Effects of Ca doping on the Curie temperature, structural, dielectric, and elastic properties of Ba0.4Sr0.6−xCaxTiO3 (0⩽x⩽0.3) perovskites

Abstract

The effects of Ca doping on the Curie temperature, structural, dielectric, and elastic properties of Ba0.4Sr0.6−xCaxTiO3 (0⩽x⩽0.3) has been studied. Powder x-ray diffraction revealed that the cubic lattice constant a decreases linearly with increasing x from 0 to 0.15, while showing an anomalous expansion between x=0.15 and x=0.18. This, together with the anomalies in the dielectric constants, Curie temperature TC, and elastic constants observed for 0.15<x<0.18, suggests that a small amount of Ca ions substituted for Ti ions. Correlated with the evolution of a with x, TC increases linearly with increasing x from 0 to 0.15; while deviating from linear behavior for x>0.15, TC increases persistently up to x=0.25 and thereafter shows a decrease. These variations of TC with x have been interpreted in terms of Ca-doping-induced A-site cation size variance, a substitution of a small amount of Ca ions for Ti ions, and structural phase separation. Upon cooling the longitudinal elastic constant CL shows drastic softening near TC, arising from the electrostrictive coupling between the polarization fluctuations and the elastic strains. Moreover, it was found that Ca doping induces the hardening of CL just below TC, and the magnitude of the relative hardening of CL (i.e., ΔCL∕CL) increases with increasing x, implying that the tetragonal ferroelectric phase was increasingly stabilized by Ca doping.