High-Pressure Synthesis, Crystal Structure, and Magnetic Properties

Feb 22, 2016 - Graduate School of Chemical Sciences and Engineering, Hokkaido University, North 10 West 8, Kita-ku, Sapporo, ... [email protected]...
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High-Pressure Synthesis, Crystal Structure, and Magnetic Properties of Sr2MnO3F: A New Member of Layered Perovskite Oxyfluorides Yu Su,*,†,‡ Yoshihiro Tsujimoto,*,§ Yoshitaka Matsushita,∥ Yahua Yuan,†,‡ Jianfeng He,†,‡ and Kazunari Yamaura†,‡ †

Superconducting Properties Unit, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Graduate School of Chemical Sciences and Engineering, Hokkaido University, North 10 West 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan § Materials Processing Unit and ∥Research Network and Facility Services Division, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ‡

S Supporting Information *

ABSTRACT: We have successfully synthesized Sr2MnO3F, a new layered perovskite oxyfluoride with a n = 1 Ruddlesden−Popper-type structure using a high-pressure, hightemperature method. Structural refinements against synchrotron X-ray diffraction data collected from manganese oxyfluoride demonstrated that it crystallizes in a tetragonal cell with the space group I4/mmm, in which the Mn cation is located at the octahedral center position. This is in stark contrast to the related oxyhalides that have squarepyramidal coordination such as Sr2MO3X (M = Fe, Co, Ni; X = F, Cl) and Sr2MnO3Cl. There was no evidence of O/F site order, but close inspection of the anion environment centered at the Mn cation on the basis of bond-valence-sum calculation suggested preferential occupation of the apical sites by the F ion with one oxide ion in a random manner. Magnetic susceptibility and heat capacity measurements revealed an antiferromagnetic ordering at 133 K (=TN), which is much higher than that of the chloride analogue with corrugated MnO2 planes (TN = 80 K). fluoride perovskites are the most widely explored mixed-anion systems, especially since the discovery of the superconducting layered copper oxyfluoride Sr2CuO2F2+δ.14 The low-temperature fluorination used to prepare such systems generally involves a simple F insertion into interstitial sites (such as O vacancy sites), a substitution of F for O, or both.15 For manganese, fluorination of the oxygen-deficient phases Sr2Mn2O5 and Sr2MnGaO5 using XeF2 as a fluorinating agent results in Sr2Mn2O5−xF1+x and Sr2MnGaO4.78F1.22 containing Mn3+/Mn4+ valence states through the filling of the O vacancy sites with F and the partial substitution of F for O, as shown in Figure 1a.16,17 Manganese oxides with the Ruddlesden−Popper (RP)-type layered structure having the general formula (AO)(AMnO3)n, LaSrMnO4 (n = 1) and Ln1.2Sr1.8Mn2O7 (n = 2), accommodate F in the interstitial sites of the (La/Sr)O rock-salt layers, not in the anion sites of the perovskite block layers, giving LaSrMnO 4 F y (y = 1, 1.7, 2) and Ln1.2Sr1.8Mn2O7Fy (y = 1, 2; Ln = La, Pr, Nd, Sm, Eu, Gd),18−21 as shown in Figure 1b. Conversely, the oxygendeficient phase Sr3Mn2O6, which contains pseudosquare and pseudohexagonal tunnels composed of corner-sharing MnO5 square pyramids, exhibits a stepwise fluorination in which the F atoms occupy first the O vacancies at the equatorial sites of the MnO5 square pyramid and then the interstitial sites in the rock-

1. INTRODUCTION Perovskite-based manganese oxides, expressed as AMnO3 (A = alkaline earth, lanthanide metals), have been extensively studied because of their interesting physical properties, such as metal− insulator transition, colossal magnetoresistance, spin-driven ferroelectricity, and charge/orbital ordering.1−6 These phenomena are very sensitive to the covalency between the Mn and oxide ions and the Mn−O−Mn bond angle, which are directly related to the superexchange interaction, charge-transfer energy, and double-exchange interaction in such materials.6−8 A large variety of electronic phases involving strong correlation between spin, charge, and orbital degrees of freedom have been achieved by fine-tuning the chemical compositions at the cation sites in manganese oxides.6,8,9 There have been a great many studies on cation substitution in manganese perovskite compounds; however, the effects of anion substitution on their physical properties have been investigated to a much lesser extent. In general, the stability of the metal−oxygen bond is high compared to that with other anions, which makes it difficult to confine mixed anions to anion sublattices under conventional high-temperature conditions. Nevertheless, much effort has been made to develop kinetic reactions for the preparation of unprecedented mixedanion phases. For example, metal hydride reductions have been used to prepare Srn+1VnO2n+1Hn (n = 1, 2, 3)10 and BaTiO0.6H0.4,11 and reductive ammonolysis reactions have been used to produce NdVO2N12 and EuNbO2N.13 Oxy© XXXX American Chemical Society

Received: December 28, 2015

A

DOI: 10.1021/acs.inorgchem.5b02984 Inorg. Chem. XXXX, XXX, XXX−XXX

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the apical sites with one apical O atom (see Figure 3a,b). Specifically, the Ni cation forms strong covalent bonds with five surrounding O anions and a significantly weaker bond with the halogen anion, leading to the low-spin configuration of S = 1/2. Thus, the Ni-centered coordination can be effectively viewed as a square pyramid rather than an octahedron. In addition to the nickel oxyhalides, Sr2CoO3F has been synthesized under highpressure conditions,26 in contrast to Sr2CoO3Cl, which can be synthesized at ambient pressure.27 These cobalt oxyhalides were found to exhibit O/X-site ordered arrangements and coordination environments similar to those of Sr2NiO3X. So far, several layered oxyhalide compounds expressed as A2BO3X (A = alkaline and alkali-earth metals; B = In, Sc, Nb, Co, Fe, Ni; X = F, Cl) have been reported.25,28−31 To our knowledge, however, the manganese oxyfluoride A2MnO3F has never been obtained. Hagenmuller et al. synthesized Ca2MnO4−yFy by a conventional solid-state reaction, but the fluorine content range in which the homogeneous sample could be obtained was limited to 0 ≤ y ≤ 0.3.32 Furthermore, Greaves et al. attempted F insertion into Sr2MnO3.5+x with a complex O vacancy-ordered structure (space group P21/c), using fluorine gas as a fluorinating agent,22 but this resulted in inhomogeneous phases corresponding to the average chemical composition Sr2MnO3.53F0.39, which consisted of a slightly fluorinated phase maintaining the Sr2MnO3.5+x structure and a highly fluorinated phase with the K2NiF4 structure.23 In the present study, we have demonstrated the successful synthesis of Sr2MnO3F by a high-pressure, high-temperature reaction. The product adopts the K2NiF4-type structure with octahedral coordination around Mn and with O/F anions disordered at the apical sites. An antiferromagnetic phase transition takes place at 133 K., which is the first observation of long-range magnetic order in a perovskite-based manganese oxyfluoride compound.

Figure 1. Crystal structures of (a) Sr2MnGaO4.78F1.22 and (b) La1.2Sr1.8Mn2O7F2. MnO6 and Ga(O,F)6 are blue and red octahedra, respectively. Gray, red, and green spheres represent Sr/La, O, and F atoms, respectively.

salt layers to form Sr3Mn2O6Fy (y = 1, 2, 3) with increasing fluorine content.22 Thus, this fluorination of the manganese oxide results in modification of the coordination environments and valence states of Mn. However, these fluorinated compounds do not exhibit a magnetic phase transition, metallic conductivity, or magnetoresistance. Another useful approach toward mixed-anion compounds is the high-pressure method, by which two new perovskite manganese compounds, i.e., PbMnO2F23 and MnTaO2N,24 have been obtained. Recently, our research group has successfully extended the RP-type layered oxyhalide series using this technique. Sr2NiO3X (X = F, Cl) containing highoxidation-state Ni3+ was synthesized at 6 GPa and 1500 °C (X = F) and 3 GPa and 1300 °C (X = Cl).25 The nickel oxyhalides crystallize in the so-called K2NiF4-type structure (n = 1 RP-type structure) but have highly distorted octahedra around the Ni centers, with the F (Cl) atoms being disordered (ordered) at

Figure 2. SXRD patterns for Sr2MnO3F measured at room temperature. Obtained, calculated and difference are presented by cross marks, upper solid lines and bottom solid lines, respectively. The vertical lines represent the Bragg peak positions. Impurity peaks which were observed in the 2θ region from 10 to 25° were excluded in the refinement. B

DOI: 10.1021/acs.inorgchem.5b02984 Inorg. Chem. XXXX, XXX, XXX−XXX

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2. EXPERIMENTAL SECTION

be assigned by bond-valence consideration. Therefore, we performed Rietveld structural refinements against the SXRD data of Sr2MnO3F to obtain its atomic coordinates. A structure model based on that of Sr2MnO4 was examined as the starting point. Because there is no evidence for O/F site order in the present sample, all of the anion sites are assumed to be occupied by O. All of the site occupancies (g) were allowed to vary during the refinement but remained fully occupied within error. The refinement converged well with reliability factors of Rwp = 2.20% and RB = 4.58%. The isotropic displacement parameter (Biso) for each site results in reasonable values. Details of the refined crystallographic data are given in Table 1.

Synthesis. Sr2MnO3F was prepared by solid-state reaction under high pressure and high temperature. A stoichiometric amount of SrF2 (99.9%, Rare Metallic Co., Ltd.), SrO prepared by heating SrCO3 in flowing O2 gas, and Mn2O3 prepared by heating MnCO3 at 800 °C overnight was thoroughly ground in an agate mortar in an argon-filled glovebox, sealed in a platinum capsule, and then set in a high-pressure cell. This was then heated to temperatures ranging from 1700 to 2000 °C at 6 GPa using a belt-type high-pressure apparatus. After heating for the prerequisite time, the sample was cooled to 1200 °C for 30 min and then quenched to room temperature by switching off the heater before pressure release. Of the reaction conditions we examined, we found that heating at 1800 °C for 45 min gave the best sample quality. Characterization. The quality of the products was assessed by powder X-ray diffraction (XRD) measurements using a PANalytical X’Pert diffractometer equipped with a graphite monochromator and Cu Kα radiation (λ = 1.5418 Å), in 0.036° increments over the range 10° ≤ 2θ ≤ 70°. Synchrotron XRD (SXRD) data were also collected at room temperature using a Debye−Scherrer camera installed on a NIMS beamline (BL15XU) at SPring-8.33 The synchrotron radiation X-rays were monochromatized to a wavelength of 0.65298 Å. The samples were loaded in a glass capillary of 0.1 mm diameter, and the profile data were recorded in 0.003° increments over the range 5° ≤ 2θ ≤ 50°. Rietveld structural refinements were performed against the SXRD data using the RIETAN-FP program.34 The split pseudo-Voigt function, which was formulated by Toraya, was employed as a profile function. The weighting R index (Rwp) and the Bragg R index (RB) are defined as follows: Rwp = [∑iwi(yio − yic)2/∑iwiyio2] and RB = ∑k|Iko − Ik|/∑kIko, where yio and yic are the observed and calculated intensities, wi is the weighting factor, and Iko and Ik are the observed and calculated integrated intensities, respectively. Magnetic measurements were conducted in the T range from 5 to 400 K, using a superconducting quantum interference device magnetometer (Quantum Design). Heat capacity measurements were carried out at zero magnetic field between 2 and 250 K using a relaxation technique with a physical property measurement system (Quantum Design). A hand-pressed pellet of the product was mounted on an aluminum plate with Apiezon N-Grease for better thermal contact.

Table 1. Crystallographic Parameters Refined from SXRD Data Collected from Sr2MnO3F at Room Temperature atom

site

ga

x

y

z

Sr Mn Oeqb Oaqb

4e 2a 4c 4e

1 1 1 1

0 0 0 0

0 0 0.5 0

0.36045(5) 0 0 0.1715(2)

Biso/Å

2

0.843(15) 1.00(3) 0.64(8) 0.68(7)

The space group is I4/mmm (No. 139), a = 3.79010(1) Å, and c = 13.28981(5) Å. R indices are Rwp = 2.20%, Rp = 1.14%, RB = 4.58%, and RF = 2.37%. ag is the site occupancy. bAll of the anion sites were assumed to be O.

Possible distribution patterns of mixed anions in a O/F molar ratio of 3:1 were investigated by bond-valence-sum (BVS) calculation using the refined atomic coordinates.38 Here, we considered two anion distribution patterns for O/F: apical anion disorder or equatorial anion disorder. For the former, the BVS values for Sr, Mn, and Oeq are 1.95, 3.20, and 2.33, respectively. In addition, the calculation of BVS for Oap/F gives an average value of 1.23. These results are consistent with values expected from the chemical formula determined by the structure refinement. In contrast, the latter model, where the apical sites are fully occupied by oxide ions and the equatorial sites by one fluoride and three oxide ions at random, results in a significantly lower value of 1.39 for Oap than that expected from full occupation of the apical sites by oxide ions. Therefore, the apical anion-disordered model better describes the structure of Sr2MnO3F. The crystal structure where a statistical 50:50 Oap/F mixture at the 4e apical site is taken into consideration is illustrated in Figure 3c, with the local coordination environment around the Sr and Mn atoms shown in Figure 4. Coordination Geometry of the Mn Center. Structural refinements reveal that the Mn cation is located at the center position of an octahedron comprising four oxide ions at the equatorial sites and oxide and fluoride ions at the apical sites. As expected from the lattice constants of Sr2MnO3F being similar to those of LaSrMnO4, the Jahn−Teller effect, which is commonly observed in trivalent Mn cations, is manifested in the local structure around the Mn center. The Mn−(Oap/F) and Mn−Oeq bond lengths are 2.274(6) and 1.89506(1) Å, respectively, and thus the ratio of the apical bond to the equatorial bond is 1.20. This value is comparable to the Mn− Oap/Mn−Oeq bond length ratio (1.20) for LaSrMnO4.36 The coordination environment around the Mn center in Sr2MnO3F is unique compared to those in related oxyfluoride compounds. For example, Sr2MO3F (M = Fe, Co, Ni) exhibits a square-pyramidal MO5 that is loosely linked by a fluoride ion, with the apical O/F anions being ordered for M = Fe and disordered for Co and Ni, as shown in Figure 3a,b.25,26,39

3. RESULTS AND DISCUSSION Synthesis and Structure. Figure 2 shows the SXRD profile collected from Sr2MnO3F prepared at 6 GPa and 1800 °C and subsequently slowly cooled to 1200 °C while the pressure was maintained. The main reflections can be readily indexed on a simple body-centered tetragonal cell with the space group I4/mmm, which is common in the related layered oxyfluorides such as Sr2NiO3F25 and Sr2CoO3F26 with n = 1 RP-type structure. Very small and broad peaks are observed, some of which can be characterized as SrF2, SrO, and MnO2 (see Figure S1). No peaks corresponding to Sr2MnO3+x with the space group P21/c are observed. The lattice parameters calculated on Sr2MnO3F are a = 3.79010(1) Å and c = 13.28981(5) Å. The c axis expands significantly compared with those of Sr2MnO4 (a = 3.787 Å and c = 12.496 Å)35 and Sr2MnO3.53F0.39 [a = 3.82069(5) Å and c = 12.6309(4) Å],22 while the a axis remains almost unchanged. In contrast, both of the lattice parameters of the product are comparable to those of the isovalent compound LaSrMnO4 (a = 3.786 Å and c = 13.163 Å)36 with a Jahn−Teller active Mn3+, which strongly suggests that the Mn center in the oxyfluoride phase is also subject to the Jahn−Teller effect. Determination of the F site in the anion lattices is challenging for oxyfluoride materials because X-ray and neutron scattering cannot distinguish between O and F atoms.25,26,37 Nevertheless, the anion environment, especially the F sites, can C

DOI: 10.1021/acs.inorgchem.5b02984 Inorg. Chem. XXXX, XXX, XXX−XXX

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atures required for the syntheses of Sr2CoO3F, Sr2NiO3F, and Sr2MnO3F with disordered O/F sites are 1900, 1500, and 1800 °C, respectively, which are much higher than those for related oxyfluorides synthesized by conventional solid-state methodology (for example, 900 °C for Sr2FeO3F39 and 1050 °C for Sr2ScO3F31). The anion-site ordering patterns have yet to be controlled in Sr2MO3F (M = Co, Ni, Mn), but we recently observed a partial O/F site disorder in Sr2FeO3F single crystals synthesized at 1800 °C and 6 GPa, which is in contrast to the fully anion-ordered Sr2FeO3F prepared by a conventional solidstate reaction.42 The high-pressure effect may be ruled out for the anion-site disordering, because the density of the aniondisordered Sr2FeO3F is lower than that of the ordered one. Magnetic Susceptibility and Heat Capacity. Figure 5 shows the temperature dependence of the magnetic suscept-

Figure 3. Comparison of metal-centered coordination and anionordered patterns in a layered oxyhalide system with the general formula Sr2MO3X (M = transition metal; X = halogen): (a) squarepyramidal coordination with anion disorder; (b) square-pyramidal coordination with anion order; (c) octahedral coordination with anion disorder. These structural features are adopted by Sr2MO3F (M = Co, Ni), Sr2MO3Cl (M = Mn, Co, Ni), and Sr2MO3F (M = Sc, Mn), from left to right.

Figure 4. Local coordination environment around the Sr and Mn atoms in Sr2MnO3F.

Sr2ScO3F31 and K2NbO3F29 containing the nonmagnetic transition metals apparently adopt a metal center coordination geometry similar to that of the Mn cation in Sr2MnO3F, i.e., a ScO5F or NbO5F octahedron with the O/F anions disordered at the apical sites. However, the ratio of the apical bond length to the equatorial bond length is 1.075 for Sr2ScO3F and 1.043 for K2NbO3F, indicating that the bond anisotropy is much smaller than that of Sr2MnO3F. The slight elongation of the octahedra is attributable to a second-order Jahn−Teller effect.40 The influence of the Jahn−Teller distortion in the Mn3+ ion on the phonon energy was investigated by comparing the heat capacity with that of Sr2ScO3F, as will be discussed later. It is interesting to compare the structure of Sr2MnO3F with the manganese oxychloride analogue, Sr2MnO3Cl, which exhibits square-pyramidal coordination around Mn and with O/Cl order at the apical sites.27 The decrease in the coordination number for the manganese oxychloride results from Coulomb repulsion between the oxide and chloride ions or the steric effect due to the chloride ion being much larger than the oxide ion (rO2− = 1.40 Å and rCl− = 1.81 Å).41 There are two important factors that cause anion disorder in the related layered oxyfluorides containing non-d0-based transition metals, as discussed previously.25,26,42 One is the relative size of the halide and oxide ions: the ionic radius of the oxide is close to that of the fluoride (rF− = 1.33 Å)41 but significantly smaller than that of the chloride. The other is the high reaction temperature associated with entropic effects, which favors a random distribution of mixed anions. The reaction temper-

Figure 5. (a) Temperature dependence of the magnetic susceptibility at H = 1 kOe for Sr2MnO3F. The red line between 200 and 400 K is a fit to a S = 2 SLHAS model obtained by high-temperature series expansion. (b) Inverse susceptibility versus T plot, where the solid lines represent the Curie−Weiss fit.

ibility χ (=M/H) and its inverse for Sr2MnO3F in the magnetic field H = 1 kOe. The χ(T) data show a smooth increase with decreasing temperature, following the Curie−Weiss law [1/χ = (T − θ)/C] in the range of 350 < T < 400 K, where C and θ are the Curie and Weiss constants, respectively. The large negative value of θ = −526(23) K suggests that a strong antiferromagnetic interaction is present in the MnO2 plane. The estimated value of the Curie constant is 3.90(10) (emu K)/mol, which is comparable to those of Sr2MnO3Cl and LaSr1−xBaxMnO4 with a trivalent Mn cation.27,43 Upon further cooling from 350 K, the susceptibility curve starts to deviate from the Curie−Weiss law because of short-range spin correlation in the MnO2 plane and exhibits a broad maximum at Tmax = 178 K. This behavior is typically seen in lowdimensional antiferromagnets. In addition, we observe a clear inflection point associated with an antiferromagnetic ordering at TN = 133 K. The Curie tail below 50 K is likely due to the free spin of impurities. It is noteworthy that the Tmax and θ values for manganese oxyfluoride are much larger than Tmax = D

DOI: 10.1021/acs.inorgchem.5b02984 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 95 K and θ = −284 K of the oxychloride counterpart,27 indicating stronger magnetic coupling in the MnO2 plane in the former. This is attributed to stronger hybridization between the Mn and O orbitals on the MnO2 plane in the oxyfluoride than that in the oxychloride, which has a corrugated Mn−O−Mn arrangement with a bond angle (163.2°) in the basal plane.27 Figure 6 shows the field dependence of magnetization at 5 K.

Figure 7. Specific heat of Sr2MnO3F. The inset shows a comparison with the diamagnetic reference compound Sr2ScO3F. The Cp(T) curves cross at around 50 K, indicating their different phonon spectra.

applicable because the low-dimensional magnetic correlation persists even at high temperatures. Therefore, we employed Sr2ScO3F as a nonmagnetic reference material, which was prepared by a conventional solid-state reaction according to ref 31. However, as shown in the inset of Figure 7, the heat capacity curve of Sr2ScO3F crosses that of Sr2MnO3F at around 60 K; thus, it is not possible to precisely subtract the phonon contribution of manganese oxyfluoride, especially in the lowtemperature region. It is likely that these behaviors result mainly from the different coordination environments of the Mn and Sc centers: the driving force of the Jahn−Teller effects is first-order for Mn and second-order for Sc.40 At present, we cannot extract the magnetic contribution from the total heat capacity. Finally, we compared the halogen-site dependence of the magnetic properties of Sr2MO3X (M = Mn, Co, Ni; X = F, Cl). As described above, both of the manganese oxyhalide compounds adopt a long-range magnetic ordered state at low temperatures, which is remarkably different from the situation with Sr2NiO3X. Both of the nickel oxyhalide compounds exhibit a low-spin configuration of Ni3+ with S = 1/2 as well as similar positive Weiss temperatures close to 20 K.25 However, the oxyfluoride undergoes a spin-glass-like transition at TSG = 11 K in contrast to the long-range magnetic ordering at 33 K for the oxychloride. It has been assumed that the unpaired electron in the Ni cation occupies a dxy orbital, giving antiferromagnetic J1 and ferromagnetic J2 on a NiO4 square. Thus, it is likely that the spin-glass behavior in nickel oxyfluoride originates from the bond randomness of J2, whose magnitude varies depending on the direction of the NiO5 square pyramids or F sites. In light of the situation in nickel oxyhalides, it is likely that two factors favor the long-range magnetic ordered state in manganese oxyfluoride with a half-filled dxy orbital: One is the lack of bond randomness along the diagonal direction (unlike nickel oxyfluoride) because the Mn center sits on the center position of a MnO5F octahedron. The second concerns the magnitude and sign of J2. If J2 as well as J1 are antiferromagnetic, the two magnetic interactions compete. Spin frustration, which suppresses long-range magnetic order, occurs, and a novel quantum spin liquid state is theoretically predicted at J2/J1 ≈ 1 46 /2. Given the small value of J2/J1 = 0.11 for LaSrMnO4, the J2 value or spin frustration in Sr2MnO3F would also be quite weak. This is consistent with the magnetic ordering observed in manganese oxyfluoride. For Sr2CoO3X (X = F, Cl) with squarepyramidal coordination, the dxy orbital is fully occupied and

Figure 6. Isothermal magnetization at 5 K for Sr2MnO3F. The inset is an expansion of the plot.

M(H) increases almost linearly in proportion to the applied magnetic field and reaches 0.05 μB at 50 kOe. No hysteresis is observed, as shown in the inset of Figure 6. The electronic configuration of the Jahn−Teller active Mn3+ ion is described as (dxz,dyz)2(dxy)1(d3z2−r2)1(dx2−y2)0. In a squarelattice system composed of corner-shared MnO4 squares, not only the exchange interactions along the edges (J1) in the MnO2 square net but also those along the diagonal direction (J2) should be considered. However, the magnitude of J2 derived mainly from the direct dxy−dxy bond is smaller than that of J1. Larochelle et al. estimated J1/kB = 39.7 K and J2/J1 = 0.11 in isoelectronic LaSrMnO4 using the neutron scattering technique.44 We estimated the exchange coupling constant in Sr2MnO3F by employing a high-temperature series expansion for the square-lattice Heisenberg antiferromagnetic spin (SLHAS) model, taking only the nearest-neighbor interaction into consideration,45 which is described as follows: Ng 2μB 2 χJ

6

= 3ξ +

∑ n=1

Cn ξn−1

(1)

where N is Avogadro constant, J is the nearest-neighbor exchange constant, kB is Boltzmann’s constant, ξ = kBT/JS(S + 1), and the Cn coefficients are C1 = 4, C2 = 1.5, C3 = 0.252, C4 = 0.258, C5 = 0.134, and C6 = 0.015. As shown in Figure 5a, we obtained a reasonable fit in the range of 200 < T < 400 K, resulting in J/kB = 32.62(2) K, which is somewhat smaller than J1 in LaSrMnO4. The heat capacity Cp of Sr2MnO3F, which was measured between 2 and 250 K in zero magnetic field, is presented in Figure 7. A λ-type anomaly in the data was detected at around 140 K. This result is consistent with the antiferromagnetic phase transition observed in the χ(T) curve. To estimate the magnetic entropy of manganese oxyfluoride, the phonon contribution is subtracted from the total heat capacity. A simple Debye model of the phonon contribution is not E

DOI: 10.1021/acs.inorgchem.5b02984 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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magnetically nonactive against ordered O/X-site arrangements.26,30,47 Therefore, it is reasonable that both compounds exhibit antiferromagnetic ordered states.

4. CONCLUSION We demonstrated the high-pressure synthesis of Sr2MnO3F, another member of the layered oxyfluoride family, at 6 GPa and 1800 °C. Manganese oxyfluoride exhibited unique structural features that have never been observed in any other related layered oxyhalides, i.e., the coexistence of anion disorder between O and F at the apical sites and octahedral coordination subject to a first-order Jahn−Teller effect. The MnO2 planes separated by nonmagnetic Sr(O/F) rock-salt layers resulted in low-dimensional magnetic correlations with TN = 133 K, which is very different from the paramagnetic behavior observed in known RP-type layered manganese oxyfluorides.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02984. Synchrotron diffraction pattern at room temperature (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Y. Katsuya, M. Tanaka, and O. Sakata for supporting the SXRD experiments (Proposals 2013B4503 and 2014A4504) and M. Miyakawa, K. Fujimaki, and T. Taniguchi for support with the high-pressure synthesis at NIMS. This research was supported in part by the Ministry of Education, Culture, Sports, Science and Technology of Japan and the Japan Society for the Promotion of Science through Grants-inAid for Scientific Research (Grants 15K17837 and 25289233).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b02984 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.5b02984 Inorg. Chem. XXXX, XXX, XXX−XXX