Local structure and medium range ordering of tetrahedrally coordinated Fe3+ ions in alkali–alkaline earth–silica glasses

Tetrahedral iron (III) environments in alkali–alkaline earth–silica glasses have been studied as functions of alkali and alkaline earth cation type and Fe2O3 content using photoluminescence and optical absorption spectroscopies. The luminescence band centered at 13 000–15,500 cm−1 is attributed to the 4T1(G) → 6A1(S) transition of tetrahedral Fe3+ ions. This band has Gaussian linewidths of 1500–3000 cm−1 but linewidths exhibit no clear compositional dependency. Ligand field strength, 10Dq, and the Racah parameters B and C are consistent with tetrahedral Fe3+ and here for the first time their linear variation with the alkali/alkaline earth ratio of ionic radii, cation field strengths or individual oxide basicities is demonstrated. This is attributed to the effects of near-neighbor cations on length and covalency of Fe3+–O bonds and on host glass structure. Alkali cations stabilize Fe3+ ions in tetrahedral coordination; stabilization increases linearly with increasing alkali ionic radius and therefore with decreasing alkali field strength. The role of alkaline earth cations in Fe3+ stabilization in these glasses is not clear, although their effect is the inverse of that of the alkalis. The structural behavior of Fe3+ is defined as selective, reflecting its strong local ordering effects.