First Principles Calculation Study of the Origin of the Tetragonal Structure of BiCoO3

We have investigated the electric state of BiCoO 3 in various crystal and magnetic structures by first-principles calculation method to clarify the origin of the tetragonal structure of BiCoO 3 . The ferroelectric displacement of Co in the tetragonal crystal is larger than that in the rhombohedral one. Because of the large ferroelectric displacement, the energy levels of the Co's d 6 localized electrons are lowered and the crystal tends to be tetragonal. Moreover, we calculated the electric state of BiCo 0.5 Fe 0.5 O 3 , BiCoO 3 –BiFeO 3 solid solution. In this solid solution, the effect of Co's d 6 localized electron is effective and the crystal also tends to be tetragonal.