Atomic and electronic structures of amorphous Ge₂Sb ₂Te₅; Melt-quenched versus ideal glasses

To investigate an amorphous structure of Ge₂Sb ₂Te₅ that satisfies the 8-N rule (so-called 'ideal glass'), we perform alternative melt-quench simulations on Si₂As ₂Se₅ and replace atoms in the final structure with Ge-Sb-Te. The resulting structures have salient features of the 8-N rule such as the tetrahedral configuration for all Ge atoms and the localized Te lone pairs at the valence top. In addition, the average Ge-Te and Sb-Te distances are in good agreement with experiment. The energetic stability of the ideal glass supports the existence of this amorphous structure that is distinct from the melt-quenched glass. From the analysis of electronic structures and optical dielectric constants, it is concluded that the electronic character of the melt-quenched amorphous Ge₂Sb₂Te₅ lies in between the resonant p-bonding of the crystalline phase and the covalent bonding of the ideal glass.