The infrared spectra of the alkali iodates and ammonium iedate are studied, as well as the Raman spectra of KIO 3 and LiIO3, using a laser source of excitation. It is concluded that these salts conrain isolated IO s" ions in their structures. The frequencies are identified by factor group analysis and deutefization. There is a sharp difference between the spectra of a- and g-LiIO3, due to a change in the symmetry of the crystal field and a rearrangement of the coordination polyhedron of lithium; there is also a reduction in v t(IO3) and in the frequency splitting of the antisymmetric valence vibration ZX v 3 in the spectra of the alkali iodates as the. polarizing capacity of the cation diminishes. It was shown in an earlier paper [1] that the uncertainty at present existing in relation to the number, magnitude, and identity of the absorption band maxima in the vibrational spectra of the iodates was due to inadequacies in the photographic procedures, the absence of information regarding the polymorphism of the iodates, and the contradictory nature of structural data. Neglect of the effects arising from the polarizing capacity of the cation, the hydrogen bonds, and the static and dynamic effects of the crystal field has frequently led to erroneous conclusions regarding the symmetry of the IO 3" ion and the coordination polyhedron of I(V) [2, 3]. In order to elucidate the effects of these factors in the present investigation, we obtained the corresponding polymorphic forms of the alkali iodates under specified conditions of temperature and concentration [4, 5] and studied their vibrational spectra. The crystal forms of the iedates were identified by x-ray diffraction and their composition was monitored by chemical analysis. The Raman spectrum of potassium iodate was obtained in a DFs-a2 instrument with a laser source of excitation, the exciting line being the 6328 A. The dispersion was 10 ~/mm, and the spectral slit width 5 cm "1. The Raman spectra of KP a- and g-LiIO s were recorded in the Cnderg H-IO Raman spectrometer, and the infrared spectra of the iodates in question, suspended in vaseline or fluorinated oils, were obtained in the UR-20 and Hitachi-225 spectrometers over the range 4000-200 cm "1. In order to separate out the weak absorption band of vaseline oil, with its maximum of 725 cm -t, which distorted the contour of the I-O valence-vibration bands in the 600-1000 cm "1 range we studied the infrared spectra of samples deposited on KC1 or KBr substrates; no interaction appeared to take place between the iodates and the substrates. We see from Fig. I that, in the spectra of iodate powders on substrates, the absorption bands in the neighborhood of the I-O valence vibrations are more sharply expressed and better reproduced than in the spectra of vaseline oil suspensions. The absorption band maxima detected in the vibrational spectra of the iodates are listed and identified in Table 1. The bands were attributed to symmetrical and antisymmetrlcal vibrations of the IO s- ion on the basis of a comparison between their relative intensities in the infrared and Raman spectra. Table 1 clearly shows that the spectra of the compounds under consideration do not contain the frequencies of the valence vibrations of the IO G octahedron (610-710 cm "t [6]). All the spectra exhibit the absorption bands characteristic of the pyramidal IO 3- ion, which refutes x-ray data indicating the existence of a regular IO s octahedron in the structures of the alkali iodates [7-10].
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Nov 1, 1978
A B Hollinger + 1
