Lattice dynamics of bismuth-deficient BiFeO3 from first principles

Lattice dynamic properties of bismuth ferrite with 6% bismuth vacancies have been investigated in present work for the first time. Hellmann-Feynman forces of each atom were calculated by direct method in the framework of density functional theory, and strong electron correlations of Fe ion were taken into account by applying the Hubbard potential U and the exchange interaction J. Based on the analysis of partial phonon density of states, effective force constants of Fe-O bond are influenced by the altered nearest coordination of oxygen due to bismuth vacancy. This behavior is connected with the soft phonon modes observed in the low energy range of phonon dispersion curve of nonstoichiometric Bi0.94FeO3. Low-temperature heat capacities are showing essential difference between nonstoichiometric and stoichiometric system that optic phonon modes begin contribute to the heat capacity from 3 K in Bi0.94FeO3, earlier than 7 K in BiFeO3.