Using first-principles calculations, we identify the mechanisms that lead to the lowest energy structures for the stable and metastable ${\left(\text{GeTe}\right)}_m{\left({\text{Sb}}_2{\text{Te}}_3\right)}_n$ (GST) compounds, namely, strain energy release by the formation of superlattice structures along of the hexagonal [0001] direction and by maximizing the number of Te atoms surrounded by three Ge and three Sb atoms (3Ge-Te-3Sb rule) and Peierls-type bond dimerization. The intrinsic vacancies form ordered planes perpendicular to the stacking direction in both phases, which separate the GST building blocks. The 3Ge-Te-3Sb rule leads to the intermixing of Ge and Sb atoms in the (0001) planes for ${\text{Ge}}_3{\text{Sb}}_2{\text{Te}}_6$ and ${\text{Ge}}_2{\text{Sb}}_2{\text{Te}}_5$, while only single atomic species in the (0001) planes satisfy this rule for the ${\text{GeSb}}_2{\text{Te}}_4$ and ${\text{GeSb}}_4{\text{Te}}_7$ compositions. Furthermore, we explain the volume expansion of the metastable phase with respect to the stable phase as a consequence of the different stacking sequence of the Te atoms in the stable and metastable phases, which leads to a smaller Coulomb repulsion in the stable phase. The calculated equilibrium lattice parameters are in excellent agreement with experimental results and differ by less than 1% from the lattice parameters derived from a combination of the GeTe and ${\text{Sb}}_2{\text{Te}}_3$ parent compounds.

• Received 5 September 2008

DOI:https://doi.org/10.1103/PhysRevB.78.224111