In order to explain perturbed angular correlation data for the tetragonal phase at 900 to 1300C, a 4-state stochastic model was developed. This simulated vacancies which trapped and de-trapped at a perturbed angular correlation probe nucleus. While trapped, the vacancies hopped around the probe in equivalent sites. The 4 states in this Winkler-Gerdau stochastic theory were 3 trapped states with equivalent electric field gradients having differing orientations, plus a de-trapped state having a weaker electric field gradient whose axis of symmetry was oriented along the diagonal between the 3 trapped electric field gradients. There were 3 hopping rates in the model. One was the rate at which a trapped vacancy hopped around the probe, a second was the de-trapping rate and the third was the trapping rate. Calculations were performed for values of the hopping rates which were implied by tetragonal zirconia data. Heuristic fitting functions were considered which summarized computer results and could be used to fit data efficiently for a wide range of parameters.

Stochastic PAC Models for Vacancy Motions with Trapping and Detrapping. W.E.Evenson, J.Lu, M.W.Winz, J.A.Gardner, M.O.Zacate, T.Lee, N.Mommer: Hyperfine Interactions, 1999, 120-121[1-4], 427-31