Ba3(Cr0.97(1)Te

Article pubs.acs.org/IC

Ba3(Cr0.97(1)Te0.03(1))2TeO9: in Search of Jahn−Teller Distorted Cr(II) Oxide Man-Rong Li,† Zheng Deng,† Saul H. Lapidus,‡ Peter W. Stephens,§ Carlo U. Segre,∥ Mark Croft,⊥ Robert Paria Sena,# Joke Hadermann,# David Walker,∇ and Martha Greenblatt*,† †

Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, New Jersey 08854, United States ‡ Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois 60439, United States § Department of Physics & Astronomy, State University of New York, Stony Brook, New York 11794, United States ∥ Department of Physics & CSRRI, Illinois Institute of Technology, 3300 South Federal Street, Chicago, Illinois 60616, United States ⊥ Department of Physics and Astronomy, Rutgers, The State University of New Jersey, 136 Frelinghusen Road, Piscataway, New Jersey 08854, United States # EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium ∇ Lamont Doherty Earth Observatory, Columbia University, 61 Route 9W, PO Box 1000, Palisades, New York 10964, United States S Supporting Information *

ABSTRACT: A novel 6H-type hexagonal perovskite Ba3(Cr0.97(1)Te0.03(1))2TeO9 was prepared at high pressure (6 GPa) and temperature (1773 K). Both transmission electron microscopy and synchrotron powder X-ray diffraction data demonstrate that Ba3(Cr0.97(1)Te0.03(1))2TeO9 crystallizes in P63/mmc with face-shared (Cr0.97(1)Te0.03(1))O6 octahedral pairs interconnected with TeO6 octahedra via corner-sharing. Structure analysis shows a mixed Cr2+/Cr3+ valence state with ∼10% Cr2+. The existence of Cr2+ in Ba3(Cr2+0.10(1)Cr3+0.87(1)Te6+0.03)2TeO9 is further evidenced by X-ray absorption near-edge spectroscopy. Magnetic properties measurements show a paramagnetic response down to 4 K and a small glassy-state curvature at low temperature. In this work, the octahedral Cr2+O6 component is stabilized in an oxide material for the first time; the expected Jahn−Teller distortion of high-spin (d4) Cr2+ is not found, which is attributed to the small proportion of Cr2+ (∼10%) and the face-sharing arrangement of CrO6 octahedral pairs, which structurally disfavor axial distortion.

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t2g6eg1) in oxides is not common;13 in some cases they appear as dopants in other host lattices.13−15 A unique local static JT distortion was observed in LaCo3+O3 perovskite since the Co3+ (IS, t2g5eg1) state can be thermally excited from the ground-state LS configuration (t2g6eg0).4,16,17 This IS state can be further stabilized by carrier doping in La1−xSrxCoO3, which is compatible with a JT glass state.16 To the best of our knowledge, there is no report of JT distortion of HS octahedral Cr2+ (d4) in oxides to date, but it has been observed in the

INTRODUCTION

The Jahn−Teller (JT) distortion effect of octahedrally coordinated high-spin (HS) d4, low-spin (LS) d7, or d9 configuration cations usually induces important properties in transition metal oxides,1−4 such as colossal magnetoresistance (CMR)5−8 and high-temperature superconductivity,9−12 which derive from the strong coupling between static or dynamic charge, orbital, and magnetic interactions. Strong JT distortion in Mn3+ (HS, d4, t2g3eg1) and Cu2+ (d9, t2g6eg3) has been extensively studied in CMR manganites5−8 and high-temperature supercondutor cuprates, respectively.9−12 In contrast, the JT distortion of intermediate-spin (IS) Co3+ and LS Ni3+ (d7, © XXXX American Chemical Society

Received: April 26, 2016

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DOI: 10.1021/acs.inorgchem.6b01047 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry hydrated state in solution18 or inorganic−organic hybrid compounds.19 Recently, the JT-active Cr2+ has also been stabilized in perovskite fluoride KCrF320 and rock-salt superstructure sulfide Lu2CrS4.21 Double perovskites A2CrWO6 (A = Ca, Sr, Ba) appear to be possible hosts for Cr2+O6, since W6+ is highly favorable at octahedral sites of perovskites and related compounds,22−24 even in reducing conditions.25 However, it appears that Cr3+/W5+ dominates over the highly unstable Cr2+/W6+ in A2CrWO6.26−28 The mixed-valence Cr2+/Cr3+ state has been claimed in LaCr0.9Ti0.1O3,29 but the 10% (by mass) La2Ti2O7 second phase makes the proposed composition of the main phase in doubt. Moreover, the average ⟨(Cr0.9Ti0.1)−O⟩ distance (1.972(3) Å) is almost identical with the average ⟨Cr−O⟩ value (1.974(2) Å) in LaCrO3 at room temperature, which is unlikely to confirm the presence of any Cr2+ state considering the ionic radius difference between Cr2+ (HS, 0.80 Å) and Cr3+ (0.615 Å) in octahedral coordination.30 These findings suggest that the JT distortion of octahedral Cr2+ is significantly more difficult to stabilize in oxides than in those of octahedral Mn3+, Co2+/3+, Cu2+, or Ni3+. JT distortion-induced low-dimensional A2BB′O6 double perovskites are of great interest, but they are rare.31−33 Only a few compounds have been prepared to date, such as the layered Ln2CuMO6 (Ln = La, Pr, Nd, Sm; M = Sn, Zr)34−36 and the quasi-two-dimensional Sr 2 Cu(Mo 1−x W x )O 6 . 37 La2CuSnO6 can be prepared at ambient pressure, while the other Ln2CuMO6 compounds can only be stabilized at 6−8 GPa. In the monoclinic Ln2CuMO6 structure (P21/m) with alternating CuO6 and MO6 octahedral layers, the JT distortion of Cu2+ gives in-plane Cu−O bond lengths between 1.93(4) and 2.06(4) Å and out-of-plane Cu−O distances of 2.22(3)− 2.39(3) Å.35 Although the CuO2 layers are similar to the hightemperature cuprate superconductors, Ln2CuMO6 are not superconducting. A possible way to achieve superconductivity is to flatten the buckled CuO2 layer with electron doping.38 The tetragonal Sr2Cu(Mo1−xWx)O6 family, prepared at ambient pressure for 0 ⩽ x ⩽ 0.6 and high pressure (∼4 GPa) for 0.7 ⩽ x ⩽ 1.0, exhibits a quasi-two-dimensional S = 1/2 square lattice with possible magnetic frustration. The tetragonal distortion decreases with increasing x in Sr2Cu(Mo1−xWx)O6, accompanied by lowering the first-order tetragonal-cubic cooperative JT ordering temperature. These studies suggest that it might be possible to stabilize the B-site JT distortion of A2BB′O6 double perovskite, such as Cr2+O6 in A2Cr2+B′O6 at high pressure. Compared with the A2Cr3+W5+O6 (A = Ca, Sr, Ba) series,26−28 the B-site octahedral Cr2+O6 might be more stable in A2Cr2+Te6+O6, since the Te6+, once formed in A2Cr2+Te6+O6, would be unlikely to be dynamically reduced to Te4+, considering the difference of charge, size, and electron structure between octahedral Te6+ (ionic radius r = 0.56 Å) and Te4+ (r = 0.97 Å with 5s2 lone-pair electron).30 When the A-site is occupied by large cations in the perovskites, hexagonal structures may be adopted as exemplified by the well-known six-layered (6H) perovskites such as the 6H-BaTiO3.39 The general formula of these perovskites can be written as A3B2B′O9 with two types of 2:1 Bsite cation ordering: (i) The BO6 face-sharing octahedral dimers connect with B′O6 octahedra via shared corners, which crystallize in P63/mmc (No. 194), such as Ba3Cr2MoO9,40 Ba3Ru2MO9 (M = In, Y),41,42 or in P-62c (No. 190), as Ba3Cr2WO9.40 Both P63/mmc- and P-62c-type structures have similar polyhedral stacking - the only difference is between the oxygen sites, i.e., 12k in P63/mmc, and 12i in P6̅2c (Figure S1a);

(ii) the atoms are stacked the same way as in (i), but the B-sites are ordered giving face-sharing B1O6−B2O6 octahedral dimers (Figure S1b, space group of P63mc (No. 186)) as in Ba3Ti2IrO9.43 In this work we obtained a new close-packed 6H-hexagonal perovskite Ba3(Cr0.97(1)Te0.03(1))2TeO9 in attempts to prepare Ba2CrTeO6 double perovskite with JTdistorted Cr2+ and report the high-pressure and high-temperature synthesis, the determination of the crystal structure, formal oxidation state of cations, and the magnetic properties of this unusual new phase.

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EXPERIMENTAL SECTION

Synthesis. The title compound was obtained in an attempt to make Ba2CrTeO6 via a two-step solid-state reaction. First, the Ba2TeO5 precursor was synthesized with BaCO3 (99.98%, SigmaAldrich) and TeO2 (99.995%, Alfa Aesar) as previously reported (Figure S2).44 Then the mixture of Ba2TeO5, Cr powder (99.99%, Alfa Aesar), and Cr2O3(99.97%, Alfa Aesar) (atomic ratio of Ba/Cr/Te = 2:1:1) was heated at 1773 K and 6 GPa for 4 h in a LaCrO3 heater lined with Ir capsule inside a MgO crucible in a Walker-type multianvil press45 and then quenched to room temperature (RT) by turning off the voltage supply to the resistance furnace. The pressure was maintained during the temperature quenching and then released slowly over 8−12 h. Laboratory and Synchrotron Powder X-ray Diffraction. The product was initially characterized by laboratory powder X-ray diffraction (PXD, Bruker D8 ADVANCE, Cu Kα, λ = 1.5418 Å) for phase identification and purity. Synchrotron powder X-ray diffraction (SPXD, λ = 0.414160 Å, instrument resolution of 0.0005°) data were collected on a sample that was diluted with BN powder (for the purpose of X-ray spectroscopy discussed below) at ambient temperature at Beamline 11-BM of the Advanced Photo Source (APS) of Argonne National Laboratory. The structure was indexed, solved, and refined with TOPAS-Academic software.46 Electron Microscopy and Microprobe Analysis. Samples for electron microscopy were prepared by crushing the powder and dissolving it in ethanol, of which a few drops were put on a copper grid covered with holey carbon. Precession-electron diffraction (PED) patterns were taken on a Philips CM20. The high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images were acquired on an FEI Titan 50−80 microscope, operated at 300 kV. The simulated HAADF-STEM were calculated using QSTEM.47 The chemical composition was determined by energydispersive X-ray (EDX) analysis on 50 different crystallites, using a JEOL 5510 scanning electron microscope equipped with an INCA xsight 6587 system (Oxford instruments). The electron microprobe analyses were performed with a 5 wavelength-dispersive spectrometer Cameca SX-100 that has an additional Princeton Gamma Tech IMIX energy dispersive spectrometer, installed at the American Museum of Natural History in New York City. BaSO4, Te, and Cr were used as standards for Ba Lα, Te Lα, and Cr Kα radiation. X-ray Absorption Near Edge Spectroscopy. X-ray absorption near edge spectroscopy (XANES) was performed on beamline 10-ID (MRCAT) at APS with a liquid nitrogen cooled Si(111) double crystal monochromator and a Pt harmonic rejection mirror. The sample was diluted to 5 wt % with BN powder, to manage self-absorption. The Cr−K and Te−K edge XANES data were collected in both the transmission and fluorescence mode with simultaneous standards. The spectra were fit to linear pre- and post-edge backgrounds and normalized to a unity absorption step across the edge.48−56 Magnetic Properties Measurements. Magnetization measurements were performed with a commercial Quantum Design superconducting quantum interference device (SQUID) magnetometer. The susceptibility was measured in field-cooled (FC) and zerofield-cooled (ZFC) modes under a 1 T magnetic field, for temperatures ranging from T = 4 to 300 K. B


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Inorganic Chemistry

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RESULTS AND DISCUSSION Composition. Micoprobe analysis on the as-prepared sample shows a Cr-enriched, Ba−Te-depleted phase with composition Ba2.14(1)Cr1.18(2)Te0.82(2)Ox and some Cr-free phases: BaTeOx and Ba2TeOx (Figure S3). PXD data of the as-prepared light-brown phase can be well-indexed with a hexagonal phase (a ≈ 5.72 Å, c ≈ 14.05 Å) and trace unknown impurity, which washes away in dilute HCl acid solution giving the pure phase shown in Figure S4. All the measurements and characterization in this work subsequent to microprobe analysis were performed on the pure washed phase. EDX analysis of this washed pure phase gives an average formula of Ba2.00(9)Cr1.24(5)Te0.68(4)Ox, indicating Cr-rich B/B′-sites and some minor uncertainty about the composition compared to the microprobe result. The BaTeOx and Ba2TeOx phases seen by the microprobe are washed away by dilute HCl acid and have little PXD signature, suggesting that they may be amorphous. Crystal Structure. Systematic absences in the SPXD and PED (Figure S5) data suggest possible space groups of P63mc (No. 186),43 P6̅2c (No. 190),40 or P63/mmc (No. 194).40−42 Simulated annealing in P63mc gave very poor results, but the other two space groups were satisfactory. Candidate solutions in the other two space groups agree on the metal sites; the difference is that P6̅2c has two oxygen sites in a general position, whereas P63/mmc has the corresponding sites on a mirror plane (Figure S6). The quality of refinements in both space groups is identical, and in P6̅2c, the oxygen in a general position refines to the mirror plane within statistical uncertainty. HAADF-STEM imaging was performed in parallel. Figure 1a,b shows the HAADF-STEM images in [100] and [111] zones, respectively. By comparing the experimental and calculated results for the three models, clearly there is agreement with the P63/mmc and P6̅2c models; moreover, there is no distinction possible between the two by TEM. There is no agreement with the P63mc model. In the [100] zone image (Figure 1a), for example, the P63mc model will

result in a different brightness for the neighboring Cr and Cr + Te columns (indicated by the white circles), while for P63/mmc these columns will have the same occupation and thus the same brightness. On the experimental image, these columns indeed have the same brightness. In the [111] zone (Figure 1b) the projected Cr/Te columns (2 × 2 columns) are also indicated. In the experimental image, these four columns are again identical. On the one hand, this agrees with the calculated image for P63/mmc: in this model the four columns all contain the same amount of mixed Cr and Te and thus have the same brightness. On the calculated image for P63mc, on the other hand, two projected atom columns are dark (seemingly absent), and two are bright, since in this model two columns contain only Cr (dark ones), while two contain a mix of Cr and Te (bright ones). In [120] and [130] a better fit with P63/mmc than with P63mc can also be seen, but there the difference is less obvious as shown in Figure S5. Refinements of the SPXD data (Figure 2) in P63/mmc yields a structural formula of Ba3(Cr0.969(5)Te0.031(5))2TeO9 (BCTO), that is, 3.1 ± 0.5% (standard uncertainty) replacement of Cr by Te, in reasonable agreement with the EDX and microprobe results (Figure S3). The standard uncertainties in Rietveld refinements are derived from the propagation of counting statistics through the least-squares minimization of residuals and are generally several times smaller than any realistic estimate of the accuracy of the derived parameter. This is typically a consequence of systematic errors in the crystallographic or line-shape models. In the present case, the refined χ2 of 1.414 is much lower than generally encountered, indicating that statistical fluctuations of detected X-ray counts are more significant than is customarily the case. In turn, this suggests that the standard uncertainty should be taken seriously, and there truly is a measurable occupancy of Te in the Cr site. This result is consistent with the EDX results above and X-ray spectroscopy results discussed below. The two crystallographically independent Ba1 and Ba2 are located at 2b (0, 0, 1/4) and 4f (1/3, 2/3, z), Te1 at 2a (0, 0, 0), the mixed (Cr/Te)2 at 4f (1/3, 2/3, z), and two oxygen sites at 6h (x, −x, 1/4) and 12k (x, −x, z), respectively. Table 1 lists the detailed crystallographic parameters and reliability factors. The relatively large R factors do not indicate a deficiency of the crystallographic model; rather, they are a consequence of the weak scattering from the small diluted sample. The difference curve in Figure 2 shows that statistical noise dominates, and the remaining features are associated with BN peaks, not the BCTO. As shown in the inset of Figure 2, the crystal structure of BCTO is isostructural with a series of 6H polymorph perovskites, such as BaRuO3,57 BaCrO3,58 and Ba3Ru2MO6 (M = In, Co, Ni, Fe, Y, La, Sm, Eu, and Lu),41,42 and it consists of face-shared (Cr0.969(5)Te0.031(5))O6 octahedral pairs interconnected by corner-sharing TeO6 octahedra to form the framework. Table 2 presents the selected interatomic distances. The average metal−oxygen distances around the 12-coordinated Ba1 and Ba2 are 2.875(4) and 2.891(5) Å, respectively, comparable with those in the high-pressure made 6H BaRuO3 (2.884(4) and 2.883(4) Å).57 The ⟨Te1−O⟩ distance (1.927(5) Å) is also in line with the observed values (∼1.92−1.96 Å) in other Te(VI)-containing double perovskites.59,60 However, the average ⟨(Cr0.97Te0.03)−O⟩ distance (2.020(5) Å) is somewhat longer than the Cr−O distances in Ba2Cr3+M5+O6, such as 1.986(2) Å for M = Nb61 and 2.006(7) Å for M = Ta,62 indicating the possible presence of Cr2+

Figure 1. HAADF-STEM images of the washed sample in (a) [100] and (b) [111] zone. Brightness comparison of the neighboring Cr and Cr + Te columns is highlighted by circles. Calculated images are indicated by a rectangular white border. C


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Inorganic Chemistry

Figure 2. Rietveld refinements of the SPXD data for BCTO in P63/mmc structure at RT. Note the square-root intensity scale in the data and model, and that the difference plot is scaled to statistical uncertainty of the data. Tick marks indicate the positions of allowed target phase and BN peaks on top and bottom, respectively. (inset) The crystal structure viewed along [110] direction. Ba atoms are shown in blue spheres; O, violet spheres; TeO6 ochatedra, light yellow; (Cr0.97(1)Te0.03(1))O6 octahedra, dark blue.

Table 1. Structural Parameters of BCTO Refined from the SPXD Data Collected at Room Temperature atom

site

occ

x

y

z

Uiso (Å2)b

Ba1 Ba2 Te1 (Cr/Te)2 O1 O2

2b 4f 2a 4f 6h 12k

1 1 1 0.969(5)/0.031(5) 1 1

0 1/3 0 1/3 0.5141(7) 0.8405(5)

0 2/3 0 2/3 −0.5141(7) −0.8405(5)

1/4 0.0998(1) 0 0.6617(1) 1/4 0.0783(3)

0.38(3) 0.66(2) 0.27(3) 0.63(5) 0.21(14) 0.73(11)

a Space group P63/mmc (No. 194), Z = 2, a = 5.7248(1) Å, c = 14.0485(4) Å, V = 398.73(1) Å3, Rp/Rwp = 7.47/9.27%, χ2 = 1.41. bThe Uiso values are multiplied by 100.

considering the size of octahedral site Cr2+ and Cr3+ (r(Cr2+) = 0.80 Å (HS)), r(Cr3+) = 0.615 Å).30 Assigning the Ba2+ and Te6+ formal oxidation states, from bond valence sums (BVS, Table 2) calculations in BCTO, Cr displays mixed valence of Cr2+/Cr3+ with at least 10% of the Cr2+ state according to charge balance, or possibly more if there is any oxygen defect, which cannot be determined from X-ray diffraction. Octahedral HS Cr2+ is rarely observed in oxides, and our attempt to prepare Ba2Cr2+Te6+O6 at higher pressure and temperature was unsuccessful but yielded Cr-rich BCTO, presumably with depletion of Te6+ to form Ba2(Cr2+0.10(1)Cr3+0.87(1)Te6+0.03)2TeO9 in the isolated reaction system. This off-stoichiometry is probably responsible for the small impurity, which is dissolvable in dilute acid (Figure S3). The three long (2.083(5) Å) and three short (1.956(5) Å) metal−oxygen bonds give an octahedral distortion parameter (Δ)63 of 9.8 × 10−4 for (Cr0.97(1)Te0.03(1))O6 in BCTO, which is, however, much smaller than those of the known JT-distorted octahedral Cr2+X6 clusters. For example, in the hybrid and multiferroic [C(NH2)3]Cr2+[(HCOO)3] (X = HCOO),19 the cooperative JT distortion (CJTD) results in strong axially anti-

Table 2. Selected Interatomic Distances (Å), Bond Valence Sums, Octahedral Distortion Parameters (Δ), and Bond Angles (deg) in BCTO at Room Temperature Ba1O12 Ba1−O1 × 6 Ba1−O2 × 6 ⟨Ba1−O⟩ BVS

2.866(3) 2.884(4) 2.875(4) 2.43

Te1O6 Te1−O2 × 6 BVS ΔTe1 (1 × 10−4)

1.927(5) 5.84 0

O2−Te1−O2

89.4(2) 90.6(2) 180.0

Ba2O12 Ba2−O1 × 3 Ba2−O2 × 6 Ba2−O2 × 3 ⟨Ba2−O⟩ BVS (Cr/Te)2O6 (Cr/Te)2−O1 × 3 (Cr/Te)2−O2 × 3 ⟨(Cr/Te)2−O⟩ BVS Δ(Cr/Te)2 (1 × 10−4) O1−(Cr/Te)2−O1 O1−(Cr/Te)2−O2 O2−(Cr/Te)2−O2

2.768(2) 2.879(3) 3.039(4) 2.891(4) 2.40 1.956(5) 2.083(5) 2.020(5) 2.73 9.8 84.1(2) 92.1(1) 174.9(2) 90.6(2)

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Inorganic Chemistry ferrodistortion of CrX6 (ΔCr2+ = 55.5 × 10−4) with long and short Cr−X bonds of 2.010 and 2.358 Å, respectively. The antiferrodistortive CJTD of CrX6 is also observed in the perovskite fluoride KCr2+F3 (ΔCr2+ = 46.2 × 10−4, short/long C−F bonds of 1.986(4)/2.294(4) Å) and rock-salt superstructure sulfide Lu2Cr2+S4 (ΔCr12+/Cr22+ = 39.5/106.6 × 10−4, short/long Cr−S bonds of 2.599(14)/3.003(12) and 2.383(10)/2.946(13) Å for Cr1 and Cr2, respectively).20,21 Although Cr2+ is for the first time stabilized in a perovskite oxide under high pressure, the CJTD is absent in BCTO, since there is only ∼10% Cr2+; moreover, the face-sharing (Cr0.97(1)Te0.03(1))O6 octahedral pairs do not favor any axial distortion. X-ray Absorption Near-Edge Spectroscopy. To further examine the formal oxidation states of cations, XANES analysis was performed. The signatures of 3d transition-metal valencestate variations can be discerned in both the main- and pre-edge regions of their K-edge X-ray absorption spectra. The main edge features at 3d transition metal K-edges are dominated by 1s to 4p transitions, riding on a step-feature continuum onset. Despite substantial variations/energy-splittings in the main edge features the chemical shift (to higher energy with increasing valence) of the main edge has been widely used to chronicle the evolution of the transition-metal valence state in oxide-based materials.48−56 In Figure 3a the Cr−K main edge for BCTO is compared to those for a series of formally Cr3+, Cr4+, and Cr6+ standard compounds. Note that spectral intensity in BCTO has been shifted into the lower energy rising edge of the spectrum relative to the higher valent standard materials. The energy below which this excess intensity is shifted is indicated by a vertical dashed line (and arrow) in the figure. A similar downward shift of K-edge spectral intensity was observed by our group for Ni-based oxide materials in which admixtures of the anomalously low Ni1+ state was stabilized.52,53 The Cr2+ state is similarly anomalously low in this solid-state oxide. Thus, the observed similar downward spectral shift in BCTO mainedge spectrum is consistent with the introduction of the Cr2+ state in this material. In an attempt to corroborate the presence of the Cr2+ state, the Cr−K pre-edge region for BCTO is compared to those for the same series of formally Cr3+ and Cr4+ standard compounds in Figure 3b. The pre-edge features at the K-edges of 3d transition metal compounds are due to quadrupole-allowed 1s/ 3d or dipole-allowed 1s/3d-p-hybridized transitions. It is the Coulomb interaction between core 1s-hole and the d-electron states that shifts these transitions down into the pre-edge region. The structure and energy shift of the pre-edge features can offer a corroborative method of identifying the transition metal valence changes.48−51,55,56 In Figure 3b the BCTO preedge spectrum the features can be seen to be shifted down in energy relative to the higher valence standards.48,52,55,56 Thus, the pre-edge downward energy shift is also consistent with the presence of the Cr2+ state in this compound. To address the Te-oxidation state in BCTO, Te K-edge measurements (see Figure 3c) were performed on the compound along with elemental-Te and Te4+O2 standards. The intense peak at the Te−oxide Te−K edges, traditionally referred to as a white line (WL) feature, involves dipole transitions from the core 1s to empty 5p states. In elemental Te there are only two 5p hole states in extremely broad itinerant bands yielding a negligible WL feature. In contrast, in the insulating Te4+O2 compound the four final 5p hole states are quite localized yielding a distinct WL feature. The BCTO

Figure 3. (a) The Cr−K main edge spectra for the BCTO compound along with those for a series of Cr compounds with varying formal valences: Cr3+, LaCrO3 and Cr2O3; Cr4+, CrO2; and Cr6+, K2Cr2O7. Here the Cr−O coordination is octahedral except for the Cr6+ standard, where it is tetrahedral (which induces the intense hybridization induced pre-edge feature). (b) The Cr−K pre edge spectra for the octahedrally coordinated compounds in (a). The vertical dashed lines underscore the downward energy shift of the BCTO pre-edge features. (c) A comparison of the Te−K edges of BCTO, to the Te4+O2 and elemental-Te standards.

spectrum manifests dramatic WL feature intensity enhancement and 4.9 eV shift to higher energy relative to that of Te4+O2. Both of these observations support the Te6+ 5p2 configuration assigned for BCTO. Indeed a very similar WL intensity enhancement and chemical shift (∼4.6 eV) have been reported for the Te4+ to Te6+ change at the Te L1-edge (2s to 5p transitions) in previous work.54 Magnetic Properties. The per formula unit (f.u.) magnetic susceptibility (χ) for the system at hand is shown in Figure 4a. On a 0−300 K scale, only the large low-temperature Curie− Weiss-type behavior is visible. However, by greatly expanding and offsetting vertical scale (see Figure 4a right), the presence of a χ component that increases with increasing temperature (above ∼75 K) indicates the presence of excited-state magnetic multiplets. As is conventional64,65 in such systems, one turns to a plot of the function χT (i.e., the effective Curie moment) versus temperature, shown in Figure 4b, to emphasize the thermally induced moment increase. Materials with quasi-isolated magnetic dimers, such as Cr3+− Cr3+, have been discussed in the literature40,64,65 in terms of the phenomenological, intradimer (A−B), Heisenberg−Dirac−Van Vleck Hamiltonian, H = −JS⃗A·S⃗B.63 In the examples of E


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Inorganic Chemistry

The most relevant facet of the χT data and fit to it is the prominent Cr dimer component. This component is represented by the b = 0.84, |J|/k = 476 K term in the fit. The separate Cr dimer contribution to the χT fit is shown in Figure 4b, and the calculated values of TχCr3+dimer (along with the magnetic multiplet energy level structure) is shown in the Supporting Information (Figure S7) for the case of |J|/k = 476 K. The Cr dimer component constitutes just the lowtemperature tail of the depopulation excited-state multiplets on passing into the singlet S = 0 ground state. The presence of this component in χ is highly consistent with the presence of the face-sharing octahedral Cr−Cr dimers (with a separation of dCr−Cr = 2.467 Å).40,64−66 The value of b would be consistent with an 84% per f.u. Cr dimer population. On the one hand, the susceptibility was measured on a per gram basis, assuming the diffraction stoichiometry. The EDX results, on the other hand, could be interpreted in terms of an excess of the heavy Ba−Te elements. This could in turn increase the per Cr susceptibility by roughly a factor of 1.07. Such an increase would increase the χT fit to a per f.u. Cr dimer population to 90%. Within this model there is no precedence for incorporating the potential role of a Cr2+ admixture onto the dimer sites. Thus, the central message from the susceptibility data is the clear confirmation of the Cr−Cr dimer sites observed in the structural refinements. The isolated nondimer Cr sites are associated with the fitted Curie χT contribution. The Curie fit parameter a = 0.201 μBK/ T fit could alternately be consistent with ∼3% Cr3+ S = 3/2 sites; or with ∼2% Cr2+ t2g3 eg1 S = 2 sites. Here these percentages are per f.u., and an increase by a factor of 1.07 would be consistent with the EDX results. These isolated site percentages are consistent, within the experimental uncertainties, with those expected from the structural refinement results. It is worth noting that there is a broad upturn (followed by a sharp downturn) in the experimental χT below ∼70 K (see region labeled “i” in Figure 4b. This region is presumably dominated by magnetic glass-type interactions between the isolated Cr sites. Inclusion of a ferromagnetic-like interaction Weiss parameter in the Curie (free moment) contribution would statistically follow this upturn; however, it would do so with a false divergent term. Consequently, the authors have chosen to fit using only a constant Curie χT term with no interactions.

Figure 4. (a) The temperature dependence of the magnetic susceptibility (χ, solid black line/left scale) of the system with the assumed Ba3Cr2TeO9 f.u. The important high-temperature (labeled HT) plot of χ is also shown as a dashed blue line on a much-expanded vertical scale (see right scale). The horizontal T-scale remains the same in the HT expanded plot. The solid thin red lines are the results of the model fit discussed in the text. The susceptibility data were collected with H = 1 T field. (b) The temperature dependence of the experimental (heavy dotted black line) and various fitted model contributions to the effective magnetic moment χT. The heavy dashed blue line is the Curie (χT) contribution, the dotted black line is the Cr dimer contribution, and the solid red line is the combination of the Curie and dimer contributions fitted to the data for T > 70 K. In the temperature region indicated by “i” nondimer Cr site interactions are presumed operative. The downturn in the χT data at the lowest temperatures is attributed to a glassy freezing of the Cr−Te site moments.

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Ba3Cr2MO9 (M = Mo, W),40 one has relatively isolated, facesharing octahedral anti-ferromagnetic-coupled, Cr3+−Cr3+ dimers (i.e., J is negative). After Kahn63 the magnetic susceptibility of such an anti-ferromagnetic-coupled Cr3+− Cr3+ dimer is given by χCr3 +dimer = 2

CONCLUSION In summary, octahedral Cr2+O6 is observed in an oxide prepared at high pressure and temperature. In the 6H-type hexagonal perovskite structure of Ba3(Cr0.97(1)Te0.03(1))2TeO9, determined by electron and synchrotron X-ray diffractions, mixed-valent Cr2+/Cr3+ oxygen octahedra form, but a Jahn− Teller distortion expected of Cr2+O6 octahedra is hindered by the small fraction (∼10%) of Cr2+ and the face-shared arrangement of (Cr0.97(1)Te0.03(1))O6 octahedral pairs. The presence of Cr2+ state is also indicated by structural analysis and X-ray absorption near edge spectroscopy analysis, giving Ba3(Cr2+0.10(1)Cr3+0.87(1)Te6+0.03(1))2TeO9. Magnetic χ indicate the presence of contributions from a singlet ground state with excited-state magnetic multiplets, along with a low-temperature Curie tail. A quasi-isolated magnetic dimer model simulation of the susceptibility data clearly confirms the Cr−Cr dimer sites observed in the structural refinements. This work demonstrates that it is difficult, but possible, to stabilize Cr2+ in oxides at high pressure and temperature. The Jahn−Teller distortion of Cr2+O6 is expected in rock salt or layered A2Cr2+Te6+O6

(g ̅ μB )2 ⎡ 3e−(|J| / k)/ T + 15e−(3 |J| / k)/ T + 42e−(6 |J| / k)/ T ⎤ ⎢ ⎥ 3kT ⎣ 1 + 3e−(|J| / k)/ T + 5e−(3 |J| / k)/ T + 7e−(6 |J| / k)/ T ⎦

After Shikano et al., g ̅ = 1.8 is used in this expression when fitting below.40 The presence of some percentage of isolated nondimer sites provides a Curie−Weiss contribution to χ; accordingly, the experimental χT of the compound (see Figure 4b) was fit to the functional form: χT = a + bTχCr 3+dimer

Here the first term represents the Curie contribution, which dominates χ at low temperature and in which interactions between the isolated moments were omitted. The fitted parameters were b = 0.84, a = 0.201 μBK/T, and |J|/k = 476 K. The separate Cr dimer and Curie components, along with and total model fit to the χT data, are shown in Figure 4b. F


Article

Inorganic Chemistry double perovskite with smaller A-site (A = Sr, Ca2+, and solidsolution of them) cations in future work.

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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.6b01047. Figures S1−S7, XRD patterns, plot showing atomic proportions of cations in three oxides, comparison of hexagonal perovskite structures, precession electron diffraction patterns, HAADF-STEM images, magnetic multiplet energy-level structure of Cr3+−Cr3+ dimer model. (PDF) Crystallographic information (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSF-DMR-1507252 grant. MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. Use of the Argonne National Laboratory and the Advanced Photon Source is supported by the U.S. Department of Energy, under Contract No. DE-AC02-06CH11357. The authors would like to thank Dr. S. W. Kim at Rutgers Univ. for making a precursor, Ms. J. Hanley at LDEO in Columbia Univ. for making the highpressure assemblies, and Dr. A. Fiege at the American Museum of Natural History in New York City for the assistance of microprobe analyses.

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Ba3(Cr0.97(1)Te

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