Intrinsic Reasons for Variations in Dielectric Properties of Ba_6-3xR_8+2xTi₁₈O₅₄(R=LA–Gd) Solid Solutions

Studies of the temperature coefficient of permittivity of Ba_6-3xR_8+2xTi₁₈O₅₄ (R=La–Gd) solid solutions revealed a high degree of correlation with the ratio of mean radii (r_A/r_B) of A- and B-site ions. Single-crystal X-ray diffraction analyses of La, Sm and Pr analogues showed that r_A/r_B can be further correlated to tilting of the TiO₆ octahedra in the network of the Ba_6-3xR_8+2xTi₁₈O₅₄ crystal structure. Changes in the tilting of the octahedra induce changes in the crystal field and subsequently, changes in the temperature coefficient of permittivity as well as dielectric loss. Variations in dielectric properties are limited by the flexibility of the crystal structure. Variations in the tilting of the octahedra resulting from changes in the composition are highly anisotropic, which explains the markedly high electrical anisotropy of Ba_6-3xR_8+2xTi₁₈O₅₄ solid solutions.