Structural characterization of glassy phases in the system Na2O-Ga2O3-P2O5 by MAS-NMR, EXAFS and vibrational spectroscopy. I. Cations coordination.

The cationic coordinations of phosphate based gallium sodium glasses in the system Na2O-Ga2O3-P2O5 have been studied by several techniques (71Ga and 23Na MAS-NMR, EXAFS and vibrational spectroscopies) in order to study the relationship between the structure and the chemical composition. We found that three different environments are available for the gallium ions while it is very difficult to get accurate information on the sodium coordinations. Our data show that in orthophosphate glasses, gallium is mainly tetrahedral but when the mean phosphate chain length increases, its coordination becomes more and more octahedral. In these glassy structure, it becomes then possible to dissolve large amounts of typically octahedral cations like Fe3+ or Cr3+.

Structural characterization of glassy phases in the system Na2O-Ga2O3-P2O5 by MAS and solution NMR and vibrational spectroscopy: II. Structure of the phosphate network.

The structure of the phosphate network of glasses in the system Na2O-Ga2O3-P2O5 has been investigated as a function of the Na/Ga molar ratio and the phosphate composition corresponding to a mono, di, tri, tetra and meta phosphate stoichiometry. The glass is made of phosphate molecular groups of different lengths linked by the cations with rather ionic (Na) or more covalent (Ga) bonds except in the case of the orthophosphate composition for which we only found isolated PO4(3-) ions. The vibrational spectra are sensitive to composition variations but the band width and the couplings between the different groups prevent any quantitative determination. 31P MAS-NMR gives an in-situ information on the P environment in the glass but the signals are often large and ill defined so that the assignment is not at all straightforward. On the other hand, 31P solution NMR gives sharper signals, allowing more quantitative determinations but the dissolution process always introduces some indeterminacy on the real glass structure which can be minimized by a careful preparation of the solution.