The effect of Pb²⁺ substitution on the quantum paraelectric behaviour of CaTiO₃

Abstract

Temperature dependent dielectric, x-ray diffraction and '1/f' conductance noise studies on (Ca_1-xPb_x)TiO₃ ceramics provide evidence for a new phase transition in this mixed system for the composition range 0.10≤x≤0.40. The dielectric anomaly corresponding to this transition is superposed on the quantum paraelectric background of the host CaTiO₃ matrix. This anomaly is not associated with a ferroelectric or relaxor ferroelectric transition, as confirmed by the absence of a polarization hysteresis loop. The Curie-Weiss temperature for this transition is found to be negative which, using Kittel's model, is proposed to be due to an antiferroelectric transition. These results are discussed in the light of recently reported soft modes in CaTiO₃. It is also shown that the nature of phase transition changes abruptly from antiferroelectric to relaxor ferroelectric type at 0.40<x<0.45 and finally to a normal ferroelectric type for 0.60<x≤1.0. The relaxor behaviour in the intermediate composition range is attributed to frustration caused by competing ferroelectric and antiferroelectric instabilities.