Berlin 2015 – scientific programme

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HL: Fachverband Halbleiterphysik

HL 24: Thermoelectricity

HL 24.4: Talk

Tuesday, March 17, 2015, 10:15–10:30, EW 202

Formation and function of vacancies in Si/Ge Clathrates: The importance of broken symmetries — •Amrita Bhattacharya, Christian Carbogno, and Matthias Scheffler — Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin

One promising material class for improved thermoelectrics are the clathrates, i.e., semiconducting host lattices encapsulating guest atoms. Even in simple clathrates, such as, Si₄₆ and Ge₄₆, the introduction of guests can result in important but not yet understood effects: In Si hosts, the addition of K (or Ba) results in defect-free K₈Si₄₆ (Ba₈Si₄₆) phases. In spite of their structural and electronic similitude, Ge hosts behave fundamentally different upon filling: Under addition of K/Ba, the most stable phases are K₈Ge₄₄ and Ba₈Ge₄₃, having two and three tetravalent framework vacancies respectively that completely/ partially balance the electrons donated by the guests. In this work, we use density-functional theory, carefully evaluating the role of exchange correlation functionals, to compute the formation energies of vacancies and vacancy complexes in Si- and Ge-hosts as function of the filling with K/Ba. By taking into account structural disorder, geometric relaxations, and vibrational entropies, we verify and explain the experimentally found vacancy concentration and the thermodynamic stabilities of these compounds. We can trace back the contrasting behavior of Si/Ge clathrates upon filling to a curious, charged vacancy induced break in symmetry that occurs in Si but not in Ge hosts.