Superparaelectric phase transition in KTaO₃:Li with a small amount of a “second admixture”

Abstract

A model of a new type phase transition in K_1-xLi_xTaO₃ (0.022 ≤ x ≤ 0.04) doped in small concentration (y ≪ x) by a “second”, additional impurity capable of diffusion is suggested. Diffusion of this “second” impurity induces diffusion of the “main” dipolar Li⁺ impurity off-centers. Under self-organization conditions, this leads to formation of a superstructure with layers enriched by Li⁺ centers, and phase transition with a strong enhancement of the value of the dielectric permittivity. The latter is caused by appearance of a new soft mode-diffusion soft mode corresponding to the Li⁺ superstructure melting. The model developed allows to explain the startling increase in the magnitude of the maximum of ∊′ (until ∼ 90 000) for K_0.97 Li_0.03TaO₃ doped by Cu (≤ 0.2 % wt.), which was observed at low temperature recently.