Perturbed angular correlation spectroscopy was used to measure static and fluctuating electric field gradients at the M sites in pyrochlore ceramics, where R was Nd, Sm, Eu or Gd and M was Zr or Hf. The samples were doped with a low concentration of 181Hf  181Ta probe ions, and M-site nuclear electric-quadrupole interactions were observed mainly at temperatures of up to about 1300K. At temperatures of less than a few hundred degrees, the perturbation functions for the Sm-, Eu-, and Gd-containing compounds exhibited broadened lines that mainly indicated the presence of static disordered O ions. At slightly higher temperatures, the perturbation functions exhibited attenuated lines that were produced by fluctuating electric field gradients that arose from the hopping motion of O ions. At very high temperatures, the perturbation functions revealed sharp lines which had shapes that reflected the presence of axial symmetry. These line-shapes indicated that the electric field gradient fluctuation rates had increased up to the motional-narrowing limit. An analysis of the attenuation rates, which were measured in the fast fluctuation regime, indicated activation energies that ranged from about 0.17 to about 0.32eV. These activation energies were associated with electrostatic barriers that O ions encountered when they jumped to a vacant site. On the other hand, measurements of Nd2Zr2O7 revealed sharp lines over the entire temperature range. This indicated that the O ions were ordered in Nd2Zr2O7 crystals, and that low-temperature kinetic pathways to disordered structures were not accessible to the O ions. The magnitudes of the measured activation energies were not consistent with the results of theoretical calculations or with the deductions of electrical conductivity measurements.

G.L.Catchen, T.M.Rearick: Physical Review B, 1995, 52[14], 9890-9