NOTES
Nov., 1963
80
0'
sewed for all mixtures up to 50 mole % p-chlorobenzonitrile, but is not observable for higher concentrations. The incongruent melting point of the complex has been observed for mixtures of 80 to 47 mole % p-chlorobenxonitrile. The diagram clearly shows the occurrence of a complex at 50 mole yo,with an incongruent melting point of 41.5", and a probable metastable melting point of 47'. Comparison of this diagram with that of the chloroform-benzonitrile system indicates that the stability of the dimethyl sulfoxide-nitrile complex is comparable t o that of the chloroform-nitrile ~ o m p l e x . ~ We feel that this phase study provides exceptiona,lly strong support of our earlier hypotheses. The implication is clear that physical and chemical properties of solutes in such aprotic solvents as dimethyl sulfoxide will be strongly affected by specific solvent-solute interactions.
I
6 0 -
.
0
'40)
0
.20 MOL
Fig. 1.-Phase
2499
.40
-60
FRACTION
.80
1.0
(7) J. E. Rioci, "The Phase Rule and Heterogeneous Equilibrium," D. Van Noatrand Co., Inc., New York, N. Y., 1951, pp. 128-129.
PCN.
diagram for the system: p-chlorobenzonitrile (PCN)-dimethylsulfoxide.
Although we felt that this interpretation was well supported by the data, and moreover, was in excellent agreement with the results of other s t u d i e ~ , we ~,~ recognized the desirability of more concrete evidence for the existence of the postulated complexes. I n order to provide this evidence, v e have carried out a phase study of the p-chlorobenzonitrile-dimethylsulfoxide system. Experimental The dimethyl sulfoxide used was Baker's A.R. grade material and was purified by fractional freezing prior to each experiment. The p-chlorobenzonitrile was Eastman White Label material. Cooling curves of various mixtures of the two materials were obtained with a Beckmann-type freezing point apparatus. The apparatus was fitted with an enclosed glass stirrer and a temperature measuring device and was immersed in an ice-salt bath. The container was insulated so that a suitably slow cooling rate was obtained. The temperature measuring device was in most instances a Sargeant thermistor and bridge attached to a 10-mv. potentiometer. Due to the somewhat limited range of the thermistor, the higher temperatureii were measured with a mercury thermometer, and the cooling curve was plotted manually. I n a few cases, supercooling occurred a t the first temperature break on the cooling curve, but such supercooling rarely exceeded 0 2". I n the majority of cases, no supercooling wm observable. Additional confirmation of the freezing point of the 70 mole % p-chlorobenzonitrile mixture was obtained by capillary melting point observation of the solid formed on freezing. The first liquid was apparent a t 45", and the material was completely liquid a t about 75'.
Results and Discussion The phase diagram constructed from the cooling curves6 is shown in Fig. 1. This diagram is of the same general nature as the freezing point diagram for the chloroform-benzonitrile system which has been reported by Muirray and Schneider.6 The diagram shown in Fig. 1 is, however, more complete than a simple freezing point diagram. The eutectic for the phases; dimethyl sulfoxide : complex (solid), liquid has been ob(3) C. D . Ritehie and E. 8. Lewis, J . Ana Chem Soc., 84, 591 (1962). (4) R. W. Taft, Jr., E. Price, I. R. Fox, 1. C. Lewis, K. K. Andersen, and G . T . Davis, ztizd., 85, 709 (1963). (5) B. J. Mair, Q. R. Glassow, and F. D . Rossini, J. Res. Natl. Bur. Std., 26, 591 (1941). (6) F. E. Murray and W. 1%. Schneider, Can. J. Chen., 38, 797 (1955).
T H E HEATS OF FUSION OF SOME RARE EARTH METAL HALIDES BY A. S. DWORKIN AND M. A. BREDIG Chemistry Diviszon, Oak Rzdge Natzonal Laboratory,1 Oak Rzdge, Tennessee Received M a y WY, 1969
We have previously measured and reported the heats of fusion of a number of lighter rare earth chlorides and iodides.2 The present report extends these measurements to other rare earth halides with a variety of crystal structures. These data are of interest in connection with our continuing program of the interpre tation of phase equilibria and electrical conductance measurements in metal-metal halide systems. The copper block drop calorimeter used for the measurements and the experimental procedure were previously described in detail.3 The rare earth halides were prepared by the reaction of the rare earth oxide with a large excess of the corresponding ammonium halide. Upon completion of the reaction, the excess ammonium halide was sublimed a t temperatures up to 500'. The rare earth halide was then sublimed under high vacuum and subsequently handled in an inert atmosphere. KOinsoluble matter or alkalinity was found on dissolving the halides in water or in alcohol. Spectrographic analysis intlicated an absence of foreign metals and less than 0.5% of other rare earth jons. Table I lists the heat and entropy of fusion, the heat content of the solid salts a t their melting points, and the heat capacities of the solid and liquid in the vicinity of the melting temperature. The heat contents were measured with a precision of 0.1-0.2% and an estimated over-all accuracy of 0.5%. The heats of fusion are believed to be accurate to a t least *1-2(% and the heat capacities to about &5%. LaBr3, PrBr3, and GdC4 (which have the UClg type structure4) and XdBr3 with the PuBra type structure4 have similar entropies of fusion to those founid2 (1) Operated by the Union Carbide Corporation for the U. S. Atomic Energy Commission, Oak Ridge, Tennessee (2) A. S. Dworkin and &I. A. Bredig, J . Phys. Chem., 6 7 , 697 (1963). (3) A. 9. Dworkin and M. A. Bredig, tbzd., 64, 269 (1960). (4) W. €3. Zaobanasen, A C ~ Cry& P X, 266 (1948).
TABLE I HEATCAPACITY AND HEATAND ENTROPY OF FUSIOXOF SOME RAREEARTHMETALHALIDES Cp solid, Tm,
LaBr, PrBs XdBrs GdBrr GdC18 HOC18 ErC18
OK.
1061 966 955 1058 875 993 1049
cal. mole-' deg.?
C p liq., cal. mole-' deg.-l
33.0 31.5 31.0 32.1 29.1 29.0 32.0
34.5 37.0 35.5 32.3 33.7 35.3 33.7
H T (solid) ~ kcal. mole-'
mole-'
ASm. e.u. mole-'
20.3 18.0 17.6 19.6 14.8 17.4 19.2
13.0 11.3 10.8 8.7 9.6 7.0 7.8
12.3 11.7 11.3 8.2 11.0 7.1 7.4
-H m ,
AHm,
ked.
for the chlorides of La, Pr, and Nd (UCL type) and the iodides of Ce and Pr (PuBr3type). On the other hand, HoCL and ErCL (YCL = A1C13 type5) and GdBr3 whose structure is not reported have unusually low entropies of fusion. h1Cl3 has a much higher entropy of fusion (18.2 e.u.), because melt'ing iiivolves transformation to a liquid that unlike the crystal is not' ionic but molecular in structure. The low entxopy of fusion of HoCL, ErC13, and GdBr3, in which the ionic radius rkla+ is greater than ?.4l8+, is in agreement with the fact that their melts are ionic like the crystals and like the melts of the lower rare earth trichlorides, LaC13, etc. The especially low value of ASiusion may, however, suggest, looking, e.g., by X-ray diffraction, for a slightly different ionic structure of the melt's, possibly including increased ion association to (MC1)2+or the like. Acknowledgment.-We are indebt'ed to R. B. Quincy for the preparation of the salts used in t'his work. (5) D.
H. Templeton and G. F. Carter, J . Phgs. Chem., 58, 940 (1954).
EFFECTS OF IMPURITIES ON THE LUMINESCENCE OF RARE EARTH CHELL4TES1 BY M. L. BHAUMIK -4ND I. R. TANNENBAUM Electro-Optical Swtems, Inc., Pasadena, California Receiaed June 9, 1969
Recently there has been a great deal of interest in the study of luminescence of rare earth chelates, primarily because of their possible use as laser materials. It has been observed during the progress of our experimental investigations that the luminescence processes in rare earth chelates, especially those parameters which govern laser operation, u i x . , the lifetime of the excited states, the spectral characteristics, the quantum efficiencies, etc., are strongly dependent on the impurities present and vary considerably with the degree of purification. These impurities usually are introduced during the synthesis of the chelates, and great care must be taken to remove them in order to obtain reliable ai2d consistent results. The effects of the impurities are illustrated in the study of the lumiiiescence in europium trisbenzoylacetonate (EuBA), europium trisdibenzoylmethide (EuDBM), and terbium trisacetylacetonate (TbA A) chelates. The extreme sensitivity of the luniiiiescence of the chelates on the impurities can be understood in the light of the origin of these emissions. The transitions giving rise to the line emission in rare earth atoms nor(1) Work supported in part by Rome Air Developmend Center under Contraot AF 80(802)-291C
mally are forbidden by Laporte's rule, but become allowed due to the mixing of energy levels produced by the surrounding perturbation field. Thus the fluorescence in the rare earth atom is determined to a great extent by its microscopic environment. The chelates were prepared according to the method of Whan, et aL2 The results of the analysis of the samples are shown in Table I . TABLE I CHEMICAL ANALYSISOF THE EUROPIUM CHELATES -Weight Sample
Eu(BA)a Theoretical Observed (pure) Observed (impure) Eu(DMB)~ Theoretical Observed (pure) Observed (impure)
percentages--
C
H
MrOa
56.70 55.61
4.25 4.78
27.80 29.30 25.01
65.80
4.02
67.04
4.85
21.40 21.80 18.06
...
.. ..
...
The emission spectra of EuBA and EuDBM consist of an intense group of lines a t about 613 nip and other weaker lines. The intense lines can be identified as the transition 5Do-7F2in europium. The peak positions due to this transition are listed in Table 11; both EuBA and EuDBM show four lines before purification and only tw.o after purification. Similar results are indicated in other transitions as well. From group theoretical considerations3 only two lines are expected in the above transition. The increased number of lines in the impure chelates may be a collection of lines arising out of different species of chelates or the impurity may, somehow, distort the symmetry of the perturbation field. In the latter case only one line is expected in the transition 6Do-7Fo occurring a t about 5801 8. Actually, more than one line is observed in this transition indicating t,hat the impure chelates consist of different species. This is also supported by the measurement of decay times. While the decay curves for the pure chelates are smooth exponential functions, those of the impure chelates seem to consist of components having different decay times. TABLE I1 PEAK POSXTIONS O F LINESDUE TO TRASSITION 'Dr'F2 EUROPIUM CHELATES Benzoyl acetonate k7ioK.,
Pure Impure
Dibenroyl methide X7T0I< , A.
A. 6125 6132 6111 6128 6140 6165
IS
Pure Impure
6130 6136 6111 6122 6133 6143
The average lifetimes of EuDBRI microcrystals before purification were measured to be about 550 and 350 psec. a t 77 and 300°K., while the corresponding values after purification are 505 and 105 psec., respectively. Terbium acetylacetonate (TbAA), also in the microcrystal form, was found to have a lifetime of 550 and 730 psec. a t 77°K. before and after purification, respectively. The efficiency of the rare earth emission after excita(2) R. E. Whan and G. A. Crosby, J . M o l . [a) M, LABhnumik, ibid., to be publidmda
Spectry., 8 ,
816 (1962),
