The local structure of mixed alkali germanate glasses by EXAFS

Interatomic distance, coordination number and degree of disorder around mobile (K, Rb) as well as network forming (Ge) cations have been determined using extended X-ray absorption fine structure (EXAFS) analysis for a 0.2[yK .(1 - y)Rb](2)O . 0.8GeO(2) mixed alkali glass series. The glasses were prepared by casting the melts (at 1200-1400 degrees C) in a metal mold at ambient temperature. Quenching stresses were relieved by annealing the glass at similar to 450 degrees C and then furnace cooling to room temperature. The X-ray absorption spectra were measured near liquid helium temperature to eliminate thermal disorder. The larger structural disorder around K and Rb indicates that the modified random network structure model is not applicable to germanate glasses. The replacement of K by Rb has little effect on the various interatomic distances, which indicates that the local static structure is not important for the mixed alkali effect. The monotonic change in molar volume or excess volume does not correlate with the variation of the interatomic distances, indicating that there exists an intermediate range spatial heterogeneity of the 'unoccupied' or 'excess' volume in the glass structure. (C) 1999 Elsevier Science B.V. All rights reserved.