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

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Santiago Rigamonti, Institut für Physik and IRIS Adlershof, Humboldt-Universität zu Berlin
When
22 August 2017 from 3:30 PM to 4:00 PM
Where
339 Davey Laboratory
Contact Name
Jorge Sofo
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A class of promising materials for thermoelectric applications are the Si-based clathrate alloys. Their complex cage-like structure containing guest endohedral dopants is regarded as a realization of the phonon-glass electron-crystal, promising a large thermoelectric figure of merit.  A first-principles approach to this materials class has been hampered by the combinatorial explosion of the number of compositions and dopant arrangements. In this work, we achieve an accurate ab-initio description of the structural stability and the electronic properties of the strongly doped compounds Ba8AlxSi46-x and Sr8AlxSi46-x by making use of a novel iterative cluster-expansion approach. Fully unbiased predictions of the ground-state phases and quasi-degenerate structures are obtained in a highly efficient way. These are found from a simulated annealing technique where millions of configurations are sampled. We find an excellent agreement with experiment regarding the structural properties of the stable phases. For 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. 

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