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-fr equency shoulder near 1200 cm(-1) to the TO component of the out-of-ph ase antisymmetric stretching vibration of the oxygen atoms along a lin e 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 stretc h, indicating that the latter assignment was wrong. This criticism is contested, based on evidence provided by a previous publication that w as overlooked [R. M. Almeida, Phys. Rev. B 45, 161 (1992)] and on the errors introduced in the Kramers-Kronig transformation by the extremel y low IR reflectivity measured by Kamitsos et al. for their gel sample s (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).