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, γ-Ni 2SiO 4 and Ni-bearing Na-disilicate glass and melt

Abstract

The coordination chemistry of Ni in NiO, γ-Ni 2SiO 4 and a Ni-bearing sodium disilicate glass and melt (with 2 wt% NiO) has been investigated between 300 and 1250 K using X-ray absorption fine structure (XAFS) spectroscopy at the Ni K-edge. In the crystalline model compounds, the phase term of the XAFS oscillations for the NiO pair arising from thermal disorder (“anharmonicity”) is found to vary simply as a function of temperature T: δφ T(k) = − ϵφΔT αR 0(k − k 0) , where ϵφ is the interatomic phase-shift correction at room temperature; α is the average linear thermal expansion (in K −1); ΔT is the temperature range (= T − 293 K); R 0 is the bond length at room temperature; and k 0 is the starting value for the photoelectron wavenumber ( k) region used during the modeling of the XAFS signal (both k and k 0 in Å −1. This formalism confirms the relationship between anharmonicity and linear thermal expansion ( α ). 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 ∼ 5 to ∼ 6 during the nucleation of the glass (∼ 800 K) and another from ∼ 6 to ∼ 4 during the melting of the nucleated sample (∼ 850 K). These observations are confirmed by new ab initio calculations of the Ni K-edge region. Our results show that the coordination chemistry of Ni in this glass is not the same as that in the melt. The predominance of 5- and 4-coordinated Ni in sodium disilicate glass and melt, respectively, is consistent with similar observations for Fe 2+, Mg and Zn in similar silicate glass-melt systems.