Diffusion phase transition and aging properties induced by B-site disorder in Na-doped barium strontium titanium ceramics

Na-doped barium strontium titanium ceramics [(Ba1-xNax)(0.9)Sr0.1TiO3-delta (x = 0.01, 0.03, 0.05, 0.08, 0.1, 0.15, 0.2, 0.3)] with perovskite tetragonal structure has been successfully fabricated by conventional solid state reaction processing. With the increasing of Na, microstructure exhibits the evolution of grains from club-shaped, square, needle-shaped and lamellate to rod-shaped. The temperature dependence of dielectric properties illustrates normal ferroelectric-paraelectric phase transition with x = 0.01, 0.03, and 0.05 and the diffusion phase transition with x = 0.08, 0.1, 0.15, 0.2 and 0.3, which implies that B-site disorder caused by composition fluctuation of Na in A-site can also induce diffused phase transition. Also, the aging properties have been investigated with decreased dielectric constant and Curie temperature, as well as constricted P-E loops compared to fresh samples, which can be explained by the symmetry-conforming principle of point defects with defect dipoles in the form of oxygen vacancies surrounding a central Na+ in the distorted oxygen octahedron.