Ordering and correlation of cluster orientations in CaCd6

In order to study the low temperature phase transition in CaCd6, which is attributed to a reordering of the innermost tetrahedral cluster shells, accurate Embedded-Atom-Method potentials are developed for this system. With these potentials, the ideal cluster structure and the couplings between neighbouring clusters are determined. From these data, an effective hamiltonian for the cluster orientations is derived. The hamiltonian is used in Monte Carlo simulations, which exhibit a sharp jump in the internal energy near the expected transition temperature.