Phase stability and transition ofBaSi2-type disilicides and digermanides

Abstract

${\text{BaSi}}_{2}$-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 ${\mathrm{BaSi}}_{2},{\mathrm{BaGe}}_{2}$, and ${\mathrm{SrGe}}_{2}$ and show that Si tetrahedra in orthorhombic ${\mathrm{BaSi}}_{2}$ 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 ${\mathrm{SrGe}}_{2}$ both energetically and kinetically. However, Ge tetrahedra in ${\mathrm{BaGe}}_{2}$ tend to convert to a ${\mathrm{ThSi}}_{2}$-type tetragonal net with bond breaking around the $c$ axis. The kinetic barriers are large for both the reaction ($\ensuremath{\sim}0.43$ eV under compression) and the counter-reaction ($\ensuremath{\sim}0.39$ eV under decompression) for ${\mathrm{BaSi}}_{2}$, which explains the stability of the trigonal and cubic phases at room temperature and the high-temperature requirement for the phase transitions.