High-pressure and high-temperature multianvil synthesis of metastable polymorphs of Bi2O3: Crystal structure and electronic properties

High-pressure and high-temperature experiments with Bi2O3 using a 6-8 type multianvil device led to the formation of a metastable polymorph (HP-Bi2O3) 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 (R-Bi2O3). Upon further annealing finally the transformation back to the known ambient phase (R-Bi2O3) takes place. Both crystal structures were solved from high-resolution x-ray and neutron powder-diffraction data. The orientation and stereochemical activity of the Bi3+ 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 HP-Bi2O3, bismuth oxide exhibits a pronounced polarization and can be considered as ferroelectric.