Lattice stability of Ca, Sr and Yb disilicides

In this paper a study of the lattice stability of Ca, Sr and Yb disilicides is reported. The structural stability of six different prototype lattices was investigated by means of density functional theory calculations and pseudopotentials within the generalized-gradient approximation using the VASP code. The high-pressure equilibria for the CaSi2 and SrSi2 have been described deriving the transition pressures, the structural properties, the bulk moduli and the heats of formation for the various polymorphs. Results are discussed together with the experimental literature. For the ytterbium disilicide a specific study of the Si vacancies in the hR3 ground state structure was carried out. Six different supercells derived from the primitive hexagonal cell were considered and full atomic and lattice relaxations were performed in order to predict the energetically most favorable structure. The formation of vacancies in the Si-sublattice is driven by the lowering of the density of states at the Fermi energy. A new mP22 lattice is proposed to describe the structure of the defective YbSi2 x phase: the resulting stoichiometry is Yb4Si7.