Prediction Of Optically-Active Transitions In Type-Viii Guest-Free Silicon Clathrate Si-46: A Comparative Study Of Its Physical Properties With Type-I Counterpart Through Principles

We have systematically studied the structural, electronic, and optical properties of two types of guest-free Si-46 silicon clathrates using first-principles density-functional theory calculations. The properties are compared between two different crystal structures with the same chemical formula of Si-46, Beeke, 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 1.36eV 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 Si-46 clathrates. It is found that optical transitions are dipole-allowed at the band-edge located along the Gamma-H line for Si-46-VIII, while the transitions for type-I Si-46 arc 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. Published by AIP Publishing.