Chemical order in Ge-Ga-Sb-Se glasses

The short range order in Ge$_{30}$Ga$_{5}$Sb$_{10}$Se$_{55}$ and Ge$_{21}$Ga$_{5}$Sb$_{10}$Se$_{64}$ glasses was investigated by X-ray (XRD) and neutron diffraction (ND) as well as extended X-ray absorption fine structure (EXAFS) measurements at the Ge, Ga, Sb and Se K-edges. Large scale structural models were obtained by fitting simultaneously the experimental data sets by reverse Monte Carlo (RMC) simulation technique. It was found that Ge, Sb and Se atoms follow the Mott-rule and have 4, 3 and 2 nearest neighbors, respectively. The average coordination number of the Ga atoms was around 4. The structure of these glasses can be described by the chemically ordered network model: the Ge-Se, Ga-Se and Sb-Se bonds are the most prominent while Ge-Ge and Ge-Sb bonds are formed only in Se-poor compositions. Models generated by RMC contained some long distances (0.3-0.4 {\AA} higher than the usual covalent bond lengths) between Ge-Se and/or Ge-Ge pairs. Dedicated simulation runs confirm the existence of these bonds.