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A perturbed angular correlation (PAC) experiment that measures dynamic damping also needs information about the fundamental quadrupole frequency to relate the damping as a function of temperature to the EFG fluctuation rate. When the experiment is unable to access slow electric field gradient (EFG) fluctuations that show the fundamental quadrupole frequency directly, one needs additional information to determine the hyperfine field parameters and thereby the connection between observed damping and EFG fluctuation rates. One way to solve this problem is to estimate the hyperfine parameters from the fluctuation rate for maximum damping (i.e. at the relaxation peak) or from the rate of maximum damping. This work relates both the maximum damping rate and the fluctuation rate at the relaxation peak to EFG magnitudes (or quadrupole frequencies) for five dynamic N-state symmetric models of fluctuating EFGs.

We report on a procedure developed to create stochastic models of hyperfine interactions for complex diffusion mechanisms and demonstrate its application to simulate perturbed angular correlation spectra for the divacancy and 6-jump cycle diffusion mechanisms in L1 2 -structured compounds.Diffusion plays an important role in materials processing and in determining suitable applications for materials. Therefore, it is of interest to refine methodologies capable of discerning individual atomic jumps that take place in the diffusion process, particularly in intermetallic compounds for which complex diffusion mechanisms can dominate the simple vacancy mechanism. Here, complex diffusion refers to cases where diffusing species change sublattices or multiple defect species cooperate in diffusion. In principle, hyperfine methods are ideal for discerning atomic-scale diffusion processes because of the short-range nature of the quadrupole interaction. More specifically, perturbed angular correlation (PAC) spectroscopy is attractive because it is sensitive to jump rates on the order of 0.1 to 1 GHz, which can be obtained in typical materials in the experimentally convenient temperature range between room temperature and 1200 • C. This work is funded in part by NSF grant DMR 06-06006 (Metals Program) and computational resources were provided in part by KY EPSCoR grant RSF 012-03.

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