AN EMPIRICAL-MODEL FOR THE ANHARMONIC ANALYSIS OF HIGH-TEMPERATURE XAFS SPECTRA OF OXIDE COMPOUNDS WITH APPLICATIONS TO THE COORDINATION ENVIRONMENT OF NI IN NIO, GAMMA-NI2SIO4 AND NI-BEARING NA-DISILICATE GLASS AND MELT

The coordination chemistry of Ni in NiO, gamma-Ni2SiO4 and a Ni-bearin g sodium disilicate glass and melt (with 2 wt% NiO) has been investiga ted between 300 and 1250 K using X-ray absorption fine structure (XAFS ) spectroscopy at the Ni K-edge. In the crystalline model compounds, t he phase term of the XAFS oscillations for the Ni-O pair arising from thermal disorder (''anharmonicity'') is found to vary simply as a func tion of temperature T: delta phi(T)(k) = -Sigma phi Delta T<(alpha)ove r bar>R(0)(k-k(0)), where Sigma phi is the interatomic phase-shift cor rection at room temperature; <(alpha)over bar> is the average linear t hermal expansion (in K-1); Delta T is the temperature range (= T-293 K ); R(0) is the bond length at room temperature; and k(0) is the starti ng value for the photoelectron wavenumber (k) region used during the m odeling of the XAFS signal (both k and k(0) in Angstrom(-1)). This for malism confirms the relationship between anharmonicity and linear ther mal expansion (<(alpha)over bar>). This model is used to determine Ni- O bond lengths in Ni-bearing sodium disilicate glass and melt at high temperature during nucleation and melting of the glass. In this system , Ni undergoes two temperature-induced coordination changes: one from similar to 5 to similar to 6 during the nucleation of the glass (simil ar to 800 K) and another from similar to 6 to similar to 4 during the melting of the nucleated sample (similar to 850 K). These observations are confirmed by new nb initio calculations of the Ni K-edge region, Our results show that the coordination chemistry of Ni in this glass i s not the same as that in the melt. The predominance of 5- and 4-coord inated Ni in sodium disilicate glass and melt, respectively, is consis tent with similar observations for Fe2+, Mg and Zn in similar silicate glass-melt systems.