D. Ail. GRUEN,R. L. MCBETH,?tl.S. FOSTER, AND C. E. CROUTHAMEL
472
Absorption Spectra of Alkali Metal Tellurides and of Elemental Tellurium in Molten Alkali Halides'
by D. M. Gruen, R. L. McBeth, M. S. Foster, and C. E. Crouthamel Argonne National Laboratory, Argonne, Illinois
(Received August 16, 1966)
Absorption spectra of solutions of LizTe in molten LiCl and LiC1-LiF and of CszTe in molten CsCl have been measured in the range 40,000 to 4000 cm-I. The LizTe and CszTe spectra are characterized by absorption bands with maxima at 21,250 and 18,200 cm-1; widths at half-maximum of 7100 and 7400 cm-l, and oscillator strengths of 0.86 X and 2.00 X respectively. The absorption bands are interpreted as arising from 5p --t 6p atom-like transitions centered on tellurium. The spectrum of Te metal in molten CsCl has also been measured and is characterized by two overlapping absorption bands with maxima at 15,500 cm-' and 20,800 cm-1 having a total oscillator strength of 3.87 X lov2. The solubility of Te in molten CsCl at 671" is 1.28 X wt '%, in molten KC1 a t 802" is 3.50 X lod2wt %, and in molten LiCl at 627" is estimated at less than 5 X wt %*
Introduction Solutions of intermetallics in molten salts were studied by Heymann and his ~ o - w o r k e r s ,who ~ ~ ~investigated distribution equilibria with reference to the stability of intermetallic compounds in melts. Recently, spectral data on solutions of LiaBi in molten LiCl-LiF were r e p ~ r t e d . ~The present study concerns itself with solutions of alkali metal tellurides and of tellurium metal in alkali chlorides. I n view of the paucity of information concerning solutions of this type, it is of interest to study their spectra in order to gain more insight into the nature of the species present in these systems. Experimental Section Materials. Tellurium used to prepare the various alkali metal tellurides was obtained from the American Smelting and Refining Co., South Plainfield, IS. J. The metal was melted in a helium atmosphere and filtered through a fine-porosity fritted disk. The sources of the alkali metals were the following: lithium metal, Foote Mineral Co., Philadelphia, Pa. ; sodium metal, Allied Chemical, Kew York, N. Y.; potassium metal, J. T. Baker Chemical Co., Phillipsburg, N. J.; and cesium metal, Dow Chemical Co., Rlidland, Rlich. No further purification of these metals was The Journal of Physical Chemistry
attempted, but only bright metal pieces were used and the metal was stored in the helium-atmosphere box. Solvents of LiCI-LiF (80 mole % ' LiC1, mp 505"), LiC1-KC1 (59 mole % LiCl, mp 352"), and LiCl (mp 607") were made from reagent grade material and purified by the method of Maricle and H ~ m e . ~ Cesium chloride, as the 99.9% material, was obtained from Atomergic Chemetals Co., Carle Place, S.Y., and was used after filtering through a quartz fritted disk. Potassium chloride (mp 776")) NaC1-SaF (50 mole % KaC1, mp 675"), and NaC1 (mp 801') were also used after filtering the reagent grade materials through a quartz fritted disk. The alkali metal tellurides were prepared directly by combining stoichiometric quantities of the elements. Lithium telluride (LizTe) was prepared in a (1) Based on work performed under the auspices of the U. S. Atomic Energy Commission. Presented in part a t the 148th National Meeting of the American Chemical Society, Chicago, Ill., Sept. 1964. (2) E. Heymann and H. P. Weber, Nature, 141, 1059 (1938); Trans. Faraday SOC.,34, 1492 (1938). (3) E. Heymann, R. J. L. Martin, and M. F. R. Mulcahy, J . P h y s . Chem., 47, 473 (1943). (4) M. S.Foster, C. E. Crouthamel, D. hl. Gruen, and R. L. McBeth, ibid., 68, 980 (1964). (5) D.L. Maricle and D. N. Hume, J . Electrochem. SOC., 107, 354 (1960).
ABSORPTION SPECTRA OF ALKALIMETAL TELLURIDES
B e 0 crucible at 950", but sodium telluride (Na2Te) and potassium telluride (K2Te) were prepared in alumina crucibles a t 950 and 725", respectively. Cesium telluride (Cs2Te) was prepared in either alumina or quartz crucibles a t 600". The solubility of tellurium metal in LiCI, KCl, and CsCl was determined by equilibrating 1 g of the metal with 10 g of the salt in the appropriate container for several hours with occasional agitation a t 627, 802, and 671 ", respectively. The saturated solutions were then dipped from the crucible with a small quartz container and solidified for analysis and subsequent use in spectral measurements. Apparatus and Procedure. Solutions of the telluride in molten salts were prepared in the helium box by heating the telluride and salt together, usually in the same container used to prepare the telluride. These solutions, too concentrated for use in spectrophotometric measurements, were decanted from the undissolved telluride, solidified, and crushed. An aliquot of the concentrated solution was diluted with an appropriate amount of the purified salt used as a solvent. Approximately 6 g of this mixture was then added to the cell apparatus used in obtaining the absorption spectrum at elevated temperatures. The cell apparatus consisted of a Fisher-Porter 1cm path length cell sealed to a 45-cm length of 12-mm 0.d. quartz tubing which contained a fine-porosity quartz frit 15 cm above the cell. To obtain shorter path lengths for the study of more concentrated solutions, suitable quartz cell spacers were employed. A standard-taper joint attached to the open end of the tube completed the cell-frit apparatus. By using appropriate vacuum line techniques, the diluted mixture was melted and filtered into the absorption cell, sealed off under 0.5 atm of argon, and transferred to the spectrophotometer furnace. Absorption spectra measurements were made from 300 to 2500 mp in a Gary Model 14 K spectrophotometer a t temperatures up to 1000". A description of the furnace and temperature controller has been given elsewhere.6 I n place of the standard tungsten source, a high-intensity Sylvania DXL quartz halogen tungsten lamp (original Sun Gun lamp) was installed. This modification enables one to use more highly absorbing samples with improved resolution. After the spectral measurements had been obtained, the cell was withdrawn from the furnace and the melt was allowed to solidify along the walls of the container. The cell was broken open inside the helium box and the solid was removed for analysis. The amount of tellurium present was determined by a colorimetric procedure using an extraction of the
473
diethyldithiocarbamate complex into carbon tetrachloride. Knowing the tellurium content and the density of the various solvents as a function of temperature, the molar extinction coefficients, E, were calculated.
Results Spectra of LGTe in LEI-LiF. Solutions of LizTe in LiC1-LiF (80 mole % LiC1, mp 505") are burgundy red and were studied spectrophotometrically a t 525". The spectrum (Figure 1, curve A) is characterized by an absorption band with a maximum at 21,250 cm-1 and a half-width of 7000 cm-l. To calculate the oscillator strength of the transition, the band envelope a t energies less than 21,250 cm-I was symmetrically reproduced at energies greater than 21,250 cm-l. The edge of what is presumed to be a charge-transfer band was obtained by subtraction of the dashed bandenvelope curve from the measured absorption in the 22,000-29,000-~m-~ region. Three different LizTe concentrations were studied. At 0.07, 0.10, and 0.13 M Li2Te concentrations, the molar absorptivities a t 21,250 cm.-l were found to be 25.1, 24.0, and 33.4, respectively. The major uncertainty in these measurements is in the analyses for the LizTe content of the melts. The differences in the values of the molar absorptivities of the three solutions are within the limits of error of the measurements and are probably not an indication of a breakdown of Beer's law. The molar absorptivities given in curve A (Figure 1) represent an average over the three determinations, and lead, by integration over the wavelength range 30,000 to 12,500 cm-I, to an oscillator strength f = 0.86 X for the transition. The low oscillator strength of the band implies that it is due to a forbidden transition. Spectra of CszTe in CsC1. Solutions of Cs2Te in molten CsCl also are red in color. The absorption spectrum of such a solution obtained a t 675" (Figure 2) is characterized by an absorption band with a maximum a t 18,200 cm-l, a half-width of 7400 cm-l, and a molar absorptivity a t the maximum of 581. By symmetrically reproducing the band envelope on the high-energy side of the maximum (dashed curve) and integrating from 9000 to 27,500 cm-l, the oscillator strength of the transition was found to be f = 2.0 X lod2. Although about 23 times as intense as the analogous transition in the LizTe solutions, the low oscillator strength of the CszTe absorption band strongly implies that it too originates from a forbidden transition. (6)
D.M. Gruen and R. L.McBeth, J. Phys. Chem., 66, 57
(1962).
Volume 70,Number 8 February 1966
D. M. GRUEN,R. L. MCBETH,M. S. FOSTER, AND C. E. CROUTHAMEL
474
- 1.8 - 1.6 - 1.4 -125
0
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30
28
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26
24
22
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1
20
18
16
14
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Figure 1. Curve A, L&Tein LiC1-LiF a t 525'; curve B, LitTe in LiCl at 650".
t
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w 700
Figure 2. CszTe in CsCl at 675".
Experiments that were performed early in this investigation gave spectra which appeared to change with changing CszTe concentration. In particular at low initial CszTe concentrations, an entirely different spectrum was obtained which later measurements showed to be the spectrum of Te metal (see below). The cause of the difficulty was traced to an impurity present in the CsCl used as a solvent in the early experiments. Although the impurity was not identified, it, reacted with CszTe to give Te metal. At high CszTe concentrations, all of the impurity was removed in this way but since CszTe was present in large excess, its spectrum could be measured by using short pathlength cells. At low Cs2Te concentrations only the Te metal spectrum was observed. However, in later experiments using the high-purity CsCl (Atomergic The Journal of Physical Chemistry
Chemetals material) as a solvent, we were able to measure the CssTe spectrum at the M concentration level. The spectrum shown in Figure 2 was obtained on a solution of CszTe in high-purity CsC1. Spectra of L&Te in CsCE and of CszTe in LiCl-LiF. The possibility that the alkali metal tellurides do not exchange with solvent cations cannot be ruled out a priori. To test this admittedly remote possibility, spectra of solutions of Li2Te in CsCl and of CszTe in LiC1-LiF were measured. The spectra were identical with those obtained on solutions of Li2Te in LiC1LiF and of CszTe in CsC1, respectively. It was thus demonstrated that the overwhelming concentration of solvent cations causes replacement of the initial alkali metal constituent and ensures that the alkali metal nearest neighbors surrounding tellurium are derived from the cations of the solvent. The nature of the solute species will be more fully discussed in a later section. Efects of Temperature on the Absorption Spectra of the Alkali Metal Tellurides. Effects of temperature on the telluride spectra were erst encountered in a study of a solution of LizTe in LiC1-KC1 eutectic and are graphically represented in Figure 3. At 400", the eutectic melt has the typical red color of the telluride solutions and the spectrum (curve A, Figure 3) has a band maximum at 20,400 cm-I. Raising the temperature to 600" has the effect of shifting the band energy to the slightly lower value, 20,000 cm-*. The edge of the charge-transfer band, however, shifts -2500 cm-l to lower energies (curve B). The effect on the spectrum of going to 800" is shown in curve C. The absorption intensity has dropped and a broad band with considerable absorption intensity in the 12,000-17,000-~m-~region has developed. This band is quite similar to the absorption spectrum of Te metal (see below). Indeed, on raising the temperature to lOOO", Te metal distilled out of the solution depositing in the unheated portion of the quartz tube. The remaining liquid had the spectrum shown in curve D. The low absorption intensity remaining in the visible region indicated that the Li2Te solute had been removed from solution. After cooling the melt and washing it out of the optical cell, examination of the quartz surface showed that it had been etched. The sequence of events deduced to explain these spectral changes can be summarized as follows. In the 400-700" region, the telluride solution in LiC1-KC1 eutectic is stable with respect to reaction with the quartz container. Beginning at about 800", the telluride solute species reacts with the quartz of the container liberating Te metal which then volatilizes out of the solution at the still higher temperature of 1000".
ABSORPTION SPECTRA OF ALKALIMETALTELLURIDES
475
at 15,500 and 20,800 cm-' is suggested by t@edashed curves. Subtraction of these dashed curves from the measured absorption band yields the edge (dotted) of a very intense absorption band. In the course of this work, Te metal was equilibrated not only with molten CsCI, but also with molten LiCl and KCI. The saturation solubilities in these alkali halides were found from subsequent analyses for Te metal content to be 1.28 X wt % in CsCl at 671"; 3.5 X 10+ wt % in KC1 at 802"; less than 5 X low4wt % in LiCl at 627", the lower limit of detection of Te by our analytical method.
Discussion l
30
l 28
l
26
f 24
l 22
l 20
i l I8 16 cm-' x IO-'
l 14
l
12
l 10
i 8
i 6
l 4
Figure 3. LhTe in LiC1-KC1 eutectic: A, 400'; B, 600'; C, 800"; D, 1000'.
At 800", the solubility of Te metal in KC1 is responsible for the increased absorption in the 12,000-17,000cm-I region. Attempts to obtain the spectra of NazTe in molten NaCl and of KzTe in molten KC1 were only partially successful presumably because of reaction of the tellurides with the quartz a t the lowest temperature (-800") at which measurements can be performed on these liquids. A qualitative spectrum of LizTe in molten LiCl at 650" (curve B, Figure 1) was obtained. Etching and reaction with the cell wall occurred at a lower temperature in this solvent because of the more corrosive nature of pure LiCl compared with LiC1-KCI eutectic. Spectrum of Tellurium Metal in CsCl. In the course of this work, EM has been mentioned, similar spectra were obtained on telluride solutions whose composition had been altered either by reaction with an impurity in the solvent (CszTe in CsCl) or by reaction with the quartz container (LizTe in LiC1-KC1 above 800"). It seemed likely that the new spectra were due to Te metal produced as a product of the reactions. To test this hypothesis, molten CsCl was equilibrated with Te metal, and the spectrum of the resultant blue solution was measured at 675". The spectrum shown in Figure 4 was iqdeed found to be virtually identical with those obtained on the telluride solutions which had undergone chepical reactions, thus lending support to the hypothesis. The Te metal spectrum (Figure 4) is characterized by a broad complex absorption band stretching from 27,000 cm-1 to about 10,OOO cm-1 and having an oscillator strength of 3.9 X 10-2. A resolution of this complex band into two overlapping bands with maxima
Absorption Spectra of LGTe in LiCl-LiF and of Cs2Te in CsCl. In attempting an interpretation of the alkali metal telluride spectra, we are cognizant of the fact that it is based on the observation of a single absorption band per spectrum. It is nonetheless instructive to consider a very simple model which takes as its point of departure the forbiddenness of the 21,250cm-I transition of LizTe df = 0.86 X and of the 18,20O-~m-~ transition of CszTe (f = 2.00 X The compound LizTe has the antifluorite structure with 4 : 8 coordination such that each tellurium is surrounded by eight lithiums at the corners of a cube.' It is likely that CssTe has the anti-CdCL structure by analogy with the structure of Cs20. In that event, each tellurium would be surrounded by six cesiums at the corners of an octahedron. On the assumption that similar coordination conditions prevail in the melts, the solution species could be formulated as
'"ti UI
i
p\
t\
h ?
I
700
IO0 30
28
26
24
22
20
18 16 cm" x IO-'
14
I2
IO
8
6
4
Figure 4. Tellurium metal in C&l at 675".
(7) A. F. Wells,"Structural Inorganic Chemistry," The Clarendon Press, Oxford, 1962, p 459.
Volume 70, Number 2 February 1966
D. M. GRUEN,R. L. &BETH, M. S. FOSTER, and C. E. CROUTHAMEL
476
[LigTe] and [CsaTe], respectively. I n the molten salt solutions, the second-nearest neighbors would, of course, not be tellurium atoms as in crystalline LizTeand Cs2Te, but rather chloride ions or a mixture of chloride and fluoride ions. The disruption of the band structure of the solid tellurides which occurs on dissolution presumably makes possible the observation of the discrete absorption spectra of the solvated species. The salt-like nature of LizTe and CszTe strongly implies that the electronic configuration of the telluride ion in the fused salt solutions is [Kr]4d1°5s25p6. The excitation of an electron from one of the 5p tellurium orbitals to a 2s lithium or 6s cesium orbital corresponds to a charge-transfer process possibly represented by the absorption edges of very intense bands occurring at energies greater than 24,000 cm-l. (The charge-transfer bands of the pure alkali halide solvents are known to occur a t considerably higher energies, 35,000-45,000 cm-l.) The low oscillator strengths of the 21,250-cm-’ LizTe and the 18,200-cm-l Cs2Te bands suggest that they are due to spin and LaPorte forbidden singlettriplet transitions corresponding to the excitation of an electron from one of the 5p tellurium orbitals to an essentially atom-like 6p orbital also centered on tellurium. To examine the reasonableness of the assignment of the bands to 5p6 + 5p56p transitions, we have compared a series of atomic transition energies in this region of the periodic table (Figure 5). The energies of the transitions from the ground state of the neutral atoms or ions to their lowest J level was taken from a compilation of “Atomic Energy Levels.”s A least-squares fit of the experimental points gave three series of lines, each of which fit the enipirical equation
E
=
[uN+ bZ - c] X
lo4 cm-’
(1)
where a, b, and c are constants, E = energy of the transition in cm-1, N = degree of ionization, and Z = atomic number. The experimental points for the 5pk + 5pk-15d transition have been omitted, but all the experimental points agree with the calculated lines t o within f5%. Both the 5pk+ 5pkT15dand 5pk+ 5pk-16s transitions are LaPorte allowed and can have large oscillator strengths as compared with the low oscillator strengths of the forbidden 5 p k + 5pk-16p transitions. As can be seen, the energies of all the transitions diminish as the degree of ionization becomes more negative. I n addition, the relative positions of the three transition energy series change in such a way that the 5pk -+ 5pk-16p transition which has the highest T h e Journal of Physical Chemistry
150
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I
yl’
’
,
,
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125
TRANSITION
EXPERIMENTAL
CbLCLlLATEO
5pk-5pX-’5d
POINTS OMITTED
---------
Jpx-5pk-’ 61 5pk--5pk-I6p
*
’ ~
iI
-I
I
I
I 25
-
I ,
O1
de i e Sb +2 +2 r2 +2
, , , , , , / . I ds $e I Te Sb Sn xe Te Sb i n I n +i +I +I +I t i tl 0 0 0 0 0 0 ELEMENTS and OXiDbTlCN STATES
i
I
,
,
,
de I Te Sb Sn -1
-1
-1
-1
-1
Figure 5. Energies of atomic transitions in various degrees of ionization.
energy for the +2 ions tends to become the lowest energy transition in the (extrapolated) region of single negatively charged ions. From the experimentally observed transition energies in the fused salt solutions, one can calculate the formal charge residing on tellurium. Substituting the energies of the transitions 21,250 and 18,200 cm-1 into eq 1 and with a = 3.44, b = 0.96, c = 44.1, and Z = 52, one obtains for N , the formal charge on tellurium, the values -1.1 and -1.2 in the case of the lithium and cesium telluride solutions, respectively. The large difference in electronegativity between the alkali metals and tellurium together with a consideration of Pauling’s electroneutrality principle make these calculated values of the formal charges on tellurium reasonable ones. Absorption Spectrum of Tellurium in CsCl. The absorption spectrum of Te metal in molten CsCl (Figure 4) is seen to consist of two overlapping bands with maxima a t 15,500 and 20,800 cm-’. The spectrum bears a qualitative resemblance to that, of Te2 vapor although the melt spectrum does not display the resolved vibronic structure seen in the v a p ~ r . ~ J ~
Acknowledgments. The authors wish to express their thanks to Mrs. Gene McCloud, who prepared the various alkali metal tellurides and molten salt solutions used in this work. Thanks are also due D. J. (8) “Atomic Energy Levels,” Vol. 111, National Bureau of Standards Circular 467,May 1, 1958. (9) M. Desirant and A. Minne, Compt. Rend., 202, 1272 (1936). (10) N. D.Prasad and P. T. Rao, I n d i a n J . Phys., 28, 549 (1954).
REACTION OF OZONE WITH PERFLUOROOLEFINS
Santelli and V. Lemke, members of the Analytical Group of th3 Chemical Engineering Division under the
477
direction of R. P. Larson, who performed the tellurium analysis.
The Reaction of Ozone with Perfluoroolefinsl
by Julian Heicklen Aeroepace Corporation, El Segundo, California (Received August 10,1966)
-
Ozone reacts with C2F4, CaF6, and C4Fs-2 at room temperature to give CF20, CFaCFO, and O2 as products. No other products were found, and, for the C2F4-03system, both CFaCFO and CF2CF20were definitely absent. The ratio of CF20 to CF3CF0 is invariant to conditions and is 4.0 and 2.8, respectively, for the CaF6 and the C4Fssystems. A reaction mechanism is presented. For the reaction R2 O3 --+ R203where R is either CF2 or CF3CF, the rate constants are 81 X lo3, 13 X lo3, and 1.1 X lo3 M-’ sec-’, respectively, for the C2F4, C3F6, and C4Fs systems. At low pressures in the C4Fs system, the rate expression becomes second order in C4Fs; this is explained in terms of the competition between the reactions R2Oa -t R2 0 3 and R203 R2 + 3 R 0 R.
+
+
+
+
I. Introduction
11. Experimental Section
The reaction of ozone with hydrocarbon olefins has been studied extensively. The results have been reviewed and summarized by Leighton2 in his excellent book. The reactions are complex, and many products are formed. However, through several investig a t i o n ~ ~the - ~ relative rate constants of some of the simpler olefins at room temperature have been obtained. The most extensive set of rate constants was obtained by Vrbagki and Cvetanovi6,6 who found values of 1.8 X lo3, 5.1 X loa, and about 15 X loa M-l sec-l for C2H4, C3H6, and C4Hs-2, respectively, based on the Cadle and Schadt value of 1.8 X lo3 M-l sec-l for C2H4. The more recent work of Wei and Cvetanovi? slightly alters the values for CBH6 and C4H8-2to 8.1 X lo3 and about 18 X lo3 M-l sec-’, respectively. As part of a continuing program of fluorocarbon oxidations, we have investigated the reactions of ozone with C2F4, C3Fe, and C4Fs-2. Those results are reported here.
A mixture of cis- and trans-perfluorobutene-2 obtained from the Matheson Co. and perfluoropropene obtained from Peninsular ChemResearch, Inc., were used without further purification. Gas chromatograms showed no impurity peaks for C3F6, but C4Fe had one impurity peak of about 1.5% in the C4-Cs fluorocarbon region. The C2F4 was prepared by slowly adding 1,2-C2F4Br2to a mixture of zinc in methanol (1) This work was supported by the U. 8. Air Force under Contract No. AF 04(695)-469. (2) P. A. Leighton, “Photochemistry of Air Pollution,” Academic Press Inc., New York, N. Y., 1961. (3) R. D.Cadle and C. Schadt, J. Am. Chem. Sac., 74,’ 6002 (1952). (4)R. D. Cadle and C. Schadt, J. Chem. Phys., 2 1 , 163 (1953). (5) P. L. Hanst, E. R. Stephens, W. E. Scott, and R. C. Doers, “Atmospheric Ozone-Olefin Reactions,” The Franklin Institute, Philadelphia, Pa., 1958. (6) T. Vrbas’ki and R. J. Cvetanovi;, Can. J. Chem., 38, 1053,1063 (1960). (7)Y. K.Wei and R. J. Cvetanovi;, ibid., 41, 913 (1963).
Volume 70, Number 1 February 1966
