Pressure-Induced Order-Disorder Transition in the Double Perovskite Oxide La<sub>2</sub>CoRuO<sub>6</sub>

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.