SHORT AND MEDIUM RANGE ORDER IN GERMANIUM SULFIDE GLASSES - A LOW-TEMPERATURE X-RAY ABSORPTION-SPECTROSCOPY STUDY

Ge K-edge EXAFS measurements were carried out at 35 K for GeS(x) (1.5 less-than-or-equal-to x less-than-or-equal-to 5) binary glasses and (1 - y)GeS(2 + y)Ag2S (0 less-than-or-equal-to y less-than-or-equal-to 0.4) glassy compositions. The three first Ge coordination shells were resolved. Thus, the short range order around germanium atoms located in tetrahedral sites has been confirmed. Then, knowledge of the medium range order existing in these glasses was improved, particularly on clustering of GeS4 tetrahedral units by edges or corners. Further, both the depolymerization rates of edge and corner sharing GeS4 tetrahedra have been estimated by reference to alpha-GeS2 structure. The (g)GeS2 structure is mainly constituted by two-dimensional clusters based on alpha-GeS2 crystalline structure. In this case the depolymerization fractions are both equal to about 40%. When the atomic composition deviates from the GeS2 stoichiometry for GeS(x) binary glasses, or when Ag2S is added to the GeS2 glass former, a significant increase of the depolymerization of both corner and edge linkages is noted. This increase implies an increase of the structural distortion in the local and mainly in the intermediate range order.