Structural and Magnetic Properties of the Iridium Double Perovskites

Article pubs.acs.org/IC

Structural and Magnetic Properties of the Iridium Double Perovskites Ba2−xSrxYIrO6 Ben Ranjbar, Emily Reynolds, Paula Kayser, and Brendan J. Kennedy* School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia

James R. Hester Australian Nuclear Science and Technology Organisation, Lucas Heights, New South Wales 2234, Australia

Justin A. Kimpton Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia ABSTRACT: The crystal structures of the series of ordered double perovskites Ba2−xSrxYIrO6 (0 ≤ x ≤ 2) were refined using a combination of high-resolution synchrotron X-ray and high-intensity neutron diffraction data. The materials displayed a sequence of structures x = 0.6

x = 1.0

x = 1.4

Fm3̅m(a0a0a0) ⎯⎯⎯⎯⎯→ I4/m(a0a0c−) ⎯⎯⎯⎯⎯→ I2/m(a−a−c0) ⎯⎯⎯⎯⎯→ P21/ n(a−a−c+) associated with increased tilting of the corner-sharing octahedra induced by increasing amount of the smaller Sr cation present. A similar sequence of transitions was induced by heating selected samples. Magnetic susceptibility measurements between 2 and 300 K showed no evidence for long-range magnetic ordering, an observation that was supported by neutron diffraction measurements, and rather strong spin− orbit coupling results in a Jeff = 0 ground state.

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INTRODUCTION The pervasiveness of perovskites in solid-state chemistry reflects the compositional and structural flexibility of the perovskite structure, and there are numerous examples where substitution of one or more of the cations significantly impacts the structure and properties,1,2 which include magnetic ordering, magnetoresistance, superconductivity, ferroelectricity, multiferroicity, thermoelectrical properties, and catalytic activity.3−5 This array of properties and inherent structural flexibility, coupled with the technological impact of devices containing perovskites, has provided the impetus for numerous studies. The A2BB′O6 double perovskites are particularly attractive for the design of new magnetic materials because the rock-saltlike arrangement of corner-sharing BO6 and B′O6 octahedra affords the possibility of two magnetic sublattices.6,7 The frustrated magnetic exchange couplings that occur within the B and B′ sublattices can be modulated by couplings between the sublattices.8 The ordering of the smaller B- and B′-type cations in the A2BB′O6 double perovskite structure alters the symmetry from Pm3m ̅ , in the ABO3 perovskite aristotype, to Fm3m ̅ (No. 225).1,9 Where the relative sizes of the cations are not optimal, as quantified by the perovskite tolerance factor t = (rA + rO)/ 2 (rB* + rO) © XXXX American Chemical Society

where rB* is the appropriately weighted averaged ionic radii of the B site cations, cooperative rotations of the corner-sharing octahedra can occur, leading to lower symmetry.9 The magnitudes of these rotations are sensitive to the temperature and pressure, and in numerous cases, perovskites exhibit one or more structural phase transitions at different temperatures and/ or pressures.10−15 Recently, perovskites containing 4d and 5d elements including Tc, Ru, Os, and Ir have emerged as fertile ground for the exploration of novel phenomena driven, in part, by the strong spin−orbit coupling (SOC) of these elements that, especially for the 5d oxides, is comparable to the on-site Coulombic (U) and crystal-field interactions. This results in a delicate balance between interactions that drive complex magnetic and dielectric behavior and result in exotic states. A dramatic demonstration of this competition is the unusual jeff = 1 /2 Mott state observed recently in the layered iridate Sr2IrO4.16,17 Another example is the ferrimagnetic double perovskite Sr2CrOsO6, which has an exceptionally high transition, TC = 725 K.18 In this case, the presence of two magnetic ions Cr3+ 3d3 and Os5+ 5d3 makes unravelling the importance of the various interactions difficult.18 Consequently, Received: August 24, 2015

(1) A

DOI: 10.1021/acs.inorgchem.5b01905 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry researchers are exploring double perovskites that contain a single magnetic ion, such as Sr2YIrO619,20 and Sr2ScOsO6.21 The two Ir-containing double perovskites Ba2YIrO622,23 and Sr2YIrO624 were first described over 25 years ago and their structures at room temperature established. Cao and coworkers19 recently reported unexpected magnetism in Sr2YIrO6 that they ascribed to a combination of strong SOC and a noncubic crystal field. Taylor et al. concluded that SOC plays only a minor role in stabilizing the high magnetic transition temperature in isostructural Sr2ScOsO6.21 Evidently, there is the need for systematic studies of 4d- and 5d-based double perovskites to establish the importance of local distortions on their physical properties, and the current study of the series Ba2−xSrxYIrO6 is a step in this direction.

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Figure 1. Portions of the S-XRD profiles for 11 samples in the series Ba2−xSrxYIrO6 from x = 0 (top) to x = 2 (bottom) highlighting the changes in the cubic 331 and 420 reflections. The Ba content was decreased in steps of x = 0.2 across the series. The asterisk illustrates a peak from trace amounts of the Y2O3 impurity. The intensity of the (331) reflection near 25° is ∼1.4% that of the strongest reflection.

EXPERIMENTAL SECTION

Polycrystalline samples of 11 members in the series Ba2−xSrxYIrO6 (x = 0, 0.2, 0.4, ..., 1.8, 2) were prepared by solid-state reaction of BaCO3 (Aldrich, 99.999%), SrCO3 (Aldrich, 99.99%), Y2O3 (Aithaca, 99.999%), and Ir metal (Aithaca, 99.9%). Prior to weighing, BaCO3 and SrCO3 were dried at 150 °C and Y2O3 was heated at 1000 °C, each for 12 h. Appropriate stoichiometric mixtures of the reactants were finely ground, by hand, in an agate mortar and then transferred to alumina crucibles. The mixtures were heated in air at 650 °C for 12 h, at 750 °C for 12 h, and at 850 °C for 12 h with intermediate regrinding. The samples were next pressed into 20 mm pellets and heated in air at 1050 °C for 24 h, at 1200 °C for 72 h, and finally at 1350 °C for 72 h. The samples were cooled to room temperature in the furnace over a period of several hours. Powder Diffraction Measurements and Analysis. Synchrotron X-ray powder diffraction (S-XRD) data were collected over the angular range 5 < 2θ < 85°, using X-rays of wavelength 0.8251 Å, calibrated using a NIST SRM 660b LaB6 standard, on the powder diffractometer at Beamline BL-10 of the Australian Synchrotron.25 The samples were housed in 0.2-mm-diameter capillaries, which were rotated during the measurements. Temperature control was achieved using a Cyberstar hot-air blower (RT-1300 K) or Oxford Cryosystems Cryostream Plus (90−400 K). Once the control sensor had reached the set-point temperature, data collection was commenced after a 2 min thermal equilibration period; thermal stability was on the order ±2.0 K for all data collection temperatures. The data were obtained using a bank of 16 Mythen detectors, each of which covers around 5 deg 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 (NPD) patterns were collected using the high-intensity powder diffractometer Wombat at ANSTO’s OPAL facility at Lucas Heights.26 For these measurements, the samples (∼3 g each) were housed in thin-walled V cans. The wavelength of the incident neutrons, obtained using the symmetric reflection from a Ge 115 monochromator at 120° takeoff angle, was 1.505 Å. Lowtemperature NPD data were recorded using a CCR cryostat. Structural refinements, using the Rietveld method, were carried out using the GSAS27 program with the EXPGUI28 front end. The peak shapes were modeled using a pseudo-Voigt function, and the background was estimated using a 12-term-shifted Chebyschev function. The scale factor, detector zero point, lattice parameters, atomic coordinates, and atomic displacement parameters were refined together with the peak profile parameters. The wavelength for the NPD data set was released to ensure that the lattice parameters were determined by the higher-resolution synchrotron data in every case. Direct-current magnetic susceptibility data were measured using a Quantum Design Physical Properties Measurement System. Data were collected from 300 to 2 K in a field of 1000 Oe using the vibratingsample-magnetometry technique.

Figure 2. Composition dependence of portions of the NPD patterns for Ba2‑xSrxYIrO6 from x = 0 (top) to x = 2 (bottom) highlighting the development of superlattice reflections associated with cooperative tilting of the corner-sharing octahedra. The indices (top) are based on the Fm3m ̅ cell, and the letters (bottom) show the positions of selected R- and M-point reflections.

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RESULTS AND DISCUSSION Room Temperature Structures. A combination of highresolution S-XRD and NPD data has been used to initially establish the most appropriate space group and then ultimately refine accurate structures for some Ba2−xSrxYIrO6 double perovskites. The narrower peak widths in the former measurements provide sensitivity to small changes in the cell metric, while NPD is more sensitive to displacements of the anions. The exceptional signal-to-noise ratio afforded by the Xray diffractometer employed in this work allows us to identify weak reflections associated with expansion of the unit cell through octahedral tilting. Although the S-XRD data allow the space group symmetry to be identified, Rietveld refinements against such data often resulted in unacceptable large errors in the refined O-position parameters. This reflects the presence of the very heavy Ir5+ cations. Consequently, NPD was essential for accurate structural refinements. Because of the high neutron absorption cross section of Ir, NPD data were recorded using the high-intensity diffractometer Wombat. With this diffractometer, peak splitting due to symmetry lowering was not observed. The strategy followed here was to establish the metric from the synchrotron data and then to verify this by B

DOI: 10.1021/acs.inorgchem.5b01905 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. Representative Rietveld refinement profiles for S-XRD and NPD (inset) patterns of (a) Ba2YIrO6 and (b) Sr2YIrO6. The symbols are the experimental data, and the solid line is the fit to the profile. In each case, the difference between the observed and calculated profiles is given in the lower solid line, and the vertical markers show the positions of the space-group-allowed Bragg reflections.

(331) reflections are observed to increase in intensity for x > 0.6. The M-point reflections are associated with in-phase tilts and are first observed for x = 1.4, and this corresponds to the transition to P21/n as noted above. Whereas the synchrotron pattern at x = 0.8 could be modeled to a tetragonal structure with c/a > 1, at x = 1.0 the pseudotetragonal ratio was c/a < 1. This reversal of the pseudocubic c/a ratio has been observed in numerous perovskites and is often associated with a reorientation of the tilts.30 In the present case, it is due to the change from tetragonal I4/m(a0a0c−) (No. 87) to monoclinic I2/m(a−a−c0) (No. 12). There is no evidence from either the S-XRD or PND data for a rhombohedral R3̅ structure, as described by Xiang and co-workers.20 On the basis of the appearance of diagnostic superlattice reflections and peak splitting, coupled with successful Rietveld refinements, it was concluded that the sequence of phases observed in Ba2−xSrxYIrO6 is

Rietveld refinement against the combined S-XRD and NPD data sets. Portions of the S-XRD profiles, recorded at room temperature, for members of the Ba2−xSrxYIrO6 series are shown in Figure 1. There is a systematic shift in the position of the peaks to higher angles as the Sr content is increased, as expected given the smaller size of Sr2+ compared with Ba2+ (ionic radii 1.44 vs 1.61 Å for a coordination of 12).29 Attention is drawn to a number of changes in Figure 1. First, the presence of the R-point (331) reflection near 25° is indicative of the rock-salt-like ordering of the Y and Ir cations, whereas the (420) reflection near 26° is a fundamental perovskite reflection. Both of these peaks are observed to broaden around x = 0.6 and appear as a poorly resolved doublet with c/a > 1 at x = 0.8. Around x = 1.4, additional reflections emerge near 2θ = 24 and 26.6° corresponding to the M-point superlattice reflections 023/211 and 131/311 [index based on the P21/n (No. 14) monoclinic cell]. These peaks are indicative of in-phase tilts of the octahedra. The NPD profiles are equally informative (Figure 2). Recall that ordering of the B and B′ cations allows intensity at the Rpoint superlattice reflections and that their intensities are dependent on the difference in the scattering length of the two cations; in this case, Y has b = 7.75 fm and Ir has b = 10.6 fm. The R-point reflections are generally weak in the NPD profiles, with the strongest being 511/333 (indexed on the Fm3̅m cell) near 2θ = 56°. The R-point reflections also gain intensity from the out-of-phase tilting of the octahedra, and the (311) and

x = 0.6

x = 1.0

Fm 3̅ m(a 0a 0a 0) ⎯⎯⎯⎯⎯→ I 4/m(a 0a 0a−) ⎯⎯⎯⎯⎯→ I 2/m(a−a−c 0) x = 1.4

⎯⎯⎯⎯⎯→ P 21/n(a−a−c+)

Typical examples of the Rietveld refinements are illustrated in Figure 3, and the results of the refinements are summarized in Table 1. The refined structures of Ba2YIrO6 and Sr2YIrO6 are in reasonable agreement with those from earlier studies.19,22,24 The refinements showed no evidence for significant antisite disorder between the Y3+ and Ir5+ cations, reflecting the C

DOI: 10.1021/acs.inorgchem.5b01905 Inorg. Chem. XXXX, XXX, XXX−XXX

space group a (Å) b (Å) c (Å) β (deg) vol (Å3) Ax Ay Az Biso (Å2) Y Biso (Å2) Ir Biso (Å2) O1 x O1 y O1 z Biso (Å2) O2 x O2 y O2 z Biso (Å2) O3 x O3 y O3 z Biso (Å2) Ir−O1 (Å) Ir−O2 (Å) Ir−O3 (Å) Y−O1 (Å) Y−O2 (Å) Y−O3 (Å) B−Oavg (Å) Ir BVS Y BVS Y−O1−Ir (deg) Y−O2−Ir (deg) Y−O3−Ir (deg) B−O−B′avg (deg) tb θ (K) μobs (μB)

D

1.9619(15)

2.2078(15)

2.08485 5.29 3.60 180(0)

180

0.991

2.2083(9)

2.08730 5.23 3.60 180(0)

180

0.997 −15.22 0.18

579.98(4) 0.25 0.25 0.25 0.045(16) 0.149(29) −0.232(15) 0.26474(18) 0 0 0.374(24)

582.012(18) 0.25 0.25 0.25 0.062(11) 0.096(19) −0.137(10) 0.26449(11) 0 0 0.237(15)

1.9663(9)

0.2

Fm3m ̅ 8.33943(30)

0

Fm3m ̅ 8.34918(14)

0.986

180

2.08235 5.49 3.52 180(0)

2.2164(32)

1.9483(32)

577.874(24) 0.25 0.25 0.25 0.103(16) −0.023(27) −0.357(13) 0.2661(4) 0 0 0.82(6)

Fm3m ̅ 8.32935(20)

0.4

0.980

176.88

2.08075 5.39 3.61 180(0) 173.76(22)

2.187(10) 2.218(6)

1.969(10) 1.949(6)

287.716(9) 0 0.5 0.25 0.420(11) 0.208(18) −0.139(9) 0 0 0.2631(12) 1.62(27) 0.2526(7) 0.2798(11) 0 0.63(15)

I4/m 5.88319(10) 8.31262(15)

0.6

0.8

0.974

175.17

2.07850 5.29 3.70 180.000(0) 170.33(14)

2.198(8) 2.198(5)

1.948(8) 1.970(5)

286.084(15) 0 0.5 0.25 0.684(12) 0.421(19) 0.083(10) 0 0 0.2650(10) 1.01(16) 0.2427(6) 0.2849(7) 0 1.11(7)

I4/m 5.87345(17) 8.29292(25)

Table 1. Structural and Magnetic Properties for the Series Ba2−xSrxYIrO6a 1

0.969 0.00 0.26

170.38

2.07850 5.30 3.72 171.3(7) 169.45(23)

2.165(11) 2.213(4)

1.985(11) 1.951(4)

I2/m 5.85264(7) 5.87543(6) 8.27538(9) 90.0967(11) 284.563(5) 0.4956(7) 0 0.2512(4) 0.948(19) 0.592(30) 0.280(15) −0.0268(21) 0 0.2391(13) 3.66(24) 0.2392(10) 0.2291(8) 0.0225(5) 0.74(6)

x 1.2

0.963

168.13

2.07700 5.40 3.70 168.1(7) 168.15(31)

2.145(13) 2.227(5)

2.004(12) 1.932(5)

I2/m 5.83517(33) 5.86467(33) 8.2523(5) 90.040(4) 282.404(27) 0.4912(10) 0 0.2493(4) 0.811(27) 0.39(4) 0.135(19) −0.0369(22) 0 0.2414(15) 2.99(28) 0.2434(11) 0.2208(8) 0.0233(7) 0.93(9)

1.4

0.957

P21/n 5.82248(32) 5.8439(4) 8.2491(5) 89.995(4) 280.682(28) −0.0058(10) 0.4864(5) 0.2538(5) 0.920(28) 0.737(35) 0.431(16) 0.0250(9) 0.0010(14) 0.2385(12) 0.51(10) 0.2142(16) 0.2553(18) −0.0362(18) 2.06(27) 0.2567(17) 0.7832(16) −0.0411(18) 0.86(24) 1.973(10) 1.967(11) 1.989(11) 2.162(10) 2.214(10) 2.204(9) 2.08483 5.09 3.75 171.90(29) 161.0(7) 159.2(7) 164.03

1.6

0.952 0.00 0.20

P21/n 5.81067(25) 5.82727(27) 8.2334(4) 90.049(6) 278.787(22) −0.0094(8) 0.4820(4) 0.2536(5) 1.100(32) 0.574(32) 0.288(15) 0.0344(11) 0.0042(12) 0.2349(12) 1.29(12) 0.2051(14) 0.2708(20) −0.0378(13) 1.49(24) 0.2559(15) 0.7823(15) −0.0402(13) 0.81(19) 1.944(10) 2.002(10) 1.983(9) 2.192(10) 2.195(9) 2.197(8) 2.08550 5.10 3.73 168.8(4) 157.3(6) 159.8(6) 161.97

1.8

0.946 0.00 0.17

P21/n 5.79657(21) 5.81158(22) 8.21327(30) 90.023(5) 276.682(18) −0.0081(8) 0.47730(28) 0.2559(5) 0.997(30) 0.387(25) 0.019(11) 0.0483(14) 0.0096(11) 0.2329(14) 2.14(17) 0.2065(12) 0.2699(14) −0.0321(9) 0.46(18) 0.2619(13) 0.7914(13) −0.0426(10) 0.75(20) 1.934(11) 1.991(7) 1.974(8) 2.212(11) 2.180(7) 2.212(8) 2.08383 5.24 3.67 164.1(4) 159.5(5) 157.2(4) 160.27

2

0.941 0.00 0.16

P21/n 5.78084(23) 5.79806(21) 8.18904(30) 90.2205(14) 274.475(18) −0.0040(6) 0.47129(25) 0.2571(5) 1.072(28) 0.572(26) 0.091(11) 0.0610(12) 0.0066(9) 0.2355(8) 1.26(10) 0.2066(10) 0.2650(10) −0.0355(9) 0.87(13) 0.2762(10) 0.8038(12) −0.0403(9) 0.89(12) 1.960(6) 1.965(6) 1.998(7) 2.198(7) 2.194(6) 2.202(6) 2.08617 5.13 3.70 159.9(4) 159.67(34) 154.19(29) 157.92

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DOI: 10.1021/acs.inorgchem.5b01905 Inorg. Chem. XXXX, XXX, XXX−XXX

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0.12 0.13 0.19 0.30 1.29

In space group Fm3̅m, the A-site cation is at 8c (1/4, 1/4, 1/4), Ir at 4a (0, 0, 0), Y at 4b (1/2, 1/2, 1/2), and O at 24e (x, 1/4, 1/4); in I4/m, the A-site cation is at 4d (0, 1/2, 1/4), Ir at 2a (0, 0, 0), Y at 2b (0, 0, 1/2), O1 at 4e (0, 0, z), and O2 at 8h (x, y, 0); in I2/m, the A-site cation is at 4i (x, 0, z), Ir at 2a (0, 0, 0), Y at 2d (0, 0, 1/2), O1 at 4i (x, 0, z), and O2 at 8j (x, y, z); in P21/n, the A-site cation is at 4e (x, y, z), Ir at 2a (0, 0, 0), Y at 2d (0, 0, 1/2), O1 at 4e (x, y, z), O2 at 4e (x, y, z), and O3 at 4e (x, y, z). bSee the text for eq 1. cSee the text for eq 3.

appreciable differences in their size (0.90 and 0.57 Å, respectively) and charges. The difference in the ionic radii (0.23 Å) is mirrored in the difference between the average Y− O and Ir−O bond distances across the series; see Table 1. The composition dependence of the unit cell parameters is illustrated in Figure 4. The observed sequence of structures is consistent with the group theory analysis9 and mimics the sequence seen in other double perovskites, where the size of the A-site cation is systematically increased including the A2YRuO6,31 A2CoWO6,32 and A2InTaO633 series. The unit cell volume decreases as the Ba is replaced by Sr; however, as is evident from Figure 4, there is a noticeable deviation from Vegard’s law. This is a consequence of the octahedral tilting, effectively reducing the volume of the unit cell. The magnitude of this effect, or volume strain, can be estimated from the difference between the values of the volume extrapolated from the cubic structure and the observed volume. As illustrated in the inset to Figure 4, this strain increases linearly as the Sr content increases and does not appear to be impacted by successive phase transitions. This is similar to the behavior observed recently in the closely related ruthenate series Ba2−xSrxYRuO6.31 The lattice parameters refined here for Sr2YIrO6 are in good agreement with those reported recently by Cao and coworkers.19 These authors described the IrO6 octahedron as “significantly flattened since the bond distance between Ir and apical oxygen Ir−O3 (=1.9366 Å) is considerably shorter than the in-plane Ir−O1 and Ir−O2 bond distances (=1.9798 and 19723 Å, respectively)”. In the setting used here, O1 corresponds to the apical Ir−O bond along the c axis and O3 is in-plane. Irrespective of this, our work demonstrates that IrO6 is actually slightly elongated rather than compressed: Ir−O1 = 1.960(6) Å, Ir−O2 1.965(6) Å, and Ir−O3 1.998(7) Å. Cao et al.19 did not provide estimated standard deviations in their bond distances; however, it is reasonable to expect them to be at least as large as those reported here. NPD studies of Sr2YRuO634 and Sr2ScOsO621 reveal distortion of the BO6 octahedra similar to that observed here; that is, the BO6 octahedra cannot be described as flattened, although recent neutron diffraction studies of Sr2YOsO6 and Sr2InReO6 demonstrate the BO6 octahedra to be flattened.35,36 The reason for the change in the nature of the distortion is unclear; irrespective of this, distortion of the BO6 octahedra will generate a noncubic crystal field and lift the degeneracy of the partially filled t2g orbitals. Cao et al.19 proposed that this will impact the nature of the ground state because of competition between the SOC and crystal-field effects. The 5d orbitals are spatially extended, resulting in strong hybridization between the 5d and O 2p orbitals. Distortion of the BO6 octahedra favors t2g(π)−eg(σ) mixing that broadens the nd(Ir/Y)−2p(O) sub-bands. The bandwidth (W) can be estimated from the empirical relationship

W ≈ cos ω/l 3.5

(2)

where l is the average of the B−O and B′−O bond lengths and ω the average (Y, Ir)−O−(Y, Ir) bond angle. This formula was originally proposed to correlate structural and magnetic properties in manganites 37 but has subsequently been successfully used to quantify changes in A2FeMoO6 double perovskites.38 Whereas the average B−O distance appears to be essentially independent of the composition (see Figure 5), there is a complex compositional dependence of the bandwidth. W initially increases as Sr is introduced into the structure,

a

5.89 4.67

TIP (×10−4 emu/mol) dc

Table 1. continued

0

0.2

0.4

0.6

0.8

1

x

1.2

1.4 4.80

1.6

5.54

1.8

5.89

2

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.5b01905 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) Composition dependence of the unit cell parameters and (b) volume of Ba2−xSrxYIrO6. The dotted line is the best fit to the volume of the three cubic samples. In the inset is the excess volume, calculated as the difference between the value estimated from the linear fit and the observed value.

Figure 5. Composition dependence of the average B−O bond distance and bandwidth W in the series Ba2−xSrxYIrO6.

overbonded with BVS ∼ 3.65, and the introduction of tilting does not significantly impact this. It is expected that the tilting would negate the impact of the reduced unit cell volume on the average bond distances, and hence effective BVSs, and this is observed to occur in a number of perovskite systems, for example, in the series Ba1−xSrxSnO3.39 To a minor extent, this occurs for the Ir cation; although the Ir cations also show slight overbonding, this appears to be moderated by the octahedral tilting, and the BVSs for the Ir cations in the monoclinic P21/n structures are unexceptional. Evidently, the bonding requirements of the Ir cation dominate, and it is speculated that this reflects the small size/high charge of the IrV cation.

reaching a maximum near x = 1.0 that is close to the composition where the structure becomes monoclinic. This behavior is similar to that described recently for the related 4d oxides Ba2−xSrxYRuO6, where the composition dependence of W mirrors that of the bond distances.31 It appears that changes in the tilting play a more significant role in tuning the electronic properties of these analogous ruthenium oxides than observed here for the Ba2−xSrxYIrO6 iridium oxides. That the average B−O distance is approximately independent of Sr doping is not surprising and could suggest that tilting optimizes the bonding of the two B-site cations. Examination of the bond valence sums (BVSs) suggests that this is not the case. In all cases, the Y cation is observed to be significantly F

DOI: 10.1021/acs.inorgchem.5b01905 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

unexpected. The monoclinic phase persisted to ∼573 K, at which point the S-XRD profiles showed evidence for coexistence of the I2/m and I4/m phases. These two phases coexisted to ∼640 K, above which the sample appeared to be single phase, initially tetragonal I4/m and ultimately cubic Fm3̅m above 800 K. The I4/m → Fm3̅m transition appears to be continuous, as allowed by group theory. The sequence of phases observed here is the same as that seen in the composition-dependent studies described above. The influence of heating was also investigated for the x = 1.0 sample. As summarized in Figure 8, this underwent a I2/m to I4/m transition near 450 K, and instrument availability precluded the study at higher temperatures. Again it was observed that a plot of (β − 90)1/4 versus T was linear. Magnetic Susceptibilities. Variation in the tilts usually results in a significant difference in the magnetic properties of perovskites, as observed, for example, in the ruthenium perovskites, where CaRuO3 is metallic with increased paramagnetism and SrRuO3 is a ferromagnetic metal below 160 K.40 The temperature dependence of the zero-field-cooled (ZFC) magnetic susceptibilities (χ m ) for selected Ba2−xSrxYIrO6 samples is illustrated in Figure 9 and shows no evidence for long-range magnetic ordering down to 2 K. This is consistent with the NPD measurements described previously. There is no significant divergence of the field-cooled and ZFC susceptibilities. In a cubic field, Ir5+ is expected to have a lowspin t2g4eg0 (S = 1) configuration and, hence, is expected to be magnetic. Lowering of the symmetry from cubic, as seen for the Sr-rich oxides, will lift the degeneracy of the t2g orbitals; however, this is not expected to result in a diamagnetic state. Strong SOC will split t2g into energetically lower 4Jeff = 3/2 and higher 2Jeff = 1/2 states. The Jeff = 3/2 band will be filled by the d4 electrons, resulting in the appearance of an electrically insulating Jeff = 0 singlet ground state; see Figure 10. As is evident from Figure 9, the inverse susceptibility data do not obey the Curie−Weiss law. The observed behavior is similar to that described by Wakeshima et al.,24 who described this as arising from a combination of strong SOC and a small amount of a paramagnetic impurity. The temperature dependence of the magnetic susceptibilities was fitted to the equation

Figure 6. NPD Rietveld refinement profiles for Ba0.6Sr1.4YIrO6 at room temperature and 5 K. The structure at 5 K was fitted to a monoclinic I2/m model and that at 298 K to a tetragonal I4/m model.

Variable-Temperature Structures. The same sequence of structures as those described above was observed upon cooling the samples to 100 K, although the critical compositions were somewhat different. S-XRD measurements showed that the x = 0.4 sample is tetragonal at this temperature and the x = 0.8 sample is monoclinic in I2/m. NPD patterns were measured for two samples x = 0.0 and 0.4 at low temperature, 5 K, and in both cases, there was no evidence for any changes associated with long-range magnetic ordering. Magnetic susceptibility measurements described below also show no evidence for magnetic ordering. Subtle changes in the intensities of the Rpoint reflections were observed for the x = 0.4 sample (see Figure 6) associated with the transition from tetragonal I4/m to monoclinic I2/m observed in the S-XRD data. The structure of the x = 1.4 sample was monoclinic in P21/n at room temperature, and heating this to 373 K resulted in a transformation to I2/m (Figure 7). Because the P21/n to I2/m transition is continuous and is characterized by the loss of the weak M-point superlattice reflections, it is difficult to estimate the precise transition temperature. The monoclinic angle is observed to vary continuously through the transition, and the temperature dependence of this initially followed a T1/4 dependence, as expected for a continuous second-order phase transition. Given that the transition to the tetragonal I4/m structure must be first-order, this observation is somewhat

⎡ ⎤ ⎛ 0.375 ⎞ C ⎟ χcal = ⎢(1 − d) + TIP⎥ + d⎜ ⎣ ⎦ ⎝ T ⎠ T−θ

(3)

Figure 7. Temperature dependence of the lattice parameters of Ba0.6Sr1.4YIrO6 (x = 1.4). The hatched area illustrates the temperatures where both the I2/m and I4/m phases coexist. The solid line is the best fit to the equation of the type (β − 90)1/4 versus T. G

DOI: 10.1021/acs.inorgchem.5b01905 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 8. Temperature dependence of the lattice parameters of (BaSr)YIrO6 (x = 1) illustrating the reversal of the pseudotetragonal c/a ratio that accompanies the first-order monoclinic I2/m to tetragonal I4/m transition near 480 K. The solid line is the best fit to the equation of the type (β − 90)1/4 versus T.

Figure 9. Temperature dependence of the magnetic (black lines) and inverse (red lines) susceptibilities for Sr2YIrO6 and Ba2YIrO6.

speculated that this is associated with local disorder within the structure that is not evident from the diffraction studies. The non-Curie−Weiss-like behavior observed here and by Wakeshima et al.24 is in stark contrast to that described by Cao et al.,19 who reported Curie−Weiss behavior between 50 and 300 K and μeff = 0.91 μβ/Ir for their single-crystal sample of Sr2YIrO6. This value of μeff = 0.91 indicates that SOC is important but suggests that the ground state may be intermediate between the two extremes illustrated in Figure 10. Because the extended 5d orbitals are expected to lead to a greater coupling of the magnetic and electronic phenomena to the lattice than seen in the 3d (or even 4d) analogues, it is possible that the difference between our results and those of Cao et al.19 reflects small changes in the lattice. The suggestion is worthy of further investigation. Likewise, while a similarly large TIP term to that observed here was also seen in some AOsO3 perovskites,41 the origin of this remains unclear.

Figure 10. Ground state for a single Ir5+ 5d4 ion when the crystal field dominates, giving a S = 1 ground state, and when strong SOC dominates, giving a Jeff = 0 ground state.

where d indicates the amount of paramagnetic impurity, TIP is a temperature-independent contribution to the magnetic susceptibility, and C and θ are Curie and Weiss constants, respectively. Fitting of the susceptibilities suggested that θ was approximately zero and that the TIP term was nontrivial ∼5 × 10−04 emu mol−1. A similar result was reported for the isoelectronic osmium(IV) perovskites AOsO3 (A = Ca, Sr, Ba).41 The effective magnetic moment per Ir is 0.16 μβ/Ir, which is much less that the value expected for a conventional S = 1 system (2.83 μβ/Ir), demonstrating that the ground state is dominated by strong SOC. For Sr2YIrO6, this was estimated to be ∼0.1% of a S = 1 impurity, with the other samples having similar amounts. It is

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CONCLUSION

In summary, the structures of 11 members in the series Ba2−xSrxYIrO6 have been refined against combined S-XRD and NPD data sets. Replacing the larger Ba cation with Sr introduces cooperative tilting of the corner-sharing octahedra, with the sequence of structures being H

DOI: 10.1021/acs.inorgchem.5b01905 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry x = 0.6

x = 1.0

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Fm 3̅ m(a 0a 0a 0) ⎯⎯⎯⎯⎯→ I 4/m(a 0a 0a−) ⎯⎯⎯⎯⎯→ I 2/m(a−a−c 0) x = 1.4

⎯⎯⎯⎯⎯→ P 21/n(a−a−c+)

Increasing the temperature results in a reduction in the tilting with the same sequence as that observed in Ba0.6Sr1.4YIrO6. Magnetic susceptibility measurements showed no evidence for long-range magnetic ordering, an observation supported by neutron diffraction measurements. Although IrV has a low-spin t2g4eg0 (S = 1) configuration and, hence, is expected to be magnetic, strong SOC results in a Jeff = 0 ground state.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or brendan.kennedy@ sydney.edu.au. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was, in part, performed at the powder diffraction beamline at the Australian Synchrotron. B.J.K. acknowledges support of the Australian Research Council for this work.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b01905 Inorg. Chem. XXXX, XXX, XXX−XXX