First-principles study of the cubic perovskites BiMO₃ (M=Al, Ga, In, and Sc)

We systematically investigated the structure, electronic properties, zone-center phonon modes, and structure instability of four cubic perovskite BiMO₃ compounds, with three of the M ions being IIIB metals (Al, Ga, and In) and one IIIA transition-metal Sc, using first-principles density-functional calculations. Optimized lattice parameters, bulk moduli, band structures, densities of states, as well as charge density distributions are calculated and compared with the available theoretical data. Our results are in good agreement with those previously reported in the literature. All the BiMO₃ oxides considered in the present work are semiconductors with an indirect band gap between the occupied O 2p and unoccupied Bi 6p states varying between 0.17 and 1.57 eV. Their electronic properties are determined mainly by Bi–O bonding, which, in turn, depends on the M–O bonding. Ferroelectric properties of these oxides come from the 6s² lone pair on the A-site Bi ion and is similarly affected by the M ions through their influence on the Bi–O bonding, as suggested by our calculations of density of state, Born effective charge, and soft modes. The existence of soft modes and eight [111] minima suggests that the phase transition in BiAlO₃ has a mixed displacive and order-disorder character. There is evidence that ferroelectricity is absent in BiGaO₃. Our investigation suggests that the BiMO₃ oxides or their modified versions are promising ferroelectric, piezoelectric, multiferroic, and photocatalytic materials.