XANES Analysis of Ni & Co in silicate glass: A Preliminary Investigation of Pressure Induced Changes in Their Coordination Environment

Jesse Jones¹, Hugh St.C. O'Neill¹and Andrew Berry²

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Department of Earth Science and Engineering Imperial College London, South Kensington London SW7 2AZ United Kingdom

The XANES region of the x-ray absorption spectrum (typically within 30-50 eV of the absorption edge) is strongly sensitive to both formal oxidation state and local coordination chemistry of the element under analysis. Pre-edge features in many transition-metal k-edge spectra provide a qualitative means to determine their coordination environment. Whilst s-p (orbital) is the primary transition for Ni and Co (1s core electron) K-edge spectra, elements of their pre-edge features reflect the degree to which local geometry around the absorbing atom allows hybridization of p-d orbitals, increasing the availability of transition states for the 1s core photo-electron and hence the pre-edge absorption intensity. The extent (if any) to which the intensity and shape of these pre-edge peaks are seen to vary with pressure, indicates a change in allowable hybridization associated with shifts between octahedral, distorted octahedral and tetrahedral symmetry.

To examine the possibility of pressure induced changes in the coordination chemistry of Ni and Co in silicate melt (taking silicate glass as the closest available analogue to a liquid melt structure) a series of high pressure experiments was conducted using a piston-cylinder apparatus (at 1500°) to produce uniform high pressure glasses at 10, 20, 30 & 40 kb for each element. XANES analysis of the experimental glasses was carried out at the KEK PF synchrotron, Tsukuba, Japan. After the appropriate data reduction, our results indicate that whilst Co shows no apparent shift in its coordination environment over the applied pressure range, a systematic change in the coordination symmetry of Ni can be seen to occur toward the upper 40 kb limit of the study. This shift to a lower pre-edge peak intensity at higher pressures suggests a transition from tetrahedral coordination, where increased p-d hybridization occurs at lower pressures, toward a more centro-symmetric octahedral symmetry reflected by the lower intensity 40 kb peak. Further experiments are planned to examine this shift over an expanded pressure range.