Crystal structure and dielectric properties of complex perovskite (Sr, Ba, Ca)(Ni_0.5W_0.5)O₃

A-site cation substitution with Ba²⁺ and Ca²⁺ was made for the complex perovskite Sr(Ni_0.5W_0.5)O ₃ to correlate with the structure evolution and the temperature coefficient of dielectric constant. The crystal structures of the solid solutions at elevated temperatures were studied using high temperature X-ray powder diffractometry. A single phase of the perovskite was obtained and ordering of the cations in B-site was observed for all compositions of the solid solutions sintered at 1400 °C for 24 h. The crystal symmetry of the solid solutions at room temperature was changed from monoclinic→rhombohedral→orthorhombic→cubic with the corresponding A-site cation species of Ca²⁺→Sr ²⁺→Ba²⁺. The orthorhombic phase of Sr(Ni _0.5W_0.5)O₃ was transformed to the cubic phase at 300°C. The phase transformation from rhombohedral to cubic at elevated temperatures was observed for each of the Ca²⁺-substituted solid solutions, and the phase transformation temperatures increased with increasing Ca²⁺ content. Another phase transformation from monoclinic to rhombohedral gradually occurred for (Sr_0.2Ca_0.8)(Ni _0.5W_0.5)O₃ between 400 and 600°C. Phase transformation was not observed for Ca(Ni_0.5W_0.5)O ₃ in the temperature range from room temperature to 1200°C. Dielectric constants of the solid solutions at room temperature were approximately 27-33. Temperature dependence of dielectric constant was measured in the temperature range from -100 to 500°C and the temperature coefficients of dielectric constant for each of the solid solutions are discussed correlating with the changes in the crystal symmetry during heating.