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

Na-doped barium strontium titanium ceramics [(Ba_1−xNa_x)_0.9Sr_0.1TiO_3−δ (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.