Predicting ground-state configurations and electronic properties of the thermoelectric clathrates Ba${}_8$Al${}_x$Si${}_{46-x}$ and Sr${}_8$Al${}_x$Si${}_{46-x}$

Abstract

The structural and electronic properties of the clathrate compounds Ba${}_8$Al${}_x$Si${}_{46-x}$ and Sr${}_8$Al${}_x$Si${}_{46-x}$ 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 Ba${}_8$Al${}_x$Si${}_{46-x}$. 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 Ba${}_8$Al${}_{16}$Si${}_{30}$ and Sr${}_8$Al${}_{16}$Si${}_{30}$ 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.

arXiv comment9 figures

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