SPECTROSCOPIC PROPERTIES OF LOW FE3-DOPED SILICA-GELS()

Trivalent iron (0.21-1.21 mol% equivalent Fe2O3) was introduced into s ilica gel monoliths prepared via sols obtained from tetraethyl orthosi licate and metal salts at different final sol pH values. The UV-visibl e spectra indicated a change from an octahedral to tetrahedral Fe3+ en vironment as the pH increased. An electron paramagnetic resonance spec tral resonance at g = 4.22 appeared, in addition to that at g = 2.0, a s the final pH increased. The resonance at g = 4.22 was also observed for some gels upon heat treatment. Colloidal precipitation of Fe2O3 wa s noted with increase in cation concentration and temperature of heat treatment. The effects of processing on fight transmission of gels wer e measured. The results indicated several processing conditions leadin g to total cut-off of ultraviolet light through the doped gel monolith s.