Electronic, elastic, vibrational, and thermodynamic properties of type-VIII clathrates Ba₈Ga₁₆Sn₃₀ and Ba ₈Al₁₆Sn₃₀ by first principles

We present the results of studying electronic, elastic, vibrational, and thermodynamic properties of type-VIII clathrates Ba₈Ga ₁₆Sn₃₀ Ba₈Al₁₆Sn₃₀ calculated from a first-principles approach. The calculations utilize the generalized gradient approximation to density functional theory. The results indicate that the Ba₈Ga₁₆Sn₃₀ and Ba ₈Al₁₆Sn₃₀ are indirect semiconductors with fundamental band gaps of 160 meV and 315 meV, respectively. It was also found that the stiffness of Al containing type-VIII clathrate does not show any significant change against the uniform pressure, shearing, and linear strains. The phonon spectrum and the phonon state densities of these compounds as well as the Raman and infrared active modes were further calculated and the effects of replacing the Ga with Al atoms on the properties of interest were discussed. The calculated elastic, vibrational, and thermodynamic properties along with Raman and IR spectra are reported for the first time. The identification of the Raman and infrared active modes will be especially useful for the experimental characterizations of these compounds. Our calculations show that the heat capacities of these clathrates increase smoothly with temperature and approach the Dulong-Petit value at about room temperature, which agrees with the existing experimental data.