Infrared Absorption Spectra of Chromate Ions in RbCl Single Crystals

Abstract

Since there are published data concerning the I R absorption spectra of chromate ions in a number of alkali halides (AVARMA, MAURING; DEM'YANENKO et al.; JAIN et al.; MIRLIN, RESHINA; RADHAKRISHNA, SHARMA), RbCl doped with this impurity is not studied till now. The results of such investigation of RbC1:CrO:-crystals are given in this paper.The studied crystals were grown in air by the Kyropoulos method in a platinum crucible from RbCl of special purity. An impurity was added to the mixture prior to melting as K,Cr,O, or K,Cr04 in the ammount of 0.5 ~n o l~~. I R absorption spectra were measured on UR-20 spectrophotonieter.Absorption spectra of crystals, grown when doping the melt with either K,Cr,O, or K,Cr04, are completely the same. Since K,Cr,O, a t 500 "C decomposes in K,Cr04 and Cr,03 (RIPAN, CHETYANU), identity of spectra permits to conclude, that in both cases an impurity enters the crystal as CrOz-.Chromate ion GO4,-in a free state has four normal vibration modes (NAKAMOTO): vl(Al) = 847 cm-l, v2(E) = 348 cm-l, v3(T2) = 884 and v4(T2) = 368 cm-l, only two latter degenerated vibrations being IR-active. When this ion enters the aklkali halide crystal, substituting for a halogen ion, the necessity appears to compensate an extra negative charge. The simplest way of such compensation is the formation of an anion vacancy. If this vacancy is far enough from (302,-ion, its symmetry remains unchanged. I n such a case only absorption bands, corresponding to y3 and v4 modes, should be observed in IR spectrum. But if synimetry of chromate ion is lowered because of neighbour vacancy or some other defect, the degeneracy of v3 and v4 modes is removed completely or partially and vl, v, modes become IR-active. IRspectrum of the crystal doped with CrOi- (Fig. 1, curve 1) shows six absorption bands in the region of the asymmetric stretching mode v3 (884, 895, 904, 917, 924, and 930 crn-l) and a band a t 852 ~n i -~, corresponding to a symmetric vibration vl, that is indicative of reduction of CrO2,-synimetry. Appearance of six absorption bands in the region of v3 mode suggests the simultaneous realization of various ways of coiiipensation of an extra negative charge. It is ascertained by investigations of other alkali halides that compensation of extra negative charge of chromate ion is possible not only by means of an anion vacancy, but also by means of divalent alkaline earth ions being present in the crystal as a contamination. To verify the possibility of this way of charge compensation in RbC1, we investigated the crystals doped simultaneously with CrOi-and Ca2+, since calcium is the most probable contamination in the used reagent. When growing such crystals, 0.05 mol% of CaCl, were doped in the melt simultaneously with K,Cr,O,. The spectrum of such crystal is shown by curve 2 in Figure 1. One can see intensification of absorption bands a t 924 and 930 cm-l and appearance of a new band a t 887 cm-l; that