Critical effects in BaTiO₃ measured by perturbed-angular-correlation spectroscopy

Abstract

We have used perturbed-angular-correlation (PAC) spectroscopy via the ¹⁸¹Hf→¹⁸¹Ta probe to measure critical effects at the Ti site in ceramic BaTiO₃. We measured the temperature dependence of the electric-field-gradient (EFG) parameters V_zz, η δ, and f at temperatures very near the tetragonal-to-cubic transition. As temperature increases toward the transition temperatures, the EFG component V_zz, which represents the primary, low-frequency Ti-site interaction, and the corresponding site-occupancy fraction f both decrease rapidly. The decrease in f is accompanied by a corresponding increase in a second site-fraction that undergoes a “zero-frequency” interaction. The parameters V_zz and f both show very similar power-law temperature dependences. The decrease in V_zz that occurs as T_c is approached can be explained by analogy to magnetic transitions. But the corresponding decrease in f does not yield a conventional explanation and may be an artifact that arises from the analysis model.