High-pressure structural evolution of HP-Bi2O3

Abstract

In situ high-pressure x-ray and neutron powder diffraction experiments on the recently reported metastable high-pressure polymorph of Bi${}_{2}$O${}_{3}$ (HP-Bi${}_{2}$O${}_{3}$) at ambient temperature has revealed a first-order translationengleiche subgroup-supergroup phase transition at a pressure of $~$2.1 GPa from $P31c$ toward space group $P{6}_{3}\mathit{mc}$ (No. 186). Density functional theory calculations were performed to rationalize the experimental observation and to gain further insight into the mechanism of the phase transition. The transition is caused by a torsion of Bi-O polyhedra and the appearance of a mirror plane. It is accompanied by a contraction of the trigonal c axis and reorientation of localized Bi related electron lone pairs, which leads to a volume drop of $~$3.3%. Both modifications were treated independently in terms of common equations of state evaluation. Bulk moduli were determined to be 32.8 GPa for HP-Bi${}_{2}$O${}_{3}$ and 60.3 GPa for the polymorph past phase transition. It was found that the observed phase transition represents a thermally triggered discontinuity in a continuous evolution of the crystal structure of HP-Bi${}_{2}$O${}_{3}$ with pressure, shifting the phase transition from ideally the second order to the observed first order.