Structural and Magnetic Properties of the Osmium Double Perovskites Ba2−xSrxYOsO6 Paula Kayser, Sean Injac, and Brendan J. Kennedy* School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia Thomas Vogt Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter Street, Columbia, South Carolina 29208, United States Maxim Avdeev, Helen E. Maynard-Casely, and Zhaoming Zhang Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia ABSTRACT: The crystal and magnetic structures of double perovskites of the type Ba2−xSrxYOsO6 were studied by synchrotron X-ray and neutron powder diffraction methods, bulk magnetic susceptibility measurements, and X-ray absorption spectroscopy. The structures were refined using combined neutron and synchrotron data sets based on an ordered array of corner-sharing YO6 and OsO6 octahedra, with the Ba/Sr cations being completely disordered. The structure evolves from cubic to monoclinic Fm3 ̅m (x ≈ 0.6) → I4/m (x ≈ 1.0) → I2/m (x ≈ 1.6) → P21/n as the Sr content is increased, due to the introduction of cooperative tilting of the octahedra. Bulk magnetic susceptibility measurements demonstrate the oxides are all anti-ferromagnets. The decrease in symmetry results in a nonlinear increase in the Neel temperature. Low-temperature neutron diffraction measurements of selected examples show these to be type-I anti-ferromagnets. X-ray absorption spectra collected at the Os L3- and L2-edges confirm the Os is pentavalent in all cases, and there is no detectable change in the covalency of the Os cation as the A-cation changes. Analysis of the L3/L2 branching ratio shows that the spin−orbit coupling is constant and insignificant across the series. ■ INTRODUCTION Transition-metal-containing double perovskite oxides, with the general formula A2BB′O6, are the subject of considerable current interest. In particular, double-perovskite-containing metals with unfilled 5d shells have been found to display fascinating and attractive physical properties, including colossal magnetoresistance (e.g., Sr2FeWO6 1), optical properties (e.g., Sr2CrReO6 2), high TN ferrimagnetism (e.g., Sr2CrOsO6 3), half metallicity (e.g., A2CrWO6 4), and metallicity (e.g., Sr2CrReO6 5). The A2BB′O6 double perovskite structure consists of a corner connected network of BO6 and B′O6 octahedra, which alternate in all three directions, so that each BO6 octahedron is connected only to B′O6 octahedra and vice versa.6,7 Consequently the structure is a combination of two interpenetrating face-centered cubic lattices, each of which exhibits intrinsic geometric frustration.8,9 Compared to simpler ABO3 ternary perovskites, the ordering of the B and B′ cations strongly influences both the electronic structure and the magnetic coupling, resulting in an increased variety of magnetic exchange interactions in the double perovskite structure. The subtle competition between different exchange inter- actions in double perovskites can lead to exotic magnetic phases, exemplified by anti-ferromagnetic (AFM) transitions in the 3d−5d double perovskites Sr2CoOsO6 10,11 and Sr2FeOsO6. 12,13 In these oxides the long-range Os−B′−Os coupling is mediated by the magnetic B′ cation Co(3d7)/ Fe(3d5) and is surprisingly large. Even when the second metal does not contain unpaired d-electrons it can play a significant role in moderating the magnetic exchange interactions between the two Os cations. This is illustrated in the two oxides Sr2YOsO6 and Sr2InOsO6 that have AFM transition temper- atures of TN = 53 and 27 K, respectively.14,15 Y3+ has a 4d0 open-shell configuration, whereas In3+ has a closed-shell 4d10 configuration. Density functional theory (DFT) calculations reveal that the short-range Os−Os interactions are much stronger in Sr2YOsO6 than the long-range ones, unlike those in Sr2CoOsO6 10,11 and Sr2FeOsO6. 12,13 Hybridization between the Os 5d and B′ 4d orbitals is much smaller in the d10 closed- Received: March 16, 2017 Published: May 17, 2017 Article pubs.acs.org/IC © 2017 American Chemical Society 6565 DOI: 10.1021/acs.inorgchem.7b00691 Inorg. Chem. 2017, 56, 6565−6575 D ow nl oa de d vi a U N IV O F E D IN B U R G H o n M ay 5 , 2 02 0 at 1 6: 15 :3 3 (U T C ). Se e ht tp s: //p ub s. ac s. or g/ sh ar in gg ui de lin es f or o pt io ns o n ho w to le gi tim at el y sh ar e pu bl is he d ar tic le s. pubs.acs.org/IC http://dx.doi.org/10.1021/acs.inorgchem.7b00691 shell case than that in the d0 open-shell case, reducing the amplitudes of Os−Os coupling in Sr2InOsO6 compared to those in Sr2YOsO6. Magnetic exchange interactions, and the consequent magnetic ground states, in double perovskites are known to be sensitive to the effect of chemical pressure that can be tuned by altering the size of the larger A-site cation. Both Sr2CrOsO6 and Ca2CrOsO6 are ferrimagnets; however, replacing the Ca with the larger Sr cation significantly increases TC from 490 to 660 K.16−18 Ca2ScOsO6 has a Neel temperature of 69 K.19 Substituting Ba for Sr in ALaNiOsO6 (A = Sr, Ba) results in a change in the Weiss temperature (ΘW) from negative to positive as a consequence of the decrease of structural distortions.20 Spin−orbit coupling (SOC) in 4d and 5d oxides has emerged as another key factor contributing to the unusual magnetic properties of oxides containing metals such as Os. In 5d oxides the relativistic SOC is of a similar magnitude to crystal field effects and electronic correlations (Hubbard U).21 A consequence of these often finely balanced interactions in 5d oxides is the emergence of unexpected phenomena, such as the SOC-dominated jeff = 1/2 Mott-like insulating state observed in Sr2IrO4 22 and magnetic metal insulator transitions in pyrochlore iridates and osmates.23 In the present work we report the synthesis and character- ization of the series of Os-containing double perovskites Ba2−xSrxYOsO6. Since the 4d and 5d orbitals are more spatially extended than the 3d orbital, in oxides containing 4d and especially 5d elements, the Coulomb interactions are typically weaker resulting in a large splitting of the crystal field and increased sensitivity to lattice distortions. In addition to the crystal-field splitting, the t2g orbitals may be modified into Jeff = 1/2, 3/2 bands through the effects of strong SOC. The combination of these factors makes this system a perfect candidate to study competition between lattice distortions and SOC. We demonstrate through the use of X-ray and neutron diffraction coupled with magnetic susceptibility measurements that chemical pressure, introduced by the use of a smaller A-site cation, alters the structure and details of the magnetic properties of these Os double perovskites. The structural studies allow us to establish the importance of geometric factors in a series with the same magnetic cation. X-ray absorption spectroscopy at the Os L-edge has been used to establish the oxidation state of the Os cation and to probe the relationship between changes in the structure and the SOC and magnetic properties across this series. ■ EXPERIMENTAL SECTION Polycrystalline samples of 11 members of the solid solution Ba2−xSrxYOsO6 (0 ≤ x ≤ 2) were prepared by conventional solid- state methods. Stoichiometric amounts of BaO2 (Sigma-Aldrich 95%), SrO2 (Sigma-Aldrich 958%), Y2O3 (Aithaca, 99.999%), and Os powder (Aithaca, 99.9%) (10% excess) were finely mixed as an acetone slurry in an agate mortar and heated in air at 1150 °C for 24 h with intermediate regrindings. Because of the possibility of producing toxic OsO4 gas, the entire synthesis was performed in a fume hood using sealed alumina crucibles. Reaction progress was followed by X-ray diffraction (XRD), using a PANalytical X’Pert PRO X-ray diffrac- tometer in Bragg−Brentano reflection geometry with Cu Kα radiation (λ = 1.5418 Å) equipped with a PIXcel solid-state detector. Synchrotron X-ray powder diffraction (S-XRD) data were collected over the angular range of 5 < 2θ < 85°, using X-rays of wavelength 0.824 530 Å, calibrated using an NIST SRM 660b LaB6 standard, on the powder diffractometer at Beamline BM-10 of the Australian synchrotron.24 The samples were housed in 0.2 mm diameter capillaries, which were rotated during the measurements. The data were obtained using a bank of 16 Mythen detectors, each of which covers 5° of data. Diffraction data were collected for 5 min at each of the two detector positions, to avoid gaps in the data from the individual modules. Neutron powder diffraction experiments were performed at ANSTO’s OPAL facility at Lucas Heights. Room- temperature neutron powder diffraction (NPD) data were acquired using the high-intensity diffractometer Wombat,25 and low-temper- ature NPD experiments were performed in the high-resolution powder diffractometer Echidna26 to determine the magnetic structure of selected samples. The wavelength of the neutrons, obtained using a vertically focusing Ge 115 and Ge 331 monochromator, was 1.498 and 2.439 Å, respectively. Approximately 1.5 g samples were placed in thin- walled vanadium holders. The S-XRD and NPD data were refined by the Rietveld method27 using the FULLPROF refinement program.28 A pseudo-Voigt function was chosen to generate the line shape of the diffraction peaks. No regions were excluded in the refinement. The following parameters were refined in the final analysis: scale factor, zero-point error, background (12 term shifted Chebyschev) coefficients, lattice parameters, positional coordinates, and isotropic atomic displacement. A Quantum Design PPMS system was used to collect direct-current (dc) magnetic susceptibility data, measured both in zero-field-cooled (ZFC) and field-cooled (FC) modes in the 2 ≤ T ≤ 300 K range under an applied magnetic field of 0.1 T. X-ray absorption near edge structure (XANES) spectra were measured at the Os L2 and L3 edges on beamline BL-12 at the Australian synchrotron.29 The measurements were performed at room temperature in transmission mode using argon-filled ionization chambers. The beam intensity (I0) was monitored by a flow through ionization chamber located upstream from the sample. XANES spectra were collected by a second ionization chamber placed immediately after the sample. A third ionization chamber was placed downstream to simultaneously measure a standard spectrum of Ba2YOsO6. Appro- priate amounts of the sample and BN were mixed, manually pressed between two Kapton tapes, and positioned directly in front of the X- ray beam. Energy steps as small as 0.25 eV were employed near the absorption edge with a counting time of 1 s per step. The energy scale of the monochromator was calibrated using the L3 edge of a Pt foil at 11 562.7 eV, and the Ba2YOsO6 reference spectra were used to align scans from different samples during the experiment. All XANES data were analyzed using the Athena software program.30 ■ RESULTS AND DISCUSION Crystal Structures. The 11 Ba2−xSrxYOsO6 (0 < x < 2) samples were obtained as black well-crystallized powders. In a first visual analysis of the S-XRD data, it was observed that all of the patterns are characteristic of a perovskite-type structure, showing sharp and well-defined superstructure reflections due to the rock-salt arrangement of the Y3+ and Os5+ cations. As detailed by Anderson et al.6 establishing long-range 1:1 B-site ordering in double perovskites requires a difference in the charge and size difference between the two cations. The combination Y3+ (0.900 Å) and Os5+ (0.575 Å) is sufficient to stabilize a high degree of cation ordering. In addition, the high covalency of the Os−O bonds, compared with the correspond- ing Y−O bond, reinforces the cation ordering in this system, since the anion polarization is maximum for Y−O−Os rather than for Y(Os)−O−Y(Os). At higher Sr compositions, additional superlattice reflections corresponding to the distortion of the oxygen sublattice were identified. These reflections indicate the presence of cooperative tilting of the corner-sharing BO6 octahedra and will be discussed in detail in the next paragraphs. To assign the correct symmetry, a combined structural refinement of high-resolution S-XRD and NPD was performed at room temperature. While the narrower peak widths in the S- Inorganic Chemistry Article DOI: 10.1021/acs.inorgchem.7b00691 Inorg. Chem. 2017, 56, 6565−6575 6566 http://dx.doi.org/10.1021/acs.inorgchem.7b00691 XRD patterns provide high precision in determining the cell metric and revealed splitting of selected reflections indicative of symmetry lowering, the refined positional parameters of the light oxygen anions were of relatively low precision reflecting the presence of heavy Os (Z = 76) and Ba cations (Z = 56). Consequently NPD was an essential technique to refine accurate and precise anion positions. As a first step, the nuclear structure was refined from the synchrotron data, and subsequently the veracity of the obtained lattice parameters was confirmed by Rietveld refinement against the combined S- XRD and NPD data sets. The difference in the number of electrons as well as in the neutron scattering length between the Y (7.75 fm) and Os (10.7 fm) atoms provided considerable sensitivity to Y−Os ordering over the two B sites. The refined antisite occupancy of Y at the Os positions and vice versa indicates that all the samples are fully ordered. It is worth recalling that for double perovskites the cation ordering results in the same additional R-point reflections as does out-of-phase tilting of the octahedra; consequently, it is necessary to consider the intensity of these to establish their origin. The in- phase tilting is associated with an M-point mode, while coupling of the out-of-phase tilts and the displacement of the A- type cation gives rise to X-point reflections.31,32 The refinement of the oxygen occupancies shows that there are no oxygen vacancies in any of the samples. Small amounts of Y2O3 (<2%) and SrY2O4 (<5%) oxides were identified in some samples, and these were included as secondary phases in the refinements. These phases are likely formed due to the evaporation of small amounts of Os during the synthesis. Four polytypes were identified in the series, and a representative Rietveld refinement for x = 1.0 is shown in Figure 1. The refined parameters, including unit-cell parameters, volume, atomic displacement parameters, atomic positions, and selected bond distances and angles are summarized in Tables 1 and 2. For the end member Ba2YOsO6 oxide the S-XRD and NPD data were fitted in the cubic space group (SG) Fm3 ̅m (No. 225), with the unit-cell related to ap (parameter of primitive cubic perovskite) as a = 2ap. As shown in Figure 2a, the doubling of the unit cell is confirmed by the presence of R- point reflections (odd-odd-odd reflections). The refined cell parameter a = 8.354 09(1) Å is in good agreement with that previously reported by Kermarrec et al. (a = 8.3541(4) Å).8 The introduction of Sr at the A site causes the tolerance factor, = + +t r r r r ( ) 2 ( ) A O B O , where rA, rB, and rO correspond to the radius of the A and average of the two B site cations and the oxygen anions respectively, to decrease resulting in cooperative tilting of the BO6 octahedra. As shown in Figure 2b, which displays portions of the room-temperature NPD profiles, there is a noticeable increase in the intensity of the R-point reflections at x = 0.6 as a result of the introduction of out-of-phase tilting of the corner-sharing octahedra. Examination of the S-XRD profile at x = 0.6 suggests that the structure is tetragonal. According to Howard’s group theory analysis,31 the tetragonal structure that only contains out-of-phase tilts is in space group I4/m (No. 87), and this was used in the analysis of the diffraction data for Ba1.4Sr0.6YOsO6. The transition from Fm3 ̅m to I4/m involves the introduction of out-of-phase octahedral tilting along the [001] direction, and the transformation is allowed to be continuous (second-order transition). Further doping with Sr at the A site results in a second phase transition, which has been identified based on the c/a ratio. At x = 0.6 the pseudotetragonal ratio is > 1 c a p p , while at x = 1.0 it reverses < 1 c a p p indicating a reorientation of the octahedral tilting.31 Again only R-point reflections are evident in the NPD and S- XRD profiles. In the composition range of x = 1.0−1.4 the crystal structure of all the compounds has been successfully refined in the monoclinic space group I2/m (No. 12). The transition from I4/m to I2/m involves a change in the direction of the octahedral tilt from about the c-axis in the former symmetry to along the [110] direction in the monoclinic structure. Group theory shows that this transition must be first- order.31 At still higher Sr contents (x ≥ 1.6), additional superlattice peaks at 2θ ≈ 34° and 40° are observed (Figure 2b). These are identified as M-point reflections, which demonstrate the existence of in-phase octahedral tilting. The structure of these samples (x = 1.6−2.0) is described in space group P21/n, which shows out-of-phase tilting along the [100] and [010] directions and in-phase-tilt along the [001] direction with respect to the pseudocubic perovskite cell. The nature of this transformation, which only introduces an extra tilt along the c-axis, is allowed to be second-order. The unit-cell parameters of the Sr2YOsO6 (a = 5.784 99(8), b = 5.800 75(7), and c = 8.1925(1) Å) obtained in this work are very similar to those reported by Paul et al. (a = 5.7817(1) Å, b = 5.8018(1) Å and c = 8.1877(1) Å).14 The observed phase transition sequence: Fm3 ̅m→ I4/m→ I2/m→ P21/n has been previously reported in the analogous solid solutions A2YBO6 for B = Ru and Ir33,34 and in a number of related systems including A2CoWO6, 35 A2NiWO6, 36 and A2InTaO6. 37 Magnetic Properties. The thermal evolution of the magnetic susceptibilities for selected members of the solid solution Ba2−xSrxYOsO6 (x = 0.2, 0.8, 1.0, 1.4, 1.6, 1.8) is displayed in Figure 3. In all of the cases the curves show a well- defined maximum corresponding to an AFM transition. Depending on the composition, and therefore on the crystal structure of each compound, the Neel temperature generally increases from 54 to 74 K as the Ba content, and hence Figure 1. Combined S-XRD-NPD Rietveld refinement profiles for Ba1.0Sr1.0YOsO6 at room temperature. The symbols are the experimental data, the solid line is the fit to the profile, the first series of Bragg reflections correspond to the main perovskite phase (SG I2/m), and the second one corresponds to Y2O3. The S-XRD data were collected at λ = 0.824 53 Å, and the NPD data (inset) were collected on the high intensity Wombat diffractometer at λ = 1.4980 Å. Inorganic Chemistry Article DOI: 10.1021/acs.inorgchem.7b00691 Inorg. Chem. 2017, 56, 6565−6575 6567 http://dx.doi.org/10.1021/acs.inorgchem.7b00691 T ab le 1. U ni t C el l, A to m ic P os it io ns ,T he rm al P ar am et er s, a an d R el ia bi lit y Fa ct or s fo r B a 2 − xS r x Y O sO 6 O bt ai ne d fr om th e C om bi ne d R efi ne m en ts A ga in st S- X R D an d N P D D at a at R oo m T em pe ra tu re SG Fm 3̅m I4 /m I2 /m P2 1/ n x 0 0. 2 0. 4 0. 6 0. 8 1. 0 1. 2 1. 4 1. 6 1. 8 2. 0 a (Å ) 8. 35 40 9( 1) 8. 37 85 2( 8) 8. 38 71 (2 ) 5. 90 63 (8 ) 5. 88 04 (4 ) 5. 87 95 7( 9) 5. 87 21 6( 4) 5. 85 47 2( 3) 5. 82 89 (2 ) 5. 80 78 8( 3) 5. 78 49 9( 8) b (Å ) 5. 88 88 (1 ) 5. 84 97 1( 4) 5. 83 46 0( 3) 5. 81 88 (2 ) 5. 80 91 4( 3) 5. 80 07 5( 7) c (Å ) 8. 36 37 (1 4) 8. 32 05 (1 2) 8. 30 37 (1 ) 8. 26 78 2( 5) 8. 24 97 8( 5) 8. 24 04 (2 ) 8. 20 95 9( 4) 8. 19 25 (1 ) β (d eg ) 90 .1 88 9( 9) 90 .1 33 4( 6) 90 .1 53 8( 4) 90 .1 46 (2 ) 90 .1 76 1( 3) 90 .1 81 1( 8) V (Å 3 ) 58 3. 03 8( 2) 58 8. 16 9( 1) 58 9. 97 (1 ) 29 1. 76 (7 ) 28 7. 72 (5 ) 28 7. 50 5( 7) 28 4. 00 3( 3) 28 1. 81 1( 3) 27 9. 50 (1 ) 27 6. 98 0( 2) 27 4. 91 7( 6) A x 0. 49 79 (6 ) 0. 49 96 (4 ) 0. 49 78 (4 ) 0. 00 30 (1 1) 0. 00 47 (3 ) 0. 00 17 (5 ) y 0. 48 68 (4 ) 0. 47 89 (2 ) 0. 47 31 1( 2) z 0. 25 02 (8 ) 0. 25 24 (2 ) 0. 25 13 (2 ) 0. 24 8 (1 ) 0. 24 80 (2 ) 0. 25 35 (5 ) B (Å 2 ) 0. 50 8( 6) 0. 46 6( 16 ) 0. 77 (3 ) 1. 09 (3 ) 0. 65 6( 9) 0. 58 (1 ) 0. 65 9( 8) 0. 92 1( 8) 0. 84 5( 16 ) 0. 71 0( 13 ) 0. 84 1( 14 ) Y B (Å 2 ) 0. 14 4( 9) 0. 04 (3 ) 0. 68 (6 ) 0. 56 (9 ) 0. 19 6( 16 ) 0. 37 (2 ) 0. 14 3( 6) 0. 25 3( 11 ) 0. 18 9( 16 ) 0. 09 4( 6) 0. 18 6( 11 ) O s B (Å 2 ) 0. 14 6( 5) 0. 13 (1 ) 0. 54 (2 ) 0. 60 (7 ) 0. 24 8( 8) 0. 23 (1 ) 0. 13 3( 13 ) 0. 21 9( 6) 0. 07 5( 9) 0. 09 4( 6) 0. 11 7( 6) O 1 x − 0. 02 2( 2) − 0. 04 0( 1) − 0. 05 49 (1 1) 0. 05 26 (1 7) 0. 06 13 (1 2) 0. 07 02 (1 5) y 0. 00 7( 2) 0. 01 03 (1 4) 0. 01 87 (1 3) z 0. 23 41 (3 ) 0. 23 27 (4 ) 0. 23 19 (5 ) 0. 23 4( 3) 0. 23 0( 4) 0. 23 65 (1 4) 0. 23 35 (9 ) 0. 23 76 (1 0) 0. 23 73 (1 3) 0. 24 55 (9 ) 0. 23 12 (1 1) B (Å 2 ) 0. 73 (2 ) 0. 82 (5 ) 1. 32 (7 ) 1. 7( 5) 2. 2( 4) 0. 8( 2) 1. 1( 2) 1. 50 (1 6) 1. 26 (1 6) 0. 42 (1 1) 0. 79 (1 6) O 2 x 0. 22 2( 2) 0. 21 7( 2) 0. 24 8( 2) 0. 22 74 (7 ) 0. 23 38 (7 ) 0. 20 67 (1 7) 0. 20 4( 2) 0. 20 61 (1 7) y 0. 24 6( 2) 0. 26 0( 2) 0. 21 8( 2) 0. 24 16 (9 ) 0. 23 42 (9 ) 0. 25 88 (1 9) 0. 25 27 (1 9) 0. 26 56 (1 6) z 0. 00 5 (1 ) 0. 02 41 (6 ) 0. 02 68 (5 ) − 0. 02 89 (1 2) − 0. 02 83 (1 3) − 0. 03 98 (1 5) B (Å 2 ) 1. 3( 2) 0. 8 (1 ) 1. 4 (1 ) 1. 22 (1 1) 1. 33 (9 ) 0. 55 (1 6) 2. 1( 2) 0. 87 (1 6) O 3 x 0. 25 86 (1 6) 0. 27 67 (1 7) 0. 27 76 (1 8) y 0. 78 97 (1 2) 0. 81 43 (9 ) 0. 79 88 (1 2) z − 0. 03 21 (1 2) − 0. 00 76 (9 ) − 0. 02 31 (1 2) B (Å 2 ) 0. 55 (1 6) 0. 28 (1 4) 0. 71 (1 6) re lia bi lit y fa ct or s χ2 5. 42 8. 83 5. 6 3. 08 5. 07 3. 82 3. 35 3. 23 10 .2 3. 72 3. 75 m ag ne tic pa ra m et er s T N (K ) 69 74 65 65 61 54 53 54 53 θ (K ) − 71 7 − 49 7 − 46 0 − 52 8 − 35 4 − 35 5 − 36 5 − 49 7 − 31 3 a T he m ag ne tic pa ra m et er s fo r se le ct ed m em be rs of th e so lid so lu tio n w er e ob ta in ed fr om th e su sc ep tib ili ty m ea su re m en ts . Inorganic Chemistry Article DOI: 10.1021/acs.inorgchem.7b00691 Inorg. Chem. 2017, 56, 6565−6575 6568 http://dx.doi.org/10.1021/acs.inorgchem.7b00691 symmetry, is increased, although this is clearly not a simple relationship. The corresponding values reported previously for Ba2YOsO6 (x = 0) and Sr2YOsO6 (x = 2) are 70 and 53 K, respectively.8,14 The data for x = 1.2 show a second weak anomaly that is believed to be due to trace amounts of a magnetic impurity that were not apparent in the diffraction data. There is no divergence between the ZFC and the FC Table 2. Main Bond Distances (Å) and Selected Angles (deg) for Ba2−xSrxYOsO6, Obtained from Rietveld Refinements against Combined S-XRD and NPD Data Sets at Room Temperature SG Fm3 ̅m I4/m I2/m P21/n x 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Os−O1 (Å) 1.956(2) 1.950(3) 1.945(4) 1.95(2) 1.91(3) 1.97(1) 1.946(7) 1.987(8) 1.979(11) 2.046(7) 1.939(9) Os−O2 (Å) 1.952(10) 1.991(12) 1.948(1) 1.954(5) 1.946(5) 1.944(11) 1.901(11) 1.976(10) Os−O3 (Å) 1.960(9) 1.937(9) 1.995(9) Os−Oav (Å) 1.956(2) 1.950(3) 1.945(4) 1.95(1) 1.96(1) 1.95(1) 1.951(6) 1.960(6) 1.961(10) 1.961(9) 1.970(9) Y−O1 (Å) 2.221(2) 2.240(3) 2.249(4) 2.21(2) 2.25(3) 2.20(12) 2.216(7) 2.188(8) 2.188(11) 2.121(7) 2.243(9) Y−O2 (Å) 2.215(10) 2.182(12) 2.221(12) 2.211(5) 2.210(5) 2.224(10) 2.252(11) 2.201(10) Y−O3 (Å) 2.211(8) 2.240(7) 2.167(8) Y−Oav (Å) 2.221(2) 2.240(3) 2.249(4) 2.21(7) 2.20(3) 2.214(12) 2.213(6) 2.202(7) 2.207(10) 2.204(8) 2.203(9) Y−O1−Os (deg) 180 180 180 180 180 172.8(5) 166.8(3) 162.2(3) 162.9(4) 160.0(3) 156.7(4) Y−O2−Os (deg) 174.3(5) 170.1(5) 172.7(5) 168.5(2) 167.7(2) 162.2(4) 162.9(4) 157.5(4) Y−O3−Os (deg) 161.7(3) 159.0(3) 159.7(3) Y−O−Osavg (deg) 180 180 180 176.2(9) 173.4(5) 172.7(5) 167.9(2) 165.8(2) 162.3(3) 160.6(3) 157.9(4) Figure 2. (a) S-XRD Rietveld refinement profile for Ba2YOsO6. The R-point reflections (odd-odd-odd) indicate the doubling of the unit cell. (b) Portions of the NPD patterns for Ba2−xSrxYOsO6 from x = 0 (top) to x = 2 (bottom) highlighting the development of superlattice reflections associated with cooperative tilting of the corner-sharing octahedra. Figure 3. Thermal evolution of (a) dc magnetic susceptibility and (b) inverse susceptibility for selected members in the series Ba2−xSrxYOsO6 measured at 0.1 T. Inorganic Chemistry Article DOI: 10.1021/acs.inorgchem.7b00691 Inorg. Chem. 2017, 56, 6565−6575 6569 http://dx.doi.org/10.1021/acs.inorgchem.7b00691 curves for any of the samples, except at very low temperatures that may indicate a slight canting of the spins. At 4.5 K the isothermal magnetization curves (Figure 4) show an almost linear response with no hysteresis or remnant magnetization. The shapes of all of the curves are characteristic of the behavior of AFM materials. We note that estimating the Neel temperature in double perovskites such as these is better achieved combining bulk susceptibility with either neutron diffraction or heat capacity measurements. Because of instru- ment availability and the challenges in preparing well-sintered samples, such measurements were not feasible for this series. Above the magnetic ordering temperature, the reciprocal susceptibility curve (Figure 3b) follows the Curie−Weiss law χ = C/(T − θ). The effective magnetic moment, obtained as 2.84√C (where C represents the Curie constant) from a linear fit in the temperature range of 200−300 K, varies between 3.0(1) and 2.96(3) μB. Kermarrec et al.8 reported a significantly higher value of the effective magnetic moment for Ba2YOsO6 (μ = 3.93 μB). These workers however noted that their Curie− Weiss fitting yielded an extremely large negative θcw (−717 K), indicating that the paramagnetic region in their sample was above 300 K. In contrast Paul et al. reported μ = 3.45 μB for Sr2YOsO6 with a much-reduced negative value of θcw (−337 K). These values are below the spin-only magnetic moment for a Os5+ (t2g 3, S = 3/2) ion of 3.87 μB. The difference between the observed and calculated magnetic moments is attributed to SOC effects, which for a half-filled t2g 3 electron configuration should lead to a reduction of the moments from the spin-only values.15 A further effect is the large covalence existing in these systems, which is expected to lead to a reduction in the magnetic moment. The large negative Weiss temperatures in the compounds, which varies between −350 and −530 K, confirms that the magnetic interactions are predominantly AFM and suggests the presence of moderate frustration with a ratio of |θcw|/TN ≈ 4−8.38 That θcw did not vary systematically with composition is believed to be a consequence of the frustration in the system. The face- centered-cubic (fcc) lattice can be viewed as composed of edge- sharing Os tetrahedra, and is geometrically frustrated in the presence of AFM interactions.8,21 While the frustration index cannot be used as the unique criteria to predict magnetic frustration, it provides a strong indication of the nature of exchange interactions in the system. Magnetic Structure. The magnetic structure of the two selected members (x = 1.0 and 1.2) of the solid solution Ba2−xSrxYOsO6 were investigated using NPD patterns meas- ured below the magnetic ordering temperature. Additional variable-temperature data were measured for Ba1.0Sr1.0YOsO6 from 3 to 75 K (inset of Figure 5). Careful examination of the NPD data measured at 3 K provided no evidence for any M- point reflections, which are diagnostic of the presence of in- phase tilting of the octahedra, and it was concluded that the structure remains monoclinic in I2/m down to 3 K. As evident in Figure 5 as the temperature is reduced from 70 K a number of additional reflections emerge at low angles (2θ ≈ 16° and 24°) as a consequence of the onset of long-range magnetic order. This magnetic ordering temperature range is in good agreement with the corresponding Neel temperature obtained from bulk susceptibility measurements described above (TN = 65 K). For both samples, analysis of the positions of the magnetic reflections, using K_SEARCH in FullProf,28 indicated Figure 4. Magnetization vs magnetic field isotherms for selected members in the series Ba2−xSrxYOsO6 measured at 4.5 K under a magnetic field ranging from −40 000 to 40 000 Oe. Figure 5. Observed (+), calculated (solid line), and difference (bottom) NPD Rietveld profiles for (a) Ba1.0Sr1.0YOsO6 and (b) Ba0.8Sr1.2YOsO6 at 3 K collected at the high-resolution Echidna diffractometer. The first and the second series of Bragg reflections correspond to the nuclear and magnetic structure of main perovskite phase, and the third one corresponds to Y2O3. (inset) Thermal evolution of NPD patterns; the red arrows indicate the position of the magnetic reflections. Inorganic Chemistry Article DOI: 10.1021/acs.inorgchem.7b00691 Inorg. Chem. 2017, 56, 6565−6575 6570 http://dx.doi.org/10.1021/acs.inorgchem.7b00691 that the corresponding magnetic structure was defined by the propagation vector k = (1/2 1/2 0), which means that the additional magnetic peaks could be indexed by a supercell where the a- and b-axes are doubled with respect to the nuclear cell. To determine the possible magnetic structure models compatible with the monoclinic I2/m space group, representa- tion analysis was employed following the procedure described by Bertaut.39 The irreproducible representations and the corresponding basis vectors associated with the Os5+ sublattice were generated using BasIrep as implemented in Fullprof. For the space group I2/m and the propagation vector k = (1/2 1/2 0), the only irreducible representation of the small group Gk is Γ2, and the basis vectors for the magnetic atoms located at (000) and (1/2 1/2 1/2) are (100), (010), and (001). The Rietveld refinement of the nuclear and magnetic structure, shown in Figure 5, yielded in a magnetic moment of 1.5(2) and 1.64(5) μB for Ba1.0Sr1.0YOsO6 and Ba0.8Sr1.2YOsO6, respec- tively, with the spins defined along the c-axis. Figure 6 displays a schematic view of the structure, which can be described as an AFM coupling of ferromagnetic (110) planes indicative of an AFM1 structure. The magnetic ground state described by k = (1/2 1/2 0) is relatively rare compared with the k = (000) ground state, although it was described recently for La2LiOsO6. 21 Given the two have similar near neighbor and next nearest neighbor interaction they are expected to be very similar in energy, and it is unclear what, if any, significance lies in this. X-ray Absorption near-Edge Structure (XANES) Re- sults. XANES spectra were collected at the Os L3 and L2 edges from all 11 samples in the Ba2−xSrxYOsO6 series. Figure 7 shows the normalized Os L3- and L2-edge XANES spectra of BaSrYOsO6 corresponding to dipole allowed transition of 2p3/2 → 5d and 2p1/2 → 5d, respectively (the continuum step at the L3 and L2 absorption edge is normalized to one and to a half, respectively, reflecting the difference in the number of initial core−electron states available for the L3 and L2 absorption processes). There is essentially no change in the XANES spectra as the content of Sr changes from 0 to 2, as evident in Figure 8, confirming that the Os oxidation state remains 5+ throughout the solid solution Ba2−xSrxYOsO6. For Os atoms coordinated to six oxygen atoms (i.e., in an octahedral or slightly distorted octahedral environment), the Os L3-edge XANES spectra consist of two components corresponding to the t2g and eg orbitals due to crystal field splitting (CFS) of the Os(5d) states, similar to the XANES spectra at the L3-edge from octahedrally coordinated Zr or Hf in pyrochlore materials.40,41 Although the two peaks corresponding to t2g and eg orbitals are well-resolved at the Zr L3 edge, they are not Figure 6. Schematic view of the magnetic structure in the I2/m structured Ba2−xSrxYOsO6 (x = 0.8 and 1.0) oxides. The large green spheres represent the A-cations, and the blue and pink spheres correspond to the Y and Os cations, respectively. The arrows indicate the direction of the Os magnetic moments. Figure 7. Normalized XANES spectra obtained from BaSrYOsO6 at the Os L3 and L2 absorption edges. Figure 8. Normalized XANES spectra collected at the Os L3 edge from Ba2YOsO6 (black), BaSrYOsO6 (red), and Sr2YOsO6 (blue) with their second derivatives plotted (inset). Inorganic Chemistry Article DOI: 10.1021/acs.inorgchem.7b00691 Inorg. Chem. 2017, 56, 6565−6575 6571 http://dx.doi.org/10.1021/acs.inorgchem.7b00691 resolved at the Hf or Os L3 edge due to larger natural widths associated with higher-energy atomic levels of Hf(5d) or Os(5d) than Zr(4d).42 The CFS of the Os(5d) states can, however, still be estimated as ∼3.9 eV from the difference between the two “valleys” in the second derivative spectra (inset of Figure 8).41,43 It should be mentioned that very slight displacements of the eg peak were observed at the Y L3 edge in the analogous system of Ba2−xSrxYRuO6 as a function of Sr content,33 which were not observed at the Os L3 edge in the current series of Ba2−xSrxYOsO6. Overlap of the t2g and eg peaks of the Os(5d) states prevent such subtle changes being detected. Furthermore, X-ray absorption spectroscopy provides useful information to probe the SOC interactions of a material, and the strength of the coupling can be quantitatively extracted from XANES spectra by determining the intensity ratio of the white lines at the L3 and L2 edges (i.e., branching ratio BR = I(L3)/I(L2)). Branching ratios significantly greater than the statistical value of 2 indicate the presence of a strong coupling between the local orbital and spin moments. This method, proposed by Thole and van der Laan,44 has been successfully used to study the influence of SOC interactions in the ground state of several iridium-based compounds.43,45 Since SOC interactions are potentially present in Os-containing com- pounds, and it was observed that the magnetic moments (as obtained from the analysis of the NPD data) are lower than the expected values in the current Ba2−xSrxYOsO6 system, the branching ratios have been calculated for all the samples in this series. It is shown that the BR remains constant throughout the series (BR ≈ 2.2 using the “moderate” method as shown in Figure 2 of Cho et al.46), and this value of the BRs corresponds to the statistical value within experimental error, which means that the SOC effect is minimal in this system. Nevertheless the inelastic neutron data analysis reported by Kermarrec et al.8 reveals a spin gap to the spin-wave excitations of 19(2) meV for Ba2YOsO6, which is unexpected for an orbitally quenched t2g 3 electronic configuration, and these authors concluded SOC is important in this compound. Evidently additional studies are required to reconcile these differences. ■ DISCUSSION High-resolution S-XRD and NPD data were measured at room temperature for 11 members in the series Ba2−xSxYOsO6. Examination of the superlattice reflections and of the diagnostic peak splitting enabled the symmetry and space groups of these oxides to be established. Subsequently the structures were refined using combined S-XRD/NPD data sets. All oxides exhibited a rock-salt-like ordering of the Y3+ and Os5+ cations. As illustrated in Figure 9, an increase in the Sr content at the A site of the double perovskite generally resulted in a reduction in the unit-cell parameters and volume, although the results for the two Ba-rich oxides Ba2YOsO6 and Ba1.8Sr0.2YOsO6 are clearly anomalous. We note that the diffraction peaks in the SXRD profiles for the x = 0.2 and 0.4 samples were somewhat broader than for other members of the series. The unit-cell volumes obtained for the two end-member oxides Ba2YOsO6 and Sr2YOsO6 in the current work are in excellent agreement with published values, and, in particular, the refined cell parameter for Ba2YOsO6 of 8.354 09(1) Å is in excellent agreement with that reported by Kermarrec et al., 8.3541(4) Å. The values for Ba2YOsO6 and Ba1.8Sr0.2YOsO6 are reproducible, demonstrating this is not an experimental artifact. Deviations from Vegards law are not uncommon and can arise from either local ordering of the dopant cation or magnetic effects. In the present work we are unable to establish if either of these effects is responsible, but the peak broadening suggests the presence of additional strains that may be a consequence of local disorder,47 although strain fields around dopant atoms typically only extend across 2−3 unit cells with effective relaxation distances of ∼16−18 Å in diameter.54 The average Os−O and Y−O bond distances (Table 2) were remarkably constant across the series, with the average Y−O distance larger than that of Os−O. The contraction in the average A site cation size is accommodated by the structure by the introduction of cooperative rotation of the corner-sharing octahedra. Four phases were identified, and the sequence of these, together with their Glazer48 tilt pattern, is Fm3 ̅m(a0a0a0) → I4/m(a0a0c−) → I2/m(a−a−c0) → P21/n(a −a−c+). The Fm3 ̅m → I4/m and I2/m → P21/n transitions involve the introduction of a tilt about [001] and are allowed by group theory to be continuous, whereas the I4/m → I2/m transformation requires a reorientation of the out-of-phase tilt from [001] to [110] and is required to be first-order. The variation of the volume in the solid solution, plotted in the inset of Figure 9, shows evidence for a small anomaly near x = 0.8− 1.0 that may be associated with the first-order I4/m → I2/m transition. The inclusion of the NPD data in the structural refinements provided relatively precise and accurate atomic coordinates for the anions and, hence, for the individual bond distances and angles. As noted above, the average Y−O and Os−O distances are remarkably constant across the series. Internal changes in the structures are expected to directly impact the magnetic properties. The effect of the octahedral tilting on the Neel temperature is observed in Figure 10. A feature of this figure is that TN is reasonably constant for samples with monoclinic symmetry (x < 1.4) and then increases above this. This suggests there is a critical angle at which the favorable AFM superexchange interactions are maximized. In particular, a decrease of the Y−O−Os bond angle leads to less effective overlap between 5d Os and 2p O orbitals and thus weakens the Figure 9. Composition dependence of the, appropriately scaled, unit- cell parameters and volume (inset) for the series Ba2−xSrxYOsO6 estimated by Rietveld refinements against combined S-XRD and NPD data sets. Where not apparent the esds in the values are smaller than the symbols. Inorganic Chemistry Article DOI: 10.1021/acs.inorgchem.7b00691 Inorg. Chem. 2017, 56, 6565−6575 6572 http://dx.doi.org/10.1021/acs.inorgchem.7b00691 superexchange interactions. This results in a lowering of the magnetic ordering temperature. There are two possible superexchange pathways, via nearest neighbor (NN) Os−O−Os (π-superexchange) and next nearest neighbor (NNN) interactions Os−O−Y−O−Os (σ- superexchange). On the one hand, since Os5+, the only magnetic cations involved in the magnetic interactions, has a half-filled t2g orbital set the NN interactions are expected to be AFM, as described by the Goodenough−Kanamori rules.49 On the other hand, the coupling of two Os cations over the NNN Os−O−Y−O−Os linkage is dependent on the connection through the nonmagnetic cation Y3+ (d0). DFT calculations indicate that the NNN Os−O−Y−O−Os interactions are weaker than the NN Os−O−Os interactions that are mediated by a π interaction.50 The resulting competition between the magnetic interactions gives rise to the well-known AFM1 magnetic structure,51,52 where eight of the NN spins are AF aligned, four are FM, and all NNN spins are also FM. This is a very common spin arrangement previously described in numerous double perovskites including Sr2YRuO6, and it is consistent with the refined magnetic structure obtained at 3 K (Figures 5 and 6). The values of the ordered magnetic moments estimated from the neutron diffraction, 1.5(2) and 1.64(5) μB for x = 1.0 and 1.2, respectively, are noticeably lower than those expected for S = 3/2 Os5+ (μ = 3 μB), which in turn is lower than the theoretical spin-only magnetic moment of 3.87 μB for the Os 5+ (t2g 3, S = 3/2) ion. Considering the effects of SOC alone a moment of 1.9−2.3 μB for Os5+ appears reasonable.8 That the observed moment is reduced from this suggests other effects such as covalency between the d and O p orbitals, which removes some fraction of the ordered moment from the transition-metal ion site and transfers it to the ligand site, or fluctuations induced by either geometrical or quantum effects may also be important. Since one of the cations placed over the B-site is nonmagnetic (Y3+), the Os5+ sublattice forms an fcc geometrically frustrated network, and the dominating NN interactions between these cations are therefore frustrated. The frustration index is estimated to be |θcw|/TN ≈ 4−8, which indicates a degree of frustration, but this alone is probably insufficient to explain the reduced magnetic moment. The large covalence of the Os−O chemical bonds caused by the spatially extended 5d orbitals results in a high delocalization of the magnetic moments, and it might contribute to lower the experimental magnetic moment. X-ray absorption spectroscopy shows that the branching ratio I(L3)/I(L2) is constant across the series. This implies that the magnitude of the SOC is also relatively constant. That SOC plays a significant role in the reduction of the magnetic moments has been identified from inelastic neutron experiment on similar systems. Kanungo et al.53 have investigated the magnetic properties of related Os- containing perovskites (Sr2BOsO6; B = Y, Sc, and In) using first-principles DFT, and they concluded that SOC is essential to describe the magnetic behavior of the mentioned compounds. Initially, they found the AFM2 structure to be the preferred magnetic ground state; however, including SOC in their calculations results in the AFM1 state being energetically lower than AFM2 by a value ∼1.2 meV/f.u. Furthermore, Taylor et al.50 have reported the presence of a large spin gap in Sr2ScOsO6. It evidenced the existence of SOC- induced anisotropy, which is indispensable to stabilize the AFM1 ground state of an fcc frustrated system with strong NN magnetic interactions and negligible NNN interactions. Since there are many factors involved in the stabilization of the magnetic ground state of the current systems it would be interesting to further analyze the system as a function of changing symmetry theoretically. ■ CONCLUSIONS Accurate and precise structures have been determined for 11 members in the series Ba2−xSrxYOsO6 using a combination of synchrotron X-ray and neutron powder diffraction methods. The results obtained from the Rietveld refinement using combined S-XRD and NPD data sets show the structure to evolve from cubic to monoclinic as the Sr content is increased due to the introduction of cooperative tilting of the octahedra. Bulk magnetic susceptibility measurements reveal a nonlinear increase in the Neel temperature demonstrating the sensitivity of the magnetic ground state to changes in the precise nature of the structure and, in particular, the Os−O−Y−O−Os geometry. The bulk magnetic moments are significantly reduced from the corresponding spin-only value. The Os L3/ L2 branching ratio = I(L3)/I(L2) ≈ 2.2 was only slightly above the statistical value of 2 and constant across the series as determined by X-ray absorption spectra, demonstrating both that the SOC does not change across the series and is relatively unimportant. This indicates that in the current series the observed reduction in the magnetic moment from the spin-only value can be attributed to covalence effects rather than SOC. This observation is inconsistent with the recent inelastic neutron study by Kermarrec et al.8 that identified an unexpected spin gap that they proposed was a consequence of SOC. Additional studies are required to reconcile these differences. ■ AUTHOR INFORMATION Corresponding Author *E-mail: Brendan.Kennedy@Sydney.edu.au. ORCID Brendan J. Kennedy: 0000-0002-7187-4579 Maxim Avdeev: 0000-0003-2366-5809 Notes The authors declare no competing financial interest. Figure 10. Variation of the Neel temperature vs the bond angle Y− O−Os for members of the series Ba2−xSrxYOsO6. Inorganic Chemistry Article DOI: 10.1021/acs.inorgchem.7b00691 Inorg. Chem. 2017, 56, 6565−6575 6573 mailto:Brendan.Kennedy@Sydney.edu.au http://orcid.org/0000-0002-7187-4579 http://orcid.org/0000-0003-2366-5809 http://dx.doi.org/10.1021/acs.inorgchem.7b00691 ■ ACKNOWLEDGMENTS We acknowledge the Australian Research Council for support of this work which was, in part, performed at the Powder Diffraction and X-ray Absorption Spectroscopy beamlines at the Australian Synchrotron. We thank Drs. P. Kappen and J. Kimpton for their assistance with these measurements. ■ REFERENCES (1) Ray, S.; Kumar, A.; Majumdar, S.; Sampathkumaran, E. V.; Sarma, D. D. Transport and magnetic properties of Sr2FeMoxW1‑xO6. J. Phys.: Condens. Matter 2001, 13, 607−616. (2) Das, H.; De Raychaudhury, M.; Saha-Dasgupta, T. Moderate to large magneto-optical signals in high Tc double perovskites. Appl. Phys. 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