Binary alloys of Ge and Te: Order, voids, and the eutectic composition

The liquid and amorphous structures of Ge(0.15)Te(0.85) and GeTe alloys are characterized using combined density functional/molecular dynamics simulations. Te is threefold coordinated, in contrast with predictions of the 8-N rule, and Ge atoms (fourfold coordinated) show octahedral and tetrahedral bonding angles. Cubic local environment occurs in both materials, and GeTe shows a pronounced alternation of atomic types. Tetrahedral Ge coordination is more common in the eutectic Ge(0.15)Te(0.85), which comprises corner- and edge-sharing GeTe(4) units surrounded by Te. There is no Te segregation, and the material resembles neither GeTe nor Te. The ubiquitous cavities (voids) have been overlooked in Ge(0.15)Te(0.85), where they comprise over 25% of the volume.