Infrared-reflectance spectra of heat-treated, sol-gel-derived silica

Kamitsos et al. [Phys. Rev. B 48, 12 499 (1993)] studied the infrared- (IR) reflectance spectra of bulk SiO2 gels and attributed the high-frequency shoulder near 1200 cm(-1) to the TO component of the out-of-phase antisymmetric stretching vibration of the oxygen atoms along a line parallel to the two adjacent Si atoms, contrary to the assignment of Almeida and Pantano [J. Appl. Phys. 68, 4225 (1990)], who attributed this shoulder to the LO component of the in-phase antisymmetric stretch, indicating that the latter assignment was wrong. This criticism is contested, based on evidence provided by a previous publication that was overlooked [R. M. Almeida, Phys. Rev. B 45, 161 (1992)] and on the errors introduced in the Kramers-Kronig transformation by the extremely low IR reflectivity measured by Kamitsos et al. for their gel samples (as low as 2% for the 1086 cm(-1) peak, compared with values in the range of 20-31 % for gels prepared in our laboratory).