Negative Thermal Expansion in the (Bi0.5Na0.5)1-x(Ba0.4Sr0.6)xTiO3 System

The (Bi0.5Na0.5)1-x(Ba0.4Sr0.6)xTiO3 (BNT-BST) is being investigated as a lead-free ferroelectric system for energy storage capacitors. At x = 0.2 BNT-BST exhibits a tetragonally distorted perovskite structure. Dielectric measurements reveal two anomalies at ca. 125 C (ferroelectric to relaxor-ferroelectric) and 360 C (relaxor-ferroelectric to paraelectric). The former is associated with a change in structure from tetragonal to cubic symmetry, which involves a 25 C range of negative thermal expansion (NTE), where a dynamic deformation of the structure appears to consume some free space leading to volume shrinkage. The aim of this work is to characterise this dynamic structural deformation in the NTE region as well as the more subtle high temperature transition.