INFRARED REFLECTION SPECTRA OF MOLTEN FLUORIDE SOLUTIONS
Infrared Refledon Spectra of Molten Fluoride Solutions. Hydrolysis of Tantalum(V) in Potassium Fluoride-Lithium Fluoride
by J. Stuart Fordyce and Ruth L. Baum Umwn Carbide Corporation, Development Department, Parma Research Laboratory, Parma, Ohio (Receiued July 20, 1966)
Infrared absorption spectra, derived from reflectivity measurements, over the range 4000 to 200 cm.-l for hydrolyzed nnelts of Ta (V) in KF-LiF have been studied. A band characteristic of a tantalum-oxygen multiple bond is found at 900 crn.-l, the intensity of which is stoichiometrically related to the amount of water reacted with the melt. On the basis of these results and spectroscopic and X-ray diffraction studies of the solidified melts, the TaOl?e3- anion is presumed t o be the stable species in these molten solutions.
Introduction A previous paperL discussed the complex anionic species present in alkali fluoride melts containing tantalum fluoride, In the KF-LiF solvent, the Tal??species was found by infrared reflection spectroscopic techniques to be the predominant one over arangeof concentrations and temperatures. The present investigation is an extension of this work to systems in which water is added to study the hydrolysis reactions which take place in molten fluorides and t o determine what species are the stable products in such melts.
Experimental Section The infrared reflectance measurements, reduction of data, and the prepa,ration of anhydrous melts were carried out as previously described.l Qualitative hydrolysis experiments were conducted by adding 1 or 2 drops of water to the cooled solid at room temperature. The wet sample was transferred to an enclosed furnace assembly, provided with an argon flow, and was heated to just above the melting point (-720"). After holding it at this temperature for 1 hr., or so, it was cooled and transferred to the spectroscopic assembly where the infrared reflectance spectrum of the melt at 720"was determined. Quantitative experiments were performed by maintaining the anhydrous melt in the closed-furnace assembly provided with an argon flow. Through a nickel tube placed about 0.64 em. above the melt
surface, argon, saturated with water vapor at 25", was passed in at a rate of 61.2 l./hr. (-0.078 mole of HzO/hr.) for various time periods. At the end of each period the wet stream was closed off and the temperature maintained for 1 hr,, or so, to drive off the hydrogen fluoride. It was then cooled, and the reflectance spectrum of the melt at 720" was obtained. Further hydrolysis was carried out the same way. Chemical analyses were carried out on the samples following grinding in a drybox and drying overnight at 110". The pyrohydrolysis method was used for fluorine, the gravimetric cupferron method for tart& lum, and flame photometry for lithium, potassium, and nickel. The inert gas fusion method (using Laboratory Equipment Corp. apparatus) ww used for oxygen which always gives high results for these samples and is not too reliable.
Results and Discussion Figure 1 shows the infrared absorption spectrum from 1000 to 200 cm.-l of a hydrolyzed melt with an initial composition of 9 mole % ' TaPB in KF-LiF, Nothing of interest was observed in the 4000- to 1000em. -l region. In particular, the hydroxyl OH stretching band in the 3500- to 3700-~m.-~ region was not observed. This spectrum closely resembles that of the (1) J. 8. Fordyce and R. L. Baum, submitted to J . Chem. Phys.; see J. Electrochem. SOC.,112, 82C (1965), and Extended Abstracts of Eleotrothermios and Metallurgy Division, Vol. 3, Electrochemical Society, New York, N. Y., 1965, p. 135.
Volume 69, Number 1.9 December 1966
J, STUART FORDYCE AND RUTHL. BRUM
Figure 1. Infrared spectra of melts: hydrolyzed Ta(Y) in
KF-Lip, . . . . . ..
Figure 2, Infrared Bpectrum (KBr disk) of solidified, hydrolyzed Ta(V) in KF-Lg.
anhydrous melt with the exception of the band at 900 cm.-l. Chemical analysis of this system gave a composition: Ta, 22.1; K, 29.2; Li, 6.1; Ni, 0.93; F, 40.1; and 0,4.8%. The nickel analysis was performed because in these hydrolysis experiments slight attack on the nickel crucible was experienced. The X-ray diffraction powder pattern of the solid showed the presence of aoface-centered-cubic material with a cell edge of 8.87 A. This cell dimension and the line inThe Journal of Phgsical Chemistry
tenaities are in good agreement with the pattern fai' KsTaOFG given in the literatureZ (a0 = 8.90 8.). The remaining lines could be assigned to K F and LiF, and the few other weak ones, to KBTaF,. The analytical r e d t s can be explained on the bask of a mixture; KaTaFs, 18-82; KaTaOFe, 31.95; LiF, 22.80; HF, 24.95; and Nil?3, 1.53. Therefore, the melt spectrum, is for a mixture of hydrolyEed and unhydrolysed material. The infrared absorption spectrum of the solid in B KBr disk is shown in Figure 2. The strong band at 888 ern.+ is apparent. The low-frequency region is characteristic of KiTaF7,l except that the 535-cm.--I band of that compound is superimposed on band peaked at 475 cm.-i. In mulls, all of the bands above ZOO0 cm,-l were very weak and appeared at different frequencies, In view of these difficulties, this spectrum will not be interpreted in detail. The appearance of a band in the 000- to 1100-cry1.-1 region is B elem indication of a metal-oxygen bond sf multiple character ad several authors have painted out, particularly Kharitonov and Buslaev, who studied a series of oxyfluorides of metals in groups IV and V of the periodic table. Field and Hardy4 give 922 cm.-l for the Nb=O stretch in KaNbOE'e and 927 cm.-l in HzNbQF~*H~0, Unfortunately, no data for the T e O stretch in similar compounds have been reported; it should lie very close to but slightly lower than the frequency for the niobium case. This is coneistent with the present findings, ie., 900 om.+ in the melt (888 in the solid), On the basis of these reaults, it c m be concluded that an oxyfluoro anion of tantalum ia the stable hydrolysis product in these melts. I n view of the excess fluoride preaent and the identity of the compound in the solid, it is Most likely that the TaOFea- species is predominant, showing that seven-coordination of the tantalum i s preserved. This ion has an octahedral arrangement of fluorine8 around the tantalum with the Qxygen.off one triangular face (paint group G v ) S and would be easy to form from the TaFT2- ion (trigonal priam with one fluorine above a square face) if the two fluorines on the apex of the trigonal prism were most labile to substitution by oxygen. If only one reacts with water, the T a O F P ion is formed. If both react, the octahedral TaOF62- ion would be formed, but in the presence of excess fluoride this may not be stable. The TaOF52- ion would also possess (2) A. E. Baker and H. M. Haendler, Inorg. Chem., 1, 127 (1962). (3) Y. Y. Kharitonov and Y . A. Buslaev, Izv. Akad. Nauk SSSR Otd. Khim. Nauk, 393 (1962). (4) B. 0. Field and C. J. Hardy, Proc. Chem. SOC.,11 (1963). (6) M. B. Williams and J. L. Hoard, J . Am. Chem. Soc., 64, 1139 (1942).
INFRARED REFLECTION SPECTRAOF MOLTENFLTJORIDE SOLUTIOKS
.06 .00 MOLES HzO
Figure 3. A. 900-om.-' band of hydrolyzed Ta(V) in KF-LiF melts for varioue,times of exposure to water-saturated argon stream: . . . . . . , 30 min.; -, 60 min.; , 90 min. €3. 900-cm,-1 band intensity plotted against the number of moles of water vapor passed into the melt.
an absorption band at about 900 cm.-l and therefore cannot definitely be rulled out as a possibility.
The quantitative hydrolysis study was undertaken
to relate the intensity of the 900-cm.-l band to the amount of water reacted. The reflection data were obtained only over the range of interest for the study of the 900-cm.-1 band. The integration was carried out from 766 to 1010 cm.-1.6 The raw reflection data gave negative values for k because a sloping background was encountered. To overcome this, the background was approximated by a curve: R = -14.6/~ 1.905 X which was subtracted from the observed refle9tance c~iive. The optical constants were determined and are shown in Figure 3A. Even though in one case the IC values are negative in the band tails, it is felt that this procedure allows a better determination of the intensity. The band intensity (peak height) is plotted vs. the number of moles of water passed over the melt in Figure 3B. The linear rela-
tionship indicates that the 900-em. -l band intensity and the concentration of the TaOFs3- species to which it belongs is stoichiometrically related to the number of moles of water added, and presumably all or at least a constant fraction of the water reacts with the melt under these conditions to form this species. On the basis of the reaction KzTaF7 KF HzO + K3TaOFs 3.2HFt and the assumption that all the water reacted in this manner, -80% conversion to the oxyfluoride is expected with the addition of 0.118 mole of water. This is consistent with the diffraction pattern of the solid which showed only KaTaOFe, KF, and LiF with a trace of K2TaF,. In one experiment a large stoichiometric excess of water vapor was added to the melt. X-Ray diffraction examination of the solid after reaction showed a precipitation of TazOs. K3TaOF6was homogeneously distributed throughout with the K F and LiF. This is consistent with the inability of KF-LiF to dissolve Ta205 at temperatures up to 800'. This was demonstrated by the infrared reflection spectrum of molten KF-LiF with TazOs present as a solid phase which remained unchanged from the pure solvent. On prolonged heating at 850" in KF-LiF, the solid Tat05 is converted to solid KTaOs in agreement with the work of Ping-hsin, et al.'
Summary The stable species in hydrolyzed molten solutions of Ta(V) in KF-LiF has been shown t o be the TaOFe3anion. This species is formed in a quantitative reaction of water with the Tal?,+ anion. Extensive hydrolysis leads to the precipitation of Ta2O6 from the melt.
Acknowledgment. The authors wish to thank the analytical group of these laboratories for their contribution and Drs. G. W. Mellors and S. Senderoff for stimulating discussions. H.J. Bowlden and J. K. Wilmshurst, J . Opt. Soc. Am., 53, 1073 (1963). (7) T.Ping-hain, V. I. Konstantinov, and N. P. Luzhnaya, Russ. J. IWTQ. Chem., 8 , 204 (1963). (6)
Volume 69,Number 1% December 1966