Abstract
In situ high-pressure x-ray and neutron powder diffraction experiments on the recently reported metastable high-pressure polymorph of BiO (HP-BiO) at ambient temperature has revealed a first-order translationengleiche subgroup-supergroup phase transition at a pressure of 2.1 GPa from toward space group (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-BiO 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-BiO with pressure, shifting the phase transition from ideally the second order to the observed first order.
4 More- Received 15 February 2011
DOI:https://doi.org/10.1103/PhysRevB.83.214102
©2011 American Physical Society