Insights into the structure of the stable and metastable (GeTe)(m)(Sb(2)Te(3))(n) compounds

Using first-principles calculations, we identify the mechanisms that lead to the lowest energy structures for the stable and metastable (GeTe)(m)(Sb(2)Te(3))(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 Ge(3)Sb(2)Te(6) and Ge(2)Sb(2)Te(5), while only single atomic species in the (0001) planes satisfy this rule for the GeSb(2)Te(4) and GeSb(4)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 Sb(2)Te(3) parent compounds.