Pressure-driven phase transitions in an antiferromagnetic double perovskite

Double perovskite oxides (A₂BB′O₆) gives various exotic magnetism mainly due to the interaction between the transition metals (B and B′). Here we studied a rather simple system, La₂CoTiO₆ (LCTO) where the magnetism comes from the Co²⁺ (d⁷) as Ti is having d⁰ electron configuration due to its 4⁺ oxidation state. The polycrystalline sample is having a monoclinic structure with space group P21/n which orders antiferromagnetically below 14.6 K. First-principles electronic structure calculations have been done for both ambient and finite pressure conditions. Calculations of the distorted monoclinic structure confirm the experimental antiferromagnetic ground state. When we theoretically removed the distortion (high-symmetric tetragonal structure with space group I4/mmm), the system transforms to a low-spin (tetragonal) state from its high-spin (monoclinic, distorted) state. Upon applying hydrostatic pressure to the monoclinic structure shows a series of transitions: from antiferromagnetic insulator (AFM-I) to antiferromagnetic metal (AFM-M), and ultimately to itinerant nonmagnetic metal. The transition from AFM-I to AFM-M is concomitant with a spin-state (high-spin to low-spin) transition. Energy level diagram shows a significant modification in the crystal field splitting between Co-t_2g and Co-e_g levels, driven by the robust hybridization of Co-d and O-p orbitals [1].

[1] S. Nandi, S. Pradhan, A. K. Nandy, and R. S. Manna, Phys. Rev. B 110, 085101 (2024).