Phase transformation and temperature dependence of relative permittivity for A-site substituted Sr(Y0.5Ta0.5)O3 perovskites

A-site cation substitution with Ba2+ or Ca2+ ions was made for the ordered complex perovskite Sr(Y0.5Ta0.5)O-3 to correlate the structure evolution and the change in the temperature coefficient of relative permittivity (TC epsilon(r)) with the cation substitution. The crystal symmetry of the solid solutions at room temperature changed as monoclinic --> rhombohedral --> cubic with the corresponding A-site cation species of Ca2+ --> Sr2+ --> Ba2+. On heating, the rhombohedral phase of Sr(Y0.5Ta0.5)O-3 was firstly transformed to the cubic phase with a lower symmetry (<F(4)over bar>3m or F432) at 1000 degrees C and further to the ideal cubic phase (Fm3m) at 1300 degrees C. The similar phase transformation behavior at elevated temperatures was observed for each of the Ba2+-substituted perovskites, both the phase transformation temperatures being lowered with increasing Ba2+ content. epsilon(r) of the ordered perovskites monotonously increased over the temperature range where the phase transformation regions from monoclinic to rhombohedral or from rhombohedral to cubic (<F(4)over bar>3m or F432) occurred. On the other hand, a subtle symmetry change in the cubic phase from <F(4)over bar>3m (F432) to Fm3m caused a remarkable change in the TC epsilon(r) from positive to negative at the transformation temperature. These results suggested that the positive TC epsilon(r) is essentially correlated to the relaxation of the tilting of the O-6 octahedra in the perovskite structure on heating.