STRUCTURE AND PROPERTIES OF (BA,CA)TIO3 CERAMICS PREPARED USING (BA,CA)CO3 PRECURSORS .2. DIFFUSE PHASE-TRANSITION BEHAVIOR

Ferroelectric phase transition in (Ba,Ca)TiO3 ceramics has been invest igated using dielectric measurements. It is shown that samples prepare d using (Ba,Ca)CO3 precursors exhibit diffuse phase transition (DPT) w hile those prepared by the conventional route show BaTiO3-type sharp t ransition. The diffuseness of the phase transition is found to increas e with increasing calcium content. The critical exponent for the diele ctric stiffness (1/epsilon') departs significantly from the mean field value of unity and is shown to increase from 1.21 to t.86 with increa sing calcium content. Arguments are advanced to show that the DPT beha vior in these ceramics is inexplicable in terms of either the internal stress model due to the absence of 90-degrees domains in fine-grain c eramics or the compositional inhomogeneity model. It is proposed that DPT behavior is intrinsic to the system and is intimately linked with the appearance of polar phase in the critical regime whose width is sh own to increase with increasing Ca content.