STRUCTURAL ENVIRONMENT OF NICKEL IN SILICATE GLASS MELT SYSTEMS .1. SPECTROSCOPIC DETERMINATION OF COORDINATION STATES

Nickel environments in silicate and aluminosilicate glasses have been investigated using X-ray Absorption Near Edge Structure (XANES) spectroscopy, Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, and optical absorption spectroscopy. Glass compositions included nickel end-members such as Ni-diopside (CaNiSi2O6) and Ni-alkali silicates (Na2NiSi3O8 and K2NiSi3O8) as well as glasses with compositions of diopside, anorthite, potassium trisilicate, and sodium trisilicate and disilicate containing 1-3 wt% NiO. Optical spectroscopy, XANES, and EXAFS show that nickel occurs as [4]Ni and [5]Ni in all the glasses investigated, [4]Ni being predominant in potassic glasses as [5]Ni prevails in glasses with higher field strength cations (Na, Ca, Mg). [4]Ni occurs in a distorted tetrahedral site and [5]Ni in a slightly distorted trigonal bipyramid. The geometry of [4]Ni and [5]Ni sites does not vary much among the glasses investigated, as shown by the constant position of [4]Ni and [5]Ni optical absorption bands. XANES parameters are intermediate between those expected for [4]Ni and [5]Ni. EXAFS-derived mean Ni-O distances and number of oxygen neighbors are 1.96-1.99 angstrom and 3.8-4.7 (+/-0.5), respectively, depending on glass composition. They also suggest a mixing of [4]Ni and [5]Ni with [4]Ni-O = 1.95 angstrom and [5]Ni-O = 2.00 angstrom, respectively. Including the data of the literature, spectroscopic data on Ni are independent on the total Ni content of the glass investigated (0.1-27 wt% NiO). However, the relative proportion of the two Ni coordination states depends upon glass composition. Silicon second neighbors were determined by EXAFS in all glasses at Ni-Si distances of 3.2-3.3 angstrom, depending on glass composition, but the number of Si neighbors is higher in the potassic glasses. Finally, there is no indication of significant amounts of [6]Ni in silicate glasses, which is consistent with the structural data and models on the sites occupied in silicate glasses by other small divalent cations, such as Mg and divalent transition metal cations.