LAUSR

The structural and electronic properties of the clathrate compounds Ba8AlxSi46–x and Sr8AlxSi46–x are studied from first-principles, considering an Al content x between 6 and 16. Due to the large number of possible substitutional configurations, we make use of a special iterative cluster-expansion approach, to predict ground states and quasi-degenerate structures in a highly efficient way. These are found from a simulated annealing technique where millions of configurations are sampled. For both compounds, we find a linear increase of the lattice constant with the number of Al substituents, confirming experimental observations for Ba8AlxSi46–x. Also the calculated bond distances between high-symmetry sites agree well with experiment for the full compositional range. For x being below 16, all configurations are metallic for both materials. At the charge-balanced composition (x = 16), the substitutional ordering leads to a metal–semiconductor transition, and the ground states of Ba8Al16Si30 and Sr8Al16Si30 ...