Prediction of optically-active transitions in type-VIII guest-free silicon clathrate Si₄₆: A comparative study of its physical properties with type-I counterpart through first-principles

We have systematically studied the structural, electronic, and optical properties of two types of guest-free Si₄₆ silicon clathrates using first-principles density-functional theory calculations. The properties are compared between two different crystal structures with the same chemical formula of Si₄₆, type-I, and type-VIII. We apply a hybrid functional approach with the Becke, Lee-Yang-Parr (BLYP) functional and compare the electronic properties with those by generalized gradient approximation (GGA-PBE). The band structures of both clathrates calculated within the GGA indicate semiconducting (insulating) properties with indirect bandgaps of 1.38 and 1.36 eV for type-I and type-VIII, respectively. These band gaps increase to 1.60 and 1.62 eV by using the BLYP hybrid functional. The calculated optical absorption of type-I clathrate is slightly larger than that of type-VIII. Lastly, we also performed a symmetry analysis of optical transitions for both Si₄₆ clathrates. It is found that optical transitions are dipole-allowed at the band-edge located along the Γ-Η line for Si₄₆-VIII, while the transitions for type-I Si₄₆ are forbidden by symmetry. It is hoped that this finding will motivate further investigations of optical capabilities of the silicon based type-VIII clathrates for photovoltaic and photonic devices.