Characterizing Phase Transitions in the Perovskites PbTiO3 and BaTiO3 Using Perturbed-Angular-Correlation Spectroscopy

Abstract

Abstract Perturbed-angular-correlation (PAC) spectroscopy was used to measure in ceramic samples of PbTiO3 and BaTiO3 the temperature dependence of the Ti-site electric-field gradients (EFGs) at temperatures very close to the ferroelectric-to-paraelectric transition temperatures Tc . The samples were doped with small amounts of Hf that carried the 181Hf - 181Ta probe radioactivity. A high-frequency nuclear quadrupole interaction that decreases very little as the temperature approaches Tc , 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 contrast, a lower-frequency interaction that decreases rapidly as temperature approaches Tc , characterizes the BaTiO3, transition. Both phases of BaTiO3 appear to coexist over an interval of about 2 K, and the thermal hysteresis is about 1 K. At temperatures above Tc , both PbTiO3 and BaTiO3 show weak, non-vanishing Ti-site EFGs. Although, for BaTiO3 , this effect limits the accuracy with which critical effects can be measured, we estimate a power-law exponent ß = 0.21 ± 0.05, which most likely is somewhat lower in magnitude than the actual critical exponent. For the explanation of our observations we assume the existence of a distribution of Tc -values. This distribution would arise because the crystals could have spatially non-uniform distributions of nucleation sites, which for PbTiO3 and BaTiO3 could be point defects.