MN2-GEL PROCESS IN SILICA XEROGELS AT 77 K( EPR INVESTIGATION OF SOL)

The EPR spectrum of the Mn2+ ion has been used as a diagnostic tool at liquid-nitrogen temperature (77 K) to investigate the structural evol ution of SiO2 xerogels as a function of temperature during their calci nation, as-well as during subsequent aging. The positions of the Mn2hyperfine lines were used to estimate the g-factor, the hyperfine inte raction constant (A) and the zero-field splitting parameter (D) for th e Mn2+ ion in the various xerogels. The values of the parameters g, A of the xerogels were found to be, within experimental error, independe nt of the firing temperature as well as that of aging. However, the sp litting of the forbidden hyperfine lines, and thus the value of D, was found to be significantly different in the samples prepared at firing temperatures of 360 degrees C and below from those prepared at firing temperatures of 440 degrees C and above; they remained independent of aging.