Structural, electronic, phonon and thermodynamic properties of hypothetical type-VIII clathrates Ba8Si46 and Ba8Al16Si30 investigated by first principles

We present the results of first principles calculations of the structural, electronic, elastic, vibrational, and thermodynamic properties of the hypothetical silicon-based, guest containing type-VIII clathrates Ba8Si46 and Ba8Al16Si30. We obtained the lattice constant, formation energy, band structure, density of states, elastic constants, sound velocity, and Debye temperature using the density functional theory with generalized gradient approximation (GGA). We calculated phonon dispersion and vibrational density of states spectra using the density functional perturbation energy within GGA. We computed the temperature dependent specific heat, vibrational entropy, and vibrational Helmholtz free energy by utilizing quasi-harmonic approximation. We found that replacing some silicon atoms in the framework with aluminum atoms leads to the decrease of the fundamental band gap from 1.0 in Ba8Si46 to 0.18 eV in Ba8Al16Si30. Moreover, the guest Ba atoms produced localized phonon modes lying below 1.2, 1.8 THz for Ba8Si46, and Ba8Al16Si30, respectively, which lead to the reduction of the acoustic bandwidth of the host material. The effect of replacing Si atoms with Al on the properties of the interest is also discussed.