Atomic ordering in ternary phases having the Al$_{4}$Ba structure

Al$_{4}$Ba is by far the most common intermetallic structure that has a 4:1 ratio of constituent elements. There is one Ba-site and two inequivalent Al-sites. Experiments were carried out to measure quadrupole interactions at dilute $^{111}$In/Cd probe atoms using perturbed angular correlation spectroscopy (PAC) for a number of ternary phases based on Al$_{4}$Ba. One was a sample having composition In$_{2}$Al$_{2}$Ba, with end-member phases Al$_{4}$Ba and In$_{4}$Ba both having the Al$_{4}$Ba structure. The PAC spectrum exhibited two broadened quadrupole interaction signals, indicating that Al and In atoms do not order on the two sublattices. Instead, they appear to locate more or less randomly on the two sublattices, making a ``pseudo-binary'' Al$_{4}$Ba phase. The other sample was BaNiSn$_{3}$, for which it has been reported that Ni-atoms order on half of one of the two Al-sublattices. Indium was expected to prefer to occupy sites of chemically-similar Sn, of which there are two types: one having a quadrupole interaction that is axially symmetric and the other one having lower symmetry. Experiment showed only one signal for a low-symmetry site, but with excellent signal coherence. It is concluded that there is a high degree of atomic ordering in BaNiSn$_{3}$ and that indium impurities in the phase occupy only one of two inequivalent Sn-sites. \textit{Supported in part by the NSF under grant DMR 09-04096 (Metals Program).}