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

抄録

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.