Paramagnetic sites in alkali germanate glasses

Electron paramagnetic resonance (EPR) spectra of sodium-germanate glasses ( xGeO(2) . (1 - x)Na2O, 0.65 < x < 1) prepared by the conventional melting t echnique are analyzed as a function of the Na2O content, before and after X -ray irradiation. Signals due to three-fold coordinated Ge sites (Ge E' cen ters) and non-bridging oxygen hole centers (NBOHCs) are observed, whereas n o signal due to paramagnetic centers related to GeO6 sites is detected. At high Na2O concentration, a broad signal at g = 2.21 is also observed. The E PR results suggest three main mechanisms for the Na+ ion insertion: interst itial; network modifier; a source of medium range structural rearrangements of GeO4 tetrahedra. (C) 2000 Elsevier Science B.V. All rights reserved.