EXAFS INVESTIGATION OF THE COORDINATION STATE OF GERMANIUM IN XBI2O3.(100-X)GEO2 GLASSES

Extended X-ray absorption fine structure (EXAFS) measurements of different compositions of glassy xBi2O3.(100-X)-GeO2 at the Ge K-edge are reported. The glasses were melted at 1200-degrees-C for 5 min and quenched in a melt-spinning apparatus or on a brass mould. It is found that the local structure around the germanium atoms has small changes with composition: the Ge-O distance shows no variations greater than the errors of measurements and therefore the coordination number is four at all bismuth concentrations. No evidence of a sixfold coordination, in analogy with alkali-germanate glasses, was detected. The EXAFS Debye-Waller-like factor shows a non-linear change, indicating an intriguing influence of the Bi atoms on the glass structure which is discussed in terms of a depolymerization of the GeO4 tetrahedral units.