NOTES
934
Vapor Pressures of Solutions of Europium and
0.71
Ytterbium in Liquid Ammonia-Evidence
of Hexaammoniates
by D. S. Thompson, 11. J. Stone, and J. S. Waugh Department of Chemistry and Research Laboratory of Electronics, Massachusetts Irwtitute of Technology, Cambridge, Massachusetts 01199 (Received September 27, 1066)
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500
1000
1500
2000
TIME (min) Figure 1. Hydrogen and oxygen sorption by BeO. The experiments were made in alphabetical sequence a t 460” and 9 m m gas pressure: 4 , B, E, and F, hydrogen sorption; C and D, oxygen sorption.
However, hydrogen species bound to the surface were never observed in many attempts using infrared spectroscopic techniques such as those described else~ v h e r e . ~The transmittance of Be0 above about 10 p decreased in the presence of hydrogen, indicating an increase in the electrical conductivity and hence implying the occurrence of some electron-transfer process. When the hydrogen-treated Be0 was degassed, the transmittance increased. The failure to detect Be-H species is inconclusive and could be brought about by their absence, small number, or weak absorption. As the characteristic infrared bands of 3 and 6.1 p of water adsorbed on the Be0 surface could be readily observed, however, it is unlikely that appreciable amounts of surface hydroxyls or water were formed on hydrogen sorption. These effects, although incomplete] lead to the speculation that hydrogen sorption produced a chemisorbed protonic species of the type postulated3 to exist on ZnO, as the surface coverage was low, a charge transfer was indicated, and infrared bands of a hydroxyl species were not observed. The species was partly or wholly desorbed as water, possibly causing the formation of anion vacancies, leading to the observed progressive decline of the hydrogen sorption that could partially be reversed by oxygen sorption.
Acknowledgment. Support for this work by USAEL Contract DA36-039-AilIC-O2170(E) and KSF Grant GP 1434 is gratefully acknowledged. (3) R. P. Eischens, W. A. Pliskin, and M. J. D. Low, J . Catalysis, 1, 180 (1962).
The Journal of Physical Chemistry
Paramagnetic resonance and electronic spectroscopy indicate that both europium and ytterbium metals dissolve in liquid ammonia to give divalent cations and solvated e1ectrons.l These dark blue, paramagnetic solutions are quite similar to those obtained by dissolving alkaline earth metals in liquid ammonia. This seems to lend further credence to the observation that europium and ytterbium are actually more like alkaline earths in many respects than they are like other lanthanides.2 It has been known for many years that the alkaline earths form compounds with ammonia-the so-called metal hexaammoniates. ?(/lore recent work shows that these hexaammoniates of the alkaline earths may not be true stoichiometric compounds, for Marshall and Hunt have shown that the apparent number of ammonia molecules associated with a given alkaline earth metal ion seems to vary somewhat with temperat~re.~ We have investigated the composition dependence of the vapor pressure of ammonia over solutions of both europium and ytterbium metals in liquid amWe find that these two metals do monia a t -75.9’. behave in a manner similar to that of the alkaline earths in that both europium and ytterbium appear to form hexaammoniates. Our experimental method was similar to that used by Rlarshall and Hunt.* The sample container was so arranged that the metal samples could be introduced under an atmosphere of purified argon. The ammonia was triply distilled from potassium before use. A rotating permanent magnet, immersed in the thermostating bath below the sample bulb, both pro(1) (a) J. C. Warf and W. L. Korst, J . Phys. Chem., 60, 1590 (1956); (b) D.S. Thompson, E. E. Hazen, Jr., and J. S. Waugh, to be published; (c) D. s. Thompson, D. W. Schaefer, and J. s. Waugh, to be published. (2) T. Moeller, “The Chemistry of the Lanthanides,” Reinhold Publishing Corp., New York, N. I-., 1963,Chapter 3. (3) (a) R. C. Meutrel, Compt. Rend., 135, 790 (1902); (b) G.Roederer, ibid,,140, 1252 (1905); (e) C. A. Kraus, J . Am. Chem. SOC., 30, 653 (1908). (4) P. R. Marshall and H. Hunt, J . Phys. Chem., 60, 732 (1956).
NOTES
935
5.0r
upon warming to room temperature and !eft a gray residue which apparently contained metal and some metal amides as found by Warf and Karst.'*
Acknowledgment. This work was supported in part by the Joint Services Electronics Program under Contract DA-36-039-AllC-O3200(E) and in part by the National Science Foundation. H
- 3.0 I
Radiolysis of Tetrafluoromethane’
I
..
i? 2 .oC
I
by J. Fajer, D. R. XacKenzie, and F. W. Bloch Brookhaven National Laboratory, Upton, New York (Received October 86,1966)
i v ~~
6
~
8
IO
12 14 16 18 MOLE R A T I O ( N H 3 / Y b )
20
22
Figure 1. Composition dependence of the vapor pressure over solutions of ytterbium in liquid ammonia at -75.9’.
vided stirring of the bath and drove a Teflon-encased stirring bar in the sample solution. The solutions were made up by weight, and small known amounts of ammonia were removed to vary the concentration. Precautions were taken to check the extent of decomposition by measuring any hydrogen pressure in the sample system. The appropriate corrections to the concentration could then be made when necessary. We observed, upon gradual removal of ammonia from europium and ytterbium solutions, that the solutions turned from deep blue to bronze in color as the ratio of metal to ammonia increased. Ytterbium is less soluble than is europium. After enough ammonia had been removed that the vapor pressure over the sample went to about zero, a bronze-colored solid remained. Figure 1 shows the composition dependence of the vapor pressure over a solution of ytterbium in liquid ammonia. A similar curve was obtained for europium. The value of n. in the formula i\!t(NHs)n was obtained by extrapolat,ion to zero pressure. The values of n thus obtained for europium and ytterbium solutions are, respectively, 6.3 and 6.4. Thus the bronze-colored solid had the approximate composition &I(NH& where M is either europium or ytterbium. This solid phase gave up ammonia
We have been investigating the radiation chemistry of fluorocarbons2 because their properties make them potentially useful in the nuclear field. In the work reported here, tetrafluoromethane was chosen first because it is the simplest compound where only carbon-fluorine bonds can be attacked and second because the elusive perfluoroacetylene, C2F2, had been reported as a product in the radiolysis of perfluoroalkanes.4r5 The CF4 radiation work reported to date includes the detection of CF3 radical in the and electron’ irradiations of CF4. Colebourne and Wolfgangs studied the hot-atom chemistry of F18 with CF, and found labeled CF4 and C2Fe, due to both thermal and hotatom reactions. Reed and llailen4 found C2F6 and CzF2in the pile and y-irradiation of CF4 a t high pressure ( ~ 6 atm). 0 C2F2was also tentatively reported by Kevan and Hamlet3 as a product in the radiolysis of C2F6. They found that the “C2F2”yields increased in the presence of oxygen.
Experimental Section Irradiations were done in a CosOsource, with doses of the order of lo9 rads. The dose rate was measured (1) This work was performed under the auspices of the U. S. Atomic Energy Commission. (2) D. R. NlacKenzie, F. W. Bloch, and R. H. Wiswall, Jr., J . Phys. Chem., 69, 2526 (1965). (3) CzFz has only recently been isolated by J. Heicklen and V. Knight, ibid., 69, 2484 (1965), who prepared it by photolysis of C Z F ~ . (4) T. M. Reed and J. C. Mailen, TID 21576 (1963). (5) L. Kevan and P. Hamlet, J . Chem. Phys., 42, 2255 (1965). (6) R. E. Florin, D. W. Brown, and L. A. Wall, J . Phys. Chem., 66, 2672 (1962). (7) R. W. Fessenden and R. H. Schuler, J . Chem. Phys., 4 3 , 2704 (1965). (8) N. Colebourne and R. Wolfgang, ibid., 3 8 , 2782 (1963).
Volume 70,Number 3 March 1966
