Local order in amorphous $Ge_2Sb_2Te_5$ and $GeSb_2Te_4$

The structure of amorphous ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ and $\mathrm{Ge}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$ phase-change alloys was investigated by high energy x-ray diffraction, extended x-ray-absorption fine structure, and neutron diffraction. The reverse Monte Carlo simulation technique was used to generate large scale atomic models compatible with all experimental data sets. Simulations revealed that Ge-Ge bonding is present already in $\mathrm{Ge}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$. Ge-Sb bonding was also found to be significant for both compositions. Within experimental uncertainties, all atomic species satisfy formal valence requirements: Ge is fourfold coordinated, Sb has three neighbors, and Te is mostly twofold coordinated. The environment of Ge atoms was investigated in detail. The predominance of $\mathrm{Ge}{\mathrm{Te}}_{4}$ or ${\mathrm{Te}}_{3}\mathrm{Ge}\text{\ensuremath{-}}\mathrm{Ge}{\mathrm{Te}}_{3}$ units in these compositions can be excluded. Instead, these alloys are characterized by a variety of local motifs.