CHARACTERIZING PHASE-TRANSITIONS IN THE PEROVSKITES PBTIO3 AND BATIO3USING PERTURBED-ANGULAR-CORRELATION SPECTROSCOPY

Perturbed-angular-correlation (PAC) spectroscopy was used to measure i n ceramic samples of PbTiO3 acid BaTiO3 the temperature dependence of the Ti-site electric-field gradients (EFGs) at temperatures very close to the ferroelectric-to-paraelectric transition temperatures T-c. The samples were doped with small amounts of Hf that carried the Hf-181 - -> Ta-181 probe radioactivity. A high-frequency nuclear quadrupole int eraction that decreases very little as the temperature approaches T-c, characterizes the PbTiO3 transition. The tetragonal and cubic phases for PbTiO3 appear to coexist over a temperature interval of 8 +/- 1 K, and the transition shows a thermal hysteresis of about 4 K. In contra st, a lower-frequency interaction that decreases rapidly as temperatur e approaches T-c, characterizes the BaTiO3 transition. Both phases of BaTiO3 appear to coexist over an interval of about 2 K, and the therma l hysteresis is about 1 K. At temperatures above T-c, both PbTiO3 and BaTiO3 show weak, non-vanishing Ti-site EFGs. Although, for BaTiO3, th is effect limits the accuracy with which critical effects can be measu red, we estimate a power-law exponent beta = 0.21 +/- 0.05, which most likely is somewhat lower in magnitude than the actual critical expone nt. For the explanation of our observations we assume the existence of a distribution of T-c-values. This distribution would arise because t he crystals could have spatially non-uniform distributions of nucleati on sites, which for PbTiO3 and BaTiO3 could be point defects.