Raman and Infrared Spectral Studies of Anhydrous Li2CO3 and Na2CO3

Abstract

The infrared and Raman spectra of anhydrous crystalline Li2C0 3 and Na 2 C0 3 were measured at 300 and at SocK. Bands observed in the vibrational spectra of Li2C03 were assigned according to the C 2h factor group symmetry. The doublet structure observed for each of the internal modes in the spectra of ::-Ja 2 C0 3 was interpreted in terms of an ordered arrangement of C03~ 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 Li 2 C0 3 and Na2C0 3 • 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. INTRODUCTIONcation) has the effect of doubling the number of vibrational bands. I4 . 15 Splitting from nonequivalent sites will Procedures for the determination of zero wave vector often be most apparent in the regions of non degenerate selection rules for optically active vibrational modes in modes that otherwise could only be split by correlation crystals are well established. I-II Although the vibra-field effects. tional frequencies of a molecular ion in the solid phaseRecently, Dubbledam and De Wolff 16 reported the remain similar to the hypothetical "free" ion fre-(average) crystal structure of anhydrous Na2C0 3 and quencies, two effects, each governed by the crystal suggested a modulated structure that could be exstructure, act to produce modification from the "free" plained by assuming an ordered arrangement of the ion spectrum: (a) symmetry of the site occupied by the C0 3 = ions over two nonequivalent orientations within molecular ion (static field effect) and (b) dynamic the primitive cell. The effects of this unusual structure coupling between identical vibrational modes of two or should be apparent in the vibrational spectrum of more molecular ions which are located on equivalent Na 2 C0 3 . Inasmuch as Li 2 C0 3 belongs to the same sites in the primitive unit cell (correlation field effect). crystal class I7 as Na2C03 but has only olle carbonate A complete factor group analysis is required to deter-orientation, it may serve to give a convenient normal mine all theoretically active normal modes. However, vibrational spectrum with which 1\a2C0 3 may be the vibrational spectrum can often be interpreted from compared. The Raman spectra of Li2C03 and Na2C0 3 simple site symmetry approximations because the have not previously been reported; however, Buijs and interactions that give rise to correlation field splittings Schutte lS and Tarte l9 have reported results from thin usually cause smaller perturbations than those due to film infrared transmission measurements on crystalline the static field. 7 -11 Li 2 C0 3 and ~a2C03. Buijs and Schutte 1S analyzed their Complications to the interpretation of the vibrational data in terms of static field effects, but no evidence was spectrum of a molecular ion can occur if the ion occupi...

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