NUCLEAR-MAGNETIC-RESONANCE STUDY OF THE CUBIC-TO-TETRAGONAL PHASE-TRANSITION IN BATIO3

Ti-47, Ti-49 and Ba-137 NMR measurements were performed on single-domain pure BaTiO3 Crystals between room temperature and 1700 K. In the tetragonal phase (T < T(c) almost-equal-to 400 K), the NMR lines show a temperature dependent quadrupole shift DELTAupsilon(rho) associated with the electrical polarization P(s). Relating P(s) with DELTAupsilon(rho), the underlying electric field gradient is found to be proportional to the square of the order parameter. The temperature variation of DELTAupsilon(rho) is shown to fit Landau's relation of first-order phase transitions. The observed nuclear spin relaxation rates consist of three contributions: A classical phonon contribution, a diffusion-induced contribution above T(c) due to the motion of oxygen vacancies, and a contribution in the vicinity of T(c) related to the phase transition. From the latter, the correlation time associated with the dynamics of the phase transition was determined.