A first principles study of constitutional and thermal defects in D8m-Si3Nb5

First principles calculations of point-defect enthalpies of formation are performed in the D8m-Si3Nb5 compound. Four sublattices are introduced to account for the D8m structure. Based on a statistical–thermodynamic model, the defect concentrations are calculated as function of temperature and deviation from stoichiometry. For the stoichiometric composition, all the point-defect atomic concentrations remain very low even at the compound melting temperature. In the Si-rich D8m-Si3Nb5, the constitutional defects are Si antisites on the Nb 4b sites of the D8m structure. The thermal defects are Si antisites on the 16k sites of the D8m structure, but their concentration remains very small even at high temperature. In the Nb-rich D8m-Si3Nb5, the constitutional defects are Nb antisites on the 8h Si positions. The formation Gibbs energy of the D8m-Si3Nb5 phase as well as the Si and Nb chemical potentials are obtained as function of the deviation from stoichiometry for various temperatures. The extension of the one-phase domain of D8m-Si3Nb5 in the Si–Nb phase diagram is discussed.

► D8m-Si3Nb5 compound.
► First principles calculations of point-defect enthalpies of formation.
► Defect concentrations calculated as function of temperature and deviation from stoichiometry.
► Off-stoichiometry of the compound in the Si-rich side due to Si antisites on the Nb 4b sites.