Temperature-dependent properties of thermoelectric clathrates*

Intermetallic clathrate compounds, among them the type-I clathrate Ba₈Al_xSi_46-x, are promising candidates for high-efficiency thermoelectric applications. They exhibit a strong dependence of the electronic properties on the atomic arrangement of the substitutional Al atoms in the crystal framework [1]. At the charge-balanced composition (x=16), the ground-state configuration is semiconducting, however, configurations higher in energy are metallic. Understanding the temperature-dependent properties is essential, as semiconducting behavior is a prerequisite for thermoelectric applications. By employing the cluster expansion technique combined with Monte-Carlo samplings and the Wang-Landau method [2] we find a semiconductor-to-metal transition at around 600 K which is driven by a partial order-disorder transition. Signatures of this phase transition are observed in the temperature-dependent band structure, specific heat, and partial occupations.
[1] M. Troppenz, S. Rigamonti, and C. Draxl, Chem. Mat. 29, 2414 (2017).
[2] S. Rigamonti, M. Troppenz, M. Kuban, A. Huebner, C. Sutton, L. Ghiringhelli, M. Scheffler, and C. Draxl. CELL: a python package for cluster expansions with a focus on complex alloys, in preparation.