Effects of Ca doping on the Curie temperature, structural, dielectric, and elastic properties of Ba[sub 0.4]Sr[sub 0.6-x]Ca[sub x]TiO₃ (0≤x≤0.3) perovskites

Abstract

The effects of Ca doping on the Curie temperature, structural, dielectric, and elastic properties of Ba[sub 0.4]Sr[sub 0.6−x]Ca[sub x]TiO₃ (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 T[sub C], 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, T[sub C] increases linearly with increasing x from 0 to 0.15; while deviating from linear behavior for x>0.15, T[sub C] increases persistently up to x=0.25 and thereafter shows a decrease. These variations of T[sub C] 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 C[sub L] shows drastic softening near T[sub C], arising from the electrostrictive coupling between the polarization fluctuations and the elastic strains. Moreover, it was found that Ca doping induces the hardening of C[sub L] just below T[sub C], and the magnitude of the relative hardening of C[sub L] (i.e., ΔC[sub L]/C[sub L]) increases with increasing x, implying that the tetragonal ferroelectric phase was increasingly stabilized by Ca doping.