A computational and Raman spectroscopic study of successive phase transitions in Co3TeO6 under high pressure and high temperature

Abstract

Co₃TeO₆ was previously found to transform from a nonpolar (point group C_2h) to polar (C₃) antiferromagnet (AFM) at 5 GPa and 1023 K, of which the transition pressure was predicted by first-principles calculations assuming ferromagnetic (FM) ground states. Here we report a computational contrast experiment on the transition pressure in Co₃TeO₆, followed by verification experiments. The C2/c to R3 transition is reproduced by both FM (4.7 GPa) and AFM (4.5 GPa) calculations. A further phase transition from R3 to P2₁/n is predicted to occur at 16.2 (FM) or 19.2 GPa (AFM). To avoid cell deformation due to reduced symmetry in the AFM calculations, an FM-optimized unit cell is used. This strategy is inspired by calculations on Mn₃TeO₆, whose magnetic structure preserves the paramagnetic point group and for which a direct AFM calculation is feasible. Refinement of ex situ powder X-ray diffraction data of Co₃TeO₆ polymorphs prepared under ambient pressure, 5 GPa, and 20 GPa (and high temperature, then quenched and decompressed to ambient pressure for the latter two) show C2/c, R3 and phase coexistence of R3 (72%) and P2₁/n (28%), respectively. Pressure-dependent Raman spectroscopy on polymorphs with C2/c and R3 phases at room temperature show no hints of phase transition. Our preliminary diffraction and spectroscopy results indicate that kinetic effects in phase transition cannot be ignored, and serve as a foundation for future in situ investigations.