Communicationsto the Editor
1834
ATIONS TO THE EDIT
On the Existence of Associated Species in Lanthanum (! I I) Chloride-Potassium Chloride Melts'
Publication cosI's assisted by the Atomic Energy Commission and the N a t i ~ n aScience l Foundation M=MOLE 7- LOCI,
Sir: Recently thermodynamic studies2 of molten lanthanum (111)chloride-potassium chloride mixtures by calorimetric and electroma'tive-force methods indicated the presence of associated species involving lanthanum(II1) ions and chloride ions. In accord with our continuing interest in the structure of binary charge-unsymmetrical fused-salt system^,^ ,* we have undertaken an investigation of the Raman speciaa of il series of LaC13-KC1 melts to establish the presence of complex species in these melts and, if possible, to characterize them from a structural viewpoint. The sources and methods of purification of the potassium chloride andL lanthanum(II1) chloride were the same as those described previously.2 The apparatus and general technique uc+ed to obtain the Raman spectra have also been described el~eivhere.~-$ Raman spectra of LaCl3-KCl melts were recorded for seven compositions in the range from 10 to 65 mol % La&. Spectra rec'xded on the Stokes side of the exciting line contained an a,ppreciably enhanced background in the region of the Raman scattering (50-300 cm-I). This background scattering, which persisted to several thousand cm- I beyond the exciting line, is presumably due to fluorescence from trace amounts of a rare earth impurity originally present in the LaQ used for the preparation of La&. Spectra recorded on the anti-Stokes side of the exciting line contained less of this background; as a result, the region of the Ramaii scattering on the anti-Stokes side was less obscured than on the Stokes side. A4nti-Stokt!sRaman spect,ra recorded to 700" for samples containing 10 to 53.3 mol % LaCI3 and just above the melting poinit (-730") for a sample containing 65 mol % Lac13 are shown in Figure 1. The spectral changes that take place over the composition range investigated indicate the presence of a complex equilibrium, but the number of species involved cannot be determined reliably from a simple inspection of the data in Figure 1. Studies of the depolarization characteristics of these spectra show that the band w'hich peaks near 250 cm-I for the mixtures containing from 10 to 33.7 mol % Lac13 is strongly polarized; hence, the complex from which it originates must be highly symn:ietriczJ. Furthermore, this band appears to reach maximum intensity in the range from 20 to 25 mol % IaaC13, in consonance with the observation of Papatheodarou and Ostvold2 that the interaction parameter, Am [An1 = AHm/,Y~(L - X i ) 9 AHm = enthalpy of mixing and Xi = mole fraction of component i], determined from mensurem.ents of the enthalpy of mixing, reaches a minimum in the range from 15 to 25 mol % La&. The Journal of Physical Cnemistry. Vol. 78. No 11. 1974
M=IOG I
800
I
600 400 200 u, cm'' (ANTI-STOKES1
Figure 1. Raman spectra of LaC13-KCI melts recorded with the exciting radiation polarized perpendicular to the direction of observation. The curves for M = 10 through 53.3 were recorded at 700". The curve for M = 65 was recorded just above its melting point (-730'): spectral slit width -4-15 c m - l , time constant = 3 sec, scan rate = 15 cm-'/min.
An attempt was made to examine the Raman data recorded for these LaC13-KCl melts by curve-resolution techniques of the type used in our previous study3 of SnClZ-KCl melts; however, the present examination was complicated by the observed fluorescence, which created difficulties in the determination of a reliable baseline. Even the anti-Stokes data consistently showed a residual background extending almost to 2000 cm -1. Compensation for this background was made difficult by an apparently large asymmetry of the fluorescence in the region of the exciting line. As a result, the curve-resolution data contained larger overall errors (by a factor of two or more) than were experienced in our previous study3 of the SnCla-KCl system. For the most part, these errors arose because of poor fitting a t the extremities of the extrapolated Rarnan curves, which in our experience is a characteristic consequence of baseline inaccuracies. Although the results of the curve-resolution analyses contained appreciable uncertainties, several reasonably accurate observations could be made. The resolved spectra consistently showed a strongly polarized band a t 250 f
Communications to the Editor
1135
4 cm-I and two bands of roughly equal integrated intensity in the range from 40 to 200 cm-I. (The computed peak positions of the latter two bands were nominally 147 f 15 and 90 f 10 CXTI-~.) There was some question regarding the states of polarization of the latter two bands, although in mmt of the calculated spectra both appeared to be totally depolarized. ased on results obtained thus far in our studies of LaCl3-containing melts, the following conclusions can be made. The spectral data clearly indicate the presence of at least one h:ghly symmetrical, associated species in LaCls-KC1 me1 ts. The maximum concentration of this species appears to occur near 25 mol '70 La&, but in the absence of interiaal intensity standards, this observation is not firmly established1 Im general, these results confirm the contention of Papatheodorou and Ostvold2 that discrete complex spxies exist in LaCla-KCl melts.
(1) Work performed, under auspices of the U. S. Atomic Energy Commission. (2) 6 . N. Papalhecrdorou and T. Ostvold, J. Phys. Chem., 7 8 , 181 (1974). (3) E. J . Hathaway and ' J . A . Maroni, J. Phys. Chem., 7 5 , 2796 (1972). (4) V. A. Maroni. E. J. Haithaway, and E. J. Cairns, J. Phys. Chem., 75, 155 (197'1). (5) V. A. Maroni and P 1'. Cunningham, Appl. Spectrosc., 27, 428 (1973). (6) V. A . Maroni and E J. Cairns in "Molten Salts: Characterization and Analysis," G Mamanto,d. Ed., Marcel Dekker, New York, N . Y . , 1969, pp 256-.263
Victor A. Maroni* Ellen J. Hathaway
Chemical Engineering Division Argonne Nati:onal k b o r a t o r y Argonne, Illinois 60439 The James Franck lnstitule University of Chicago Chicago, iilinois 60637
G. N. Papatheodorou
Received February 4 , 1974
Reversible Trap to arbanion Electron Transfer in y-lrradiated HydrocairbonGlasses' Publicatiori costs assi.sted by [he U . S. Atomic Energy Commission
Sir: y -Irradiation of hydrocarbon glasses produces trapped electrons, radicals, andl cations.2 Experiments3 in which the et,- ir spectrum and photoconductivity4 are regenerated by 375-nm light after complete bleaching by ir light imply formation and subsequent photoionization of carbanions (or, less probably, impurity anions) (Re + eR:- ; R:- + hv ---*R - -+- e-). Thus, anion formation and the retrapping pjrocess compete with the cations for mobile electrons. We report here (Table I) experiments which show that this competition is maintained through at least seven ir-av bleaching cycles, and does not change markedly over a 12-fold range of dose and radical concentration. The probabiiity for a, trapped electron to escape neutralization in the first ir-uv cycle is not revealed by the data of Table I, because the yield of anions produced during y-irradiation and present before the first ir illumina-
-
TABLE I: Yields of Reversible Transfer of Electrons from Trapped State to Carbanion State in ?-Irradiated 3MP and 3EP Glasses. Dose, eV g-1 X 10-1s 2.'7
CYele
2.7
1.6
4.8
4.8
3.2
3.2
11
20
20
% of eti- from y-irradiation which remain following ir-uv bleaching cycles i _ _ _ l l _ _ _ _ _ _ _ _
0 100 100 100 100 100 100 100 100 100 1 33 33 37 30 31 38 32 35 35 2 15 15 9 12 17 1 7 15 3 12 11 Ei 4 9 7 4 5 7 5 6 6 5 n 5 5
100 37
a Columns 2 and 3, 3MP a t 72'K; all other columns for 3EP at 77'K. Uv illumination for columns 2, 3, and 4 was at 360 n m with high-intensity Bausch and Lomb monochromator with HBQ 200-W mercury lamp and
Corning 7-37 filter; other uv illuminations were filtered light (Corning 7-37) from a n A H 4 medium-pressure mercury lamp. Ir illuminations were made with a tungsten light and Corning 7-56 filter. Yields assumed proportiorral to height of the etr- esr singlet a t 4 IW. T h e 3MP and 3EP were purified by passage through silica gel degassing on the vacuum line and storage over a sodium mirror.
tion is not known. Attempts to determine this yield by measuring the growth of the esr singlet of et,- during uv irradiation of a freshly y-irradiated sample a t various wavelengths from 320 to 375 nm have been unsuccessfuul. A decrease rather than an increase in the signal height always occurs, implying that the ir spectvuni of et,- has a tail extending into the uv (unless these wavelengths are able to activate cations to react with electrons; or photoinduced fluorescence from the Suprasil tube detraps electrons). There is no effect of dose on the percentage yields of the eV ir-uv bleaching cycles from 1.6 x 1019 t3 20 X g-1, within the experimental accuracy (Table 1). 'This i s of particular interest since it is k n o ~ n 3 2that ~ the et,Concentration rises to a maximum a t ca. i x 1 P eV 8-l and then falls, approaching zero a t ca. 3 X IOz0 e'd g-I9 while the concentration of radicals continues to increase. The constancy of the et,- regeneration yields with dose implies that the ratio of the concentration of radicals to that of cations changes with dose in a way such. that the fractional loss o f electrons to trapping during the ir-uv cycles remains constant. Neither the concentration of cations ([M+] = [et,-] + [R:-]) nor of carbanions can, at present, be measured directly, but the concentration of cations must be as large or larger than that of e r r - . The concentrations of etr- a t the maxima of the concentration u s . y-dose curves are ca. 3 x 10-5 and 1 x 10--5mole fraction in 3MP and 3EP, respectively, as estimated from earlier data,3 assuming G(e- ) initial in 3MP = 0.7,2 The concentration of radicals is 4 x mole fraction, assuming G(R.1 = 3.0 The regeneration of et,- by photoionization of carbanions allows evaluation of the R:- decay ra.te. Samples of 3MP bleached with ir immediately after a y dose of 2.7 X 1019 eV 8-1 at 72°K were exposed a t 72°K to a mediumpressure Hg arc with 7.4 mm of Corning 7-37 filter after 0, 30, 90, 240, and 2900 min of standing a t 77°K. The yields of et,- relative to the population before the first ir bleaching were 33, 27, 22, 17, and 16%, respectively. The decay of ca. 50% in the first 4 hr and only a few per cent in the next 44 hr is similar to that of 3-methylpentyl radicals under the same conditions,e implying that both intraspur and random diffusive combinations occur at about the The Journal of Physical Chemistry. Vol 78. No. 1 7 . 1974
