Anal. Chem. 1995, 67,2129-2135
Use of Vibrational Spectroscopy To Determine Oxide Content of Alkali Metal Fluoride-Tantalum Melts N. J. B j e r " , * R. W. Bwg, E. Christensen, and D. H. K d d g e Materials Science Group, Chemistty Department A, Technical Univefsiiy of Denmark, DK-2800 Lyngby, Denmark
J. H. von Bamer Institute of Mineral Industv, Building 204,Technical Univefsfty of Denmark, DK-2800 Lyngby, Denmark
I&ared and Raman spectroscopic measurements on a series of quenched alkali metal fluoride (Flinak) melts containing tantalum and oxide have led to the development of new methods of oxide analysis, which have an important use in the field of molten fluoride chemistry. The tantalum concentration was maintained at 2.7 mol %, with the oxide concentration varyingfrom 0 to 5.4 mol %, over 22 samples. 'Ihe oxide content of a melt can be determined to within fO.l mol % from the intensity ratio of two vibrations, For example, a melt consisting of purified Flinak, after addition of bTaF7 and heating to 700 "C for 5 h for equilibration, was shown by Mared spectroscopy to contain 0.3 f 0.1 mol % oxide and by Raman spectroscopy to contain 0.2 f 0.1 mol %. The frequency and relative intensities of the bands enable the assignment of Ta-F and Ta-0 bands for both fluorotantalate and oxofluorotantalate entities. In addition a new, dimeric, species with a Ta-0-Ta unit is postulated. Raman spectra of molten samples showed bands of the Same frequency and relative intensities, indicating that crystal dects were not important and that the equilibrium melt species were retained on quenching. Upgrading the properties of cheaper materials is clearly needed as the resources of rarer and more exotic materials become progressively exhausted. A primary way to enhance desirable properties of a lowcost base material such as steel is to plate it with high-melting and chemically inert metallic layers. The use of molten fluoride solutions, e.g., eutectic LiF-NaFKF (Flinak), for the electrolytical deposition of refractory metals has been known for many years.' Despite their toxicities and high melting points, such melts are still important, in some cases irreplaceable,for plating of certain metals, including niobium and tantalum. Numerous studies on the electrochemicalreactions of refractory metals in halide melts have been done. It is of great importance in this connection to know the content of oxide in the melts since this strongly influences which species are formed (Le., fluoro or oxofluoro complexes), as discussed in detail by Christensen et ale2for the case of niobium in Flinak. Many electrochemical investigations are performed with minor concentrations of the solute refractory metal, which makes it even more (1) Senderoff, S.;Mellors, G. W. J. Electrochem. Soc. 1965,77, 266. (2) Christensen, E.; Xindong Wang; von Barner, J. H.; Ostvold, T.; Bjerrum, N. J. J. Electrochem. Soc. 1994, 141, 1212.
0003-2700/95/0367-2129$9.00/0 0 1995 American Chemical Society
important to know the amount of oxide present. If the oxide content is not taken into account, erroneous conclusions may easily be drawn. Such problems are very common for investigations in molten salts, because these solvents are in most cases hygroscopic. However, the number of papers describing methods for oxide determination in melts or solidiiled melts is very small. Important examples (for chloroaluminate melts) are given by Kurayasu and Inokuma3and Laher et ale4 In the latter investigation: T a m was used as a probe for oxide. However, the plating of niobium and tantalum has proved to be a complex process and difiicult to optimize. Among the parameters that are important is the oxide content of the fluoride melt. In the case of tantalum solutions, the oxide is present as oxofluoro species which would seem to be electrochemically active. Because of the importance of electrodepositionof tantalum onto steel from a melt of fixed oxide content, and the inherent variability of residual oxide in a puritied eutectic melt of Flinak, two rapid quantitative analytical methods were devised, based on the intensity ratios of chosen vibrational bands to give the tantalum/oxide ratio. Much of our current knowledge concerning tantalum oxofluoride species is derived from crystalline solids. Much less is known about such species in solution and especially in high-temperature melt solutions. Vibrational spectroscopy, both infrared and Raman, is particularly useful in these cases because it can be applied both to melts and to their quenched solids at room temperature. In this study, a large number of samples of closely spaced compositions were prepared under conditions excluding both moisture and gaseous oxygen. In this way the appearance of vibrational bands with increasing oxide content could be followed closely and deductions made from their relative intensities about the tantalum/oxide ratios present. The intensity ratios also clearly indicate which bands arise from the same, or closely related, species. EXPERIMENTAL SECTION All handling and weighing was carried out in gloveboxes filled with dry nitrogen (dew point --45 "C). Each alkali metal fluoride was purified separately by recrys-
tallization from the melt. The salt, contained in a platinum crucible, was evacuated for 8 h at 400 "C to remove moisture and then heated to 20 "C above its melting point. The salt was then (3) Kurayasu, H.; Inokuma, Y. Anal. Chem. 1993,65, 1210. (4)Laher, T. M.; McCuny, L. E.; Mamantov. G. Anal. Chem. 1985,57, 500.
Analytical Chemistty, Vol. 67, No. 13, July 1, 1995 2129
KBr discs
KBr discs
FLINAK
FLlNAK 2.7 m/o K2TaF7
2.7 m/o K2TaF,
m/o oxide
h
w
u
a
m K
0 v)
m
a
L
k €00
800
1000
1200
1
10
WAVE NUMBER (cm-7
Figure 1. infrared absorption spectra of KBr disks containing solidified KZTaFdFlinak samples with added oxide ( d o , mol Yo).02-/
Ta(V) molar ratios 51. The maximum absorption (at 534 or 545 cm-') is typically in the range of 2.0. cooled slowly (4 "C/h), until 20 "C below the freezing point, from which temperature cooling was more rapid. The solidilied lump was broken up, and the outer pure crystals were removed manually from the impurities concentrated at the center. Flinak was weighed out, in the glovebox, in the required proportions (LiF 46.5 mol %, NaF 11.5 mol %, KF 42.0 mol %). Sodium monoxide was obtained by heating sodium peroxide (Merck, AR) in an alumina crucible at 700 "C for 12 h under vacuum; analysis by titration gave 98.0%of the theoretical amount for NazO. Potassium heptanuorotantalate (Phaltz and Bauer) was analyzed potentiometrically (Found: F, 33.4. Calcd for kTaF7: F, 33.9). &TaF7 was also prepared at our laboratory by mixing hot solutions of TazOs in 40 wt % HF and of KF in 15 wt % HF. A white precipitate was obtained, which was then recrystallized in 40 wt % HF. Analytical results for this material were very similar to those obtained for the commercial product. Glassy carbon crucibles (V25, Carbone Lorraine) were filled with the appropriate NazO/&T~&/FZinak mixture, in that order, to promote homogenity after melting, heated under dried argon for 5 h at 700 "C, and then cooled. The solidified pellet was broken up and stored in a glass ampule that was sealed under purified argon. For infrared measurements, 15 mg samples were taken, well ground, mixed 2130
Analytical Chemistry, Vol. 67, No. 13,July 7 ,
7995
I
1
1
I
I
600
800
1000
lzoo
1' 00
WAVE NUMBER k " ' 1 Figure 2. infrared absorption spectra of KBr disks containing solidified K~TaFdFlinaksamples with added oxide ( d o , mol %). 02-/ Ta(V) molar ratios '1.
with 300 mg of KBr (SpectraTech), and pressed at 17 tons for 3 A n . Infrared spectra were taken from the disks (12.5 mm in diameter) immediately, using a Bomem DA3.01 interferometer, averaging 500 scans. Spectra of samples taken from different places in the same solidified melt (top and bottom) were identical. The spectrum of pure Flinak was also recorded. From this spectrum, baselines could be estimated for the spectra of the tantalum species. The absorption intensities of the maxima of the bands were measured relative to these baselines. Disks made from Flinak heated with oxide, but without tantalum, showed the same absorptions as Flinak alone. Raman spectra were measured on coarsely crushed samples in sealed ampules with a JEOL JRS400D spectrometer using filtered 514.5 nm argon ion laser radiation. Signalswere detected with a cooled extended S20 PM tube and a photon counting system. The excitation power was -500 mW. The spectra were recorded in the depolarized mode, digitized every 2 cm-', and then stored in a computer for calculations and plotting. RESULTS
Infrared Spectra. The infrared spectra of a series of 22 solid samples, formed by cooling melts containing 2.7 mol % tantalum (added as kTaF7) and varying amounts of oxide (0.0-5.4 mol % 0, added as NazO) were found to form a regular series, with bands growing, or diminishing, relative to each other as the tantalum/ oxide ratio changed. Some of the spectral curves are given in Figures 1 and 2. Unavoidably there was some variation in the
Table I,Variation of Ratios of Intensities of Infrared Bands with Oxide Concentration In Fllnak Contalnlng 2.7 Mol % K2TaF.r and Na2O
band ratios % oxide 889/534 889/545 889/732 889/777 889/851 889/917
0.0 0.0 0.0 0.25 0.5 0.5 0.75 0.8 1.0 1.2 1.5 1.7 1.9 2.1 2.4 2.7 3.0 3.5 4.0 4.5 5.0 5.4
0.067 0.25 0.062 0.27 0.25 0.29 0.54 0.30 1.4 1.2 3.0 4.1
2.4 17 1.9 1.1 3.0 2 .o 0.90 2.8 0.87 0.81 5.7
0.80 6.0 1.5 0.82 1.2 1.1 1.4 1.2 1.7
0.75 1.7 3.0 2.2 2.4 3.0 6.5 10 5.0 21 5.0 5.8 17 4.2 19
0
- 0.5 1.4 0.81 0.87 0.81 0.61
2.7 1.9 1.7 1.2 1.1
_---0-00e -e05
I
1.0
1.5
0
NoZO ADDED (mol%)
Figure 3. Ratios of the intensities of the 889 and 534 cm-' infrared absorption bands against added oxide. The doughnut signature represents a coincidence of two data points.
(TaFs)has an absorption at rather higher frequencies (581 and intensities of the bands due to physical factors, such as differences in number, orientation, and size of particles, surface configuration, disk formation, etc. To standardize comparison, the ratios of the intensities of two bands from the same spectrum were employed. The intensity of the 889 cm-I band was used, in each case, as the numerator since this band arises from an oxofluoro species and was the only absorption that persisted throughout the whole range of oxide concentrations. Several bands were tested as denominators. Average values of the ratios obtained are given in Table 1. Plotting these ratios against the percentage of added oxide shows that the 889/534 ratio is by far the best measure of low oxide concentrations. This curve (Figure 3) could be used to estimate the total oxide content of an unknown sample after similar thermal treatment as discussed below. The effect of moisture on the infrared spectrum of a melt has been studied by Fordyce and B a ~ mwho , ~ reported that a 900 cm-I band due to the formation of tantalum oxofluoride products was found for fluoride melts (equimolar LiF/KF) containing tantalum pentafluoride that have been in contact with water. In our work, the absorption background from 900 to 1400 cm-I increased when a disk was aged for 2 months in a closed tube, with definite bands appearing at 1230 and 1255 cm-I. With freshly prepared disks, the background absorption over this range was very small. The absorption bands may be assigned as follows. The 534 cm-' band is in the range expected for tantalum-fluoride stretching vibrations, and it probably arises from a species not containing oxygen, because it is closest to that found by other workers for heptafluorotantalate anions in the solid potassium compound (535 cm-I in &TaF79 and the solid sodium compound (530 cm-I in NazTaF7 9). The regular octahedral anion (5) Fordyce, J. S.; Baum, R L. JPhys. Chem. 1965,69, 4335. (6) Agulyanskii, A I.; Kirillov, S. A, Prisyazhnyi, V. D. LRtr. Khim. Zh. 1980, 46,457. (7) Fordyce, J. S.; Baum, R L.J. Chem. Phys. 1966,44, 1159.
) and is, in any case, unlikely to be found in the 590 cm-l presence of excess fluoride, as is the case here where it is diluted with Flinak, because the equilibrium should be displaced toward the heptafluoro species." The eightcoordinated anion 17aF*~-) absorbs at 510 cm-1.12J3 Similarly, the bands at high frequencies (889 and 917 cm-I, which could not be completely resolved, even at the highest sensitivity) are attributed to tantalum-oxide double bond vibrations, possibly arising from two different tantalum oxofluoro species. In view of their appearance at low oxide content, the suggestion can be made that the oxide/tantalum ratio is unlikely to be higher than 1. A very weak absorption band at 732 cm-I (Table 1) may also arise from the tantalum-fluoride vibrations of such a species. The 545,777,807, and 851 cm-I bands all evidently increase in intensity with oxide/tantalum ratios greater than 1and are thus to be attributed to tantalum oxofluoro compounds, with the first arising from a tantalum-fluoride vibration. However, the number of fluorides in these species is more conjectural. Two bands (545 and 889 cm-I) are close to those of known oxohexafluorides and oxopentanuorides: (NH&TaOFs shows Raman bands at 540 and 862 cm-l,'O and CszTaOF5absorbs at 898,590,540,and 520 cm-l l4 in the infrared wavenumber range investigated here (Raman bands for CsTaOFs occur at 898 and 595 cm-I 14). In contrast to this, the two bands mentioned are less close to those of an oxotetratluoride (CsTaOF4, bands at 475,530,580,730,and 955 cm-I '5). (8) Agulyanskii,A I.; Zalkind, 0.A; Masloboev, V. A Zh. Prikl. Spektmsk. 1993, 39,960. (9)Agulyanskii,A I. Zh.Neorg. Khim. 1980,25, 2998. (10) Keller, 0.L;Chetham-Strode, A Inoq. Chem. 1966,5,367. (11) Matwiyoff, N. A;Asprey, L B.; Wageman, W. E. Znoq. Chem. 1970,9, 2014. (12) Hartman, IC 0;Miller, F. A Spectrochim. Acta 1968,244, 669. (13) Tsikaeva, D.V.;Nikitina, S. D.; Agulyanskii, A I.; Kalinnikov, V. T. Koord. Khim. 1986,12,929. (14) Pausewang, G.;Schmitt, R; Dehnicke, K 2.Anoq. Allg. Chem. 1974,408, 1.
Analytical Chemistry, Vol. 67,No. 73,July 7, 7995 2131
A number of species are likely to be present in a particular solution depending on the tantalum/oxide/fluoride ratio. Also, structural isomers may be present in some cases. The equilibria are also sensitive to temperature and to the kind of alkali metal cation. (These points may be illustrated by TaFi2-,which is probably the most studied anion, whose 535 cm-I band moves to 546 cm-* on melting6 and then to 610 cm-I at 850 "C due to its dissociation to TaFs-. With added KF, the band moves to 550 cm-l. Addition of other alkali metal fluorides gives a different pattem of band shiftsag In the present case with Flinak it seems that the absorption bands are closest to those found for the potassium salt, perhaps indicating that these cations surround the tantalate anion. Despite this complicated situation and the uncertainty conceming the species actually present, the ratios of the various absorp tion bands offer a method of linking several bands to a particular species. If the ratio of two intensities remains approximately constant as concentration is varied (in this case oxide concentration is varied while tantalum concentration and other conditions are held constant), then it follows that both bands arise from the same species, or at least from two closely related species, with the same tantalum/oxide ratio and related by a simple equilibrium such as
\ 3
From Table 1it seems likely that the 889 and 917 cm-l bands fall into this category. Likewise, the 777 and 851 cm-' bands, which possibly arise from a species with an oxide/tantalum ratio of 2, are probably also closely related since the ratio of their intensities is almost independent of concentration (Table 1). In Figure 4 are shown infrared spectra of different samples containing &TaF7 at room temperature. It is interesting to note that the pure salt, when it has not been heated, shows no IR bands due to vibrations of the Ta=O bond, i.e., no TaO-oxofluoro complexes are present. However, if the salt is heated to 750 "C for 5 h under an argon atmosphere and subsequently cooled to ambient temperature, bands appear at 889 and 917 cm-'. Since IR bands around 900 cm-1 are indicative of Ta=O vibrations, a part of the salt must have undergone a transformation to a T a O oxofluoro compound. In this light, it is likely that the bands at 889 and 917 cm-l observed for 2.7 mol % KzTaF7 dissolved in Flinak at 700 "C do not arise from the Flinak solvent itself, but rather have their origin from an oxide impurity (e.g., HzO) in the &TaFi. This view is supported by the last spectrum shown in Figure 4, where the &TaF7 concentration has been increased to 8.1 mol % &TaF7 (dissolved in Flinak at 750 "C). In this case, it appears that the 889 and -917 cm-' bands increase in intensity (absolutevalue). If the oxide impurity originated from the solvent, band intensities similar to those seen at the lower concentration should have been observed. If we now plot the intensity ratio of the 889 and 534 cm-l bands as obtained from Table 1vs the concentration of added oxide, an almost linear relationship is obtained (Figure 3). The straight line drawn under this assumption allows us to carry out an extrapolation, so that the oxide impurity in the chemicals used (Flinak solvent and &TaF7) can be estimated from the intersection of this line with the abcissa. If the purpose is to determine the (15) Buslaev, Y. A; Kokunov,Y.V. Do. Akod. Nauk. SSSR Neotg Mater. 1968, 4. 537.
2132 Analytical Chemistry, Vol. 67, No. 13, July 1, 1995
600 800 1000 1200 1 30 WAVE NUMBER (cm-')
Figure 4. Infrared absorption spectra of K2TaF7treated in different ways (KBr disks at room temperature).
amount of oxide in the Flinak solvent alone, it is important to subtract any amount of oxide originating from the &TaF7 salt, as previously discussed. In this particular case, the error in the data is not only related to the intensity ratio measured but also to the composition of the melts. This is because during preparation of the melts, and also during preparation of the KBr pellets, there is always a risk of contamination with oxide by hydrolysis. Because of this, we carried out linear regression according to the method of Bartlett16 in which the oxide content and the intensity ratio both have error variance. The resulting line for the seven points of lowest oxide content is shown in Figure 3 as a dashed line. Thus, the oxide impurity level after addition of &TaFi is estimated to be 0.3 f 0.1 mol %. If the chemicals are pudied in an appropriate way, the oxide content in the solvent melt is normally rather low. However, for electrochemical investigations, where low concentrations (often in the range of 0.1 mol % or less) are usually applied, the oxide content should be almost negligible. In such cases it is the data for the lower range of oxide concentrations, Le., the linear range in Figure 3, that are the most useful. It is also important to note that oxide contents in the order of magnitude of 0.1 mol % (in total) will easily be detected. Taking into account the correction for the oxide impurity arising from KTaF7 (0.3 mol %),a standard curve for the oxide content in the Flinak melt can now be obtained by changing the starting point of the curve to 0.0 mol %. Raman Spectra. It was evident that the Raman spectra of the same tantalum/oxide/fluoride solidified samples showed a ~~
~
(16)Bartlett, M. S. Biometrics 1949, 207.
l
FLI NAK
I
\
FLINAK
2.7m/o 0 K2ToF, Q) Q
(3
z
a W
+
t
u
v)
f
a
WAVE NUMBER (cm-') Figure 5. Raman spectra of solid K~TaF,/Flinak samples with added oxide (m/o, mol Yo).02-/Ta(V) molar ratios 50.56.
similar regular pattern of the intensity ratios for 13 frequencies as a function of the oxide content F i r e s 5-7). The chief difference is the complete absence of the 534 cm-I band, which is evidently Raman inactive. Interestingly, similar spectra were obtained from these samples in the molten state,17 so that the solid state signals cannot be ascribed purely to crystal effects. There was some small variation in the intensity ratios obtained from spectra of different crystal fragments of the same sample. Probably this is due to orientation effects or some segregation of component compounds occurring during the cooling and freezing of the melt solutions. Spectra of several crystal fragments of a sample were summed and normalized to provide representative values. Ratios of these intensitiesagain illustrate a regular pattern of variations (Table 2), which can assist the assignment of the vibrations to particular tantalum/oxide ratios as well as be used to estimate the oxide content, for example, of supposedly pure Flinak. Again the measured intensitiesof several bands were tried as the denominator, and the 890 cm-I band, which was present over almost the whole oxide concentration range studied, turned out to be most usefully employed as the numerator. The signal at 645 cm-I is clearly a Ta-F vibration and has a maximum intensity at 0 mol % added oxide, diminishing to zero intensity at 2.4 mol %. Hence it is ascribed to a tantalum fluoride (17) von Barner, J. H.; Berg, R W.; Bjemm, N. J., Christensen, E., Rasmussen, F.Mater. Sci. Forum 1991,73-75,279.
IO WAVE NUMBER Icm-')
Flgure 6. Raman spectra of solid tantalum/Flinak samples with 02-/Ta(V) molar ratios in the range 0.56added oxide ( d o , mol YO). 1.o.
species without oxide, most probably the symmetric stretch vibration of potassium heptanuorotantalate (reported to give bands at 641,7JO645,11J8J9and 635 cm-'2o ). The heptanuoride is more likely than either the hexafluoride anion (with a stretch vibration band at 692 cm-l for CsTaF6 ro,zo) or the octafhoride anion (371, 411, and 614 cm-' lo or 377, 426, and 622 cn-l,l2 both for Nar TaF8). A similar assignment is made for the vibration at 280 cm-1 observed at the same frequency by Torardi et al.19 for a Ta-F bending mode. It may be noted that Fordyce and Baum5 found a strong infrared band at 285 cm-I for solid KZTaF7 and at 292 cm-l for T a m in W/KF melts? The vibration at 380 cm-I may also be due to a Ta-F species without oxide (cf. 3921° and 395 cm-l,19 both for KZTaF7), but interference from a signal due to Pyrex glass sometimes occurred near this frequency and also at 420 cm-I. This was confirmed using an empty tube. Thus, these vibrations and their ratios were not used for oxide estimations. The positions of the 890 and 910 cm-I bands clearly show that they are due to tantalum-oxide double bond vibrations (e.g., 888 cm-l was found for & T ~ O F Sand , ~ 890 cm-' for (NH&TaOF5 lo (18) Tsikaeva,D. V.; Nikitina, S. D.; Agulyanskii,A. I.; Kalinniiov,V.T. Zh.Obshch. Khim. 1987,57,974. (19) Torardi, C. C.; Blixner, L. H.; Blasse, G. J. Solid State Chem. 1987,67, 21. (20) Babkin, A. G.; Nikolaev, A. I.; Shevyreva, E. V. Zh.Neorg. Khim. 1986,31, 2444.
Analytical Chemistry, Vol. 67,No. 13, July 7, 7995
2133
Table 2. Variations of Ratios of Intensfties of Raman Bands with Oxide Concentration In Fiinak Containing 2.7 Mol % KzTaF7 and Na&
band ratios % oxide
890/910
890/645
890/600
0.0 0.0 0.0 0.25 0.50 0.50 0.75 0.80 1.0 1.2 1.5 1.7 1.9 2.1 2.4 2.7 3.0 3.5 4.0 4.5 5.0 5.4
0.50 0.71 1.0 1.0 1.4 0.83
0.03 0.12 0.03 0.18 0.29 0.20 1.3 0.64 2.6 1.9 8.3 12 23 36
0.37 0.80 0.36 0.70 1.3 1.0 1.5 1.2 1.7 1.5 2.5 2.8 4.1 5.4 61
m m
m
m
49
m
m
m
m
890/545 m
4.0 2.0 4.7 7.3 5.0 5.5 4.4 6.0 5.1 6.4 5.0 5.9 5.1 5.4 5.5 5.8 7.9 10
FLINAK 2.7 m/o K2TaF7
a
za
w I-
s
u In
f
K
200
400 600 800 WAVE NUMBER (em-’)
1000
1
Figure 7. Raman spectra of solid tantalum/Flinak samples with added oxide (m/o, mol %). O*-/la(V) molar rarios 1.
(880 cm-I was quoted for @t4N)zTazOFlo,21but this may well be due to impurities containing tantalum terminal oxygen groups)). (21) Sala-Pala, J.; Calves, J. Y.; Guerchais, J. E.; Brownstein, S.; Dewan, J. C.; Edwards, A. J. Can. J. Chem. 1978,56,1545.
2134 Analytical Chemistry, Vol. 67, No. 13, July 1, 1995
890/310 m
m m m m m
6.6 m
4.1 5.8 3.8 4.8 2.8 2.4 2.6 3.4 3.8 2.5 2.8
890/280
870/795
870/565
870/310
9.4 11
2.3 3.8
0.15 0.80 0.17 1.8 4.4 1.2 11 m
54 m
9.2 m
31 25 m
55 m m
m
7.9 7.0 6.7 6.9 5.6
m
m
The 890 and 910 cm-l bands may be due to very similar species in slightly diflerent environments. The band at 545 cm-I varies in its intensity from very small for 0.0 mol % added oxide, through a maximum at 2.4 mol %, and decreases to zero for 5.4 mol %, respectively. Hence, this band seems to be associated with a species with a 1:l tantalum/oxide ratio. A band at 540 cm-I has been reported by Keller and Chetham-Strodelofor solid (NH& TaOFe. The constant intensity ratios found for the 545 cm-l band to the 890 cm-I band support the suggestion that they arise from the same species. The vibration at 600 cm-l was present from 0.0 to 2.4 mol % added oxide and very clearly reaches a maximum around 1.4 mol % in the normalized spectra. This change is also seen in Table 2 as a ratio (890/600) vs oxide concentration break; Le., when the intensity ratios are plotted, a distinct break between two nearly straight lines is seen. The simplest explanation of this tantalum/ oxide ratio equal to 2:l is the existence of a dimer species containing an oxygen bridge (Ta-0-Ta) and with the 600 cm-l band due to a tantalum-fluor0 stretch vibration. The formation of such a species would also explain the very rapid decrease of the tantalum-fluoride stretching band intensity (at 645 cm-l, attributed to the TaF+- anion), as was also found for the infrared stretch at 534 cm-l attributed to the same species. In F i i e s 5 and 6 the summed experimental spectra have been rescaled so that the characteristic vibration of the presumed predominant species in the sample is proportional to the concentration of that species. Thus Figure 5 is drawn so that the tantalum-fluoride vibration at 645 cm-l (considered to be due to a Ta-F stretch of a species without oxide) is normalized to the concentration of oxide-free tantalum (i.e., to Ta mol % minus oxide mol %),whereas Figure 6 is drawn with the 890 cm-I vibration (considered to be due to a Ta=O vibration of a monooxo TaOF,@--”species) made proportional to the concentration of TaOF species (i.e., to mole percent oxide). At high added oxide contents, e.g., above 4.0 mol %, new vibrations appear, at 620, 700, and 795 cm-I, while the hitherto ubiquitous 890 cm-1 signal shifts to 870 cm-I, as can be seen in
I
645 ratios were found to be particularly useful. The latter ratio is shown in Figure 8. The linear regressionI6 was carried out on the first six points, and the confidence limits were calculated for the first seven points as in the case of the infrared ratio. From this analysis (dashed line, Figure 8), the oxide impurity level in Flinak with &TaF7 added is estimated to be 0.2 0.1 mol %. It can be seen that the values obtained from the two vibrational methods are closely similar. As for the infrared spectra, a standard curve starting from 0.0 mol % oxide can also be obtained by displacementof the m e in F i r e 8 by an amount corresponding to the background impurity, here 0.2 mol %.
I
*
0 c-
.Dd,/ -
0
1.0
,
/ 0
2.0
No20 ADDED (mol%) Figure 8. Ratios of the intensities of 890 and 645 cm-I Raman bands against added oxide. For convenience, two different scales are used. Filled circles refer to the ordinate on the right side; open circles refer to the ordinate on the left side.
Figure 7 where the spectra have been rescaled so that the 870 or 890 cm-I vibration (Le., of Ta=O in a TaOF species) is of constant height. These vibrations are all attributed to new tantalumoxofluoro species with a tantalum/oxide ratio of 1:2. It is inherent in the above discussion of all spectra that the melts (both liquid and solidified) contain more than one kind of species, and probably often more than two kinds, of different stoichiometries and/or geometries, particularly when the oxide/tantalum ratios are high. The intensity ratios from different species may again be used for quantitative determination of the oxide content of fluoride melts or of Flinak itself. In the case of Flinak, the 890/600 and 890/
CONCLUSION Both infrared and Raman spectroscopies can be used for the determination of oxide contents of fluoride melts if suitable additions of tantalum0 are made and the melts equilibrated. The two techniques give identical values for the oxide content of the purified Flinak with KTaF7 added. By comparison of the spectra used for the standard curves with the spectra of pure KT TaF7,or of Flinak with &TaF7 added in a higher concentration, it can be stated that the oxide is mainly introduced via the & TaF7 Salt. Thus, the standard curves obtained can be used in different ways: By correcting for the amount of oxide arising from the tantalum salt, a standard curve for determination of oxide in the Flinak can be obtained. Alternatively, by using very pure alkali fluorides, the oxide content in the &TaF7 may be determined. The total oxide content, Le., in Flinak and &TaF7, may of course also be determined, when proper correction is carried out. The infrared method has an inherent advantage because it measures an average of several crystallites whereas the Raman method requires recording of at least two spectra to minimize crystal orientation effects or inhomogenities. Received for review October 20, 1994. Accepted March 9,
1995.m AC9410272 @
Abstract published in Advance ACS Abstracts, April 15, 1995.
Analytical Chemistry, Vol. 67,No. 13, JuW 1, 1995
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