5) 279.58(1) 280.513(9) 280.869(5) 281.526(9) 286.578(6) 291.426(5) 295.110(5) 295.729(5) 296.45(1)

2.235 2.349 2.536 2.805 1.780 2.723 2.664 2.687 3.394 1.852 1.964 1.944 1.771 6.867

0.0233 0.0503 0.0517 0.0576 0.0175 0.0549 0.0514 0.0210 0.0517 0.0181 0.0171 0.0172 0.0165 0.0846

I4/mmm. Sr: 4e (1/2, 1/2, z(Sr)); Mn: 2a (0, 0, 0); Cu: 4d (1/2, 0, 1/4); O: 4c (1/2, 0 0); Ch: 4e (0, 0, z(Ch)). bPOLARIS. cD2B.

(No. 72) and Iba2 (No. 45) as possible space groups. Streaks along the c* direction are present on the [120] and [010] patterns, indicating the order along the c-axis is not perfect, and we return to this point below. A search of the Inorganic Crystal Structure Database (ICSD) for compounds with the centrosymmetric choice of space group, Ibam, revealed that SrY2CuFeO6.516 containing Y2O1.5 fluorite-type layers which are the antitype of the Cu1.5S2 layers in Sr2MnO2Cu1.5S2 might provide a suitable model. The tetrahedral sites in the sulfide layers in the Ibam model for Sr2MnO2Cu1.5S2 are 4a, 4b, and 8f. A model for Cu ordering similar to that corresponding to the oxide ion/vacancy ordering in SrY2CuFeO6.5 corresponds to the 4a and 8f sites being occupied by Cu and the 4b site being vacant. This model was found to account quantitatively for the new reflections measured at 100 K on D2B (Supporting Information, Figure S3). The structural model is shown in Figure 5. This ordering scheme for the Cu1.5S2 layers is also adopted in the metal chalcogenide layers of the Cs2Zn3S4 structure type17,18 which is a defective variant of the common ThCr2Si2 type and is adopted by some relatives of the ironbased superconductors such as Tl2Fe3S4.19 The crystallographic group/subgroup relationships between vacancy disordered and vacancy ordered compounds, including Sr2MnO2Cu1.5S2, and compounds in which there are electronic or magnetically driven structural distortions, such as the pnictide superconductors, have recently been described in detail by Johrendt et al.20 The rationalization of the observed ordering scheme for a twodimensional square lattice with one-quarter (as well as other fractions) of the sites vacant has been described in detail by González et al.21 We analyzed the low temperature Cu/vacancy ordered structure of Sr2MnO2Cu1.5S2 in detail using measurements on separate phase pure samples using the higher resolution instruments ID31 and HRPD in order to investigate whether there were further structural distortions at low temperatures and in order to correlate the behavior of the crystal structures with the magnetic ordering as described in more detail below. These measurements also enabled us to investigate whether Cu/vacancy ordering was complete. Suitable constraints were applied to the refinement of the site occupancy factors of the three copper sites to maintain the correct stoichiometry making the assumption that occupancy of the 4b site was at the expense of both the 4a and 8f sites. We constrained the displacement ellipsoids to be similar for crystallographically distinct but

Figure 3. Room temperature structural parameters for tetragonal Sr2MnO2Cu1.5(S1−xSex)2 phases (see also Supporting Information, Tables S2 and S3). (a) The lattice parameters and unit cell volumes determined from laboratory XRPD experiments and normalized to the values in Sr2MnO2Cu1.5S2 (x = 0). Error bars are smaller than the point markers. (b) Mn−O (= a/2), Mn−Ch, Cu−Ch, Sr−O, and Sr− Ch distances determined from refinement against NPD data and normalized to their values in Sr2MnO2Cu1.5S2 (x = 0). (c) Ch−Cu− Ch angles.

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Figure 5. Rietveld refinement of Sr2MnO2Cu1.5S2 against ID31 XRPD data collected at 5 K (λ = 0.3982 Å) (lower) and HRPD NPD data collected at 7 K (upper). Black tickmarks indicate the structural reflections and red tickmarks the magnetic reflections in the NPD data. Some of the reflections which arise as a consequence of long-range order of Cu ions and vacancies are indicated (↓).

from the XRPD or NPD data of a further structural distortion beyond that which produces the intense superstructure reflections in the low temperature electron diffractograms. Selected bond lengths at 7 K are shown in Figure 6; these and refined atomic parameters for the Ibam model are given in Tables 2, S4, and S5 (Supporting Information). In the Cu1.5S2 layers the Cu−S distances are 2.39077(2) Å for Cu on the 4a site, 2.414 Å (mean) for Cu on the 8f site, and a much longer 2.46122(2) Å for the almost completely vacant 4b site. The measurements on HRPD and ID31 were of sufficient quality to enable anisotropic refinement of displacement ellipsoids to be carried out. Since the distortion of the structure accompanying Cu/vacancy ordering is small, constraints were applied to the displacement ellipsoids of the atoms so that the ellipsoid shapes had similar freedom irrespective of whether the I4/mmm or Ibam models were used. The refinements revealed that the anisotropic displacement ellipsoids of the oxide ions (Supporting Information, Table S4) are elongated significantly toward manganese at low temperatures as shown in Figure 6. This elongation is also apparent at ambient temperature but is found to increase as the temperature decreases, and furthermore the mean in-plane displacement for oxide decreases more slowly with temperature than the displacement parameters for the other atoms as shown in Figure 7. The elongation was much less extreme than that which we have described for the isostructural compounds Ba2ZnO2Ag2Se2 and Ba2ZnO2Cu2Se222 and attempts to model the ellipsoids using split oxide sites with isotropic displacement ellipsoids did not lead to any further improvements in the quality of the refinement. It is plausible that this elongation of the O ellipsoid is evidence for charge ordering of the manganese ions, and we return to this possibility below. The degree of elongation of the oxide ellipsoids along the Mn−O

Figure 4. Electron diffraction patterns of Sr2MnO2Cu1.5S2 obtained at 100 K: (a) Patterns recorded from a single (i.e., untwinned) domain and indexed in the new orthorhombic supercell. (See Supporting Information, Figure S2, for the indexing of the subcell reflections). Additional weak reflections discussed in the text are indicated with the arrows (red arrow: (010) subcell reflection; green arrow: (−1/4 7/4 0) subcell reflection). (b) The commonly encountered diffraction pattern of the [001] zone affected by twinning.

chemically similar sites (this applies to the O and Cu sites). Occupancy of the Cu 4b site at the 6−8% level improved the statistical quality and also the visual appearance of the refinements against both ID31 and HRPD which are shown in Figure 5. Subsequently we carried out further unconstrained refinements which showed that the assumption that the 4a and 8f sites were equally Cu deficient was reasonable. These results show that the Cu/vacancy ordering is not quite complete, although we cannot rule out some correlation of the absolute values of the site occupancy factors with displacement parameters or profile parameters. There was no evidence 2806



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similar in the pure selenide (Supporting Information, Figure S4). Temperature dependent measurements on warming on ID31 and HRPD showed a sharp increase in the occupancy by Cu of the 4b site (and a corresponding decrease in the occupancies of the other sites) above 200 K and a concomitant decrease in the intensity of the superstructure reflections which remain observable up to 242 K and then disappears once 244 K is reached according to our ID31 measurements (Figure 8). Below 40 K the basal lattice parameters reach a plateau and then drop sharply below about 25 K while the lattice parameter c increases below 25 K (Figure 9). These changes are contrasted with those in Sr2MnO2Cu1.5Se2 below. Further electron diffraction measurements were used to investigate the Cu/vacancy ordering on a shorter length scale than that probed in the XRPD or NPD experiments. On warming from 100 K in the TEM the superstructure reflections remain evident as diffuse scattering at temperatures as high as 280 K (Figure 10). Furthermore these reflections are evident as diffuse scattering in grains which had never been cooled below room temperature (Figure 11). These results show that there is short-range Cu/vacancy ordering at ambient temperatures in Sr2MnO2Cu1.5S2. In addition to the intense supercell reflections which arise from Cu/vacancy order and which become diffuse at room temperature, some very tiny and sharp reflections are present in the [001] electron diffraction patterns of Sr2MnO2Cu1.5S2 at all temperatures with either hk0: h + k = 2n + 1 or (h + 1/2)(k + 1 /2)0 subcell indices (e.g., the (010) is indicated with a red arrow in Figure 4a). These reflections may be the intersection points of this plane with the streaks visible in the [11̅ 0](subcell indexing) diffraction pattern in Figure 11 which we propose could arise from a small concentration of defects in which thicker Cu2m−δSm layers10,12 separate the MnO2 sheets giving disorder along the c axis. This scenario is consistent with the fact that these reflections appear at ambient temperatures. Furthermore, there are additional weak reflections evident in the [001] zone of Sr2MnO2Cu1.5S2 at 100 K which can be indexed using a (1/4 1/4 0) modulation vector applied to the tetragonal subcell indices (the reflection with subcell indices (−1/4 7/4 0) (i.e., (200) − (1/4 1/4 0)) is indicated by a green arrow in Figure 4a) which are not present in the [001] patterns collected at higher temperatures (Figure 11) nor in the diffractograms obtained from other zones. These are similar to new reflections indexed with the modulation wave vector of ( 1 / 4 1 / 4 0) which appear in the related compound La0.5Sr1.5MnO4 in what is described as a charge and orbitally ordered state below about 220 K (see Figure 2b in ref 24). The model in space group Ibam for Sr2MnO2Cu1.5S2 which enables Cu/vacancy order and which is fully consistent with the NPD and XRPD data does not simultaneously allow for charge order (CO) of different Mn oxidation statesthere is a single Mn site in the Ibam model. It is possible that the extra weak reflections observed in the electron diffraction patterns are the consequence of the response of the crystal structure to in-plane CO on the Mn sublattice which might not occur in an ordered fashion perpendicular to the planes. The possibility of CO is also suggested by the elongation and thermal evolution of the oxide displacement ellipsoids described above. This hypothesis requires further investigation in relation to the magnetic structure which we describe below. Sr2MnO2Cu1.5Se2. In contrast to the case of the sulfide, low temperature XRPD or NPD measurements revealed no

Figure 6. Crystal structure of Sr2MnO2Cu1.5S2 at 7 K obtained from refinement against HRPD data (Figure 5). The Cu1.5S2 layers (middle panel), in which the 4b site (shown as a small green circle) has a site occupancy factor of about 6−8% and the 4a and 8f sites are almost fully occupied, and MnO2 layers (bottom panel) are shown in detail projected down the c axis. Selected bond lengths are given in Å. The bonds from the sparsely occupied Cu 4b site to the surrounding anions are shown using dotted lines. Anisotropic displacement ellipsoids are shown at the 99% level. The shape of the O ellipsoid in the basal plane at 300 K is depicted in the bottom panel showing how the ellipsoid becomes relatively elongated along the Mn−O direction on cooling.

bonds and its increase on cooling is similar to that observed in the Mn3+/4+ analogue La0.5Sr1.5MnO423 which does exhibit charge ordering. The elongation of the oxide ellipsoids is 2807



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Table 2. Refined Atomic Parameters for Sr2MnO2Cu1.5S2 at 7 K (HRPD data; see also Tables S4 and S5, Supporting Information)a

a

atom

site

x

y

z

100 × Uiso,eq (Å2)

occupancy

Mn Sr1 O1 O2 O3 S1 Cu1 Cu2 Cu3

8j 16k 4c 4d 8j 16k 4a 4b 8f

0.1264(3) 0.1228(1) 0 1/2 0.2503(1) 0.1263(3) 0 1 /2 0.2553(1)

0.2469(4) 0.2567(2) 0 0 0.0020(7) 0.2398(4) 0 0 0

0 0.59957(2) 0 0 0 0.17062(4) 1 /4 1 /4 1 /4

0.04(1) 0.22(1) 0.59(2) 0.59(2) 0.59(2) 0.31(3) 0.19(1) 0.19(1) 0.19(1)

1b 1b 1b 1b 1b 1b 0.976(2) 0.082(2) 0.976(2)

Ibam. a = 11.3255(2) Å, b = 5.6651(1) Å, c = 17.0954(1) Å. χ2 = 5.98; Rwp = 0.034. bNot refined.

Figure 8. Fractional occupancy as a function of temperature of the 4b tetrahedral Cu site in the Ibam model of Sr2MnO2Cu1.5S2. Measurements on ID31 and HRPD were made on two different samples. In the disordered I4/mmm model the site has an occupancy of 0.75. The inset shows the evolution of the low angle superstructure reflections on ID31.

This gives reflection conditions 0kl: k = 2n, l = 2n; h0l: h + l = 2n; hk0: h = 2n, k = 2n; hkl: h + l = 2n suggesting Bb2b and Bbmb as possible space groups. The model generated in Bbmb (Supporting Information, Table S6, Figure S5) retains the

Figure 7. Thermal evolution of displacement ellipsoids in Sr2MnO2Cu1.5S2 from refinement against HRPD data. Constraints were applied to the displacement ellipsoids so that the atoms had similar freedom in Ibam (230 K and below) and I4/mmm (300 K). The behaviors of equivalent isotropic ellipsoids are shown together with the mean in-plane displacement (Ubasal) and the out of plane displacement (U33) which were constrained to be similar for all the oxide sites. The upper panel shows the increasing elongation of the oxide ellipsoid along the Mn−O bonds using parameters U⊥ and U∥ equivalent to the U11 and U22 parameters in the tetragonal phase.

evidence for new Bragg reflections showing that there is no long-range Cu/vacancy order in this compound. Investigations of Sr2MnO2Cu1.5Se2 using TEM were used to probe the structure on a shorter length scale. A sample made with composition Sr2MnO2Cu1.56Se2 (CFS086B) was cooled from room temperature to 103 K over the course of 1 h. The ED patterns of the main zones at this temperature are shown in Figure 12. Clear additional spots which are not consistent with the room temperature I4/mmm model are present on the [−110]T zone only (this corresponds to the [010] pattern in Figure 12 which is indexed in the appropriate supercell explained below). These additional reflections correspond to h + 1/2, k + 1/2, l when indexed in the tetragonal subcell (compare Figure 11) and disappear around 220 K upon reheating. Indexing the [1̅10]T zone necessitates a cell which is a √2a × √2a × c expansion of the room temperature cell.

Figure 9. Behavior of the lattice parameters (derived from ID31 XRPD data) with temperature for Sr2MnO2Cu1.5S2 (x = 0) (closed symbols) and Sr2MnO2Cu1.5Se2 (x = 1) (open symbols). The behavior of the basal lattice parameters a (black) and b (red) (as appropriate), the lattice parameter c (green), and the cube root of the unit cell volume (blue) are shown. The values shown for each parameter are normalized to their values at 100 K. 2808



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Figure 10. ED patterns of the [001] zone of Sr2MnO2Cu1.5S2 obtained during warming from 200 to 290 K.

Figure 11. ED patterns at room temperature from a sample of Sr2MnO2Cu1.5S2 which had not been cooled below room temperature. These patterns are indexed in the tetragonal subcell.

radiation. Stable refinement against ID31 data at 5 K in Bbmb required atomic coordinates and profile parameters to be fixed and the basal lattice parameters a and b were similar within the uncertainty in the refinement. Refinements against D2B data in Bbmb were all unstable. Neither the refinements against ID31 data nor D2B data indicated any difference in the refined occupancies of the two Cu sites, which was significantly larger than the uncertainty in these values (0.763(2) and 0.776(2)). Accordingly, refinements against ID31 data were carried out using the room temperature I4/mmm model with a single partially occupied tetrahedral site at all temperatures. The changes in the lattice parameters and cell volume with temperature are compared with those of Sr2MnO2Cu1.5S2 in Figure 9, and the changes are discussed below in the context of the models for long-range magnetic ordering in these compounds. The [001] electron diffraction pattern for Sr2MnO2Cu1.5Se2 shown in Figure 12 shows extremely weak diffuse reflections in similar positions to those in the low temperature phase of Sr2MnO2Cu1.5S2 (see Figure 4b). The majority of the [001] patterns resemble that in Figure 12, but occasionally (Figure 13) clearer weak superstructure reflections appear in the [001] zone diffraction patterns. These electron diffraction studies suggest that Cu/vacancy ordering similar to that which occurs on the long-range in Sr2MnO2Cu1.5S2 may be adopted in the selenide analogue, but only on a much shorter length scale so it is not evident from X-ray or neutron diffraction measurements. Furthermore the [010] ED patterns of Sr2MnO2Cu1.5Se2 were frequently found to exhibit streaking along the c-axis that persisted on heating back to room temperature. This was probed using high resolution lattice images at room temperature. An image of the [110] zone is shown in Figure 14a, together with its Fourier transform (Figure 14b). The streaks in reciprocal space at 1/2, 1/2, l, suggest an alternation in periodicity of some columns of atoms that are expected to be

Figure 12. Electron diffraction patterns of Sr2MnO2Cu1.5Se2 at 100 K, indexed in the B-type orthorhombic supercell. Correspondence to the indexation in the tetragonal subcell is [100] = [110]T, [010] = [1̅10]T, [001] = [001]T, [101]̅ = [100]T.

number of distinct Mn, Sr, O, and S crystallographic sites while the Cu atoms now occupy two fourfold sites which would enable partial ordering of Cu ions and vacancies. However, this orthorhombic model fails to give an improved fit to NPD data from D2B nor to high resolution XRPD from ID31, providing evidence that any possible ordering only occurs on the length scale probed by electron diffraction and not on the length scale probed using longer wavelength 2809



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our knowledge these have only been reported in the sulfide systems.10,12 The overall conclusion from these measurements is that the oxide−selenide Sr2MnO2Cu1.5Se2 is very different from the analogous oxide−sulfide. While Sr2MnO2Cu1.5S2 exhibits longrange Cu/vacancy ordering which is almost complete, the selenide analogue shows evidence only for short-range ordering. Since the Cu+ ion mobilities are similar in Cu2−xS and Cu2−xSe phases,25 the difference in behavior presumably reflects a lower enthalpic driving force for ordering in the selenide, so that the entropically favored disordered state is favored at temperatures below those at which the Cu ions are reasonably mobile. Electrostatic Cu−Cu repulsions would be better screened by the more polarizable selenide ions and also diminished by the increase in Cu−Cu distance of 1.4% when sulfide is substituted completely by selenide, which might account for the observed difference. The advantage of Cu/ vacancy order in the chalcogenide slab in terms of the Cu−Ch, Cu−Cu, and Ch−Ch covalent bonding interactions involving the well-mixed Cu-3d and Ch-np states6 may also be less in the case of the selenide than the sulfide. There is also evidence from extremely weak spots in the electron diffractograms of the sulfide that further structural modulation, possibly resulting from charge ordering of Mn ions, occurs at low temperatures, but this is not evident in the bulk diffraction measurements and requires further examination. Magnetic Susceptibility Measurements. Figure 15a s h o w s t h e m a g n e t i c s us c e p t i bi li t ie s f o r s e v e r a l Sr2MnO2Cu1.5(S1−xSex)2 (0 ≤ x ≤ 1) samples. The sharp cusp in the susceptibilities is consistent with long-range antiferromagnetic ordering for all these compositions as reported previously for Sr2MnO2Cu1.5S2.2,10 Plots of the inverse

Figure 13. Electron diffraction pattern of the [001] zone of Sr2MnO2Cu1.5Se2 at 100 K showing diffuse reflections which resemble the superstructure reflections in Sr2MnO2Cu1.5S2 (Figure 4b).

Figure 14. (a) High resolution TEM image of the [110] zone of Sr2MnO2Cu1.5Se2 at room temperature. (b) The Fourier transform of this image. (c) Modulation in the MnO2 layers; the periodicity of the lower row (red) is half of that of the upper row (green). The view of a MnO2 layer is also showndashed lines indicate where the (110) planes cut the layer.

equivalent. Profile plots along two adjacent MnO2 layers (Figure 14c) show this more clearly: one layer shows an alternation in peak heights and a periodicity √2a while the other has the periodicity of (√2/2)a expected for the I4/mmm model of the average structure. Comparable analysis of several Cu1.5Se2 layers is shown in the Supporting Information (Figure S6) and also shows a modulation which is evident only over short length scales. There is evidently modulation in the MnO2 and Cu1.5Se2 layers of the structure. The presence of Cu/ vacancy order in the sulfide analogue suggests that the local structural feature in Sr2MnO2Cu1.5Se2 is driven by a modulation in the Cu/vacancy distribution in the Cu1.5Se2 layers at room temperature, which is not evident on the length scale probed by XRPD and NPD measurements. Streaking in the electron diffraction patterns would result from this short-range modulation. However, our measurements do not allow us to rule out some other origin for this feature such as charge ordering in the MnO2 layers. Further HRTEM images (Supporting Information, Figure S7) reveal no other types of defects, intergrowths, or stacking faults with, for example, thicker copper selenide layers, and to

Figure 15. (a) Zero-field-cooled magnetic susceptibilities of several Sr2MnO2Cu1.5(S1−xSex)2 samples measured in an applied magnetic field of 0.1 T (LHS). (b) The variation of TN obtained from the peaks in (a) with composition for all samples investigated. See also Figure S8 (Supporting Information). 2810



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susceptibilities (Figure S8 in the Supporting Information) show that these compounds appear to obey the Curie−Weiss law between 200 and 300 K with effective moments of 5.4(1)μB consistent with a mixture of Mn2+ and Mn3+ ions. The Weiss constants are positive: 17(1) K for x = 0 increasing fairly smoothly to 43(1) K for x = 1 (Supporting Information, Figure S8c), showing that the net interactions between moments are ferromagnetic and these become increasingly important with increasing x, consistent with the change in magnetic structures described below. The increase in Néel temperature with x as Se is substituted for S is not monotonic (Figure 15b). Instead, TN initially decreases with x reaching a minimum at x = 0.1 and 0.125 before increasing to reach its maximum value in Sr2MnO2Cu1.5Se2. The 5 K magnetization isotherms (Supporting Information, Figure S9) show that, like Sr2MnO2Cu1.5S2,10 the other members of the series show an upturn in the measured moment in the high field region of the isotherm. The analogous compounds Sr2MnO2Cu3.5S3 and Sr2MnO2Cu5.5S4 with thicker copper sulfide layers have ferromagnetic MnO2 sheets which are coupled antiferromagnetically in zero applied magnetic field but behave as metamagnets entering the fully ferromagnetic regime well below 5 T.10 The behavior of the Sr2MnO2Cu1.5(S1−xSex)2 series suggests that at higher applied fields these compounds too might be driven into the fully ferromagnetic regime. Long Range Magnetic Ordering in Sr2MnO2Cu1.5S2 and Sr2MnO2Cu1.5Se2. Low temperature NPD measurements revealed quite different sets of magnetic Bragg reflections for the end members Sr2MnO2Cu1.5S2 and Sr2MnO2Cu1.5Se2. The selenide end member (Figure 16) adopts the A-type magnetic structure in which Mn moments of 4.1(1) μB directed along the c direction are coupled ferromagnetically within each MnO2 plane and the planes are coupled antiferromagnetically like in Sr2MnO2Cu3.5S3 and Sr2MnO2Cu5.5S4.10 However, the sulfide end member exhibits a much more complex magnetic structure, and analysis of the magnetic scattering using magnetic unit cells1 which are an expansion of the low temperature orthorhombic structural cell in Ibam showed that the magnetic scattering could be accounted for satisfactorily using the CEtype magnetic ordering scheme described by Goodenough26 and first observed in the La1−xCaxMnO3 system27 with zigzag chains of ferromagnetically coupled Mn ions coupled antiferromagnetically as shown in Figure 17. The magnetic unit cell is a a × 2b × c expansion of the orthorhombic structural cell. This arrangement of moments can also be described by two interpenetrating lattices of manganese ions, one in which moments are aligned ferromagnetically along the a axis and antiferromagnetically along the b axis, and the other in which moments are aligned antiferromagnetically along both a and b. In the classic charge-ordered CE-type structure one set of magnetic Bragg reflections arises from the ions of charge n + δ and the remainder from ions of charge n − δ because these charges are ordered according to the two sublattices described above.27 Refinement of independent moments on the two sublattices against D2B data produced values of 3.85(8) μB and 4.05(5) μB, which are not significantly different from one another at the 2σ level. This suggests that if charge order is present in this compound that it is very far from complete; alternatively, the dx2−y2 electrons are delocalized and do not contribute to the localized moments. Several possible alternative models describing the magnetic ordering were investigated using the SARAh Refine package29 interfaced with

Figure 16. Rietveld refinements against D2B data at 5 K on (a) Sr2MnO2Cu1.5S2 and (b) Sr2MnO2Cu1.5Se2. Insets show the low angle data gathered above and below the Neél temperatures in each case. The sample of Sr2MnO2Cu1.5S2 used was synthesized with 2 mol of Cu per Mn, yielding a corresponding amount of excess Cu in the product; tick marks and % by mass are as follows. (a) From bottom: Sr2MnO2Cu1.48(2)S2 (nuclear) (88%), Cu (8%), V (sample container), SrS (2.5%), MnO (nuclear) (0.7%), MnO (magnetic), 15 Sr4Mn3O7.5Cu2S228 (0.7%), and Sr2MnO2Cu1.5S2 (magnetic). (b) From bottom: Sr2MnO2Cu1.5Se2 (nuclear) (99.8%), Sr2MnO2Cu1.5Se2 (magnetic), MnO (nuclear) (0.2%), and MnO (magnetic).

Figure 17. Alignment of moments in the CE-type magnetic phase of Sr2MnO2Cu1.5S2. The structural unit cell resulting from the Cu/ vacancy order is shown by the solid line, and the magnetic cell exhibits doubling of this cell as indicated by the dashed line. The ferromagnetic zigzag chains of nearest neighbor moments are indicated.

GSAS; however, none of these alternative models were successful in accounting satisfactorily for the magnetic scattering compared with the CE-type model. To summarize: the two end members of the solid solution Sr2MnO2Cu1.5(S1−xSex)2 adopt significantly different crystal structures and magnetic structures at low temperatures: Sr2MnO2Cu1.5S2 exhibits long-range structural order between Cu+ ions and tetrahedral vacancies, which results in a quadrupling of the unit cell volume. The moments on the Mn ions order with the CE-type magnetic structure which has a unit cell with twice the volume of the low temperature structural unit cell. Sr2MnO2Cu1.5Se2 in contrast shows no 2811



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probably a slight overestimate of the axial field strength given that sulfide is a weaker field ligand than oxide. This degree of anisotropy in the ligand field strength is similar to that in LaSrMnO4 (all Mn3+) where the two Mn−O distances parallel to c are 2.267 Å at room temperature compared with the four basal Mn−O distances of 1.893 Å23 due to the Jahn−Teller distortion of the Mn3+ ions. In contrast, in La0.5Sr1.5MnO423 the distortion of the MnO6 polyhedra is relatively small: (Mn− O)axial/(Mn−O)equatorial = 1.04 so we estimate that the axial ligand field strength is about 80% of the equatorial field strength. The axially distended MnO4S6 environment in Sr2MnO2Cu1.5S2 means that for a Mn3+ ion the configuration should be dxy1, dxz1, dyz1, dz21 with the empty dx2−y2 orbital located much higher in energy.11 For a Mn2+ ion each orbital will be singly occupied, and the situation is as in the cartoon in Figure 18. So the oxide−sulfide ordering in Sr2MnO2Cu1.5S2

long-range ordering of Cu+ and vacancies, and this compound has a magnetic unit cell equal in volume to the structural unit cell with ferromagnetic MnO2 sheets. The low temperature magnetic structures confirm the importance of ferromagnetic interactions between Mn centers which are suggested by the positive Weiss temperatures. Mn is in a highly distended MnO4S2 octahedral environment which contains corner-linked MnO4 squares. Ferromagnetism in these mixed-valent Mn2+/3+ systems may be understood from two extreme viewpoints: if the moments are localized, then given that the Mn environment approaches a square plane, the dominant σ-type superexchange interactions via dx2−y2 orbitals on nearest neighbor Mn ions should be ferromagnetic if there is checkerboard charge order between Mn2+ and Mn3+ (Mn2+ (d5) has a half-filled dx2−y2 while Mn3+ (d4) has an empty dx2−y2). The other superexchange interactions between nearest neighbors via the other d orbitals should be antiferromagnetic. If the dx2−y2 electrons are itinerant and there is no charge disproportionation of Mn, then ferromagnetism is also expected. These extreme views can both account for the Atype magnetic structure with ferromagnetic MnO2 sheets. The CE-type magnetic structure is observed in many Mn3+/4+ manganites when the Mn3+:Mn4+ ratio is close to 1:1. Of these the closest structural analogue to Sr2MnO2Cu1.5S2 is the n = 1 Ruddlesden−Popper phase La0.5Sr1.5MnO4 with a single layer of vertex-linked MnO6 octahedra.30 Perovskite-type phases such as Nd 0.5 Sr 0.5 MnO 3 ,31 La 1−xCa x MnO 3 , and Pr1−xCaxMnO332,33 are also well characterized materials in which the CE-type magnetic structure is observed and the n = 2 Ruddlesden−Popper type compound LaSr2Mn2O7 is close to this regime.34,35 The adoption of the CE-type magnetic structure in the pure oxide manganites is explained by both checkerboard-type charge order of Mn ions (conveniently described as Mn3+ and Mn4+, but several experimental and theoretical works suggest that the difference in electron count is about 0.1)24,36,37 and in-plane orientational ordering of the first order Jahn−Teller distortion in the Mn3+ octahedra (so-called orbital ordering). In La0.5Sr1.5MnO4 the charge ordering and orbital ordering is evident below a transition at about 220 K, and several experiments have probed the two types of order directly.38−41 In Sr2MnO2Cu1.5S2 the only long-range structural order apparent in bulk diffraction measurements arises from Cu/vacancy order. These measurements also show that the oxide ellipsoid is elongated along the Mn−O bonds, and this could signal an electronically driven structural ordering within the MnO2 sheets. Electron diffraction measurements show extremely weak reflections at 100 K in the [001] zone (Figure 4) which are similar to those arising from the effect on the crystal structure of the charge and orbital ordering transition in La0.5Sr1.5MnO4.24 However, we do not have unequivocal evidence for charge order in Sr2MnO2Cu1.5S2. The MnO4S2 polyhedron in Sr2MnO2Cu1.5S2 is extremely distended with inplane Mn−O distances of 2.00608(5) Å and axial Mn−S distances of 2.9200(9) Å. The difference between the Mn−O and Mn−S distances is approximately double that which could be attributed to the difference of about 0.4 Å in radii of oxide and sulfide,42 and so the ligand field is highly anisotropic. If we use the estimate of Demazeau et al.43 that the ligand field strength falls off roughly as r−5 where r is the internuclear distance, then making allowance for the difference in radii of oxide and sulfide produces a ratio of the axial and equatorial ligand field strengths of about [(r(Mn−S)−0.4)/r(Mn−O)]−5 = [(2.92−0.4)/2.01]−5 ∼ 0.3 in Sr2MnO2Cu1.5S2. This is

Figure 18. Cartoons showing the d-orbital manifold appropriate to the distended octahedral coordination of Mn.

produces an anisotropic ligand field which mimics the effect of a Jahn−Teller distortion parallel to the crystallographic c axis. But the system contains mixed-valent Mn and is not purely a Mn3+ system like LaSrMnO4. The extreme anisotropy of the MnO4S2 polyhedron apparently precludes the orbital ordering observed in La0.5Sr1.5MnO4 which, along with charge ordering, is thought to be required for the CE magnetic structure to be adopted. Hence the precise origin of the CE-type magnetic ordering in Sr2MnO2Cu1.5S2 requires further examination. The sulfides Sr2MnO2Cu2m−0.5Sm+1 (m = 1, 2, 3) are bulk semiconductors when measured in the form of sintered powders,10 so there is no evidence from our data that the dx2−y2 electrons are itinerant. The behavior of the lattice parameters at low temperatures is quite different for Sr2MnO2Cu1.5S2 and Sr2MnO2Cu1.5Se2 as shown in Figure 9. In the oxyselenide with ferromagnetic sheets there is an expansion of the basal lattice parameter a below the Néel temperature (53 K) which we ascribe to the effect of exchange striction, and this is compensated by a decrease in the lattice parameter c perpendicular to the layers. In contrast, the sulfide, in which the layers exhibit net antiferromagnetism, exhibits a plateau in the basal lattice parameters below about 40 K and then a decrease in the basal lattice parameters below TN (28 K), which we again ascribe to the effect of exchange striction and a compensating increase in c. Evolution of Long-Range Structural and Magnetic Order in Sr2MnO2Cu1.5(S1−xSex)2. Low temperature NPD and XRPD measurements were used to probe the transition from the structural and magnetic behaviors of the oxide−sulfide Sr2MnO2Cu1.5S2 to the different behaviors of the oxide− selenide Sr2MnO2Cu1.5Se2 in order to investigate whether the different magnetic structures are dictated by the effect on the Mn−Mn interactions of Cu/vacancy ordering. Measurements 2812



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Refinements against the data collected on samples with x = 0.075, 0.10, 0.125, and 0.15 all resulted in satisfactory fits against the D2B data when the two magnetic phases were included (Supporting Information, Figure S12). In performing these refinements we made the assumption that all the Mn ions were participating in long-range magnetic order. The refined magnetic phase fractions for all the compositions measured are plotted in Figure 20, along with the Neél temperatures. For x =

at 2 K on POLARIS or 5 K on D2B showed that for samples of Sr2MnO2Cu1.5(S1−xSex)2 with x ≥ 0.25 the behavior resembled that of the selenide end member: the A-type magnetic structure was adopted with a mean ordered moment per Mn ion of 4.2(1) μB, and there was also no evidence for long-range Cu/ vacancy ordering from X-ray or neutron diffraction. The behavior of Sr2MnO2Cu1.5Se2 persists well into the sulfide-rich end of the solid solution. In contrast, at the sulfide-rich end of the phase diagram, the magnetic and nuclear Bragg peaks from Sr2MnO2Cu1.5(S0.975Se0.025)2 (x = 0.025) (Supporting Information, Figure S10) and Sr2MnO2Cu1.5(S0.95Se0.05)2 (x = 0.05) at 5 K required only the Cu/vacancy ordered model and the CEtype magnetic structure found for Sr2MnO2Cu1.5S2. These results show that the transition in the crystal and magnetic structural behavior occurs in the range 0.05 ≤ x ≤ 0.25 which is the region where TN reaches a minimum (Figure 15b). Inspection of the data and preliminary Rietveld refinements for compositions in the range 0.075 ≤ x ≤ 0.25 showed that both magnetic phases were required to account for the low temperature NPD data. Refinements were performed with the magnetic moments and magnetic profile parameters fixed at the values obtained in the refinements against the x = 0.025 (CEtype magnetic structure) and x = 0.25 (A-type magnetic structure) members. For x = 0.075 the magnetic scattering cannot be fully accounted for by the CE-type magnetic structure (Figure 19). There is an additional peak to the low

Figure 20. Refined magnetic phase fractions (by volume) and the Néel temperature as a function of x for Sr2MnO2Cu1.5(S1−xSex)2.

0.125 roughly equal numbers of Mn ions are participating in each structure, while for x = 0.15 the A-type magnetic structure is dominant. Nuclear superstructure reflections arising from Cu/vacancy ordering were also evident in all these patterns. The D2B data show that these nuclear superstructure reflections are somewhat broader than the reflections arising from the substructure (i.e., the ambient temperature model). The two samples with x = 0.125 and x = 0.15, between which there is a rapid change in the ratio of the two magnetic structures, were analyzed using the X-ray powder diffractometer ID31 which has an extremely high resolution across the entire d-spacing range. The ID31 data revealed that the superstructure reflections arising from the Cu/vacancy order in these samples were 4−6 times broader than the Bragg peaks arising from the subcell (i.e., the room temperature structural model). The integrated intensities of the superstructure reflections were comparable with those expected for complete Cu/vacancy order. In order to interpret the data we adopted the strategy of Ibberson et al. in the analysis of the electroceramic barium zinc tantalate (BZT) doped with strontium gallate.44 The ID31 data were divided into two histogramsone incorporating all the reflections from the substructure and a second incorporating just the regions of the pattern containing the broadened superstructure reflections. A single structural model in Ibam was refined against the two data sets with the Cu occupancy of the largely vacant 4b site allowed to increase at the expense equally of the occupancies of the 4a and 8f sites. Different profile functions, both of the Stephens type45 incorporating anisotropic strain broadening terms were used for each histogram. The refinement results are shown in Figure 21 for the case of x = 0.15 which has almost exclusively the A-type magnetic structure. The refined site occupancies of the Cu sites were 0.13(3) (4b) and 0.96(1) (8f and 4a) (i.e., there is almost

Figure 19. Refinement of Sr2MnO2Cu1.5(S0.925Se0.075) with exclusively the CE-type magnetic phase (lower) and with about 10% of the magnetic scattering from the A-type magnetic phase (upper) (red points are data from D2B, green line is the fit). The locations of the intense (100) and (102) magnetic reflections are indicated (↓), and the inset shows the increasing contribution from the A-type magnetic phase as the selenide content increases.

angle side of the 211 nuclear reflection which was not seen in the data collected for x = 0.025 (Supporting Information, Figure S10) and 0.05 (Supporting Information, Figure S11), which can be modeled by including an additional A-type magnetic phase in the structure. For the composition x = 0.125 the A-type magnetic structure of the selenide end member was clearly evident, but the refined magnetic moment of 3.2 μB obtained by assuming that this was the only magnetic phase present was much reduced relative to the values obtained for the more selenide-rich compositions, and further magnetic reflections were evident which were accounted for using the CE-type magnetic structure. 2813



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tetragonal structural model in I4/mmm did not account for broad features which were observed at d-spacings corresponding to the superstructure reflections arising from Cu/vacancy ordering, and the data were again modeled most satisfactorily (Figure 22) using a single Ibam model with full Cu/vacancy

Figure 21. Rietveld refinement against ID31 data for Sr2MnO2Cu1.5(S1−xSex) with x = 0.15. The two small insets shown over similar 2θ ranges show the contrast in the widths of peaks due to the substructure (upper inset) and the width of those which are solely due to the superstructure which arises through Cu/vacancy order (lower inset). The two sets of reflections were modeled using a single structural model in Ibam with almost complete Cu/vacancy order, but with different peak profile parameters as described in the text and elsewhere.44 See also Figure S13 (Supporting Information).

Figure 22. Rietveld refinement of the crystal and magnetic structures of Sr2MnO2Cu1.5(S1−xSex) with x = 0.15 against OSIRIS data at 2 K. The inset shows the main superstructure reflections arising from Cu/ vacancy order which are broad compared with the substructure reflections or the reflections from the single A-type magnetic structure (the 004 magnetic reflection is the narrow reflection in the inset) and were modeled using a separate profile function. The inset shows that the intensity of these superstructure reflections is consistent with almost complete Cu/vacancy order, although their widths are not isotropic as our model assumes. The reflection markers are, from bottom, the single Ibam structural phase, the A-type magnetic phase, a SrS1−xSex impurity, MnO(nuclear) and MnO(magnetic) impurities. See also Figure S13 (Supporting Information).

complete Cu/vacancy order) and the intensities of the superstructure reflections were well modeled. Quantitatively similar results were obtained for x = 0.125 (Supporting Information, Figure S13). The interpretation of this result is that in compounds in which the A-type magnetic structure accounts for almost all the magnetic scattering, there is longrange order of Cu ions and vacancies on the long length scale probed by diffraction methods and there is no evidence for phase separation into more than one crystallographic phase. However, the coherence length for the Cu/vacancy ordering is much smaller than that of the underlying substructure in these selenide-containing phases. In Sr2MnO2Cu1.5S2 the superstructure and substructure reflections have similar widths as is evident from the inset to Figure 5 above, while in the sample with x = 0.15 (Figure 21) the full width at half-maximum of the 112 (superstructure) reflection was 6.0(5) times that of the 211 (substructure) reflection. Making the assumption that the substructure reflections may be used as a highly crystalline standard, use of the Scherrer formula allows us to estimate a coherence length for Cu/vacancy order of about 50 nm in the sample with x = 0.15. These results show that for x = 0.125 the appearance of two coexisting magnetic structures involving approximately equal numbers of Mn ions in the sample is not a consequence of the existence of distinct domains in which there is either complete Cu/vacancy order or Cu/vacancy disorder; instead there is almost complete Cu/vacancy order throughout the sample, although at these values of x the coherence length of the Cu/ vacancy ordering is clearly diminishing. Cu/vacancy order also persists into the regime where the A-type magnetic structure dominates (x = 0.15). Data for a sample with x = 0.175 were collected at 2 K on OSIRIS. This instrument offers significantly higher resolution and signal-to-noise ratio than D2B at long d-spacings. In this case the magnetic scattering is described by the A-type structure alone with no measurable scattering due to the CEtype structure. The refined magnetic moment of 4.58(5) μB is larger than those obtained in the refinements against data collected on both POLARIS and D2B, and this larger moment is likely to be due to correlation with sample absorption. The

order and with the broad reflections modeled with a different profile function to those of the substructure reflections as described above for the treatment of the ID31 data for the x = 0.125 and 0.15 members. Similar behavior was observed for the composition with x = 0.2 measured on OSIRIS. For samples with x ≥ 0.25 no superstructure reflections could be reliably distinguished from the background using bulk diffraction measurements, and there is only evidence from the electron diffraction measurements described above that short-range Cu/ vacancy ordering may occur in Sr2MnO2Cu1.5Se2. These results show that the evolution of the magnetic structure and the evolution of the degree of Cu/vacancy order are not coupled in a trivial way. Full ordering of Cu ions and vacancies (as in Sr2MnO2Cu1.5S2 (x = 0) with the CE-type magnetic structure) persists on the length scale probed by diffraction methods up to about x = 0.2, but once x has reached 0.175 the magnetic scattering can be fully accounted for by the A-type magnetic model. Table S7 (Supporting Information) summarizes the results of low temperature Rietveld refinements against data collected on these Sr2MnO2Cu1.5(S1−xSex)2 samples. Summary and Comparison with Oxide Manganites. The behavior of the low temperature crystal and magnetic structures of the solid solution Sr2MnO2Cu1.5(S1−xSex)2 may be summarized as follows. First the length scale for ordering of Cu+ ions and tetrahedral vacancies in the chalcogenide layers decreases as sulfide is substituted by selenide. For x = 0 the superstructure reflections arising from the Cu/vacancy ordering give rise to Bragg peaks which are similar in width to those arising from the substructure (Figure 5). However, the detailed analysis of the samples with 0.125 < x < 0.2 on ID31 and OSIRIS shows that, while there remains long-range order of Cu 2814



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the CE-type magnetic structure of Nd0.5Sr0.5MnO3 and the Atype magnetic structure of Pr0.5 Sr0.5MnO3. Theoretical investigations show that the CE-type magnetic structure should be stable with respect to alternatives in mixed-valent Mn3+/4+ systems when the formal Mn oxidation state in the MnO6 octahedra is +3.5 but that alternatives compete strongly at other doping levels47 and to an extent that depends upon the terms used in the calculations.48 The sensitivity of the perovskite systems to composition is shown by the fact that hole doping of CE-type Nd0.5Sr0.5MnO3 leads to the A-type magnetic structure in Nd0.45Sr0.55MnO3.31 In principle the n = 1 Ruddlesden− Popper type system La0.5Sr1.5MnO4 is structurally the most similar to the oxide sulfide systems described in this paper, but as described above the ligand field around Mn is much more anisotropic in the oxide chalcogenides, and the oxide chalcogenides are Mn2+/3+ systems rather than Mn3+/4+ systems and have the added complication of the structural order in the chalcogenide layers which may influence the electronic behavior of the manganite layers. Further experiments to probe the physics of Sr2MnO2Cu1.5S2 directly are in progress.

ions and vacancies, the coherence length has decreased and the superstructure reflections are 4−6 times the width of the superstructure reflections and the two classes of reflection cannot be modeled satisfactorily using a single profile function in the Rietveld refinement (Figures 21 and 22). For x ≥ 0.25, reflections due to Cu/vacancy order are not evident in bulk diffraction measurements. For the x = 1 member, Sr2MnO2Cu1.5Se2, the only evidence for structural order is in the electron diffractograms (Figure 14). Second, there are two magnetic structures which compete at the sulfide-rich end of the phase diagram. The CE-type model of ferromagnetic zigzag chains accounts for the magnetic scattering in the sulfide end member (x = 0). An A-type magnetic structure with ferromagnetic Mn planes develops and coexists with the CE-type phase until x ∼ 0.15 and for x ≥ 0.0175 the A-type structure accounts for all the magnetic scattering. The relationship between the structural order in the chalcogenide layers and the magnetic structure is complex. The refinements against ID31 data of the two phases with x = 0.125 and x = 0.15 show that the extent of structural ordering is very similar to that in Sr2MnO2Cu1.5S2 and the domains of Cu/ vacancy ordering are still large enough to produce fairly sharp Bragg reflections; that is, there is still “long range” Cu/vacancy order. Yet these two compositions are in the region where the ratios of the two magnetic structures are changing very rapidly. The adoption of the A-type magnetic structure as the ground state may instead be driven by both the increase in the mean Mn−O distance as the Se/S ratio increases and/or by the presence of chemical disorder in the chalcogenide sublattice which will introduce local distortions of the MnO2 sheets. It is also reasonable to assume that the length scale of the Cu/ vacancy ordering will be decreased by the introduction of sulfide/selenide disorder, and increasing the Cu−Cu distance should lower the thermodynamic tendency for ordering. In the region where there is competition between the magnetic structures it is possible that the different magnetic structures could be propagated by the effect on the manganese oxide layers of slight compositional inhomogeneity on the chalcogenide sublattice and/or by modulations in the Cu content. The origin of the CE-type magnetic structure in the sulfiderich end of the phase diagram and the competition with the Atype magnetic structure require further investigation. The possibility of local or even long-range charge and/or orbital ordering in these systems and the relationship of these to the modulation of the structure by the Cu/vacancy order in the chalcogenide layers also remain to be elucidated. The literature on Mn3+/4+ oxide manganites with mean oxidation states around Mn3.5+ reveal complex physics associated with charge ordering, orbital ordering, and competition between magnetic ordering schemes, most notably between the CE-type and Atype magnetic ordering. The bilayer Mn3+/4+ manganite LaSr2Mn2O7 (n = 2 Ruddlesden−Popper structure type) exhibits charge and orbital ordering compatible with the CEtype ordering, but ultimately at low temperatures the A-type antiferromagnetic structure is adopted34 showing that there are subtle competitive factors in these mixed valent systems. A more recent examination of LaSr2Mn2O7 shows that the competition between the CE-type ordering and the A-type ordering in this bilayer system is very finely balanced and very tiny doping levels are sufficient to tip the system one way or the other.35 In the perovskites Pr0.5−xNdxSr0.5MnO3 there is evidence from EPR investigations46 for competition between

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CONCLUSIONS

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ASSOCIATED CONTENT

The solid solution Sr2MnO2Cu1.5(S1−xSex)2 offers mixed-valent Mn2+/3+ systems to complement Mn 3+/4+ oxides. The consequence of the mixed valence is that magnetic structures are adopted in which there are strong ferromagnetic interactions between Mn near neighbors within the MnO2 planes. In Sr2MnO2Cu1.5S2 the CE-type magnetic structure is adopted which is similar to that exhibited by many perovskite and Ruddlesden−Popper manganites with a Mn3+:Mn4+ ratio close to 1:1. This magnetic structure is adopted in the absence of obvious charge-ordering and with the intrinsic anisotropy of the crystal structure apparently precluding the “eg” orbital ordering found in Mn3+/4+ oxides. Sr2MnO2Cu1.5Se2 adopts a simpler magnetic structure with purely ferromagnetic MnO2 sheets as found for Sr2MnO2Cu3.5S3 and Sr2MnO2Cu5.5S4.10 The vacancies in the tetrahedral sites in the chalcogenide layers are coupled with the mixed valence of Mn and are disordered at room temperature. Long range Cu/vacancy order is found below about 240 K for sulfide-rich compositions, and the coherence length for this order decreases rapidly as the selenide content increases. In the solid solution there is competition between the two magnetic structures adopted by the end member compounds, and some samples exhibit regions with both magnetic structures. In the solid solution disorder and compositional inhomogeneity on the chalcogenide sublattice may be responsible for the decrease in the size of the regions of the structure with full Cu/vacancy order and may also allow the A-type magnetic structure to compete well against the CE-type structure. The relationship between the many related Mn3+/4+ oxide manganite systems with fairly regular MnO6 octahedra and the Mn2+/3+ systems reported here with highly distended MnO4S2 octahedra requires further examination in order to investigate whether both sets of systems can be treated by the same theoretical framework.

S Supporting Information *

Further experimental details, additional tables of refined parameters, diffractograms showing further Rietveld refinements, and further TEM data are available. This information is available free of charge via the Internet at http://pubs.acs.org/. 2815



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(25) Ishikawa, T.; Miyatani, S. J. Phys. Soc. Jpn. 1977, 42, 159. (26) Goodenough, J. B. Phys. Rev. 1955, 100, 564. (27) Wollan, E. O.; Koehler, W. C. Phys. Rev. 1955, 100, 545. (28) Hyett, G.; Barrier, N.; Clarke, S. J.; Hadermann, J. J. Am. Chem. Soc. 2007, 129, 11192. (29) Wills, A. S. Physica B 2000, 276−278, 680. (30) Sternlieb, B. J.; Hill, J. P.; Wildgruber, U. C.; Luke, G. M.; Nachumi, B.; Moritomo, Y.; Tokura, Y. Phys. Rev. Lett. 1996, 76, 2169. (31) Kawano, H.; Kajimoto, R.; Yoshizawa, H.; Tomioka, Y.; Kuwahara, H.; Tokura, Y. Phys. Rev. Lett. 1997, 78, 4253. (32) Jirák, Z.; Krupicka, S.; Simsa, Z.; Dlouha, M.; Vratislav, S. J. Magn. Magn. Mater. 1985, 53, 153. (33) Tomioka, Y.; Asamitsu, A.; Kuwahara, H.; Moritomo, Y.; Tokura, Y. Phys. Rev. B 1996, 53, R1689. (34) Argyriou, D. N.; Bordallo, H. N.; Campbell, B. J.; Cheetham, A. K.; Cox, D. E.; Gardner, J. S.; Hanif, K.; dos Santos, A.; Strouse, G. F. Phys. Rev. B 2000, 61, 15269. (35) Lee, J.-S.; Kao, C. C.; Nelson, C. S.; Jang, H.; Ko, K.-T.; Kim, S. B.; Choi, Y. J.; Cheong, S.-W.; Smadici, S.; Abbamonte, P.; Park, J.-H. Phys. Rev. Lett. 2011, 107, 037206. (36) Herrero-Martin, J.; Blasco, J.; Garcia, J.; Subias, G.; Mazzoli, C. Phys. Rev. B 2011, 83, 184101. (37) Ma, C.; Yang, H. X.; Zeng, L. J.; Li, Z. A.; Zhang, Y.; Qin, Y. B.; Li, J. Q. J. Phys.: Condens. Matter 2009, 21, 045601. (38) Wilkins, S. B.; Spencer, P. D.; Hatton, P. D.; Collins, S. P.; Roper, M. D.; Prabhakaran, D.; Boothroyd, A. T. Phys. Rev. Lett. 2003, 91, 167205. (39) Dhesi, S. S.; Mirone, A.; De Nadaï, C.; Ohresser, P.; Bencok, P.; Brookes, N. B.; Reutler, P.; Revcolevschi, A.; Tagliaferri, A.; Toulemonde, O.; van der Laan, G. Phys. Rev. Lett. 2004, 92, 056403. (40) Huang, D. J.; Wu, W. B.; Guo, G. Y.; Lin, H.-J.; Hou, T. Y.; Chang, C. F.; Chen, C. T.; Fujimori, A.; Kimura, T.; Huang, H. B.; Tanaka, A.; Jo, T. Phys. Rev. Lett. 2004, 92, 087202. (41) Staub, U.; Scagnoli, V.; Mulders, A. M.; Katsumata, K.; Honda, Z.; Grimmer, H.; Horisberger, M.; Tonnerre, J. M. Phys. Rev. B 2005, 71, 214421. (42) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751. (43) Demazeau, G.; Buffat, B.; Pouchard, M.; Hagenmuller, P. J. Solid State Chem. 1984, 54, 389. (44) Ibberson, R. M.; Moussa, S. M.; Rosseinsky, M. J.; Fitch, A. N.; Iddles, D.; Price, T. J. Am. Ceram. Soc. 2006, 89, 1827. (45) Stephens, P. J. Appl. Crystallogr. 1999, 32, 281. (46) Ling, L. S.; Tan, S.; Pi, L.; Y. H. Zhang, Y. H. Europhys. Lett. 2007, 79, 47008. (47) van den Brink, J.; Khaliullin, G.; Khomskii, D. Phys. Rev. Lett. 1999, 83, 5118. (48) Bała, J.; Horsch, P.; Mack, F. Phys. Rev. B 2004, 69, 094415.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +44 1865 272690. Tel: +44 1865 272600. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the UK Engineering and Physical Sciences Research Council for funding under Grant EP/E025447 and for the award of studentships to P.A., C.F.S., G.H., and O.J.R. and the award of a PhD+ award to P.A. We are grateful to Dr. R. I. Smith, Dr. K. Knight, Dr. R. M. Ibberson, and Dr. M. Telling (ISIS facility) and Dr. A. Hewat and Dr. E. Suard (ILL, Grenoble) for assistance with the NPD measurements; Dr. A. N. Fitch and Dr. C. Curfs are thanked for assistance with measurements on ID31. The TEM experiments were performed under the financial support from the European Union within the Framework 6 program under a contract for an Integrated Infrastructure Initiative, Reference 026019 ESTEEM.

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REFERENCES

(1) Clarke, S. J.; Adamson, P.; Herkelrath, S. J. C.; Rutt, O. J.; Parker, D. R.; Pitcher, M. J.; Smura, C. F. Inorg. Chem. 2008, 47, 8473. (2) Zhu, W. J.; Hor, P. H. J. Solid State Chem. 1997, 130, 319. (3) Zhu, W. J.; Hor, P. H.; Jacobson, A. J.; Crisci, G.; Albright, T. A.; Wang, S.-H.; Vogt, T. J. Am. Chem. Soc. 1997, 119, 12398. (4) Otzschi, K.; Ogino, H.; Shimoyama, J.; Kishio, K. J. Low Temp. Phys. 1999, 117, 729. (5) Brechtel, E.; Cordier, G.; Schaefer, H. Z. Naturforsch. B: Anorg. Chem., Org. Chem. 1979, 34, 777. (6) Vajenine, G. V.; Hoffmann, R. Inorg. Chem. 1996, 35, 451. (7) Ueda, K.; Hirose, S.; Kawazoe, H.; Hosono, H. Chem. Mater. 2001, 13, 1880. (8) Liu, M.-L.; Wu, L.-B.; Huang, F.-Q.; Chen, L.-D.; Chen, I.-W. J. Appl. Phys. 2007, 102, 116108. (9) Scanlon, D. O.; Watson, G. W. Chem. Mater. 2009, 21, 5435. (10) Gál, Z. A.; Rutt, O. J.; Smura, C. F.; Overton, T. P.; Barrier, N.; Clarke, S. J.; Hadermann, J. J. Am. Chem. Soc. 2006, 128, 8530. (11) Smura, C. F.; Parker, D. R.; Zbiri, M.; Johnson, M. R.; Gál, Z. A.; Clarke, S. J. J. Am. Chem. Soc. 2011, 133, 2691. (12) Barrier, N.; Clarke, S. J. Chem. Commun. 2003, 164. (13) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS); Los Alamos National Laboratory Report LAUR 86− 748; Los Alamos National Laboratory: Los Alamos, NM, U.S.A., 2004. (14) Toby, B. H. J. Appl. Crystallogr. 2001, 34, 210. (15) Goodwin, A. L.; Tucker, M. G.; Dove, M. T.; Keen, D. A. Phys. Rev. Lett. 2006, 96, 047209. (16) Kim, J. S.; Lee, J. Y.; Swinnea, J. S.; Steinfink, H.; Reiff, W. M.; Lightfoot, P.; Pei, S.; Jorgensen, J. D. J. Solid State Chem. 1991, 90, 331. (17) Bronger, W.; Hendriks, U. Rev. Chim. Miner. 1980, 17, 555. (18) Bronger, W.; Boettcher, P. Z. Anorg. Allg. Chem. 1972, 390, 1. (19) Zabel, M.; Range, K. J. Rev. Chim. Miner. 1980, 17, 561. (20) Johrendt, D.; Hosono, H.; Hoffmann, R.-D.; Pöttgen, R. Z. Kristallogr. 2011, 226, 435. (21) González, S.; Perez-Mato, J. M.; Elcoro, L.; García, A. Phys. Rev. B 2011, 84, 184106. (22) Herkelrath, S. J. C.; Saratovsky, I.; Hadermann, J.; Clarke, S. J. J. Am. Chem. Soc. 2008, 130, 14426. (23) Senff, D.; Reutler, P.; Braden, M.; Friedt, O.; Bruns, D.; Cousson, A.; Bourée, F.; Merz, M.; Büchner, B.; Revcolevschi, A. Phys. Rev. B 2005, 71, 024425. (24) Zeng, L. J.; Ma, C.; Yang, H. X.; Xiao, R. J.; Li, J. Q.; Jansen, J. Phys. Rev. B 2008, 77, 024107. 2816


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