Regensburg 2016 – wissenschaftliches Programm

Bereiche | Tage | Auswahl | Suche | Aktualisierungen | Downloads | Hilfe

HL: Fachverband Halbleiterphysik

HL 95: Novel Functional Materials II

HL 95.7: Vortrag

Freitag, 11. März 2016, 11:45–12:00, H10

Formation and function of vacancies in Si/Ge Clathrates: The importance of broken symmetries — •Amrita Bhattacharya, Christian Carbogno, and Matthias Scheffler — Fritz Haber Institute of the Max Planck Society

Inclusion compounds, such as clathrates, are cage-like crystal structures that can encapsulate guest atoms. Since this allows to granularly tune their electronic and vibrational properties, they are regarded as interesting materials for thermoelectric applications. Progress in this field is, however, hindered by the fact that filling of group-IV clathrates often results in complex and unexpected structural changes, e.g., the spontaneous formation of vacancies in certain hosts: In Ge₄₆ clathrates filled with K or Ba, the most favourable phases K₈Ge₄₄/Ba₈Ge₄₃ feature two/three vacancies. Conversely, the framework of the isoelectronic Si₄₆ clathrate remains intact (K₈Si₄₆/Ba₈Si₄₆) upon filling with the exact same guests. Our first-principles calculations of the formation energies (including structural disorder as well as geometric and lattice relaxations) and of the thermodynamic phase stabilities (including harmonic free energies and configurational entropies) confirm this experimental scenario and shed light on the underlying mechanisms. Due to the spatially more delocalized 4sp³ orbitals in Ge, fundamentally different symmetry breaking distortions become possible to stabilize the vacancies compared to the more localized 3sp³ orbitals in Si. Eventually, we discuss the implications of these findings for the thermoelectric properties of clathrates.