April 5 , 1954
NEW COhlPOVXDS
OF
QUADRIVALENT A\MERICIUhI
[ C O N T R I B U T I O S FROM CNIVERSITY OF CALIFORNIA, LOS
ALAMOSSCIENTIFIC
2019
LABORATORY]
New Compounds of Quadrivalent Americium, AmF,, KAmF,l BY L. B. ASPREY RECEIVED NOVEMBER 23, 1953 Two new compounds, KAmFj and AmF4, were prepared by reaction of fluorine gas with Am(III), Am(1V) or A m ( V ) compounds. The fluorinations proceed readily at 500" and one atmosphere of fluorine.
The known oxidation states of americium are ( I V ) , 2 a (V)2b and (VI),3 of which (111), (V) and (VI) have ,been found in solution. The only compound of Am(1V) known prior to this work has been the dioxide, Am02.2a The composition of this compound was found to be AmOl 98 0.02.5 ,4t the time this work was carried out, thermal data derived from the heat of solution of AmOa indicated t h a t the free energy for the reaction 1/2F2(g) = A m S ( c ) was -10.5 dmFa(c) kcal./mole a t 298'K., or -0.5 kcal./mole at 1000° K.6 More recent work indicates that the free energy of the above reaction may be considered more negative, of the order of -22 kcal./mole a t 1000°~.7 Similar calculations indicate that AmF6 may be capable of existence at 298°K. and one atmosphere of fluorine gas, b u t not a t much higher temperatures. Since the values used in the calculations are obtained indirectly, the successful production of AmF4would serve as a check on the general validity and order of magnitude of these thermodynamic quantities. Previous attempts t o prepare AmF4 by treatment of the Xm(I1I) fluoride with fluorine gas a t elevated temperatures yielded no tetrafluoride. * (III),2a
+
Experimental Purity of the Americium Stock.-The americium stock solutions used in this study were purified by J . P. Nigon. Spectrochemical analysis of this stock by Oliver Simi of this Laboratory showed no amounts of cations above the limits of detection. Preparation of Starting Materials .-Americium dioxide was prepared by precipitation of the Am(II1) oxalate from a solution 0.1 M in nitric acid and 0.1 ,M in ammonium oxalate. This oxalate was heated in a stream of oxygen to350' for one hour to convert t o black AmOz. Americium trifluoride was prepared by precipitation from a solution 1 M in nitric acid and 2 M in hydrogen fluoride. This precipitate was washed with water, then with acetone and dried on a water-bath a t 85". The material prepared in this way appears t o be much more reactive than t h a t prepared by treatment with gaseous hydrogen fluoride at 600700". The preparation of NaAmO?(C2H3O2)?was carried out in the manner previously d e ~ c r i b e d . ~The compound was washed with acetone and air-dried. (1) This work was sponsored by the AEC. (2) (a) B. B. Cunningham and L. B. Asprey, AECD-2946 (1950); see also B. B. Cunningham, "The Transuranium Elements," National Xuclear Energy Series, McGraw-Hill Book Co., Inc., N. Y.,1949, Vol. 14B, Part 11, p. 1363-1370; (b) L. B. Werner and I. Perlman, THIS JOURNAL, 79, 495 (1951); see also AECD-2898 (1950). (3) L. B. Asprey, S. E. Stephanou and R. A. Penneman, ibid., 75,
5715 (1951). (4) First identified by W. H. Zachariasen from X-ray diffraction studies. (5) L. B. Asprey, UCRL-329 (1949). (6) L. Eyring, H. R. Lohr and B. B. Cunningham, THIS JOURNAL,
74, 1186 (1952).
(7) Private communication from B. B. Cunningham. (5) S. Pried, TIIISJOI:RNAT.. 73, 416 (1951)
The solid Am( I-) compound was prepared by oxidation with SaOCl in saturated K 2 C 0 350lution.~ This compound was washed with water until partial peptization occurred, then twice with acetone and dried on a water-bath. Tank fluorine was used with no further purification. Apparatus.-The fluorination was carried out in a 1 inch i.d. nickel tube 12 inches in length. The inlet end of this tube was welded to a 3 inch length of 0.25 inch nickel tubing which, in turn, was silver soldered to the copper tubing supplying the gases used. A brass flange with screw threads was silver soldered t o the exit end of the tube. A threaded brass cap with a Teflon gasket was used t o close the end of the tube, and a small Hoke valve was connected t o this t o allow sealing off of the system. The sample, in the form of a powder, was placed in a small nickel holder to which was affixed a long nickel wire as a handle. This container was inserted through the exit end of the reaction tube to the heated zone near the inlet. The nickel reaction tube was heated a t the inlet end by means of a 1 inch nichrome heating element. The temperature was measured by means of a Cr-A1 thermocouple between the heating element and the nickel tube. A Micro-max recorder-controller was used to control the temperature. The connecting tubing was of copper with either flared or silver soldered connections. Small brass Hoke diaphragm needle valves controlled gas flow, except for fluorine, where Monel valves with Teflon seats were used. A Welch DuoSeal mechanical pump without trap was used t o establish a vacuum in the system. The pressure was read by means of a thermocouple gage. Procedure.-The substance to be fluorinated was placed in the nickel sample holder and this placed in the nickel reaction tube. The system was then evacuated t o 20 p or less, fluorine gas added to 1 atmosphere arid allowed to flow through the apparatus :it about 3 cc. per minute, while the temperature was raised to 500' over a period of about 0.5 hour. The heating was continued for one to two hours, after which the tube was allowed to cool slowly in the stream of fluorine. Argon was added and allowed t o flow through rapidly for 10-20 minutes. The system was then opened and the sample removed.
Results X-Ray powder patterns of the product of fluorination of either AmF3 or 9 m 0 2 show i t to be isomorphous with UF4, NpF4 and PuF4.10z11 I t is monoclinic with a0 = 12.47 kX., bo = 10.45 kX., co = 8.1.8 kX., /3 = 126 .'1 The salt is microcrystalline, tan in color, and in large aggregates has an orange cast. No single crystals exceeding 3 have been made. I n common with UF4l2 and PuFI,*"the compound is biaxial negative with a moderate optic axial angle. The refractive indices are slightly higher than those of the corresponding PuF4. Upon addition of water to the compound, ther- is a slow evolution of gas, and the tetrafluoride is converted into very fine-grained birefringent aggregates which give the characteristic absorption spectrum of Am(1II).
*
(9) J P. Piigon, R. A. Penneman, E. Staritzky, T . K. Keenan and L. B Asprey, J . P h y s . Chem., in press. (10) W. H. Zachariasen, MDDC-1152 (1947). ( 1 1) XX'. H. Zachariasen, "The Transuranium Elements," National Nuclear Energy Series, hIcGraw-Hill Book Co., A-ew York. N. Y., 194!J, Vu1 14B, Part 11, pp. 1462-1472. (12) 13. Stnritzky, t o lie p i i l > i i ~ i i u sli
