Local Structure Analysis of Li-substituted (Bi_0.5Na_0.5)TiO₃ and NaNbO₃

抄録

(Bi_0.5Na_0.5)TiO₃ and NaNbO₃ are ABO₃ perovskite ferroelectric materials and they are considered to be promising lead-free ferroelectric materials because of their relatively high piezoelectric properties in lead-free piezoelectric ceramics. Furthermore, disordered rhombohedral phases are included in their phase transition sequences. In these disordered phases it is possible by substituting Na for Li to induce ordered ferroelectric phases. The purpose of this work was to clarify the local structures of Li-substituted (Bi_0.5Na_0.5)TiO₃ and NaNbO₃ to elucidate the mechanism of the stabilization of the ordered ferroelectric phases. A local structure analysis was carried out using the X-ray absorption fine structure (XAFS) the X-ray pair-distribution function (PDF) obtained from high-energy synchrotron X-ray diffraction. Although XAFS and PDF are the local structure analysis techniques, the Li substitution effect was observed clearly in PDF, but not observed in XAFS. The difference in the coherence of the local structure analysis in XAFS and PDF indicates that the ordered structure of (Bi_0.5Na_0.5)TiO₃ and NaNbO₃ is stabilized by excluded volume effect.