BST solid solutions, temperature evolution of the ferroelectric transitions

Solid solutions Ba1 xSrxTiO3 (BST) are of high technological importance, particularly in microwave domain. Barium titanate has ‘‘naturally’’ three transitions, between four stable ferroelectric phases: (C) cubic, (T) tetragonal, (O) orthorhombic, (R) rhombohedral. Jaffe et al. [B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press, 1971] has given the dependence of the transition temperatures up to 30% of Sr content. We have extrapolated these temperatures and we have found that some phases might disappear at higher Sr concentrations. A family of solid solutions with x = 25, 50, 75, 90% was prepared by standard solid-state reaction and sintered at 1230 and 1260 8C, respectively. The permitivities and the dielectric losses were measured with a self-acting bridge (1 kHz), on a large temperature range ( 200 8C). The composition x = 25% shows three peak values of permittivity as expected, while the composition x = 50%, only two peak values, corresponding to phase transitions cubic–tetragonal–rhombohedral, phase orthorhombic being excluded. Compositions with x 63%, Sr shows only one peak value corresponding to a genuine transition cubic–rhombohedral. The cubic transition to several lower phases shows almost a linear decrease of the Curie point with the increase of Sr fraction. For Sr concentration x 80%, the Curie point appears to fall more rapidly than linear. To our best knowledge, there is for the first time, this effect is reported.