Local Structure and chemical bonding in La_0.63Ti_0.92Nb_0.08O₃ with A-site-deficient perovskite-type structure

Abstract

La₂/3TiO₃-based compounds with an A-site-deficient perovskite-type ABO3 structure exhibit high ionic conductivities and dielectric constants. The orthorhombic (Cmmm) - tetragonal (P4/mmm) phase transition occurs reversibly and continuously at 623(2) K in the La_0.63Ti_0.92Nb_0.08O₃. Cation vacancies in the compounds are ordered in a lattice site at lower temperature. The temperature dependence of EXAFS Debye-Waller factors in La_0.63Ti_0.92Nb_0.08O₃ perovskite with cation deficiency was investigated with the cumulant expansion method. The measurements of the Ti and Nb K-edge and La L3-edge EXAFS spectra were carried out in the transmission mode at temperature up to 800 K. The obtained local bond distances of 1.95(1) A for Ti-O, 1.98(1) A for Nb-O and 2.65(1) A for La-O are significantly smaller than the expected values on the Shannon's radii. This characteristic is attributable to the presence of the cation deficiency. The effective pair potentials, V(u)=au2/2+bu3/3!, were evaluated and the potential coefficients a and b for the Ti-O bond in La_0.63Ti_0.92Nb_0.08O₃ are 6.6(1) eV/A2 and -42(3) eV/A3, respectively. The Ti-O bond in the A-site-deficient La_0.63Ti_0.92Nb_0.08O₃ has relatively larger anharmonic and soft potential coefficient than that in ordinary CaTiO₃ perovskites.