Effect of dopant size and dopant concentration on the crystallization pressure of phase change materials: The role of local order and non-local interactions

Ge_0.15Sb_0.85 is a phase change material that undergoes a transition from a semi-conducting glass to a crystalline metal when being densified. In this work, we investigate some parameters controlling the crystallization pressure P_c in related compounds by conducting high-pressure experiments on initially amorphous, germanium- and silicon-doped antimony. We find that the amorphous phase is stabilized to higher pressures when the dopant size is decreased. This result can be easily rationalized under the assumption that in the glass, Ge and Si atoms occupy tetrahedral or related small coordination shells. When pressure increases, the larger Ge atoms move sooner into the larger octahedral shells, which are characteristic for the crystal, than the smaller Si atoms do. We also find that P_c increases quickly with Ge and Si concentrations. This observation implies that the pressure-induced change of coordination cannot be a local, elementary event, but that the four-coordination of the group-14 atoms is stabilized by the presence of other four-coordinated atoms.