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Formation of Vacancies in Si- and Ge-based Clathrates: Role of Electron Localization and Symmetry Breaking

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Böhme,  Bodo
Bodo Böhme, Chemical Metal Science, Max Planck Institute for Chemical Physics of Solids, Max Planck Society;

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Baitinger,  Michael
Michael Baitinger, Chemical Metal Science, Max Planck Institute for Chemical Physics of Solids, Max Planck Society;

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Grin,  Yuri
Juri Grin, Chemical Metal Science, Max Planck Institute for Chemical Physics of Solids, Max Planck Society;

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Citation

Bhattacharya, A., Carbogno, C., Böhme, B., Baitinger, M., Grin, Y., & Scheffler, M. (2017). Formation of Vacancies in Si- and Ge-based Clathrates: Role of Electron Localization and Symmetry Breaking. Physical Review Letters, 118(23): 236401, pp. 1-5. doi:10.1103/PhysRevLett.118.236401.


Cite as: http://hdl.handle.net/11858/00-001M-0000-002D-7919-8
Abstract
The formation of framework vacancies in Si- and Ge-based type-I clathrates is studied using densityfunctional theory as a function of filling the cages with K and Ba atoms. Our analysis reveals the relevance of structural disorder, geometric relaxation, and electronic saturation as well as vibrational and configurational entropy. In the Si clathrates, we find that vacancies are unstable, but very differently, in Ge clathrates, up to three vacancies per unit cell can be stabilized. This contrasting behavior is largely driven by the different energy gain on populating the electronic vacancy states, which originates from the different degree of localization of the valence orbitals of Si and Ge. This also actuates a qualitatively different atomic relaxation of the framework.