First-Principles Study of the Thermal Properties and Phonon Anharmonicity in the Type II Clathrate Alloys Si_34-xGe_x (0≤x<34)

Alloy clathrates are a class of materials possessing a crystalline framework containing more than one Group IV element (eg. Si, Ge and Sn). The guest-free lattice frameworks of these materials are sp³ hybridized covalently bonded structures. Others [1] have studied the electronic properties of Type II silicon-germanium clathrate alloys Si_34-xGe_x (0 ≤ x <34) for some compositions. Motivated by this previous work, we have performed our calculations using the same x values as reference [1]. Our calculations have explored the effect of phonon anharmonicity on the thermal expansion properties of these materials. Our calculations have focused on the acoustic phonon frequency shift due to clathrate framework deformation. This has led us to explore the anharmonic features of NTE via an analysis of the mode Gruneisen parameters (MGP). To explore the underlying mechanisms governing NTE behavior, we have used a first-principles method which is based on the quasi-harmonic approximation (QHA) and which uses the Vienna Ab initio simulation package (VASP).
[1] Koji Moriguchi, Shinji Munetoh, and Akira Shiniani, Phys. Rev. B 62, 7138 (2000).