Pressure-Induced Order-Disorder Transition in the Double Perovskite Oxide La₂CoRuO₆

Abstract

We investigated pressure-induced order-disorder transition in the B-site-ordered double perovskite oxide La₂CoRuO₆. La₂CoRuO₆ crystallized in a double perovskite structure with rock-salt-type B-site ordering of Co²⁺ and Ru⁴⁺ ions in the sample synthesized under ambient-pressure and high-temperature (1373 K) conditions. The ambient-pressure B-site-ordered phase of La₂CoRuO₆ underwent phase transition from monoclinic to orthorhombic structure by treating high-pressure and high-temperature conditions of 8 GPa and 1373 K. Structure refinement based on the synchrotron X-ray powder diffraction data demonstrates that the B-site cationic ordering was completely destroyed in the high-pressure phase of La₂CoRuO₆, accompanied by a slight reduction in lattice volume (ΔV = −0.25%). X-ray absorption spectroscopy revealed that the valence states of Co²⁺ and Ru⁴⁺ were retained in the high-pressure phase, indicating that the primary driving force for the pressure-induced order-disorder transition in La₂CoRuO₆ is the negative PΔV term in the Gibbs free energy. The order-disorder transition pressure in La₂CoRuO₆ (8 GPa) was lower than that in isoelectronic oxide Y₂CoRuO₆ (15 GPa), suggesting that the compressibilities of A-site metal ions play a crucial role in the transition. The high-pressure disordered phase of La₂CoRuO₆ exhibited a short-range magnetic ordering below 22 K because of the absence of the cationic ordering, whereas the ambient-pressure ordered phase exhibited an antiferromagnetic long-range ordering below 25 K.

1. Introduction

Double perovskite oxides A₂BB′O₆, in which two kinds of transition metal ions at an equal ratio occupy the B-sites in octahedral coordination, crystallize in completely B-site-ordered (typically, rock-salt and layered types), partially ordered, and completely disordered structures (see Fig. 1).¹⁾ Since the degree of order substantially affects various properties,^2–4) its precise control is demanded. The degree of order changes with the difference in ionic radius and valence, in addition to the synthesis conditions. Pressure is another factor inducing or breaking the B-site ordering in the double perovskite oxides. Sr₂FeReO₆ undergoes a pressure-induced disorder-to-order transition.⁵⁾ The sample with partial ordering of Fe and Re ions is obtained under ambient pressure (AP), whereas the Fe and Re ions are completely ordered in the sample synthesized under high pressure (HP) of 3.5 GPa. The partially B-site-ordered phase exhibits a ferrimagnetic transition temperature (T_C = 445 K), although the completely B-site-ordered phase demonstrates a lower ferrimagnetic transition temperature (T_C = 400 K) with a larger saturation magnetization. On the other hand, it has been recently reported that Y₂CoIrO₆ and Y₂CoRuO₆ display pressure-induced order-to-disorder transitions.⁶⁾ Y₂CoIrO₆ with B-site-ordered structure prepared under AP conditions transforms to a partially ordered structure by HP treatment at 6 GPa, and the further compression up to 15 GPa leads to a completely disordered structure. Y₂CoRuO₆ also becomes a completely disordered phase at 15 GPa. The stark contrast between disorder-order and order-disorder transitions induced by HP treatment is explained by the discrepancy in ionic radius and compressibility of the B-site ions. When the ordered (or disordered) structures are more compressible to lead to the smaller volumes under HP, pressure-induced disorder-order (or order-disorder) transitions occur because of the contribution of PΔV term to the Gibbs free energy: ΔG = G_disordered − G_ordered (or G_ordered − G_disordered) = ΔE − TΔS + PΔV < 0, where E, T, S, P, and V are internal energy, temperature, entropy, applied pressure, and volume, respectively. In the previous studies, the compressibilities derived from the bonds between B-site metals and oxygens were considered,⁶⁾ but those around A-site metals were not well examined.

Fig. 1

Schematic crystal structures of La₂CoRuO₆ with (a) B-site-ordered ambient-pressure phase and (b) B-site-disordered high-pressure phase.

In this study, we focused on the probable difference in compressibility between larger La³⁺ ions (Shannon effective ionic radius (CN = 8): 1.16 Å)⁷⁾ and smaller Y³⁺ ions (1.019 Å). La³⁺ ions may possess larger compressibility according to the general tendency,⁸⁾ in which larger ions in the same column in the periodic table are more compressive. The difference in the compressibility of A-site ion is expected to change the magnitude of PΔV under HP, affecting the order-disorder transitions. We investigated the pressure-induced structural phase transition in the B-site-ordered double perovskite oxide La₂CoRuO₆. Synchrotron X-ray powder diffraction (SXRD) study demonstrated that the B-site Co and Ru ions are completely disordered by the application of HP up to 8 GPa, which is lower than that (15 GPa) reported in Y₂CoRuO₆. X-ray absorption spectroscopy revealed that the valence states retained La₂Co²⁺Ru⁴⁺O₆, regardless of B-site ordering/disordering. The B-site-disordered HP phase of La₂CoRuO₆ exhibited a short-range magnetic ordering below 22 K, whereas the B-site-ordered AP phase was an antiferromagnet with T_N = 25 K.

2. Experimental

La₂O₃ (99.99%), CoO (>90%), and RuO₂ (99.9%) were mixed at a molar ratio of 1:1:1 to prepare the stoichiometric mixture of La₂CoRuO₆. La₂O₃ powder was pre-calcined at 1173 K for 13 h to decompose lanthanum hydroxide and lanthanum carbonate to obtain the pure reagent. We confirmed that all raw materials included no impurity by X-ray diffraction (XRD). The mixture was pelletized and then calcined twice at 1073 K for 12 hours, once at 1373 K for 36 hours, and once at 1373 K for 24 hours in Ar flow, obtaining the AP phase.⁹⁾ The AP phase was filled into a platinum capsule and placed in an octahedron-shaped (Mg, Co)O pressure-transmitting medium. The pressure-transmitting medium was compressed up to 8 GPa using a Walker-type high-pressure apparatus in 30 min. After the compression, the sample was heated to 1373 K for 15 min, held at this temperature for 30 min, rapidly cooled to room temperature, and quenched to ambient pressure in 20 min, obtaining the HP phase. SXRD patterns at room temperature were collected using Debye-Scherrer cameras at the BL02B2 and BL19B2 beamlines of SPring-8.^10,11) The powder sample was filled into a Lindemann glass capillary with an inner diameter of 0.2 mm. The wavelength was determined to be 0.41996 Å using a CeO₂ standard. Structure parameters were refined by using the Rietveld refinement program RIETAN-FP.¹²⁾ The schematic crystal structure in Fig. 1 was depicted using the VESTA-3 software.¹³⁾ X-ray absorption near edge structure (XANES) spectra at K-edges of Co and Ru at room temperature were collected in the transmission mode at the BL14B2 beamline of SPring-8. Magnetization measurement was conducted using a Physical Properties Measurement System (PPMS, EverCool II, Quantum Design Inc.) with a vibrating sample magnetometer (VSM) option. Magnetic susceptibility was measured in a temperature range between 5 and 300 K in an external field of 10 kOe on zero-field-cooling (ZFC) and field-cooling (FC) modes.

3. Results and Discussion

Figure 2 shows the SXRD patterns at room temperature and the Rietveld refinement result for the AP and HP phases of La₂CoRuO₆. The SXRD pattern of the AP phase was indexed with the space group of P2₁/c (No. 14), whereas the HP phase was indexed with the space group of Pnma (No. 62).^6,9) Substantial differences between AP and HP phases were evident at the Bragg reflection corresponding to the existence/absence of the B-site ordering. In addition to the $10\bar{2}$ and 100 reflections at 2θ∼5.3°, the 011 reflection was observed at 2θ∼5.25° in the AP phase (see the inset of Fig. 2(a)), indicating a high degree of order at the B-site.⁹⁾ In contrast, the relevant reflection was not observed in the HP phase (the inset of Fig. 2(b)), confirming the absence of B-site ordering between Co and Ru ions. The AP and HP phases contained unidentified impurities with weak intensities (<8.5% in relative intensity), we excluded the 2θ ranges including the peaks of the impurities from the Rietveld refinement. The structure parameters of AP and HP phases obtained from the final refinement are listed in Table 1 and 2, respectively. The occupancy factors of the two distinct B-sites (2a and 2d) in the AP phase were close to the unity: g(Co, 2d) = g(Ru, 2a) = 0.952(10), indicating that the B-site ions are almost completely ordered. The refinement of g(Co, 4b) for the HP phase in the conditions of g(Co, 4b) + g(Ru, 4b) = 1 gave g(Co) = 0.504(10), which is consistent with the value expected from the fully disordered structure with the nominal composition of La₂CoRuO₆. Thus, the occupancy factors were fixed to g(Co, 4b) = g(Ru, 4b) = 0.5 in the final refinement. The lattice volume decreased from the AP phase to the HP phase by −0.25%, indicating that the PΔV term is a driving force for the pressure-induced order-disorder transition in La₂CoRuO₆. The BVSs ($ = \sum\nolimits_{i}^{n}\exp (\frac{r_{0} - r_{i}}{b_{0}})$: r_i and n represent the bond length and the number of bonds, respectively.) were calculated from the refined structures using the following bond-valence parameters: b₀ = 0.37 Å for all atoms, r₀ = 2.172 Å for La, r₀ = 1.692 Å for Co, and r₀ = 1.834 Å for Ru.^14,15) The BVSs were +2.94 (La), +2.10 (Co), and +3.85 (Ru) for the AP phase, indicating the valence state of La³⁺₂Co²⁺Ru⁴⁺O₆. The BVSs for the HP phase were calculated to be +2.87 (La) and +2.96 (Co/Ru). The latter does not indicate the trivalent states of Co and Ru ions, i.e., La³⁺₂Co³⁺Ru³⁺O₆ in the ionic model, because the BVS at the B-site was just tentatively estimated by the weighted average BVSs assuming the Co and Ru full occupancies.

Fig. 2

Observed SXRD patterns (λ = 0.41996 Å) and Rietveld refinement results of (a) B-site-ordered AP phase (space group: P2₁/c) and (b) B-site-disordered HP phase (space group: Pnma). The circles (black) and solid lines (red) represent the observed and calculated patterns, respectively. The differences between the observed and calculated patterns are displayed at the bottom (blue). The vertical marks (green) indicate the Bragg reflection positions of La₂CoRuO₆. The insets show enlarged patterns between 5° and 5.5°.

Table 1 Structure parameters for the AP phase of La₂CoRuO₆ at room temperature obtained from the Rietveld refinement.

Table 2 Structure parameters for the HP phase of La₂CoRuO₆ at room temperature obtained from the Rietveld refinement.

The valence states of Co and Ru ions for the AP and HP phases were examined by X-ray absorption spectroscopy. Figure 3 exhibits the XANES spectra of K-edges of Co and Ru for La₂CoRuO₆. The Co K-edge absorption edges for the AP and HP phases were located at the same energy (∼7710 eV) as the Co²⁺-reference CoO. The Ru K-edge absorption for the AP and HP phases almost overlapped with the same absorption energy as the Ru⁴⁺-reference RuO₂. These observations conclude that the valence states are represented as La₂Co²⁺Ru⁴⁺O₆ for both AP and HP phases.

Fig. 3

XANES spectra of (a) Co K-edge and (b) Ru K-edge for La₂CoRuO₆ and references.

Figure 4(a) shows the temperature dependence of the magnetic susceptibility for La₂CoRuO₆. The AP phase exhibited a clear cusp at T_N = 25 K, indicating an antiferromagnetic transition as reported previously.⁹⁾ In contrast, the HP phase demonstrated a broader maximum near 22 K, followed by the branch between ZFC and FC below this temperature, unlike the identical ZFC and FC curves in the AP phase. This indicates that the short-range magnetic ordering is predominant in the HP phase because of the magnetic frustrations among the spatially disordered magnetic ions at B-site, agreeing with previous studies in the B-site-disordered double perovskite oxides.^4–6) The inverse magnetic susceptibility is shown in Fig. 4(b). The data between 150 and 300 K were fitted by using the Curie-Weiss law: χ⁻¹ = (T − Θ_AP/HP)/C_AP/HP, where C_AP/HP is the Curie constant and Θ_AP/HP is the Weiss temperature. The parameters obtained from the fitting were C_AP = 4.390(3) emu K mol⁻¹ and Θ_AP = −116.9(2) K for the AP phase, and C_HP = 4.287(2) emu K mol⁻¹ and Θ_HP = −182.6(2) K for the HP phase. The theoretical Curie constant (C_theo) based on the ionic model of La₂Co²⁺Ru⁴⁺O₆, where the following electron configurations with spin quantum numbers (S) were adopted: Ru⁴⁺ [S = 1, μ_eff(Ru) = 2.82 μ_B], Co²⁺ [S = 3/2, μ_eff(Co) = 3.87 μ_B], calculated to be C_theo = N_A[μ²_eff(Ru) + μ²_eff(Co)]/3k_B = 2.875 emu K mol⁻¹, where N_A is the Avogadro constant and k_B is the Boltzmann constant. The C_AP and C_HP values were larger than C_theo probably because of the unquenched orbital moments of the Co²⁺ ions, as reported in Sr₂CoWO₆.¹⁶⁾ The negative Weiss temperatures indicate that antiferromagnetic interactions are predominant for La₂CoRuO₆, regardless of the degree of cationic ordering.

Fig. 4

(a) Temperature dependence of the magnetic susceptibility of field-cooling (solid lines) and zero-field-cooling (dashed lines) modes in an external field of 10 kOe for La₂CoRuO₆. (b) Temperature dependence of the inverse magnetic susceptibility of field-cooling mode. The green lines represent the fitting results using the Curie-Weiss law.

In the above experiments, we demonstrated that the B-site ordering in the La₂CoRuO₆ double perovskite was disturbed by high-pressure and high-temperature treatment, whereas the aliovalent states of Co²⁺ and Ru⁴⁺ ions were retained in the B-site disordered phase. The B-site disordering altered the magnetism to the short-range magnetic ordering because of the magnetic frustration in the randomly distributed different magnetic ions. The primary driving force of the pressure-induced order-disorder transition is estimated to be volume reduction (negative PΔV in ΔG). It is notable that the magnitude of ΔV = V_{disordered, HP} − V_{ordered, AP} in La₂CoRuO₆ (−0.25%) is smaller than that in Y₂CoRuO₆ (−0.91%). Apparently, this is not in line with that the order-disorder transition pressure was lowered in La₂CoRuO₆ (8 GPa) compared to Y₂CoRuO₆ (15 GPa),⁶⁾ because a larger negative ΔV should be needed to lower the transition pressure. One of the possible reasons is that the compressibility of HP phase is much larger than that of AP phase for La₂CoRuO₆, which can increase ΔV under high pressure. Further study under HP is necessary, but probably larger compressibility of La³⁺ ions compared to Y³⁺ ions may contribute to increasing ΔV under high pressure.

4. Conclusions

In summary, we demonstrated that the B-site-ordered La₂CoRuO₆, transformed to the B-site-disordered structure by treatment at high-pressure and high-temperature conditions of 8 GPa and 1373 K, whereas the valence state was retained at La₂Co²⁺Ru⁴⁺O₆ after the phase transition. The volume reduction in the B-site disordered high-pressure phase contributed to the driving force of the pressure-induced order-disorder transition. The long-range magnetic ordering in the B-site-ordered phase was disturbed in the B-site-disordered phase, leading to the short-range magnetic ordering below 22 K. The possibly larger compressibility induced by La ions instead of Y ions may be the origin of the lowered order-disorder transition pressure for La₂CoRuO₆.

Acknowledgments

The synchrotron X-ray experiment was performed at SPring-8 under the approval of the JASRI (proposal numbers 2022A1493, 2022B1619, and 2022B1816). This work was supported by JSPS KAKENHI (grant number JP20H02825) and the Tanikawa Fund Promotion of Thermal Technology.

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