Ge136 type-II clathrate as precursor for the synthesis of metastable germanium polymorphs: A computational study

The response to compression of the clathrate type-II structure Ge(cF136) is investigated by means of ab initio small-cell metadynamics at different temperatures and pressures. Amorphous intermediates obtained from the collapse of Ge136 rapidly crystallize into different end products, depending on the choice of thermo-dynamic parameters. At lower pressure p = 2.5 GPa the metastable metallic bct-5 phase competes against beta-Sn [Ge(tI4)], which forms at higher pressures. Upon lowering the temperature, amorphous intermediates are instrumental to the formation of denser structural motifs, from which metallic bct-5 can form. Therein, anisotropic box fluctuations promote phase formation. The metadynamics runs are analyzed in depth using a set of topological descriptors, including coordination sequence and ring statistics. Differences in the structural landscape history and amorphous intermediates are critical for the selective formation of particular metastable polymorphs. Their understanding opens possibilities towards turning crystal structure predictions into actual materials, via identifying viable kinetic routes.