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 C_s through C_2ν to D_4h and possibly to O_h. 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.