Structures and energetics of Bi2O3 polymorphs in a defective fluorite family derived by systematic first-principles lattice dynamics calculations

A series of dynamically stable structures of the defective fluorite-Bi2O3 is found by first-principles lattice dynamics calculations. The crystal symmetry is lowered and local distortion is included systematically along imaginary modes of lattice vibrations. A clear band gap appears when local distortion is included in this way, which is consistent with experimental results. Many theoretical calculations in the past indicated metallic or semimetallic electronic structures. The appropriate inclusion of the symmetry breaking and local distortion are therefore essential in reproducing the electronic structures of Bi2O3. The three stable structures are found to have an energy of 0.07-0.08 eV/f.u. relative to alpha-Bi2O3. The arrays of oxide-ion vacancies in two of these structures are the same as those of beta-Bi2O3 and epsilon-Bi2O3, which were experimentally identified as stable phases. A bixbyite-type vacancy-array structure is also found to be another stable phase in the present study, which is named eta-Bi2O3. It is suggested that the three stable structures of Bi2O3 are ordered low-temperature polymorphs of the disordered delta-Bi2O3 in the defective fluorite-Bi2O3 family.