Electronic structure and lattice dynamical properties of different tetragonal phases of BiFeO3

From first-principles calculations based on the plane-wave pseudopotential method within the density-functional +U scheme, we have investigated the atomic geometry, electronic band-structure, and lattice dynamical properties of four tetragonal phases of the multiferroic BiFeO3. In contrast to the indirect Kohn-Sham band gap of the rhombohedral phase, the most stable of the single-phase stable tetragonal structure is semiconducting with a smaller and direct band gap. It is found that the highest optical phonon branch is split from the rest of the phonon continuum by a small gap. The presently calculated zone-center optical modes have been compared and contrasted with the available polarized Raman-scattering studies and previous calculations based on a simple short-range force-constant method. The highest phonon mode with frequency similar to 662 cm(-1) of A(1) representation in the tetragonal phase (P4mm) can be readily distinguished from the frequency similar to 590 cm(-1) of A(2) representation in the rhombohedral phase (R3c).