Dresden 2014 – wissenschaftliches Programm

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HL: Fachverband Halbleiterphysik

HL 49: Energy materials: Thermoelectrics

HL 49.7: Vortrag

Dienstag, 1. April 2014, 15:30–15:45, POT 051

First-principles Study of Vacancies in Thermoelectric Clathrates — •Amrita Bhattacharya, Christian Carbogno, and Matthias Scheffler — Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin

For the development of improved thermoelectrics, one promising material class may be clathrates, i.e., semiconducting host lattices that can encapsulate guest atoms. Even in the simplest clathrates, such as Si₄₆ and Ge₄₆, the introduction of guests can result in surprising electronic and structural changes: In the case of Si, the addition of K results in a K₈Si₄₆ clathrate that exhibits metallic behavior due to the electrons donated by the guests [1]. Conversely, Ge-based clathrates remain semiconducting, since two spontaneously generated vacancies balance the donated electrons in K₈Ge₄₄. But even more intriguingly, filling the Ge-clathrate with Ba does not generate four, but three vacancies, whereby the resulting Ba₈Ge₄₃ exhibits a metal-semiconductor transition at ∼ 280 K [2]. In this work, we use density-functional theory to investigate these effects and to unravel their puzzling mechanism. We compute the formation energies for vacancies and vacancy complexes in Si- and Ge-hosts as function of the filling with K and Ba, whereby we take into account structural disorder as well as geometric and lattice relaxations. Furthermore, we study the dynamics in these compounds (also analyzing non-harmonic effects) to clarify the role the vacancies play for the thermodynamic properties.

[1] G. K. Ramachandran, et al., J. Sol. St. Chem. 154, 626 (2000)

[2] U. Aydemir et al., Dalton Trans. 39, 1078 (2010).