Raman spectroscopic investigations of Mn2+ doping effects on the densification of acid-catalyzed silica xerogels

Undoped and Mn2+-doped silica xerogels were prepared from hydrolysis and co ndensation of tetramethyl orthosilicate (TMOS). The xerogels were character ised by density measurements and fluorescence and Raman spectroscopies. Ram an measurements over the range 4-1200 cm(-1) showed that the number of thre e- and four-membered rings in the xerogel network depends on the thermal tr eatment and on the concentration of Mn2+ ions. Indeed, both structures are found to be more numerous in the gel network of the doped samples than in t he undoped one, showing that doping with Mn2+ hampers the destruction of th ree- and four-membered rings. In the low-wave number region (4-100 cm(-1)), doping with manganese ions was found to affect the position of the boson p eak. The boson peak profiles were used to deduce that the sizes of the cohe sive domains in the gel-derived silica network are much larger for doped sa mples (11 nm for 500 ppm) than for undoped ones (2.1 nm). (C) 1999 Elsevier Science B.V. All rights reserved.