Ab Initio Structural Energetics of Beta-Si3N4 Surfaces

Motivated by recent electron microscopy studies on the Si3N4/rare-earth oxide interfaces, the atomic and electronic structures of bare beta-Si3N4 surfaces are investigated from first principles. The equilibrium shape of a Si3N4 crystal is found to have a hexagonal cross section and a faceted dome-like base in agreement with experimental observations. The large atomic relaxations on the prismatic planes are driven by the tendency of Si to saturate its dangling bonds, which gives rise to resonant-bond configurations or planar sp^2-type bonding. We predict three bare surfaces with lower energies than the open-ring (10-10) surface observed at the interface, which indicate that non-stoichiometry and the presence of the rare-earth oxide play crucial roles in determining the termination of the Si3N4 matrix grains.Comment: 4 Pages, 4 Figures, 1 tabl

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