First-Principles Investigations of Pb_0.5Ba_0.5TiO_3 Alloys Based on Structure Predictions

Crystal structure predictions of Pb_0.5Ba_0.5TiO_3 alloys under different pressures are performed based on the particle swarming optimization algorithm. The predicted stable ground-state and high-pressure phases are tetragonal ferroelectric (I4mm) and cubic para-electric (Fm\bar3m), respectively, whose structural details have not been reported. The pressure-induced colossal enhancements in piezoelectric response are associated with the mechanical and dynamical instabilities instead of polarization rotation. The band gap of the tetragonal phase is indirect and that of the cubic phase is always direct. As pressure increases, the alloy displays the similar band-gap behaviors to PbTiO_3, while different from BaTiO_3, which is attributed to the different orbital contributions to the valence bands. Our calculated results are in good agreement with the available data.