Predicting Ground-State Configurations and Electronic Properties of the Thermoelectric Clathrates Ba8AlxSi46-x and Sr8AlxSi46-x

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 exhibit indirect Kohn-Sham band gaps of 0.36 and 0.30 eV, respectively, while configurations higher in energy are metals. The finding of semiconducting behavior is a promising result in view of exploiting these materials in thermoelectric applications.