TEMPERATURE-DEPENDENCE OF THE NUCLEAR-QUADRUPOLE INTERACTIONS AT TI SITES IN FERROELECTRIC PBTIO3 AND IN ILMENITE AND PEROVSKITE CDTIO3 - EVIDENCE FOR ORDER-DISORDER PHENOMENA

Perturbed angular correlation (PAC) spectroscopy was used to measure electric-field gradients (EFGs) at Ti sites in the tetragonally distorted perovskite PbTiO3, which is ferroelectric, and in the orthorhombically distorted perovskite CdTiO3, and the ilmenite CdTiO3, which are not ferroelectric. The PAC probe Hf181/181Ta was substituted into Ti sites in these phases at concentrations of approximately 1% of the Ti concentrations. Nuclear quadrupole interactions were measured from 77 to 730 K for the PbTiO3 phase and from 77 to 1260 K for the CdTiO3 phases. The perturbation functions for PbTiO3 are characterized by high frequencies that show near-axial symmetry and that decrease rapidly at temperatures near the ferroelectric-to-paraelectric transition temperature Tc. Also, they are characterized by extensive line broadening that increases at temperatures near Tc. The perturbation functions for the perovskite CdTiO3 are characterized by low frequencies that show much asymmetry 0.4 and by little line broadening, and the associated EFG decreases linearly with temperature. For the ilmenite CdTiO3, the perturbation functions are characterized by moderate frequencies that show near-axial symmetry and by very little line broadening, and the associated EFG decreases with the three-halves power of the temperature. For both of the CdTiO3 phases, the asymmetries derived from the measured frequencies are consistent with the crystal-structure symmetries, and the line shapes could be attributed to static inhomogeneities. For the ferroelectric PbTiO3 phase, the measured frequencies are consistent with the tetragonal crystal structure. But, the line shapes of the perturbation functions measured at temperatures near Tc could not be explained by purely static inhomogeneities. Instead, in contrast to the CdTiO3 phases, the line broadening at these temperatures was attributed primarily to a dynamic spin-relaxation mechanism. This particular interpretation supports an order-disorder model of the ferroelectric phenomenon. © 1990 The American Physical Society.