Long-Range Antiferromagnetic Ordering in B-Site Ordered Double

Feb 17, 2016 - The crystal structure of the novel Os-based double perovskite comprises FCC arrangement of magnetic ions, which could potentially exhib...
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Long-Range Antiferromagnetic Ordering in B‑Site Ordered Double Perovskite Ca2ScOsO6 David D. Russell,† Abbey J. Neer,§ Brent C. Melot,§ and Shahab Derakhshan*,† †

Department of Chemistry and Biochemistry, California State University, Long Beach, 1250 Bellflower Boulevard, Long Beach, California 90840, United States § Department of Chemistry, University of Southern California, 3620 McClintock Avenue, Los Angeles, California 90089-1062, United States S Supporting Information *

ABSTRACT: A new Os-based B-site ordered double perovskite with the chemical composition of Ca2ScOsO6 was successfully synthesized. The crystal structure of the title compound was determined by employing the powder X-ray diffraction method and was found to crystallize in the monoclinic P21/n space group with the cell constants of a = 5.4716(1) Å, b = 5.6165(1) Å, c = 7.8168 (1) Å, and β = 89.889 (2)°. The temperature-dependent magnetic susceptibility data suggest that this novel S = 3/2 compound undergoes an antiferromagnetic transition at ∼69 K. Fitting the hightemperature susceptibility data (100−300 K) to Currie−Weisse behavior showed C = 1.734 emu·K/mol (μeff = 3.72 bohr magnetons) and θ = −341 K, which is indicative of dominant antiferromagnetic interactions. Temperature-dependent specific heat measurements exhibit a λ shape anomaly at 69 K, which is consistent with a long-range ordering of the spins. Because of a triangular arrangement of antiferromagnetically ordered magnetic ions, the system exhibits some degree of geometric magnetic frustration (GMF), but not strongly. Spin-dimer analysis, employing extended Hückel theory, reveals that a dominant exchange interaction exists (along the a crystallographic axis in perovskite layer), which violates the perfect condition for GMF.

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INTRODUCTION Antiferromagnetic (AFM) materials with triangular arrangements of magnetic ions exhibit spin correlations that cannot be simultaneously accommodated. This results in a phenomenon known as geometric magnetic frustration (GMF), where exotic ground states with enormous degeneracies are present (Figure 1a).1 Nonetheless, this condition may be violated when exchange interactions of different strengths lift the degeneracy where the dominating interaction(s) result in low-dimensional magnetism (LDM) with a nonfrustrated ground state (Figure 1b,c). The presence of GMF is generally manifested by large absolute values of Weiss constant (θ), indicating the presence of strong exchange interactions, and are accompanied by a low transition temperature (TN). Comparing the ratio between these two quantities |θ | represents the frustration index (f), f = T . Materials with f ≥ 10

DP systems with face-centered cubic (FCC) arrangement of magnetic ions (B′) and various triangular and tetrahedral substructures (Figure 1d) are potential candidates for GMF. Various structural and electronic parameters contribute to the resultant magnetism in ordered DP systems. Crystal systems from cubic to triclinic exist, and Woodward et al. have established the group theoretical relationships for these systems.3 Decorating one set with magnetic ions entirely with competing AFM exchange interactions results in GMF. Accordingly, DP lattices permit a systematic exploration of the role of spin quantum numbers and crystal symmetry in determination of the magnetic ground state in FCC arrangement. For systems with S = 3/2 and 5 /2, AFM long-range order is always found. For instance, in S = 3 /2 Ba2YRuO6 (TC = 36 K and f ≈ 16), the Y/Ru site disorder is not sufficient to induce spin freezing.4 For S = 1 systems, the properties are not easily predictable, and different magnetic behaviors may be identified for systems with the same spin multiplicities. While 3d systems such as Sr2NiWO6 and Sr2NiTeO6 exhibit long-range AFM order,5 Ba2YReO6 undergoes spin freezing below 40 K with f = 12, and La2LiReO6 may be a spin liquid with singlet ground state.6 Very recently Ca2MgOsO6 and Sr2MgOsO6 were prepared by high-pressure synthesis method. While the tetragonal Sr2MgOsO6 exhibits a

N

are considered highly frustrated.2 The B-site ordered double perovskite (DP) structure is described with the general formula of A2BB′O6. The crystal structure is composed of layers made of alternating corner-shared BO6 and B′O6 octahedra arranged in a checkerboard fashion. The systems are designed such that large differences between the oxidation states of B and B′ result in crystallographic ordering between them. The large empty space between the BO6 and B′O6 octahedra are occupied by large A-site cations. The ideal situation occurs when only one ion, B or B′, is magnetic. Accordingly, such © XXXX American Chemical Society

Received: November 16, 2015

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DOI: 10.1021/acs.inorgchem.5b02630 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Geometric magnetic frustration in an AFM system with equilateral triangle motif (a). One-dimensional nonfrustrated AFM system in sideshared triangular arrangement (b). Two-dimensional nonfrustrated AFM system in side-shared triangular arrangement (c). Edge-shared tetrahedral in FCC arrangement of magnetic ions in B-site ordered DP systems (d).

clear AFM transition at ∼110 K in both zero field cooled (ZFC) and field cooled (FC) data, magnetic susceptibility data for monoclinic Ca2MgOsO6 shows only a cusp at ∼19 K where the ZFC and FC diverge.7 For S = 1/2 systems, examples include two Os7+ compounds, Ba2LiOsO6 and Ba2NaOsO6, which exhibit long-range order with AMF and ferromagnetic transitions, respectively, with some degree of frustration.8 While the Ba2YMoO6 with singlet ground state is highly frustrated ( f > 100), another Mo5+ compound, La2LiMoO6, shows long-range order and has low frustration index.9 Sr2CaReO610 and Sr2MgReO611 are the only two known representatives, which were reported to show spin freezing with f ≈ 32 and f ≈ 9, respectively. Here we report on the synthesis, crystal structure, and physical properties of a new Os-based compound, Ca2ScOsO6, which is an S = 3/2 analogue of recently discovered Ca2MgOsO6.7 In addition, to better understand the magnetic ground state of the S = 3/2 system, the computed relative strengths of various exchange interaction pathways are presented.

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(GSAS) suite and EXPGUI interface.12,13 The X-ray diffraction data were collected at room temperature in the range of 17° ≤ 2θ ≤ 100° with ∼0.008° intervals. The structural data from Ca2MgOsO67 were used as a starting model for refinement. A pseudo-Voigt peak shape profile was chosen, and the selected parameters were allowed to change to obtain the best fit to the experimental data. When 2d and 2b Wyckoff positions were allowed to mix occupancies between Os and Sc, there was no improvement in refinement quality, and it was concluded that the cations tend to reside in their own crystallographic positions. Isotropic displacement factors for cations were refined; however, an arbitrary value was selected for oxygen ions. The powder X-ray diffraction pattern along with the fit to the structural model is presented in Figure 2.

EXPERIMENTAL SECTION

Synthesis. The sample was synthesized in two different steps. In the first step, stoichiometric amounts of CaO, Sc2O3 (Alfa Aesar, 99.99%), and OsO2 (Alfa Aesar 83% metal) were weighed, thoroughly ground, and pressed into a pellet inside an argon-filled glovebox. The pellet was then placed in a small alumina crucible, which was subsequently sealed in an evacuated fused silica tube (eq 1). The tube was then placed in a box furnace where it was heated to 900 °C for 24 h. Fresh CaO was prepared by heating CaCO3 (Alfa Aesar, 99.5%) in a tube furnace at 1000 °C for 10 h under a dynamic vacuum condition, followed by heating with hydrogen flame under vacuum. Sc2O3 was also heated at 1000 °C prior to being used for 24 h to ensure the decomposition of possible carbonates or hydroxides. 2CaO +

1 Sc 2O3 + OsO2 ⇒ Ca 2ScOsO5.5 2

Figure 2. Room-temperature powder X-ray diffraction pattern of Ca2ScOsO6. The black crosses indicate the experimental data, while the Rietveld refinement fit is shown as a solid red line. The difference between these values is represented by the bottom thin green solid line, and the peak positions are located by the blue vertical tick marks. Magnetic Susceptibility Measurements. The temperature- and field-dependent direct-current magnetization was measured on polycrystalline powders using a 14 T Quantum Design PPMS equipped with a vibrating sample magnetometer. Samples were stored in an Arfilled glovebox to prevent any oxidation prior to characterization. Heat Capacity Measurements. Specific heat measurements were performed using a semiadiabatic heat-pulse technique in the same PPMS. A well-ground powder of the title compound was mixed with equal parts by mass of Ag powder and pressed into a pellet to improve thermal coupling. The contribution from Ag was measured separately and subtracted. Theoretical Calculations, Spin Dimer Analyses. The relative values of the various spin exchange pathways were estimated by performing the extended Hückel, spin-dimer analyses.14 The OsO67− octahedral units in Ca2ScOsO6 are separated from each other by the diamagnetic ScO69− octahedral units. Accordingly, the paramagnetic Os5+ centers interact with each other only by supersuper exchange (SSE) mechanisms. For each pathway, which involves two isolated OsO67− octahera (Os2O1214‑ dimer), the intersite hopping energy (Δe) was assessed using the CAESAR package.15 For this purpose, double-ζ Slater-type orbitals (STO’s) were employed for the oxygen s and p and the osmium d states, whereas single-ζ STO’s were selected for osmium s

(1)

Since the oxidation state of osmium did not change during the Reaction 1 a subsequent controlled oxidation process (eq 2) was designed to oxidize osmium ions to +5. For this purpose Ca2ScOsO5.5 was placed in a fused silica tube, where a stoichiometric amount of PbO2 was also present in a separate container. The tube was sealed under vacuum and was heated to 950 °C for another 24 h in a box furnace. Ca 2ScOsO5.5 +

1 1 PbO2 ⇒ Ca 2ScOsO6 + PbO 2 2

(2)

Phase Analyses and Crystal Structure Determination Using Powder X-ray Data. To examine the phase purity of the product, powder X-ray diffraction data were collected by a PANalytical X’Pert Pro MPD diffractometer, equipped with a linear X’Celerator detector, with Cu Kα1 radiation. The diffraction pattern resembled that of monoclinic Ca2MgOsO67 with the position of experimental peaks generally shifted to the left. The crystal structure was determined by Rietveld refinement of the powder X-ray data, using the General Structure Analysis System B

DOI: 10.1021/acs.inorgchem.5b02630 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry and p states. The values of the ζi and ζ′i coefficients and valence shell ionization potentials Hii used for the calculations are presented in Table 1. The hopping energy for each exchange pathway is linked to the

Table 2. Cell Constants and Refinement Parameters of Ca2ScOsO6 P21/n

space group

Table 1. Values for the ζi Coefficients and Valence Shell Ionization Potentials Hii of the Atomic Slater-Type Orbitals Employed for the Spin-Dimer Calculations for Ca2ScOsO6 atom

orbital

Hii (eV)

ζi

C

ζ′i

C′

O O Os Os Os

2s 2p 6s 6p 5d

−32.300 −14.8000 −8.1700 −4.810 −11.840

2.688 3.694 2.400 1.770 4.504

0.7076 0.3322 1 1 0.6066

1.675 1.659

0.3745 0.7448

2.391

0.5486

a = 5.4716(1) Å b = 5.6165(1) Å c = 7.8168(1) Å β = 89.889(2)° V = 240.217(4) Å3 2 20 Rp = 0.0602 and wRp = 0.0806

lattice parameters

Z number of refined parameters agreement factors

Table 3. Atomic Coordinates, Occupancy Factors, and Equivalent Isotropic Displacement Parameters for Ca2 ScOsO6 Refined in P21/n

< (Δe)2 >

. Since the coulomb magnetic exchange interaction by J ≅ U force values U are identical for all these pathways one can estimate the relative magnitudes of the various J values by only comparing ⟨(Δe)2⟩ values.

Wyckoff

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4e 2d 2b 4e 4e 4e

RESULTS AND DISCUSSION Crystal Structure. The crystal structure of Ca2ScOsO6 is presented in Figure 3. It crystallizes in the monoclinic P21/n

Ca Os Sc O1 O2 O3

x

y

z

Uiso (Å2)

0.982(1) 0.5 0.5 0.090(2) 0.708(2) 0.195(2)

0.0525(6) 0 0 0.477(3) 0.309(2) 0.221(2)

0. 2519(4) 0.5 0 0.240(1) 0.045(3) 0.959(3)

0.020(1) 0.0148(3) 0.010(1) 0.02 0.02 0.02

Table 4. Coordination Sphere and Bond Valence Sumsa of Cations in Ca2ScOsO6 Ca−O1 Ca−O1 Ca−O1 Ca−O2 Ca−O2 Ca−O2 Ca−O3 Ca−O3 Ca−O3 BVS (Ca)

Figure 3. Polyhedral representation of the crystal structure of Ca2ScOsO6. The blue and green octahedra represent OsO67− and ScO69− units, respectively. The large yellow spheres represent Ca2+ ions.

2.383(11) 2.457(15) 3.157(11) 2.338(17) 2.637(14) 2.718(18) 2.456(16) 2.586(15) 2.736(17) 1.87

2 × Os−O1 2 × Os−O2 2 × Os−O3 BVS (Os)

1.943(10) 1.956(12) 1.922(12) 4.94

2 × Sc−O1 2 × Sc−O2 2 × Sc−O3 BVS (Sc)

2.095(10) 2.105(12) 2.104(12) 3.03

BVS = ∑i N= 1vi, vi = exp[(R0 − di)/0.37], where N is coordination number, and di is bond distance. R0 for Ca2+ = 1.967, for Sc3+ = 1.849, and for Os5+ = 1.868.17 a

space group, and the cell parameters are a = 5.4716(1) Å, b = 5.6165(1) Å, c = 7.8168 (1) Å, and β = 89.889 (2)°. The ab crystallographic plane is composed of corner-shared units of OsO67− and ScO69− in a checkerboard fashion. The empty voids between the octahedra are occupied by the rather large Ca2+ ions. The residual factors are Rp = 0.0602 and wRp = 0.0806. The cell constants along with the fitting parameters are presented in Table 2. The atomic positions along with their thermal displacement factors are summarized in Table 3. Coordination environment along with the bond valence sums (BVS)16 of Ca2+, Sc3+, and Os5+ are listed in Table 4 and are consistent with the expected states of the cations. The calculated tolerance factor for rCa 2 + + r O2 − Ca2ScOsO6 (t = ) is ∼0.88, which is even

For this purpose, molar diamagnetic constants for Ca2+, Sc3+, Os5+, and O2− were summed based on the corresponding stoichiometry to 1.22 × 10−4 emu/mol,18 and the corrected data sets are presented in Figure 4a. A sharp maximum in both ZFC and FC is indicative of an AFM transition at ∼69 K. Inverse ZFC susceptibility data are presented in Figure 4b. The paramagnetic section of data (100−300 K) fits very well (R2 = 0.9998) to the C Curie−Weiss law, χ = T − θ . The parameters that were obtained from the fit are C = 1.734 emu K/mol and θ = −341 K (Figure 4b). The calculated effective magnetic moment μeff is 3.72 μB, which is slightly lower than the spin-only value for a S = 3/2 ion, μSO = 3.87 μB. The difference between the μSO and μeff in Ca2ScOsO6 is much smaller than what was reported for Ca2MgOsO6 (1.84 vs 2.83).7 Note that for the Os5+ system with an nd3 electronic configuration in nearly octahedral symmetry the orbital angular momentum L in the electronic ground state is zero, and therefore the spin−orbit coupling is not a significant effect. However, for the Os6+ system with an nd2 electronic configuration orbital contributions can be mixed into the ground state, and for less than half filling, these subtract from

2 (0.5rSc3 + + 0.5rOs5 + + r O2 −)

smaller than that of Ca2MgOsO6 (t = 0.896). This is in agreement with the high degree of buckling between the cornershared octahedra. Magnetic Susceptibility Data. The diamagnetic contribution (χd) to both ZFC and FC temperature-dependent magnetic susceptibility were removed based on eq 3: χmeasured = χd + χcorrected (3) C

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Figure 6. Temperature-dependent heat capacity data for Ca2ScOsO6. The λ shape anomaly is indicative of long-range order at the transition temperature.

neighbor magnetic exchange pathways. If the relative magnitudes of the pathways along the sides of the triangles are comparable, the condition for geometric frustration is present. However, when all the interactions are not of the same magnitude, due to the lack of competing interactions the condition for GMF may not be met. All the interactions in DP systems are SSE-type, as each octahedron is sharing a corner with an octahedron with a different metal center. The interactions between Os−Os distance of up to 7.84(1) Å were analyzed, and the four most significant of these along with the angles of the shortest SSE interactions are shown in Table 5. For Os5+ in an octahedral crystal field with 5d3 electronic contribution, Jahn−Teller distortion is not expected; therefore, assuming that all t2g orbitals contribute equally to the exchange interactions is reasonable, and we have

Figure 4. Temperature-dependent ZFC/FC magnetic susceptibility data for Ca2ScOsO6 (lower left). Inverse magnetic susceptibility data as a function of temperature and Curie−Weiss fit in the paramagnetic region, 100−300 K (upper right).

the spin component. Furthermore, one should note that the |θ| value for Ca2ScOsO6 (341 K) is remarkably larger than that reported for Ca2MgOsO6 (71 K), which is indicative of stronger overall AFM exchange interaction in the former system. This can be assigned to both higher-spin multiplicity and perhaps more effective exchange pathways in Ca2ScOsO6. The field-dependent magnetization data (Figure 5) shows a linear trend without any evidence of hysteresis, which is also indicative of predominant AFM interactions.

⟨(Δe)2 ⟩ ≈

1 N2

N

∑ (Δeμμ)2 μ=1

(4)

In these systems there are three states that are involved, and therefore eq 4 can be rearranged as 1 ⟨(Δe)2 ⟩≈ [(Δe11)2 + (Δe 22)2 + (Δe33)2 ] (5) 9 and finally the spin exchange interaction constants will be given by J≅

⟨(Δe)2 ⟩ U

(6)

The relative exchange interactions were calculated, and the results are presented in Table 6. J1 and J4 pathways are the primary interactions, which appear in a nearly two-dimensional rectangular network in the ab crystallographic plane (perovskite layers). These layers are magnetically correlated with each other via rather weak J2 and J3 interactions, which contribute to interlayer rectangular and triangular arrangements. As it is shown in Figure 7, J1 and J4 interactions involve two oxygen types (O2 and O3), which reside in the same plane and enable strong interactions. On the other hand, all three types of oxygen atoms (O1, O2 and O3) contribute to the weaker interplanar J2 and J3 exchange pathways and result in rather twisted dimers. The various alignments between the octahedra in different pathways seem to have a considerable impact on the relative strengths of exchange

Figure 5. Field dependence of the magnetic moment for Ca2ScOsO6 at 2 K. The linear response is in agreement with AFM exchange interactions.

Specific Heat Measurements. The temperature-dependent specific heat measurements for Ca2ScOsO6 (Figure 4) reveals a λ-type anomaly near 69 K, in agreement with the maximum in the magnetic susceptibility, and is indicative of the onset of longrange magnetic order. This is again in contrast with what was reported for Ca2MgOsO6, where no evidence of ordering was seen down to 2 K. Computational Analysis: Spin-Dimer Calculations. The Os5+ sublattice is shown in Figure 7, along with all the nearestD

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Figure 7. Supersuper exchange interactions along various pathways. Coplanar oxygen ligands involved in in J1 and J4 facilitate the AFM interactions.

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CONCLUSION The novel Os-based compound, Ca2ScOsO6, was successfully synthesized. The crystal structure was refined by Rietveld method from powder X-ray data, based on the structural model of isostructural Os-based compound, Ca2MgOsO6. Temperature dependent magnetic susceptibility data revealed that the title compound exhibits an AFM ordering. Heat capacity data indicates that a long-range ordered ground state is in place. The frustration index for this material is ∼5, so the compound is considered moderately frustrated. Spin dimer analysis, employing extended Hückel theory, suggests that this is due to the unique preferred J1 exchange interaction, which is in contrast with the competitive nature of the AFM interactions within the triangular network of highly frustrated systems. More advanced theoretical DFT studies and also low temperature neutron diffraction studies are to better understand the ground state magnetic structure of this novel system.

Table 5. Relevant Distances to the Four Identified Major Exchange Pathways J1, J2, J3, and J4 Os···Os, Å ∠Os−O2···O3 ∠O2···O3−Os ∠Os−O1···O2 ∠O1···O2−Os ∠Os−O3···O1 ∠O3···O1−Os

J1

J2

J3

J4

5.47(1) 147(1)° 111(1)°

5.53(1)

5.54(1)

5.62(1) 113(1)° 154(1)°

111(1)° 152(1)° 143(1)° 113(1)°

151(1)° 110(1)° 118(1)° 147(1)°

Table 6. (Δe)2 and the relative J values for the various exchange pathways in Ca2ScOsO6 Pathway

(Δe)2/ meV2

Relative

J1 J2 J3 J4

503.10 54.53 89.78 263.37

1 0.11 0.18 0.52

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ASSOCIATED CONTENT

S Supporting Information *

interactions. All the interaction pathways between Os5+ ions are presented in Figure 8, which is composed of two different types

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02630. One powder X-ray crystallographic file. (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

U.S. Army Research Office under Contract/Grant W911NF1210076 and the Office of Naval Research Grant No. N00014−15−1−2411

Figure 8. Schematic representation of all nearest neighbor Os−Os exchange pathways. The blue spheres denote Os5+ ions. J1 is represented by the thick black lines, J2 pathways are shown by thin black lines and J3 and J4 are indicated by the green and red lines, respectively.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS DDR and SD acknowledge the support from the U.S. Army Research Office under Contract/Grant No. W911NF1210076. A.J.N. and B.C.M. acknowledge support from the Office of Naval Research Grant No. N00014-15-1-2411.

of triangular networks. One is composed of J1, J2 and J3 with the relative strengths of 1:0.11:0.18 where the predominant J1 interaction rules out the possibility of GMF. However, the other type of triangle, which involve one J4 and two J3 pathways with the relative strengths of 0.52:0.18:0.18 (normalized as 1:0.35:0.35), resemble the condition for moderate GMF. This is in agreement with the experimentally obtained frustration index of ∼5.

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DOI: 10.1021/acs.inorgchem.5b02630 Inorg. Chem. XXXX, XXX, XXX−XXX