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High-pressure and high-temperature experiments with Bi 2 O 3 using a 6-8 type multianvil device led to the formation of a metastable polymorph ͑HP-Bi 2 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 ͑R-Bi 2 O 3 ͒. Upon further annealing finally the transformation back to the known ambient phase ͑␣-Bi 2 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 Bi 3+ lone pairs ͑or inert pairs͒ 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-Bi 2 O 3 , bismuth oxide exhibits a pronounced polarization and can be considered as ferroelectric.

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