A monoclinic to tetragonal crossover in (Na_0.5Bi_0.5TiO₃)_(1−x)(BaZrO₃)_x ceramic: a lead-free ferroelectric material

A single-phase solid solution of Na_0.5Bi_0.5TiO₃ (NBT) and BaZrO₃ (BZ), i.e. (NBT)_(1−x)(BaZrO₃)_x (x = 0.00, 0.01, 0.03, 0.05 and 0.07), was synthesized using the solid-state reaction method. It is found that the temperature for single-phase formation increases with increasing BZ content. A detailed structural investigation was carried out as a function of composition using Rietveld analysis of powder X-ray diffraction data. The structural refinement reveals a crossover from monoclinic (Cc) to tetragonal (P4mm) symmetry in the composition range 0.03 < x < 0.05. The intensity of superlattice reflection due to a⁻a⁻c⁻ tilting of oxygen octahedra in the Cc phase was found to decrease with increasing BZ content. The unit-cell volume was also found to increase with BZ content, which was attributed to the higher ionic radii of Ba²⁺ and Zr⁴⁺ in comparison to Bi³⁺ (Na⁺) and Ti⁴⁺. The structural phase boundary observed in (NBT)_(1−x)(BaZrO₃)_x is concurrent with the observation of morphotropic phase boundary like behaviour.