X-ray and phenomenological study of lanthanum-modified lead zirconate-titanates in the vicinity of the relaxor phase transition region

X-ray diffraction analyses of chemically derived Pb1−xLax(Zr0.65Ti0.35)1−x/4O3 powders were combined with a phenomenological theory to investigate the corresponding single-crystal thermodynamic properties of compositions approaching the relaxor ferroelectric phase transition region (x=0–0.04). The temperature dependence of the electrostrictive strain component, x4, could be described by the Landau–Ginsburg–Devonshire phenomenological theory taking Tc = 357 °C independent of La content (x). The single-domain, single-crystal elastic Gibbs free-energy density was calculated as a function of temperature and composition. The calculated results were discussed in relation to a simple superparaelectric model of relaxor behavior involving the temperature stabilization of noninteracting polar microregions.