N. Sato1, Y. Takagiwa1, A. Kuwabara2, N. Uchida3 and K. Kimura1
1Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa 277-8561
TiSi2-type intermetallic compounds RuGa2 and RuAl2 have narrow band gaps of ∼ 0.3 eV and a relatively large power factor at 600–900 K. However, the maximum dimensionless figure of merit, ZT, are 0.5 and 0.2 for RuGa2 and RuAl2, respectively, due to a high lattice thermal conductivity. We investigated the phonon properties of these compounds by using a first-principles calculation and Raman spectroscopy to develop a further reduction scheme of the lattice thermal conductivity. Phonon dispersion relations and density of states were obtained from a real-space force constants method based on supercells with finite displacements. The calculated zone-center wavenumbers were comparable to the experimental Raman wavenumbers. Besides, the phonon group velocities of acoustic branches were in accord with the experimental transverse and longitudinal speed of sound. The differences of phonon dispersion between RuGa2 and RuAl2 are explained by the fact that Ga is heavier and weaker bonding element than Al. According to the calculated partial phonon density of states, acoustic phonon modes of RuGa2 and RuAl2 are attributed to Ga site and Ru site, respectively. Heavier atom substitution for these sites can effectively reduce averaged phonon group velocity and lattice thermal conductivity.
thermoelectric material, narrow band gap intermetallic compound, first-principles calculation, phonon dispersion, Raman spectroscopy
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