Vibrational properties of hexagonal Ge(2)Sb(2)Te(5) from first principles.

Phonons at the Γ point and the Raman spectrum of the hexagonal Ge(2)Sb(2)Te(5) were computed within density functional perturbation theory. The three different stackings of the Ge/Sb planes proposed in the experimental literature were considered. The theoretical Raman spectrum is similar for the three stackings with a marginally better agreement with experiments for the structure proposed by Matsunaga et al (2004 Acta Crystallogr. B 60 685) which assumes a disorder in Ge/Sb site occupation. Although the large broadening of the experimental Raman peaks prevents discriminating among the different stackings, the assignment of the Raman peaks to specific phonons is possible because the main features of the spectrum are rather insensitive to the actual distribution of atoms in the Sb/Ge sublattices. On the basis of the energetics (including configurational entropy) two stackings seem plausible candidates for GST, but only the mixed stacking by Matsunaga et al reproduces the spread of Ge/Sb-Te bond lengths measured experimentally.