THE MEDIUM-RANGE ORDER IN LI AND AG BORATE GLASSES (WITH AGI)

Models are proposed for the medium-range order (MRO) in ion-conducting Ag and Li borate glasses, (Ag2O . 2B(2)O(3))(1-x)(AgI)(x) and (Li2O . B2O3). The analysis of the Ag diborate glass was made on the basis of both neutron and X-ray scattering data. The model comprises an arrangement of diborate cages at a distance of similar or equal to 6 Angstrom, the Ag atoms being attached to these cages by Ag-O bonds 1.8 and 2.1 Angstrom. The large X-ray scattering peak located at 2.2 Angstrom(-1) then corresponds to the Ag-Ag ordering distance of 3.1 Angstrom. In the sequence Ag diborate, tetraborate, octaborate, the shift in the position of the large X-ray peak from 2.25 to 1.87 Angstrom(-1) is explained by an increase in the Ag-Ag separation from 3.10 to 3.75 Angstrom. For the Li diborate glass there is a complete disappearance of the MRO as characterized for example by cages and/or tetraborate units.