# 21.10.2016 M.Sc. Ville Härkönen (Faculty of Mathematics and Science, Physical Chemistry)

21.10.2016 12:00 — 15:00

Location: Ylistonrinne , KEM4

M.Sc. **Ville Härkönen** defends his doctoral dissertation in Physical Chemistry "Computational and Theoretical studies on Lattice Thermal Conductivity and Thermal Properties of Silicon Clathrates". Opponent Professor **Esa Räsänen** (Tampere University of Technology) and custos Academy Researcher **Gerrit Groenhof** (University of Jyväskylä). The doctoral dissertation is held in Finnish.

**Abstract **In this thesis, the lattice thermal conductivity and other thermal properties of several silicon clathrates, which are known to be promising candidates for the thermoelectric applications, are studied by theoretical and computational techniques. The studied clathrates were the silicon clathrate frameworks I, II, IV, V, VII, VIII (Si23), H and the semiconducting clathrates [Si19P4]Cl4 and Na4[Al4Si19]. The relevance of seemingly unrelated phenomena such as the negative thermal expansion on the lattice thermal conductivity was studied.

At 300 K, the lattice thermal conductivity of the clathrate Na4[Al4Si19] was found to be about ten times smaller than obtained for the clathrate [Si19P4]Cl4 which possess the same space group symmetry than the former. It appears that the main reason for the preceding is in the second-order interatomic force constants of the clathrate Na4[Al4Si19], which change the phonon spectrum such that the phonon group velocities are lower and the anharmonicity of the lattice increases. The preceding, in turn, leads to the reduction in the relaxation times of acoustic phonons. The results indicate, that the effect of harmonic quantities can be rather large on the anharmonicity of two similar crystals and may lead to one-order lower lattice thermal conductivities, even when there are no such large differences in the third-order interatomic force constants.

In the study of thermal expansion of the silicon clathrate frameworks I, II, IV, V, VII, VIII and H, an anomalous negative thermal expansion temperature range was found for the framework VII, which appears to be mostly due to stronger third-order interatomic force constants.

Mathematical expressions to calculate different elastic and thermal properties of crystal were also derived by using the technique of many-body Green's functions and many-body perturbation theory. The mathematical expressions derived extend the existing results and allow a systematic study of elastic and thermal properties of crystals with arbitrary symmetry.

Keywords: lattice thermal conductivity, clathrates, thermal expansion, many-body perturbation theory, Green’s functions