Ab initio study of the structural, vibrational and thermal properties of Ge2Sb2Te5

Abstract

The structural, vibrational and thermal properties of hexagonal as well as cubic Ge 2 Sb 2 Te 5 (GST) have been calculated from first principles. The relative stability of the possible stacking sequences of hexagonal GST has been confirmed to depend on the choice for the exchange-correlation (XC) energy functional. It is apparent that without the inclusion of the Te 4d orbitals in the valence states, the lattice parameters can be underestimated by as much as 3.9% compared to experiment and all-electron calculations. From phonon dispersion curves, it has been confirmed that the hexagonal phase is, indeed, stable whereas the cubic phase is metastable. In particular, calculations based on the quasi-harmonic approximation (QHA) reveal an extra heat capacity beyond the Dulong–Petit limit at high temperatures for both hexagonal and cubic GST. Moreover, cubic GST exhibits a residual entropy at 0 K, in agreement with experimental studies which attribute this phenomenon to substitutional disorder on the Sb / Ge / v sublattice.