FLUCTUATION-SWAMPED DISCONTINUOUS PHASE-CHANGES IN LIGHTLY DOPED FERROELECTRIC BARIUM-TITANATE

X-ray scattering experiments on lightly doped powders of BaTiO3 using the diffractometer on line 9.1, SRS, Daresbury, have been extended to investigate transitions in the barium-rich solid solutions (Ba,Sr)TiO3 and (Ba,Pb)TiO3. Samples studied earlier were 'impure systems', that is pure material doped with ions with a valency different from those they replaced. Qualitatively the ferroelectric transitions near 130-degrees-C in the solid solutions and impure systems are similar - they are 'smeared out' and appear continuous, but are in fact fluctuation-swamped, discontinuous changes. There are important quantitative differences between the transitions in the two categories. Analysis shows that impure systems behave as 'host'- pure BaTiO3 - plus point-like defects, with parameter independent of the dopant and concentration. However, for (Ba,Sr)TiO3 the parameters describing the critical behaviour depend on the concentration of Sr.