Investigation of the coordination of lead in PbO–PbF2 glasses using XANES

Abstract

The L-edge spectra of lead in PbO–PbF2 glasses have been investigated. Since PbO–PbF2 glasses contain [PbO2F4] units throughout the glass-forming compositions and since PbF2-rich glasses are considerably more ionic than PbO-rich glasses, the symmetry of the [PbO2F4] units increases from Cs through C2ν to D4h and possibly to Oh. The changes in symmetry have been ascertained from XANES using appropriate molecular-orbital diagrams. We have shown that the splitting of the L2 and L3 edges, the variation of the half-widths of all three edges and the L-edge intensities are indeed consistent with symmetry changes in the [PbO2F4] units and the suggested molecular-orbital diagrams. While the extent of ionicity of bonding undergoes a continuous increase as the PbF2 content in the glass is increased, the white-line intensities at the edges decrease in the same manner. This has been shown to be a consequence of the deshielding of final states responsible for the L-edge spectra by evaluating the dipole-matrix elements using Slater orbitals. The evolution of the symmetry of the [PbO2F4] units, the variation of L-edge absorption intensities and the variation of ionicity have thus been shown to be directly related.