Abstract
The structural evolution of the silica xerogels as a function of temperature during their heat treatment under a nitrogen atmosphere is investigated by EPR spectroscopy. Detailed analysis of the Mn(II) and Cu(II) EPR spectra recorded at RT and 77 K show that an additional resonance, singlet (g(0)=2.0014, Delta Bpp congruent to 10x10(-4) T), which corresponds to free radicals from organic fragments, is superimposed on the Mn(II) central sextet (g(eff)=2.000, A(av)congruent to 95(.)0x10(-4) T), as well as on the composite Cu(II) EPR spectra, (which further consist of the differently distributed Gamma(1)-, Gamma(2)-, and Sigma-type of individual Cu(II) EPR signals). In contrast to the previously obtained Mn(II) and Cu(II) EPR spectra of silica xerogels, which were calcined in air and under oxygen atmosphere, this additional, sharp resonance appears only in the Mn(II) and Cu(II) EPR spectra of silica xerogels that were calcined at temperatures over 400 degrees C under a nitrogen atmosphere.