Anharmonic Effects in Ge2Sb2Te5 and Consequences on Thermodynamic Stability

Chalcogenides are an important class of phase change material (PCMs) due to their application in digital memory solutions. Owing to their ability to reversibly cycle between crystalline and amorphous states, their use as phase change random access memory (PCRAM) is of interest, and of the many chalcogenide materials Ge2Sb2Te5 (GST) is a promising candidate owing to its stability and low crystallization temperature. GST possesses two stable crystalline polymorphs, cubic and hexagonal. Studies show that phenomena such as heat transport and thermal lattice expansion drive the phase-change nature of these materials. These phenomena are not incorporated in the harmonic approximation, which is a popular model for describing vibrations in solids. Through ab initio density functional theory (DFT), we computationally investigate the anharmonic behaviour of pristine hexagonal GST, i.e. without vacancies or defects, while considering the various stacking models that exist and inclusion of van der Waals (vdW) interactions in our modelling. We present the vibrational analysis of different stacking models in GST; Petrov and Kooi-De Hosson (KDH) models and the quantification anharmonic behaviour. Our calculations find that the KDH model is the most stable stacking sequence, being 88 meV more stable than the Petrov model when considering anharmonicity, where this difference is underestimated using a purely harmonic framework (65 meV). These results demonstrate the importance of incorporating anharmonic and dispersion effects when modelling GST, especially in the choice of stacking models, along with implications for phenomena relating to phase-change behaviour.