Structure of Glassy Ag-Ge-Se by Neutron Diffraction with Isotope Substitution

The structure of glassy Ag0.077Ge0.212Se0.711, which lies at y = 0.077 on the Ag-y(Ge0.23Se0.77)(1-y). tie-line, was investigated by using the method of neutron diffraction with silver isotope substitution. Two glass transition temperatures were found from a characterisation of the material using modulated differential scanning calorimetry, which indicates a mixed phase material. The diffraction method provides site-specific information on the Ag coordination environment, and gives an average of 3.5(1) Ag-Se nearest-neighbours with a bond distance of 2.65(1) angstrom together with 0.9(1) Ag-Ag next nearest-neighbours at a distance of 2.9(2) angstrom. The incorporation of silver does not appear to have a marked effect on the coordination number of Se to other matrix (Ge or Se) atoms, which supports the notion that Ag forms dative bonds with Se lone-pair electrons. A model is given for predicting the change in the Se to matrix atom coordination number when a monovalent metal such as Ag is added to a Se rich Ge-Se base glass.