L. M. Toth, A. S.Quist, and G.E. Boyd
i384
figure. They roughly coincide with the inflection points of the curves. A similar result was found by Allen18 in the case of Mn2+ in glassy methanol. Conclusions [Cr(H20)6I3+ and [Mn(HzO)6]2+ ions embedded in a polymeric matrix of PAN behave in many respects like true liquid solutions. This is manifested both in the ligand exchange mechanism and in the surprisingly short correlasec at room temperature. The tion times -2 X mechanism for the modulation of the zfs is probably due to rotational tumbling of [ C r ( H ~ 0 ) 6 ] ~ and + [Mn(HzO)sl2+ ions in the polymeric matrix. The esr method of studying the dynamics of these ions observes only the effects of the very close vicinity of the paramagnetic species, and to a first approximation does not reflect the macroscopic rheological properties of the systems. The Debye formula which correlates the viscosity to the tumbling time T~ is
17
= zc3kT/4~a3
where a is the radius of the hydrated complex ( a 3A for [Cr(H20)6l3+).Its application in our system leads to a viscosity of 6 x P. This is many orders of magnitude
smaller than the macroscopic value in glasses or rubberlike systems, and reflects the “local viscosity.” Also, the glass transition, which is accompanied by a drastic change in the macroscopic viscoelastic parameters, is only weakly reflected in the esr line shape. There is strong evidence for Cr-Cr interactions in the polymeric matrix. Such interactions probably lead to the formation of Cr pairs, a phenomenon that was previously found in solid solutions. The interaction is concentration dependent and may be the main factor that determines the esr line width at low temperature. The fact that the esr line shape of [Cr(H20)6]3+ in PAN is concentration dependent when the Cr3+ to water ratio is kept constant points to the conclusion that we deal with a true solid solution and not with a microphase separated system. In conclusion we would like to suggest that PAN is a good matrix for the study of esr spectra of small ions in a solution-like environment over a wide temperature range.
Acknowledgment. We wish to thank Mrs. T. Robinson for very careful and tedious work in preparing the samples and for many helpful suggestions.
Raman Spectra of Zirconium(iV) Fluoride Complex Ions in Fluoride Melts and Polycrystalline Solids L. M. Toth,* A. S. Quist, and G. E. Boyd Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
(Received January 15, 7973)
Publication costs assisted by the Oak Ridge National Laboratory
Raman spectrum measurements were performed with molten LiF-NaF-ZrF4 mixtures at 650” in an investigation of the coordination chemistry of zirconium(1V) in alkali metal fluoride melts. The spectra of several MxZr,F4,+x polycrystalline compounds were also examined to assess the utility of using crystalline spectra as a means of identifying species present in molten fluoride solution. Octahedral coordination for Zr(1V) in a 31-36-33 mol % LiF-NaF-ZrF4 melt was established by comparing its Raman spectrum with that of crystalline LizZrF6 where X-ray diffraction measurements have shown the presence of unbridged z r F ~ ~ions. - The occurrence of eight-, seven-, and five-coordinated zirconium in molten fluoride was inferred from frequency shifts accompanying changes in “free” fluoride ion concentration caused by varying the mole per cent ZrF4.
Introduction The occurrence of a variety of discrete complex ions formed by magnesium and by cadmium in alkali metal halide melts has been inferred from recent measurements of the Raman and infrared sDectra of these svstems.l The relative concentrations of the complex ions and the equilibria between them has been shown to depend on the amount of alkali metal halide present. Studies of equilibria in fluoride melts between species of differing coordinanumber using spectroscopy have not been reported, however. The Journal of Physical Chemistry, Vol. 77, NO, 1 I , 1973
Several different alkali metal fluorozirconate complexes are known in the crystalline compounds MzZryF4y+x. Octahedral, six coordination occurs in c ~ z Z r F 6 , ~ RbzZrFO,2 and L i 2 z r F ~ .Seven ~ coordination is found in (NH4)3ZrF7,4 and Na3ZrF7;5 and eight coordination is ob(1) ( a ) V. A . Maroni, E. J. Hathaway, and E. J. Cairns, J. Phys. Chem., 75, 155 (1971);(b) J. H. R. Clarke, P. J. Hartley, and Y . Kuroda, J. Phys. Chem., 76,1831 (1972). (2) V. H. Bode and G . Teufer, Z. Anorg. Chem., 283,18 (1956). (3) R. Hoppe and W. Dahne, Nafurwissenschaffen, 47,397(1960). (4) H. J. Hurst and J. C. Taylor, Acta Crystallogr., Sect. 6, 26, 417
(1970).
Raman Spectra of Zirconium(1V) Fluoride Complex Ions served in K ~ z r F 6 ,N~a ~ Z r s F 3 1 ,and ~ ZrF4.8 All of these coordination numbers are found in R b s Z r 4 F ~ 1 ,which ~ contains four crystallographically independent ions. Aqueous and molten salt solution measurements have proved to be less definitive for the identification of zirconium complexes. Only ZrFs2- ions were reported to exist in aqueous solutions as a result of Raman,lo nmr,ll and other techniques, whereas infrared studies12 on molten LiF-KF-ZrF4 and NaF-KF-ZrF4 indicated a band near 480 cm-1 possibly from a single zirconium complex. Evaluation of sur face tension measurements with LiF-KFZrF4 and LiF-NaF-ZrF4 melts at several temperatures indicated the presence of ZrF73- in both salt mixtures with some ZrF5- ion also reported in the latter.1aa However, interpretation of these measurements in terms of complex ion formation has been q ~ e s t i o n e d . l 3 ~ Despite the uncertainties cast on previous experiments with molten fluorides, the X-ray crystal data and experience with other halide melts suggest that an equilibrium between several coordination species of zirconium could exist. Because the interaction of Zr4+ with the solvent to form complex ions is effective in reducing the concentration of available fluoride ions in solution, the coordination number of the zirconium complexes determines the extent of the effect. Molten mixtures of LiF-BeF2 have found application in the molten salt reactor experiment, MSRE, where ZrF4 is added as a scavenger for oxide impurity. This, therefore, is the practical importance of understanding coordination behavior of zirconium in fluoride melts. A method frequently employed in the past for establishing the existence of a complex ion in a molten salt has been to compare melt spectra with those of crystalline compounds where presumably the same species exists. Unfortunately, this method i s often subject to important limitations, especially when the species possess more than five atoms, or are of low symmetry. In these cases there are many bands in the crystal and melt which often cannot be matched, and there are frequency shifts which cannot be explained. Complications of this type are found with zirconium(1V) fluorides where coordination numbers greater than six are common in the crystalline state and are expected also in molten fluoride mixtures. However, the frequency of the symmetric stretching vibration may be used to distinguish between complexes which differ only in coordination number. The limitations of this method are examined in this paper by comparing the Raman spectra of several zirconium fluorides. The method is then used within its recognized limitations to examine the coordination chemistry of Zr(1V) in the molten LiFNaF-ZrF4 system. Experimental Section The advantages of windowless cells for laser Raman spectroscopy of melts which are corrosive toward the usual optical window materials have been described.14915 These cells were contained in a furnacel6 which was positioned in the sample chamber of a Jarrell-Ash 25-300 Raman spectrometer; 4880-A radiation from an argon ion laser (Coherent Radiation Laboratories Model 52B) was used to excite the spectra. The scattered light collection system and sample handling techniques have been described.14-16 Polycrystalline fluorozirconate compounds were prepared by mixing stoichiometric amounts of alkali metal fluoride salt with ZrF4 and melting in a platinum crucible using a simple resistance furnace. Since zirconium melts
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hydrolyze readily when exposed to the atmosphere, thLe above procedure was performed in a helium-filled glope box of
