Transition elements in water-bearing silicate glasses/melts. Part I. A high-resolution and anharmonic analysis of Ni coordination environments in crystals, glasses, and melts

The local structural environment around Ni(II) in a series of crystalline model compounds and K-Ni-, Na-Ni-, and Ca-Ni-silicate glasses and melts has been evaluated by using high-resolution Ni K-edge XANES spectroscopy and anharmonic EXAFS data analysis. The glasses show NBO/T contents ranging from 0 (albitic composition) to 1 (alkali-disilicate composition) and Ni contents ranging from 40 ppm to 2 wt.%. The pre-edge feature for model compounds vary not only in normalized height as a function of Ni-coordination but also in position (by similar to0.5 eV) as confirmed by ab initio XANES calculations. In addition, the pre-edge position is indirectly correlated with the average Ni-O distance. By using these correlations, Ni is predicted to be 4-coordinated, on average, in the potassic glasses, similar to5-coordinated in the sodic glasses, and >5-coordinated in the calcic glasses. The EXAFS-derived distances and coordination numbers are consistent with these conclusions. with the average Ni-O distances ranging between 1.96 and 2.03(2) Angstrom. In contrast, NiO4 units are dominant in the melts examined (Na-disilicate composition with 4000 ppm of Ni). Variation in NBO/T or in Ni concentration does not affect the local environment of Ni in these silicate glasses whereas the type of network modifier present in the glass (K vs. Na vs. Ca + Mg) does. This finding explains the overall good agreement between this study and a previous XAFS study on silicate glasses with higher Ni concentrations and less polymerization (Galoisy and Calas, 1993a). In Part 2 of this series, these new correlations between Ni K-pre-edge features and Ni coordination number are used to characterize Ni structural environments in sodium trisilicate and albitic glasses containing 2000 ppm of Ni and exposed to pressures up to 5 kbars with 0 to 8 wt.% water. These hydrated glasses show major changes in their crystal held spectra. which correspond to major changes in the local coordination environment of Ni as revealed by XAFS spectroscopy. Copyright (C) 2001 Elsevier Science Ltd.