The Crystal Structure and Magnetic Behavior of Quinary Osmate and

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

The Crystal Structure and Magnetic Behavior of Quinary Osmate and Ruthenate Double Perovskites LaABB′O6 (A = Ca, Sr; B = Co, Ni; B′ = Ru, Os) Ryan Morrow,*,†,∥ Michael A. McGuire,‡ Jiaqiang Yan,‡,§ and Patrick M. Woodward*,† †

Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210-1185, United States Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States ‡

S Supporting Information *

ABSTRACT: Six LaABB′O6 (A = Ca, Sr; B = Co, Ni; B′ = Ru, Os) double perovskites were synthesized, several for the first time, and their crystal structures and magnetic behavior were characterized with neutron powder diffraction and direct-current and alternating-current magnetometry. All six compounds crystallize with P21/n space group symmetry, resulting from a−a−c+ octahedral tilting and complete rock salt ordering of transition-metal ions. Despite the electronic configurations of the transition-metal ions, either d8−d3 or d7−d3, not one of the six compounds shows ferromagnetism as predicted by the Goodenough−Kanamori rules. LaSrNiOsO6, LaSrNiRuO6, and LaCaNiRuO6 display long-range antiferromagnetic order, while LaCaNiOsO6, LaCaCoOsO6, and LaSrCoOsO6 exhibit spin-glass behavior. These compounds are compared to the previously studied LaCaCoRuO6 and LaSrCoRuO6, both of which order antiferromagnetically. The observed variations in magnetic properties can be attributed largely to the response of competing superexchange pathways due to changes in B−O−B′ bond angles, differences in the radial extent of the 4d (B′ = Ru) and 5d (B′ = Os) orbitals, and filling of the t2g orbitals of the 3d ion.

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INTRODUCTION Double perovskites are site-ordered variants of the simpler ternary perovskite structure. With an empirical formula of A2BB′O6, the structure consists of corner-sharing BO6 and B′O6 octahedra, most commonly ordered in a rock salt fashion.1 Hundreds of double perovskites2 have been successfully synthesized and studied, some of which possess technologically desirable properties such as spin-polarized transport,3 multiferroicity,4 and high-temperature magnetism.5−7 The vast compositional range of double perovskites makes them an excellent playground to test the Goodenough− Kanamori (G-K) rules,8,9 which have been used for decades to predict the magnetic coupling of transition-metal cations though a shared ligand. As shown in Figure 1, each B cation sublattice forms its own pseudo-face-centered cubic (FCC) lattice, whose magnetic interactions take place between nearest neighbor (NN) cations through a B−O−O−B pathway (JNN), and between next nearest neighbors (NNN) through a B−O− B′−O−B pathway (JNNN). The magnetic ground state that results from the competition of these two superexchange pathways has been extensively studied. Theoretical mean-field treatments10,11 predict a phase diagram, where a type II antiferromagnetic (AFM-II) structure is stabilized when antiferromagnetic JNNN interactions are dominant.12 When the JNN interactions are antiferromagnetic and dominant the © XXXX American Chemical Society

ground state depends on the sign of the JNNN interactions: type I antiferromagnetic (AFM-I) if JNNN is ferromagnetic or type III antiferromagnetic (AFM-III) if JNNN is antiferromagnetic. More recently Taylor et al. argue that the AFM-I structure is stabilized by a combination of antiferromagnetic nearest neighbor JNN interactions and anisotropy induced by spin−orbit coupling.13 Experimentally the AFM-I structure is often seen in double perovskites, where the only magnetic ion is a 4d or 5d cation,14,15 while the AFM-II structure is common in the cases where it is a 3d cation with a half-filled set of eg orbitals.16,17 In double perovskites, where both B and B′ are magnetic ions, superexchange between the B and B′ sublattices (JBB′) further complicates the situation, as it can stabilize either ferrimagnetic (FiM) or ferromagnetic (FM) behavior, depending upon its sign. The numerous available magnetic ground states, as well as the potential for competition between the various exchange pathways, creates a rich landscape of magnetic behaviors that depends sensitively on the electronic configuration of the magnetic cations and subtle distortions of the lattice. Received: September 5, 2017

A

DOI: 10.1021/acs.inorgchem.7b02282 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

understanding, even a qualitative one, of the magnetism in these complex mixed metal oxides. In this study, we report the structure and magnetic properties of a series of double perovskite compounds with formula LaABB′O6 (A = Ca, Sr; B = Co, Ni; B′ = Ru, Os). As these compounds have electronic configurations of either d8−d3 (B = Ni) or d7−d3 (B = Co), the G-K rules would predict an FM ground state, but as shown in this study the reality is much more complex. The title series of compounds is completed by comparison to the previously studied LaCaCoRuO6 and LaSrCoRuO6.29,30 The magnetic behavior of these compounds sheds light on how factors such as the filling of the t2g orbitals of the 3d ion, the spin−orbit coupling and spatial extent of the 4d versus the 5d orbitals, and the degree of bending of the B− O−B′ bond angles impact the magnetic properties of oxides containing transition-metal ions from different rows of the periodic table.

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EXPERIMENTAL SECTION

Powder osmate samples of a maximum size of 1.6 g and ruthenate samples of ∼3.0 g were prepared via solid-state reaction by combining stoichiometric quantities of La2O3 (99.99%, GFS, dried overnight at 1000 °C prior to use), SrO2 (98%, Sigma-Aldrich), CaO (99.9%, Sigma-Aldrich), SrCO3 (99.9%, Sigma-Aldrich), CaCO3 (99.846%, Mallinckrodt), Co3O4 (99.8%, Fischer Scientific), NiO (99.998%, Alfa Aesar), Os (99.98%, Alfa Aesar), and RuO2 (99.95%, Alfa Aesar) according to the following chemical equations: 3La 2O3 + 6SrO2 + 2Co3O4 + 6Os + (10PbO2 ) → 6LaSrCoOsO6 + (10PbO) + 3/2O2 3La 2O3 + 6CaO + 2Co3O4 + 6Os + (16PbO2 ) → 6LaCaCoOsO6 + (16PbO) + 3/2O2 La 2O3 + 2SrO2 + 2NiO + 2Os + (4PbO2 ) → 2LaSrNiOsO6 + (4PbO) + 1/2O2 La 2O3 + 2CaO + 2NiO + 2Os + (6PbO2 ) → 2LaCaNiOsO6 + (6PbO) + 1/2O2

Figure 1. (upper half) AFM-I and AFM-II structures commonly found on the double perovskite pseudo-FCC when only either B or B′ is a magnetic transition-metal cation. For AFM-I, any given magnetic cation has eight NN AFM pairs and four FM NN pairs while having six FM NNN pairs. For AFM-II, it has six NN AFM pairs and six FM NN pairs while having six of six NNN AFM pairs. (lower half) Ferro- and ferrimagnetism that result from heteroatomic coupling. Compound examples that manifest each type of magnetic behavior are given.

La 2O3 + 2SrCO3 + 2NiO + 2RuO2 → 2LaSrNiRuO6

La 2O3 + 2CaCO3 + 2NiO + 2RuO2 → 2LaCaNiRuO6 For osmate samples, each ground mixture was contained in an alumina tube and placed into a silica tube (volume of ∼40 mL with 3 mm thick walls) along with an additional vessel containing PbO2. The PbO2 was included as an in situ source of O2 gas, as it decomposes at elevated temperatures to form O2 and PbO. One quarter mole excess O2 gas per mole of product was produced to ensure complete oxidation of the product. The silica tube was subsequently evacuated and sealed before being placed into a box furnace located within a fume hood and heated for 1000 °C for a period of 48 h. Great care must be used when heating Os-containing mixtures or compounds due to the possibility of producing toxic OsO4 gas. Also note that larger batch sizes necessitate formation of more O2 gas, resulting in an increased risk of exploding the sealed tube. Ruthenate samples were produced by heating ground mixtures in air at 1250 °C for a period of 48 h. Laboratory X-ray diffraction diffraction (XRD) measurements were collected utilizing a Bruker D8 Advance instrument equipped with a Ge(111) monochromator. Neutron powder diffraction (NPD) data sets were collected at 10 and 300 K for each sample at the Oak Ridge National Laboratory’s POWGEN31 beamline. For Sr-containing compounds high and low d spacing frames of 0.2760−3.0906 and 2.2076−10.3019 Å were collected, while Ca-containing compounds

Ferromagnetism in insulating double perovskites is quite rare, with A2NiMnO6 (A = Bi, La) and A2CoMnO6 (A = Bi, La) being the best-known examples.18−21 As these compounds have a high-spin 3d7−3d3 or a 3d8−3d3 electronic configuration, with Co/Ni and Mn cations being in +2 and +4 oxidation states, respectively, FM eg-based superexchange is anticipated from the G-K rules. However, double perovskites that have similar electronic configurations, yet contain transition metals from different rows of the periodic table, exhibit considerably different properties. Two such examples are Sr2CoOsO6 (3d7−5d2) and Sr2FeOsO6 (3d5−5d3), both of which undergo multiple AFM transitions on cooling from room temperature.23−26 Furthermore, upon substitution of Ca for Sr in both series, a crossover to ferrimagnetism is observed, demonstrating strong sensitivity to relatively subtle distortions of the crystal structure.26−28 Obviously further study is needed to develop an B


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Inorganic Chemistry utilized high and low d spacing frames of 0.2760−4.6064 and 2.2076− 15.3548 Å. The difference in frame scales is due to the addition of detectors to the instrument between the periods when each sample set was measured. Data were collected for periods of 1 and 2 h for low and high d spacing frames, respectively. Rietveld refinements were conducted using the GSAS EXPGUI32,33 software package, and representational analysis of magnetic structures were performed using SARAh.34 The temperature dependence of the direct-current (dc) magnetization of each sample was measured under both field-cooled (FC) and zero-field-cooled (ZFC) conditions with applied fields of 1 and 20 kOe using a Quantum Design Magnetic Property Measurement System (MPMS) superconducting quantum interference device (SQUID) magnetometer. The measurements with a 1 kOe field were collected in the temperature range from 5 to 300 K for LaCaNiOsO6 and LaCaNiRuO6. A somewhat larger temperature range of 5−400 K was utilized for the other compounds. The 20 kOe measurements were performed over a temperature range of 2−320 K for all samples. Fielddependent dc demagnetization experiments were conducted at temperatures of 2 and 200 K for each compound, beginning at maximum fields of 55 and 70 kOe and measuring the magnetization on incrementally adjusting the applied field to zero. The low-temperature field-dependent measurements were conducted following both FC and ZFC conditions. Polycrystalline samples were contained in gel capsules and mounted in straws for insertions into the device. Corrections due to the diamagnetic signal of the sample holder were not necessary due to the magnitude of the response of the sample. Heat capacity data were collected using the relaxation method with a Quantum Design Physical Property Measurement System (PPMS) instrument equipped with the heat capacity option. The alternating-current (ac) magnetization as a function of temperature and frequency was collected with an applied dc field of 0 Oe and ac field of 10 Oe. Sintered chunks of each compound (sintered at synthesis temperatures overnight, in evacuated tubes for osmate samples) were used for these measurements. Data were collected using the ac susceptibility option of a PPMS instrument.

contrast between transition-metal cations. In all cases full B/B′ order was attained, while the La and Sr/Ca cations were randomly distributed. Refinements of site occupancies did not indicate detectable loss of Os nor the presence of O vacancies. Refined XRD data sets are available in the Supporting Information. Refined 300 K NPD patterns for each of the compounds are displayed in Figure 3, while the results of those refinements are given in Tables 1 and 2. The B−O bond lengths can be used to determine the approximate oxidation state of the transitionmetal cations. The bond valence sums39 for Co or Ni, given below the B−O bond lengths of Tables 1 and 2, are consistent with the assignment of a +2 oxidation state to the Ni and Co ions. To balance the charge, Os or Ru must therefore be in the +5 oxidation state. Comparison of the average bond length for Co, Ni, Ru, and Os to those in similar double perovskites5,23−30,40,41 as well as the distances predicted by ionic radii42 confirm the assignment of high spin (HS) Co2+, Ni2+, Os5+, and Ru5+ throughout the series of compounds. Therefore, the electronic configuration of the transition metals in the LaACoB′O6 compounds is (HS) d7−d3, and the electronic configuration in the LaANiB′O6 compounds is d8−d3. For a given B cation radius (or BB′ combination average), there is an ideal A cation size, as defined by a tolerance factor equal to 1, where the A cation perfectly fits the cubic network of corner-sharing octahedra.43 When the A cation size is smaller, and the tolerance factor becomes less than 1, an octahedral tilting distortion typically occurs to reduce the effective coordination number of the A cation and alleviate its underbonding. The ionic radius42 of eight coordinate La3+ is only 0.04 Å greater than that of Ca2+, while Sr2+ is 0.10 Å larger than La3+. The 12-coordinate radii, 1.34 Å (Ca2+), 1.36 Å (La3+), and 1.44 Å (Sr2+), follow a similar trend. Hence, the tolerance factors of the LaCaBB′O6 compounds closely resemble similar Ca2BB′O6 osmates, such as Ca2NiOsO6 and Ca2CoOsO6. As such, the magnitude of octahedral tilting and average value of ∠B−O−B′ bond angles should be comparable to those compounds allowing for direct comparison of their structure−property relationships.28,40 For LaSrBB′O6 compounds the magnitude of tilting and average ∠B−O−B′ bond angles are intermediate between the monoclinic Ca2BB′O6 and the tetragonal Sr2BB′O6 osmate double perovskites.40,44 The temperature dependence of the dc magnetization is shown in Figure 4. Each of the data sets exhibits features likely corresponding to antiferromagnetic or glassy transitions, while LaSrNiOsO6 and LaCaCoOsO6 have additional secondary features at higher temperatures that appear to be weak and ferromagnetic (FM) in nature. The temperature-dependent dc magnetization curves can be modeled with a linear Curie− Weiss fit in the higher-temperature paramagnetic regime. Higher-field dc magnetization versus temperature plots and their Curie−Weiss fits are available in the Supporting Information. The parameters extracted from the Curie−Weiss fits are given in Table 3. The effective paramagnetic moments can be compared to the spin-only calculated values also found in this table. The spin-only values are calculated as μspin = [μspin(B)2 + μspin(B′)2]1/2 using the electronic configurations of HS d7−d3 for the Co-containing compounds and d8−d3 for the Ni-containing compounds. For the compounds that contain Co, the effective moments are slightly higher than the calculated spin-only values. This can be explained by a positive orbital contribution16 for HS Co2+, while the orbital contribution for the d3 cation (Os5+/Ru5+)15,41

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RESULTS Using a previously outlined strategy35 of indexing double perovskite diffraction data, each of the six compounds could be readily assigned to the common monoclinic double perovskite space group P21/n.36 This space group symmetry comes about from the combination of octahedral tilting described by the Glazer tilt system37 a−a−c+ and rock salt ordering of the octahedral cations (Figure 2). Because of the very similar neutron scattering lengths38 of Ni and Os, cation ordering was quantified using the laboratory XRD data to provide maximum

Figure 2. Example crystal structure of a double perovskite in the P21/n space group showing views down the (a) 110 and (b) 001 directions highlighting the a−a−c+ tilt system used to model each of the reported structures. Green, red, gray, and purple spheres correspond to B, B′, A, and O sites for a given A2BB′O6 formula. C


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Inorganic Chemistry

Figure 3. Refined low d-spacing NPD patterns collected at room temperature. The black symbols, red curve, and blue curve correspond to the observed data, calculated pattern, and difference curve, respectively. The upper hashes correspond to allowed reflection of the main phase, while the lower hashes in (f) are attributed to a 2.7(3)% NiO impurity phase.

recent report of this compound, including the high-temperature divergence.45 The higher d-spacing NPD frames were examined for the presence of magnetic scattering at low temperatures. A set of Bragg reflections not present at room temperature was observed in three of the six compoundsLaSrNiOsO6, LaSrNiRuO6, and LaCaNiRuO6while no observable magnetic scattering intensity was found in the remaining three compounds. Plots highlighting the magnetic scattering by comparison of the 300 and 10 K data sets, as well as the difference of the two, are available as Supporting Information. The magnetic structures of the three long-range spin-ordered compounds were determined to be similar, each with a k vector of (1/2, 0, 1/2) and moments oriented in the ac plane, very similar to the AFM-II ordering of the Co-sublattice found in Sr2CoOsO6.24 Refinements were conducted using a variety of potential combinations of moments on the Ni and B′ sublattices, and the results of each fitting are included as Supporting Information. Refinements with moments exclusively on the Ni2+ site were marginally better than analogous refinements, where the moments were either constrained to be entirely on the Ru/Os site or spread over both transition-metal

should be relatively small. For the Ni-containing compounds, the orbital contribution from both Ni2+ and Os5+/Ru5+ cations should be small, because the orbital angular momentum is zero for the ground-state electron configuration of both ions. However, the experimentally determined effective moments are observed to be somewhat decreased from the calculated spinonly values. As any orbital contribution involving the Ni2+ ion would result in positive orbital moment, the reduction in effective moment can be attributed to Os5+/Ru5+ orbital contribution, which would be expected to be negative in sign. This is supported by the greater reduction in effective moment in the osmates presented here with respect to their ruthenate counterparts, where the strength of the spin−orbit coupling is lessened. The compounds containing Ca2+ have negative Weiss constants with values ranging from −123 K (LaCaNiOsO6) to −156 K (LaCaCoOsO6). The Weiss constant becomes significantly less negative when Ca2+ is replaced by Sr2+, signaling either a reduction in AFM coupling or an increase in FM coupling in response to increasing the tolerance factor. The temperature dependence of the magnetization of LaSrNiOsO6 and resulting Curie−Weiss parameters are quite similar to the D


Article

Inorganic Chemistry Table 1. Refinement Parameters of the LaSrBB′O6 Compounds from NPD LaSrCoOsO6 10 K

LaSrNiOsO6 300 K

10 K

LaSrNiRuO6 300 K

10 K

300 K

space group

P21/n

P21/n

P21/n

P21/n

P21/n

P21/n

a (Å) b (Å) c (Å) V (Å3) β (deg) Rwp B−O1 (×2) B−O2 (×2) B−O3 (×2) B BVS B′−O1 (×2) B′−O2 (×2) B′−O3 (×2) ∠B−O1−B′ ∠B−O2−B′ ∠B−O3−B′ La/Sr x La/Sr y La/Sr z O1 x O1 y O1 z O2 x O2 y O2 z O3 x O3 y O3 z

5.6023(2) 5.5948(1) 7.9027(2) 247.70(1) 90.003(5) 3.24% 2.076(4) 2.068(4) 2.062(4) 2.17 1.956(4) 1.959(4) 1.967(4) 158.1(4) 158.8(4) 157.4(1) 0.0062(5) −0.0303(2) 0.2508(6) 0.2077(7) 0.2230(9) 0.0337(9) 0.2268(7) 0.2128(9) 0.4644(9) 0.5700(4) 0.0089(5) 0.2561(5)

5.6103(2) 5.5952(2) 7.9186(3) 248.57(1) 90.020(4) 3.11% 2.070(4) 2.050(3) 2.087(4) 2.17 1.957(4) 1.977(3) 1.949(4) 159.4(3) 159.4(3) 157.6(2) 0.0041(7) −0.0271(3) 0.2510(6) 0.2118(8) 0.225(1) 0.0341(8) 0.2245(8) 0.2152(9) 0.4675(7) 0.5672(5) 0.0095(6) 0.2593(6)

5.586 10(7) 5.561 20(7) 7.8735(1) 244.595(7) 90.006(4) 2.51% 2.040(4) 2.029(4) 2.039(7) 2.14 1.973(4) 1.974(4) 1.970(7) 158.3(3) 159.7(3) 158.2(1) 0.0056(3) −0.0275(1) 0.2513(6) 0.2122(6) 0.2204(8) 0.0345(7) 0.2242(7) 0.217(1) 0.4662(7) 0.5673(3) 0.0088(4) 0.2546(9)

5.594 82(7) 5.567 64(7) 7.8841(1) 245.590(7) 90.016(3) 2.97% 2.046(4) 2.037(4) 2.031(6) 2.13 1.959(4) 1.973(4) 1.979(6) 160.4(3) 159.6(3) 158.8(1) 0.0061(3) −0.0238(2) 0.2516(8) 0.2141(7) 0.2250(9) 0.0305(7) 0.2269(7) 0.218(1) 0.4641(6) 0.5655(3) 0.0067(4) 0.2534(8)

5.567 25(5) 5.538 43(5) 7.840 22(7) 241.744(5) 90.026(2) 3.89% 2.037(3) 2.032(2) 2.034(3) 2.15 1.953(2) 1.949(2) 1.952(3) 159.5(3) 161.0(3) 159.1 (3) 0.0048(2) −0.0252(1) 0.2497(5) 0.2154(5) 0.2261(6) 0.0345(5) 0.2278(5) 0.2168(6) 0.4686(5) 0.5644(2) 0.0081(3) 0.2554(5)

5.578 17(6) 5.543 84(6) 7.851 17(9) 242.791(7) 90.033(2) 3.03% 2.037(3) 2.030(3) 2.038(4) 2.14 1.953(3) 1.952(3) 1.950(4) 160.5(2) 161.9(2) 159.68(9) 0.0042(3) −0.0218(2) 0.2494(5) 0.2177(6) 0.2282(7) 0.0334(6) 0.2291(6) 0.2187(6) 0.4698(6) 0.5627(3) 0.0067(4) 0.2557(5)

LaSrNiRuO6, and LaCaNiRuO6, respectively. Furthermore, these temperatures correspond to the ZFC maximum in LaSrNiOsO6, the inflection in the middle of the FC/ZFC divergence in LaSrNiRuO6, and the single cusp in LaCaNiRuO6 found in the dc magnetization data of Figure 4b,c,f. The apparent lack of magnetic reflections in the lowtemperature NPD patterns for the other three compounds, as seen in Figure 5a,d,e, suggests the possibility of spin-glass behavior. To explore this further, ac magnetization data as a function of frequency and temperature were collected (Figure 7). In each compound, features in the ac magnetization data were apparent at temperatures analogous to those observed in the dc magnetization data. The temperature at which the ac magnetization peaks is frequency-dependent for LaCaNiOsO6, LaCaCoOsO6, and LaSrCoOsO6 but not for the other three compounds. The division into two subsets corresponds directly to those that demonstrated magnetic neutron scattering and those that did not. Thus, it can be concluded that LaCaNiOsO6, LaCaCoOsO6, and LaSrCoOsO6 are spin glasses, while LaSrNiOsO6, LaSrNiRuO6, and LaCaNiRuO6 are AFM (or at least have partial AFM order). The frequency dependence of the ac magnetization peak for LaCaNiOsO6, LaCaCoOsO6, and LaSrCoOsO6 is shown in Figure 8. Fitting of this dependence to the form f(Tpeak) = f 0[exp(−EA/kBT)], where f is frequency, and kB is the Boltzmann constant, allows for the extraction of the activation energy EA. These fits are represented by black lines through each data set in Figure 8, yielding values of 1661, 2374, and 2139 K for LaSrCoOsO6, LaCaCoOsO6, and LaCaNiOsO6, respectively.

ions. While we cannot definitively rule out the presence of a small moment on the Ru/Os site, the size of the moment and the observation of an AFM-II structure are consistent with analogous A2NiB′O6 perovskites,17 where B′ is a diamagnetic d0 ion. Therefore, we choose to report the results of refinements where the Ni2+ ion carries the moment. In the related double perovskites LaSrCoRuO6 and LaCaCoRuO6,29,30 an AFM-II structure was seen, but similar ambiguities prevented a definitive assignment of the individual Co2+ and Ru5+ moments. The refined Ni moments were 0.77(6), 1.13(3), and 1.62(8) μB for LaSrNiOsO6, LaSrNiRuO6, and LaCaNiRuO6, respectively. The increasing size of the ordered Ni moment is directly evident in the series from the increasing relative intensity of the (1/2, 0, 1/2) magnetic reflection near 9 Å in Figure 5b,c,f. Previous studies of Sr2NiMoO6 and Ba2NiMoO6, both of which also adopt an AFM-II configuration, revealed Ni2+-ordered moments of 1.92(6) and 2.04(6) μB, respectively, both near the expected value for a spin-only Ni2+ cation.17 To identify the Néel temperature of the three compounds demonstrating long-range magnetic ordering through Bragg reflections of magnetically scattered neutrons, heat capacity measurements were conducted as a function of temperature. This was necessary due to the possibility of multiple interpretations of the features of the dc magnetization versus temperature data. For each of the three compounds, as shown in Figure 6, lambda anomalies corresponding to second-order phase transitions normally associated with the onset of longrange magnetic ordering were observed. These corresponded to Néel temperatures of 65, 93, and 94 K for LaSrNiOsO6, E


Article

Inorganic Chemistry Table 2. Refinement Parameters of the LaCaBB′O6 Compounds from NPD LaCaCoOsO6 10 K

LaCaNiOsO6 300 K

10 K

LaCaNiRuO6 300 K

10 K

300 K

space group

P21/n

P21/n

P21/n

P21/n

P21/n

P21/n

a (Å) b (Å) c (Å) V (Å3) β (deg) Rwp B−O1 (×2) B−O2 (×2) B−O3 (×2) B BVS B′−O1 (×2) B′−O2 (×2) B′−O3 (×2) ∠B−O1−B′ ∠B−O2−B′ ∠B−O3−B′ La/Ca x La/Ca y La/Ca z O1 x O1 y O1 z O2 x O2 y O2 z O3 x O3 y O3 z

5.5143(2) 5.5996(2) 7.8104(3) 241.17(2) 89.951(8) 2.99% 2.083(3) 2.076(3) 2.064(3) 2.14 1.967(2) 1.967(3) 1.960(3) 152.0(2) 152.8(2) 152.1(1) 0.0091(5) −0.0450(2) 0.2504(5) 0.1972(6) 0.2138(6) 0.0440(5) 0.2146(5) 0.2016(6) 0.4561(4) 0.5850(4) 0.0227(4) 0.2568(4)

5.5239(2) 5.5946(2) 7.8342(2) 242.11(2) 89.981(7) 2.81% 2.086(2) 2.083(3) 2.067(3) 2.11 1.964(2) 1.957(3) 1.962(3) 152.2(2) 153.3(2) 152.9(1) 0.0064(6) −0.0423(2) 0.2510(5) 0.1978(6) 0.2148(6) 0.0440(4) 0.2159(6) 0.2005(6) 0.4579(5) 0.5825(4) 0.0222(4) 0.2568(4)

5.4949(2) 5.5609(2) 7.7940(3) 238.16(2) 90.141(5) 3.20% 2.043(5) 2.062(4) 2.035(6) 2.08 1.968(5) 1.971(5) 1.973(5) 154.1(3) 151.5(3) 152(2) 0.0089(7) −0.0433(2) 0.2483(7) 0.2144(8) 0.205(1) 0.0411(6) 0.1991(8) 0.212(1) 0.4242(7) 0.5831(5) 0.0196(4) 0.2460(7)

5.5053(2) 5.5629(2) 7.8084(3) 239.09(2) 90.104(6) 3.23% 2.038(5) 2.062(6) 2.029(7) 2.10 1.979(5) 1.967(6) 1.981(7) 153.9(3) 152.4(3) 153.6(2) 0.0089(7) −0.0404(2) 0.249(1) 0.212(1) 0.205(1) 0.0411(7) 0.200(1) 0.214(1) 0.4563(9) 0.5809(6) 0.0190(5) 0.2469(9)

5.4864(1) 5.5546(1) 7.7780(2) 237.09(1) 89.929(5) 4.75% 2.051(3) 2.035(3) 2.011 (5) 2.16 1.963(3) 1.971(3) 1.985(5) 153.1(3) 154.1 (3) 153.5 (2) 0.0088(5) −0.0419(2) 0.2504(6) 0.2148(7) 0.2045(7) 0.0444(6) 0.2047(6) 0.2141(7) 0.4592(6) 0.5810(5) 0.0195(3) 0.2482(6)

5.4981(2) 5.5544(2) 7.7965(2) 238.09(2) 89.929(4) 4.26% 2.054(4) 2.040(3) 2.007(5) 2.15 1.953(4) 1.979(3) 1.994(5) 154.4(3) 153.5(3) 153.9(2) 0.0074(6) −0.0394(2) 0.2478(7) 0.2163(8) 0.2039(8) 0.0404(8) 0.2038(6) 0.2132(6) 0.4582(7) 0.5799(5) 0.0186(3) 0.2490(6)

spin d7−d3 electronic configurations should lead to a ferromagnetic (FM) ground state, as observed for isoelectronic double perovskite analogues containing only 3d ions: A2NiMnO6 (A = Bi, La) and A2CoMnO6 (A = Bi, La).18−21 Clearly FM heteroatomic coupling is not dominant here, due in part to the diminished strength of JBB′ resulting from the energetic mismatch of the eg orbitals of the dissimilar transitionmetal cations. The compounds studied here are not anomalies either. Antiferromagnetism is observed for a number of 3d/5d double perovskites, including Sr 2 FeOsO 6 (3d 5 −5d 3 ), Sr2CoOsO6 (3d7−5d2), and Sr2NiOsO6 (3d8−5d2).23−26,40 That is not to say that B−O−B′ coupling between 3d and 4d/5d ions is always weak. Heteroatomic coupling involving transition-metal t2g orbitals and O 2p π orbitals leads to hightemperature ferrimagnetic ordering in Sr2 CrOsO6 and Ca2CrOsO6.5,6 However, the completely filled t2g orbitals of the 3d8 Ni2+ ion prevent effective coupling through this orbital pathway, and while it might play some role in those compounds containing Co2+ it is still likely to be relatively weak. To explain the ferrimagnetism observed in Ca2BOsO6 (B = Fe, Co, Ni)22,26−28,40 double perovskites, a predominantly AFM superexchange between the 3d B eg orbitals and 5d B′ t2g orbitals has been proposed.22 Because these orbitals are orthogonal in the undistorted cubic structure, such an exchange pathway is strengthened by bending the B−O−B′ bond further away from 180°. The recent report of a modulated magnetic structure with a metamagnetic transition to ferromagnetism above an applied field of 21 kOe in cubic Ba2NiOsO6 lends further support to the claim that the sign and strength of 3d−

To further investigate the nature of the FC and ZFC divergences in the temperature-dependent dc magnetization data, field-dependent demagnetization measurements were conducted above (T = 200 K) and below (T = 2 K) the magnetic transition temperature for each compound. These data are shown in Figure 9. Each of the designated spin-glass compounds exhibited a significant level of remanent magnetization at low temperatures, as has been seen for similar spinglass compounds.28 It is interesting to note that the size of the remanent magnetization of the spin-glass compounds reduces substantially if pretreated with the ZFC condition (see Supporting Information). The remaining compounds that demonstrated long-range AFM order had a small yet nonzero remanent magnetization. The level of remanence in each case could correspond to canting of the AFM-II structures proposed of less than 1°, which would explain the FC and ZFC divergence. Because the degree of canting is very small, the magnetic structure refinements are not sensitive enough to shed further light on this hypothesis.

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DISCUSSION In these double perovskites the shortest distance between magnetic ions is the ∼4 Å separation between Co2+/Ni2+ and Ru5+/Os5+ ions. If the coupling between these ions (i.e., heteroatomic coupling) is dominant, either ferromagnetism or ferrimagnetism should result, depending on the sign of the heteroatomic superexchange coupling constant JBB′. If the coupling is through transition-metal eg orbitals and O 2p σ orbitals, as assumed in the G-K formalism, the d8−d3 and highF


Article

Inorganic Chemistry

Figure 4. dc magnetization vs temperature of (a) LaSrCoOsO6 and (b) LaSrNiOsO6, (c) LaSrNiRuO6, (d) LaCaCoOsO6, (e) LaCaNiOsO6, and (f) LaCaNiRuO6 under FC 1 kOe (red ●) and ZFC (blue ○) conditions, measured under an applied field of 1 kOe. The ZFC data for LaCaNiRuO6 lies directly under the FC data with only a slight divergence at 5 K.

while only one of the four osmates, LaSrNiOsO6, follows suit, albeit with a very reduced ordered moment that likely indicates incomplete AFM order. Given the larger spatial extent of the 5d orbitals it is likely that the magnitude of JNN, and possibly JBB′, is stronger in the osmates than it is in the ruthenates. As JNN and/ or JBB′ become competitive with JNNN, frustration will increase, which would help to explain the spin-glass state seen for LaSrCoOsO6, LaCaCoOsO6, and LaCaNiOsO6.

5d heteroatomic coupling is highly sensitive to changes in the B−O−B′ bond angles.46 Given the absence of either ferromagnetism or ferrimagnetism one must conclude that the longer-range homoatomic coupling interactions are comparable to, if not stronger than, heteroatomic coupling in these double perovskites. If we ignore heteroatomic coupling altogether, AFM-II is expected when JNNN is AFM and significantly stronger than JNN.16,17 It is notable that all four ruthenates adopt an AFM-II structure, G


Article

Inorganic Chemistry Table 3. A Summary of the Key Structural and Magnetic Properties of LaABB′O6 Double Perovskitesa d −d LaCaCoRuO6 LaSrCoRuO6 LaCaCoOsO6 LaSrCoOsO6 d8−d3 LaCaNiRuO6 LaSrNiRuO6 LaCaNiOsO6 LaSrNiOsO6 7

ave 300 K ∠B−O−B′

transition temperature

ordered momentb

μeffc

153.5 160.3 152.8 158.8

AFM (TN = 96 K) AFM (TN = 85 K) spin glass (Tg = 32.5 K) spin glass (Tg = 37.5 K)

2.80 μB 2.22 μB

5.84 5.37 6.06 5.79

153.9 160.7 153.3 159.6

AFM (TN = 94 K) AFM (TN = 93 K) spin glass (Tg = 30 K) AFM (TN = 65 K)

1.65 μB 1.04 μBd

4.71 4.77 4.41 4.18

θ

ref

μB μB μB μB

−191 K −15 K −156 K −84 K

29 30 this work this work

μB μB μB μB

−147 K −24 K −123 K −13 K

this this this this

3

0.75 μBd

a

work work work work

b

The bond angles are determined from NPD refinement results in Tables 1 and 2. Ordered moments are obtained assuming the moment is entirely localized on the Co2+/Ni2+ ion. cThe spin only effective moments are 5.48 μB for the LaACoBO6 compounds and 4.79 μB for the LaANiBO6 compounds. dThe relatively small magnetic moment observed in the AFM state via analysis of NPD data suggests some degree of magnetic inhomogeneity in these samples.

Figure 5. High d-spacing NPD patterns collected at 10 K. The black symbols, red curve, and blue curve correspond to the observed data, calculated pattern, and difference curve, respectively. Black hashes refer to nuclear reflections, while pink hashes refer magnetic reflections of compounds with long-range magnetic order (b, c, f). The asterisk in (f) refers to an unfit magnetic reflection that orginates from a NiO secondary phase. The increased background noise seen in (d, e, f) is due to changes in the POWGEN instrument over time rather than differences in the samples.

H


Article

Inorganic Chemistry

Figure 8. Dependence of ac magnetization inverse peak temperature on the applied ac field frequency for suspected spin-glass compounds. Black lines are linear fits as described in the text.

Figure 6. Heat capacity divided by temperature vs temperature of the three compounds demonstrating long-range magnetic order.

Figure 7. ac magnetization data as a function of temperature and frequency of (a) LaSrCoOsO6 and (b) LaSrNiOsO6, (c) LaSrNiRuO6, (d) LaCaCoOsO6, (e) LaCaNiOsO6, and (f) LaCaNiRuO6 measured with an applied dc field of 0 Oe and ac field of 10 Oe. I


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Inorganic Chemistry

Figure 9. Field dependence of the dc magnetization of each material at 2 and 200 K. The 2 K data sets were collected following an FC procedure. Analogous data following a ZFC procedure is available as Supporting Information.

the refined neutron moment of 1.62(8) μB is plausible for a Ni2+ ion. In fact, the ordered moment is smaller than the values of 1.92(6) and 2.04(6) μB observed in Sr2NiMoO6 and Ba2NiMoO6, respectively.17 If the diffraction data are modeled with a magnetic structure where both Ni and Ru ions are assumed to be magnetically ordered, the fit does not improve. A similar scenario exists in the LaACoRuO6 compounds previously studied by Bos and Attfield.29,30 Further studies with a technique like muon spin resonance would be required to confirm or refute this hypothesis. In all cases studied here the LaCaBB′O6 compounds have more negative Weiss constants than their analogous LaSrBB′O6 counterparts. The reasons for this behavior are not entirely clear. In the LaSrBReO6 system, Thompson et al. found positive Weiss constants for B = Mn, Co, and Ni and explained

The shape of the temperature-dependent dc magnetization plots for LaSrNiRuO6 and LaCaNiRuO6 are fundamentally different than the corresponding curves for the Os-containing compounds (Figure 4). Whereas all four Os compounds have a cusp or maximum at finite temperature, the onset of AFM just above 90 K in LaSrNiRuO6 and LaCaNiRuO6 is more of an inflection point on a rising curve. One possible explanation for this behavior would be if the Ni2+ spins order, while the Ru5+ spins remain paramagnetic even in the AFM state. While the susceptibility data hint at a second magnetic transition near 25 K in LaSrNiRuO6, there is neither a clear frequency dependence of the ac magnetization data nor a heat capacity anomaly to reveal the nature of this magnetic transition. It is not possible from the NPD data to decouple the moments on Ni and Os (or Ru) while refining the magnetic structure, but J


Inorganic Chemistry

■

the observation with the fact that FM exchange is anticipated in these compounds according to the G-K rules.47 If heteroatomic coupling between 3d eg orbitals and 4d/5d t2g orbitals is significant, one would expect its strength to diminish as the B− O−B′ bonds become more linear, eventually becoming FM, as in Ba2NiOsO6.46 This could help explain the less negative Weiss constants seen for the LaSrBB′O6 compounds. Another possible explanation for the shift in Weiss constants hinges on the difference in cation variance48 among the A cations. The A cation variance is greater in those compounds containing La and Sr, due to the size mismatch of Sr2+ and La3+, than it is in the compounds containing La and Ca. This may cause the longer-range exchange pathways, which are more sensitive to localized strains, to become disrupted. Cation variance may also underlie the observed reduction of ordered magnetic moments for Ni and Co. While the ordered moments in LaCaNiRuO6 and LaCaCoRuO6 are not far from the expected values for Ni2+ and high-spin (HS) Co2+ ions, respectively, each of the three LaSrBB′O6 compounds that magnetically order have reduced moments. It is very likely that this reduction in moment is due to disorder in the magnetic structure, rather than a true reduction in the size of the local moment. A final mystery is the lack of ferrimagnetism in LaCaNiOsO6 and LaCaCoOsO6, which are isostructural and have very similar B−O−B′ bond angles to Ca2NiOsO6 and Ca2CoOsO6, both of which have been shown to order ferrimagnetically.22 The major difference is the change from Os6+ in Ca2BOsO6 to Os5+ in the compounds studied here. When the electron count was changed from 5d2 (Os6+) to 5d3 (Os5+), both the heteroatomic coupling JBB′ and the homoatomic nearest-neighbor Os−Os coupling JNN should increase. If JNN increased by a greater extent, frustration from competing superexchange pathways could be responsible for the spin-glass behavior of LaCaNiOsO6 and LaCaCoOsO6. However, Ca2FeOsO6, which also contains the 5d3 Os5+ ion, is a ferrimagnet, not a spin glass.26,27 This raises the possibility that disorder induced by La/Ca alloying, though subtle, may be enough to disrupt long-range AFM order. The jury is still out on this question.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02282. Refined XRD patterns, Curie−Weiss fits to temperaturedependent dc magnetization data, dc demagnetization data sets for LaSrCoOsO6, LaCaCoOsO6, and LaCaNiOsO6, 300 and 10 K NPD comparison and difference plots, and a table of magnetic refinement models fit qualities (PDF) Accession Codes

CCDC 1572131−1572136 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (R.M.) *E-mail: [email protected]. (P.M.W.) ORCID

Ryan Morrow: 0000-0001-9986-3049 Michael A. McGuire: 0000-0003-1762-9406 Patrick M. Woodward: 0000-0002-3441-2148 Present Address ∥

Leibniz Institute for Solid State and Materials Research Dresden IFW, D-01171 Dresden, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support for this research was provided the Center for Emergent Materials an NSF Materials Research Science and Engineering Center (DMR-1420451). A portion of this research was performed at Oak Ridge National Laboratory’s Spallation Neutron Source, which is sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences. M.A.M. acknowledges support from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. R.M. acknowledges support from the Alexander von Humboldt Foundation. The authors thank A. Huq and P. Whitfield for assistance with the neutron powder diffraction experiments.

CONCLUSIONS

The structure and magnetic properties of six double perovskites, each containing a 1:1 mixture of transition-metal ions from different rows of the periodic table, have been studied. LaSrNiOsO6, LaSrNiRuO6, and LaCaNiRuO6 order antiferromagnetically with TN values of 65, 93, and 94 K, respectively, with ordered moments that appear to be associated primarily with Ni2+ ion. In contrast, LaSrCoOsO6, LaCaCoOsO6, and LaCaNiOsO6 have spin-glass ground states with freezing temperatures of 37.5, 32.5, and 30 K, respectively. The variations in magnetic ground state and transition temperature arise from a competition between heteroatomic superexchange interactions and longer-range homoatomic superexchange coupling interactions. In those compounds containing Ru5+ homoatomic, next nearest-neighbor coupling between 3d ions (Co2+ or Ni2+) dominates leading to antiferromagnetic ordering with a type II structure. In those compounds containing Os5+ multiple superexchange pathways are competitive, leading to frustration, which destabilizes long-range magnetic ordering.

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REFERENCES

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M


The Crystal Structure and Magnetic Behavior of Quinary Osmate and

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