Structure and Raman scattering study on Ba8GaxSi46-x (x=10 and 16) type I clathrates

Structure and vibrational properties of Ba8GaxSi46-x (x = 10 and 16) clathrates were studied by X-ray diffraction and Raman scattering measurements. The temperature dependent electrical resistivity measurement on Ba8Ga10S36 has shown semiconducting nature of that clathrate with an energy band gap value of 0.31 eV. On the other hand the measurement on Ba8Ga16Si30 has shown metallic like electrical conductivity of that clathrate. The origin of semiconductivity in Ba8Ga10Si36 was found to be due to the vacancy disorder in the framework sites. Room temperature Raman scattering measurements resolved several Raman vibrational modes, including low frequencies ones corresponding to the rattling motion of Ba atoms. The low frequency positions of Ba in the respective clathrates at 49.4, 73.7 and 97.3 cm(-1) for Ba8Ga10Si36 and at 43.7, 74.5 and 92.4 cm(-1) for Ba8Ga16Si30 were found to be in agreement with the reported density functional (DF) calculated low frequency modes of Ba8Ga16S30. The framework gallium difference and vacancy disorders were found to influence the position and widths of frequency modes. Room temperature lattice thermal conductivity of Ba8Ga10Si36 and Ba8Ga16Si30 were 1.128 and 1.071Wm(-1)K(-1), respectively, and this low value was attributed to the resonant scattering between the framework acoustic and Ba rattling modes. (C) 2004 Elsevier Inc. All rights reserved.