Crystalline GeTe-based phase-change alloys: Disorder in order

Through the combined use of x-ray absorption and scattering experiments and ab initio simulations, we demonstrate that the metastable cubic phase of GeTe-based phase-change alloys, e.g. Ge ₂Sb ₂Te ₅, is significantly more disordered than is generally believed, with a large number of Ge atoms located off octahedral resonantly bonded sites. The stochastic off-octahedral locations of Ge atoms, that are invisible to Bragg diffraction probing the average structure, lead to disruption of the continuous resonance bonding network of the crystalline phase, resulting in localization of charge carriers. It is proposed that the degree of coherency of local rhombohedral displacements, that may be varied, e.g., by doping, can serve as means to control electrical properties of Ge-Sb-Te alloys.