Item

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 similar to 2.1 GPa from P31c toward space group P6(3)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 similar to 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.