High-pressure and high-temperature multianvil synthesis of metastable polymorphs ofBi2O3: Crystal structure and electronic properties

Abstract

High-pressure and high-temperature experiments with ${\text{Bi}}_{2}{\text{O}}_{3}$ using a 6--8 type multianvil device led to the formation of a metastable polymorph $({\text{HP-Bi}}_{2}{\text{O}}_{3})$ with noncentrosymmetric trigonal symmetry. This phase relaxes during the course of several months at ambient temperature or more rapidly via annealing, to a second intermediate modification $({\text{R-Bi}}_{2}{\text{O}}_{3})$. Upon further annealing finally the transformation back to the known ambient phase $(\ensuremath{\alpha}{\text{-Bi}}_{2}{\text{O}}_{3})$ takes place. Both crystal structures were solved from high-resolution x-ray and neutron powder-diffraction data. The orientation and stereochemical activity of the ${\text{Bi}}^{3+}$ lone pairs (or inert pairs) is discussed in terms of crystal-chemical considerations and density-functional theory calculations. Whenever suitable, results were verified by experimental determination of the respective properties. The results of the theoretical analyses show that within the structure type of ${\text{HP-Bi}}_{2}{\text{O}}_{3}$, bismuth oxide exhibits a pronounced polarization and can be considered as ferroelectric.