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

Temperature dependent dielectric, x-ray diffraction and '1/f' conductance noise studies on (Ca1-xPbx)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 anti ferroelectric 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 anti ferroelectric instabilities.