Ferroelectric-to-relaxor crossover and oxygen vacancy hopping in the compositionally disordered perovskites KTa1-xNbxO3: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 KTa1-xNbxO3: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 relaxer 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.