Structural stability of intermetallic phases in the Si-Ti system. Point defects and chemical potentials in D8_8-Si_3Ti_5 phase

摘要

The total energies of intermetallic compounds in the Si-Ti system are calculated employing electronic density functional theory (DFT). The calculations are performed for the experimentally observed compounds and selected structures at their ideal stoichiometry. The calculated formation enthalpies are in good agreement with the available experimental data. For the stable intermetallic compounds, the calculated zero-temperature lattice parameters agree well with those obtained experimentally at ambient temperature. The point defect formation energies in D8_8-Si_3Ti_5 (hP16, P6_3/mcm, prototype Mn_5Si_3) are obtained from first principles calculations. Four sublattices are introduced to account for the D8_8 structure and for the possibility of inserting atoms (2b Wyckoff positions of P6_3/mcm space group). The total energies of supercells containing one defect are calculated. A statistical model based on a mean-field approximation is used to obtain the defect concentrations, the chemical potentials and the Gibbs energy of formation as functions of temperature and deviation from stoichiometry. Analytical expressions of the defect concentrations, chemical potentials and Gibbs energy as functions of composition for various temperatures are provided. The off-stoichiometric domain of D8_8-Si_3Ti_5 is discussed.