First-principles-based phonon calculation and Raman spectroscopy measurement of RuGa2 and RuAl2 with high thermoelectric power factors

Abstract

TiSi2-type intermetallic compounds RuGa2 and RuAl2 have narrow band gaps of approximately 0.3 eV and relatively large power factors of between 600-900 K. However, the maximum values of the dimensionless figure of merit, ZT, for RuGa2 and RuAl2 are 0.5 and 0.2, respectively, due to a high lattice thermal conductivity. We investigated the phonon properties of these compounds using first-principles calculation and Raman spectroscopy and developed a method to reduce the lattice thermal conductivity of both compounds. 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. The phonon group velocities of the acoustic branches agreed with the experimental transverse and longitudinal speeds of sound. Differences between the phonon dispersion of RuGa2 and RuAl2 were attributed to the fact that Ga is a heavier and weaker bonding element than Al. According to the calculated partial phonon density of states, acoustic phonon modes of RuGa2 and RuAl2 are strongly influenced by the Ga site and Ru site, respectively. Substitution of heavier atoms onto these sites would effectively reduce the averaged phonon group velocity and lattice thermal conductivity of these compounds.