Soft Mode Driven Monoclinic Phase In Multiferroic Bifeo3

A monoclinic phase (Pm) BiFeO3 corresponding to the softening behavior of the E mode of the tetragonal phase (P4mm) was characterized by using first-principles method and factor group analysis. The calculated atomic structures, elastic constants and vibration frequencies at the Gamma point of rhombohedra, tetragonal and monoclinic phases have an agreement with the available experimental and theoretical results. The results show the monoclinic phase is more dynamically stable than the tetragonal phase, and its ferromagnetic state more energetically stable than the non-ferromagnetic state, implying it might be a candidate of the multiferroic phase linking the R3c and P4mm symmetry of BiFeO3.