Lattice stability of intermediate phases of the Sr-Si system

Acomputational study of the lattice stability of the intermediate phases of the Sr–Si system is presented. Nine compositions have been considered, investigating 26 different crystal lattices by means of density functional theory calculations and pseudopotentials within the generalized-gradient approximation using the VASP code. The heats of formation of the various polymorphs have been derived for all the investigated compositions and found to be in excellent agreement with the available experimental data in the literature. The Sr2Si, Sr5Si3 and SrSi phases have been predicted to undergo high pressure transitions: the lattice transitions Sr2Si(oP12→hP6), Sr5Si3(tI32-Cr5B3→tI32-Mo5Si3) and SrSi(oC8→oP8→tP2) have been calculated to occur at 5.5, 19.9, 11.8 and 60 GPa, respectively. Electronic structure of the computed ground states and the predicted four new high pressure polymorphs of Sr2Si, Sr5Si3 and SrSi phases are calculated and discussed in relation with their corresponding crystal structures and heats of formation. The band gap of the semiconducting oP12 Sr2Si ground state structure has been calculated to be 0.29 eV. The bonding of the Sr–Si phases is found to be mainly ionic, as expected from Pauling’s electronegativities, although there is evidence of formation of directional covalent bonds between neighboring Si atoms in the silicon richest phases.