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 nonpolar (point group C2h) 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 experiment. 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 AFM calculation, FM-optimized unit cell is used. This strategy is tested and works well for Mn3TeO6, whose magnetic structure preserves the paramagnetic point group and a direct AFM calculation is feasible. Refinement of powder 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, indicating that high-temperature is indispensible for pressure-induced phase transition in Co3TeO6.