Predicting Ground-State Configurations and Electronic Properties of the Thermoelectric Clathrates Ba₈Al_<i>x</i>Si_{46–<i>x</i>}and Sr₈Al_<i>x</i>Si_{46–<i>x</i>}

Abstract

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.