Raman and Infrared Spectral Studies of Anhydrous Li2CO3 and Na2CO3

The infrared and Raman spectra of anhydrous crystalline Li₂CO₃ and Na₂CO₃ were measured at 300 and at 80°K. Bands observed in the vibrational spectra of Li₂CO₃ were assigned according to the $C_{\text{2h}}$ factor group symmetry. The doublet structure observed for each of the internal modes in the spectra of Na₂CO₃ was interpreted in terms of an ordered arrangement of CO₃⁼ ions over two nonequivalent orientations within the primitive cell. Multiple internal reflection and polarized specular reflectance techniques were used to determine transverse optical (TO) and longitudinal optical (LO) mode frequencies in Li₂CO₃ and Na₂CO₃. It was shown from these measurements that the infrared transmission spectra of these compounds exhibit band maxima which are admixtures of LO and TO modes.