Crystal structure and dielectric properties of complex perovskite (Sr, Ba, Ca)(Ni0.5W0.5)O3

A-site cation substitution with Ba2+ and Ca2+ was made for the complex perovskite Sr(Ni0.5W0.5)O3 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+→Sr2+→Ba2+. The orthorhombic phase of Sr(Ni0.5W0.5)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 (Sr0.2Ca0.8)(Ni0.5W0.5)O3 between 400 and 600 °C. Phase transformation was not observed for Ca(Ni0.5W0.5)O3 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.