  The structure of Ge$_{20}$Sb$_{10}$S$_{70}$, Ge$_{23}$Sb$_{12}$S$_{65}$ and
Ge$_{26}$Sb$_{13}$S$_{61}$ glasses was investigated by neutron diffraction
(ND), X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS)
measurements at the Ge and Sb K-edges as well as Raman scattering. For each
composition, large scale structural models were obtained by fitting
simultaneously diffraction and EXAFS data sets in the framework of the reverse
Monte Carlo (RMC) simulation technique. Ge and S atoms have 4 and 2 nearest
neighbors, respectively. The structure of these glasses can be described by the
chemically ordered network model: Ge-S and Sb-S bonds are always preferred.
These two bond types adequately describe the structure of the stoichiometric
glass while S-S bonds can also be found in the S-rich composition. Raman
scattering data show the presence of Ge-Ge, Ge-Sb and Sb-Sb bonds in the
S-deficient glass but only Ge-Sb bonds are needed to fit diffraction and EXAFS
datasets. A significant part of the Sb-S pairs has 0.3-0.4 {\AA} longer bond
distance than the usually accepted covalent bond length (~2.45 {\AA}). From
this observation it was inferred that a part of Sb atoms have more than 3 S
neighbors.
