Phase stability and transition of BaSi₂-type disilicides and digermanides

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.