Predicting ground-state configurations and electronic properties of the thermoelectric clathrates Ba$_{8}$Al$_{x}$Si$_{46-x}$ and Sr$_{8}$Al$_{x}$Si$_{46-x}$

Maria Troppenz; Santiago Rigamonti; Claudia Draxl

doi:10.1021/acs.chemmater.6b05027

paper

Predicting ground-state configurations and electronic properties of the thermoelectric clathrates Ba $_{8}$ Al $_{x}$ Si $_{46 - x}$ and Sr $_{8}$ Al $_{x}$ Si $_{46 - x}$

arXiv:1708.08126 · doi:10.1021/acs.chemmater.6b05027

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.

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Predicting ground-state configurations and electronic properties of the thermoelectric clathrates Ba8​Alx​Si46−x​ and Sr8​Alx​Si46−x​

Abstract

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Predicting ground-state configurations and electronic properties of the thermoelectric clathrates Ba $_{8}$ Al $_{x}$ Si $_{46 - x}$ and Sr $_{8}$ Al $_{x}$ Si $_{46 - x}$