New Ferroelectric Perovskite—Materials Design

Abstract

Based on scientific knowledge gained from BaTiO₃ and PbTiO₃ perovskites, it is attempted to design new ferroelectric perovskite. They have tetragonal structure, which implies Ti–O–Ti bond length in one specific direction is different from other two directions. In quantum bonding (QB) motion, when stabilisation energy in one specific direction is larger than other directions, ferroelectric QB motion occurs. Hence, as our strategy, we consider to introduce different bond length in one specific direction. First, molecular orbital calculation is performed to investigate QB motion in ideal LaTiO₂N perovskite. Though Ti–N–Ti bond length is shorter than Ti–O–Ti bond length, ferroelectric QB motion along Ti–N–Ti bond occurs. Next, we consider SrTiO₂C perovskite, where carbon atoms are replaced by oxygen atoms in SrO layer. As same as LaTiO₂N perovskite, Ti–C–Ti bond length is different from Ti–O–Ti bond length. Covalent bonding is formed between titanium and carbon atoms. Hence, different type of QB motion along Ti–C–Ti bond is expected in the application of electric field. In the QB motion along Ti–C–Ti bond, Ti–C bond is elongated, instead of shrinking. In addition, though periodic QB motion is not stabilised, aperiodic QB motion occurs. On the other hand, in perpendicular QB motion, larger stabilisation energy is given. As the result, when controlling the magnitude of electric field, ferroelectric QB motion can be caused. Owing to smaller stabilisation energy, high-speed polarisation reversal, low electric power consumption and improvement of endurance can be expected in ferroelectric SrTiO₂C perovskite. QB motion in TiOC layer is also investigated. In this case, as larger stabilisation energy is given along Ti–C–Ti bond, the QB motion is advantageous.