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

Abstract Tetrahedral iron (III) environments in alkali–alkaline earth–silica glasses have been studied as functions of alkali and alkaline earth cation type and Fe 2 O 3 content using photoluminescence and optical absorption spectroscopies. The luminescence band centered at 13 000–15,500 cm −1 is attributed to the 4 T 1 (G) →  6 A 1 (S) transition of tetrahedral Fe 3+ ions. This band has Gaussian linewidths of 1500–3000 cm −1 but linewidths exhibit no clear compositional dependency. Ligand field strength, 10 Dq , and the Racah parameters B and C are consistent with tetrahedral Fe 3+ 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 Fe 3+ –O bonds and on host glass structure. Alkali cations stabilize Fe 3+ 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 Fe 3+ stabilization in these glasses is not clear, although their effect is the inverse of that of the alkalis. The structural behavior of Fe 3+ is defined as selective , reflecting its strong local ordering effects.