No abstract

The plane-wave pseudo-potential method within the framework of first principles is used to investigate the structural and elastic properties of Mg 2 Si in its intermediate pressure (Pnma) and high pressure phases (P 6 3 /mmc). The lattice constants, the band structures. The bulk moduli of the Mg 2 Si polymorphs are presented and discussed. The phase transition from anti-cotunnite to Ni 2 In-type Mg 2 Si is successfully reproduced using a vibrational Debye-like model. The phase boundary can be described as P = 24.02994 + 3.93 × 10 −3 T − 4.66816 × 10 −5 T 2 − 2.2501 × 10 −9 T 3 + 2.33786 × 10 −11 T 4 . To complete the fundamental characteristics of these polymorphs we have analysed thermodynamic properties, such as thermal expansion and heat capacity, in a pressure range of 0-40 GPa and a temperature range of 0-1300 K. The obtained results tend to support the available experimental data and other theoretical results. Therefore, the present results indicate that the combination of first principles and a vibrational Debye-like model is an efficient scheme to simulate the high temperature behaviours of Mg 2 Si.

No abstract

The high-temperature lattice constants and elastic moduli of the silicon nitrides are calculated using the plane-wave pseudo-potential method combined with the vibrational Debye-like model. β-Si3N4 is ductile at low temperature and brittle at high temperature. γ-Si3N4 is found to be brittle and covalent in nature. We find a positive slope of the β→γ phase boundary, hence, at higher temperatures it requires higher pressures to synthesize γ -Si3N4. The α → γ phase boundary may be expressed as P=16.29-1.835-10-2 T+9.33945-10-5T2-2.16759-10-7 T3+2.91795-10-10T4. We also obtain the electronic structures and energy bands of Si3N4 with and without pressure. The interaction between Si-s, p and N-s, p plays a dominant role in the stability of α-Si3N4. The α- and β-Si3N4 have the ΓV-ΓC indirect band gaps (4.9~eV and 4.4~eV) while γ-Si3N4 has a direct band gap(3.9~eV). The tops of the valence bands for α- and β-Si3N4 are along the Γ-M and Γ-A direction, respectively. Our results are consistent with the experimental data and the theoretical results.