A Comparative Study of the Effect of Strain on the Electronic and Optical Properties of Filled and Unfilled Ba₈Si₄₆ Type I-clathrate

A first principles calculations study is carried out to theoretically investigate the effect of compressive and tensile strains on the structural, electronic and optical properties of type-I guest-free Si₄₆ and barium-filled Ba₈Si₄₆ clathrates. The electronic band structure of the unfilled Si₄₆ clathrate revealed a semiconducting behaviour with a quasi-direct band gap of 1.36 eV. Under hydrostatic pressure, the bandgap magnitude of the guest-free Si₄₆ behaves monotonously. For the Ba doped Si₄₆ clathrate (Ba₈Si₄₆) structure, the strain has no significant effect on the electronic band structure, while its impact on the optical properties is appreciable. The optical properties, such as the dielectric function and the absorption were computed for different strain variations, which are clearly enhanced for both the unfilled Si₄₆ and Ba-filled Ba₈Si₄₆ clathrates when the pressure is 1 GPa in the direction of a compressive state.