BaSi2-type disilicides and digermanides hold great promise for solar-cell applications, but their structural stability and phase transition mechanisms remain unresolved. Here we present ab initio calculations of pressure-induced structural phase transitions of BaSi2, BaGe2, and SrGe2 and show that Si tetrahedra in orthorhombic BaSi2 tend to convert to corrugated layers in the trigonal phase under high pressure with bond breaking along the b axis, and a three-dimensional Si net in the cubic phase is stabilized energetically at low pressure. The orthorhombic semiconductor-to-trigonal metal conversion is also preferred for SrGe2 both energetically and kinetically. However, Ge tetrahedra in BaGe2 tend to convert to a ThSi2-type tetragonal net with bond breaking around the c axis. The kinetic barriers are large for both the reaction (∼0.43 eV under compression) and the counter-reaction (∼0.39 eV under decompression) for BaSi2, which explains the stability of the trigonal and cubic phases at room temperature and the high-temperature requirement for the phase transitions.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - 2015 Feb 17|