Ferroelectric-to-relaxor crossover and oxygen vacancy hopping in the compositionally disordered perovskites KTa_1-xNb_xO₃:Ca

It is shown that lattice disorder induced by Nb and Ca substitution has a strong influence on the dielectric and relaxational properties of KTaO₃. Both substituents are believed to occupy off-center positions at the Ta site, and the difference in valence between the Ca²⁺ and Ta⁵⁺ ions leads to the formation of oxygen vacancies (V_O). Specifically, for a KTa_1-xNb_xO₃:Ca crystal with x=0.023 and with a 0.055 at. % Ca doping, we observe (i) a ferroelectric transition at atmospheric pressure (1 bar); (ii) a large enhancement of the transition temperature by Ca doping; (iii) a pressure-induced crossover from ferroelectric-to-relaxor behavior; (iv) the impending vanishing of the relaxor phase at high pressure; (v) the reorientation of the Ca-oxygen vacancy (Ca:V_O) pair defect; and (vi) the variation of the energetics and dynamics of this reorientation with pressure. Most of these effects are associated with Nb- and Ca-induced dipolar entities and appear to be general features of soft mode ferroelectrics with random-site polar nanodomains. The ferroelectric-to-relaxor crossover can be understood in terms of a large decrease with pressure in the correlation length among polar nanodomains—a unique property of soft ferroelectric mode systems.