Density functional study of amorphous, liquid and crystalline Ge(2)Sb(2)Te(5): homopolar bonds and/or AB alternation?

The amorphous, liquid and crystalline phases of the phase change material Ge(2)Sb(2)Te(5) (GST) have been studied by means of density functional/molecular dynamics simulations. The large sample (460 atoms and 52 vacancies in the unit cell) and long simulations (hundreds of picoseconds) provide much new information. Here we extend our original analysis (2007 Phys. Rev. B 76 235201) in important ways: partial coordination numbers and radial distribution functions, bond angle distributions, new local order parameters, vibration frequencies, and the charges on atoms and vacancies. The valence band densities of states in amorphous and crystalline GST are compared with ones from x-ray photoemission spectroscopy. The results for the liquid phase are new and those for the crystalline phase much expanded. GST shows pronounced AB alternation (A: Ge, Sb; B: Te), especially in its amorphous phase, and ABAB squares play a central role in the amorphous to crystalline transition. We comment on earlier speculations concerning the nature of the amorphous to crystalline transition.