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One of the first applications of SHIML (or rather a developmental version of SHIML) was the simulation of spectra from perturbed angular correlation spectroscopy (PAC) for the special case of non-impurity PAC probes on the Cu-sublattice of Cu3Au- (or L12-) structured compounds [34]. In the context of the 5-frequency model, a vacancy that starts in a first neighbor Cu position next to a PAC probe on a Cu site can experience three different jump rates (before making the assumption that the PAC probe is a host element). A vacancy starting in a second neighbor position to the probe can jump to a first neighbor position with a fourth jump rate. The four types of jumps are labeled according to their jump frequencies, w 1, w 2, w 3, and w 4 in the figure to the right (Cu atoms in black and Au in white). When the PAC probe is a host element (Cu in Cu3Au), then all four jump frequencies are equal w1= w2= w3= w4 = w. A hyperfine without a nearby vacancy experiences an axially symmetric electric field gradient (EFG), which arises because of the non-cubic point symmetry at the Cu-site. The EFG's symmetry axis at a particular Cu site is perpendicular to the plane formed by the nearest neighbor Au atoms, and there are three possible Cu-sites, labeled 1, 2, and 3 in the figure, corresponding to the possible directions of the symmetry axis. The symmetry of the EFG that a probe experiences is reduced further when a vacancy is located near the probe. The disturbance to the EFG falls off as 1/r 3, and in the simulations below, it is assumed for simplicity that only vacancies in first neighbor positions to the probe will have a non-negligble effect. In addition, it will be assumed that the vacancy concentration is small enough that at most one vacancy will be located next to the probe. Under these assumptions, there are 15 possible EFGs that probes can experience: 3 due to each Cu site's defect-free environment and 12 with a near neighbor vacancy (4 for each Cu site). Illustrations of the sources of two EFGs are shown below.
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