Soft-mode driven tetragonal-to-monoclinic phase transition in multiferroic BiFeO3

► Tetragonal phase BiFeO3 with fully relaxed structural parameters is dynamically unstable resulting from the softest E phonon. ► Sequence phase of the soft-mode driven phase transition of tetragonal BiFeO3 is monoclinic (Bm, No. 8). ► The tetragonal-to-monoclinic phase transition occurs at about 13.3 GPa with a second-order character. ► The tetragonal-to-monoclinic phase transition involves the presence of ferroelectric distortion in the a-b plane of BFO.