EFFECTS OF ISOVALENT SUBSTITUTIONS ON LATTICE SOFTENING AND TRANSITION CHARACTER OF BATIO3 SOLID-SOLUTIONS

The transition character of paraelectric-to-ferroelectric phase transition and its relation to lattice soft-mode behavior in the solid solutions of (Ba,Sr)TiO3, (Ba,Ca)TiO3, Ba(Ti,Zr)O3, and Ba(Ti,Sn)O3 were investigated by the measurements of dielectric constants in the temperature range from -30 to 150°C, and x-ray line breadth of 100, 110, and 111 Bragg reflections at temperatures above the transition point. From the result of dielectric measurement, it was found that the specimens containing Ti-site substituents have shown a diffuse phase transition character, whereas those having Ba-site substituents have preserved a sharp phase transition. It was also noted from the change of line breath of Bragg reflections that for the solutions of Ba(Ti,Zr)O3 and Ba(Ti,Sn)O 3, the high dynamic strain at the 〈1 0 0〉 axis, induced from the soft-mode of lattice vibration, become repressed when a large amount of solutes is contained, but for the solutions of (Ba,Sr)TiO3 and (Ba,Ca)TiO3, it is recovered at temperatures near the transition point. Moreover, the dynamic strain at the 〈1 1 1〉 axis can be rapidly repressed by the addition of Ti-site substituents. The different effects on the dynamic strains are attributed to the disparate distortions of oxygen octahedra in the perovskite lattice, induced by the solutes of different site substitution. It is also believed that the influence on the soft-mode of lattice vibration has caused the different phase transition characters of the four BaTiO3 solid solutions.