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) spectr oscopy, Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, and optical absorption spectroscopy. Glass compositions included nick el end-members such as Ni-diopside (CaNiSi2O6) and Ni-alkali silicates (Na2NiSi3O8 and K2NiSi3O8) as well as glasses with compositions of di opside, anorthite, potassium trisilicate, and sodium trisilicate and d isilicate containing 1-3 wt% NiO. Optical spectroscopy, XANES, and EXA FS show that nickel occurs as [4]Ni and [5]Ni in all the glasses inves tigated, [4]Ni being predominant in potassic glasses as [5]Ni prevails in glasses with higher field strength cations (Na, Ca, Mg). [4]Ni occ urs 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 var y much among the glasses investigated, as shown by the constant positi on of [4]Ni and [5]Ni optical absorption bands. XANES parameters are i ntermediate 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 angst rom 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 an gstrom and [5]Ni-O = 2.00 angstrom, respectively. Including the data o f the literature, spectroscopic data on Ni are independent on the tota l Ni content of the glass investigated (0.1-27 wt% NiO). However, the relative proportion of the two Ni coordination states depends upon gla ss composition. Silicon second neighbors were determined by EXAFS in a ll 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 a nd models on the sites occupied in silicate glasses by other small div alent cations, such as Mg and divalent transition metal cations.