G. W. MELLORSAND S. SENDEROFF
1110
Vol. 63
As noted earlier, all parts of the right-hand member are tixed for a given composition either by the known value of UO or by the known magnetic moment. Thus f i A can be calculated and from it, the valence and the per cent. d character. The results of these calculations are also listed below in Table I. It is rather surprising that it appears to make little difference in the values of 6 obtained whether these laborious calculations are used or whether the simple assumption is made that the valence is a linear function of composition. I n these calculations the lattice parameter data of Substitution of (8) and (9) into (7) and rearranging Owen and are used. It should be added yields that, although both Pauling and Owen and Pickup quote figures and equations in terms of Angstrom units, both works actually are in terms of kX units. (36) E. A. Owen and L. Pickup, Z. Xrisl., 88, 116 (1935).
THE PHASE DIAGRAM OF THE CERIUM-CERIUM TRICHLORIDE SYSTEM BY G. W. MELLORSAND S. SENDEROFF IZesearch Laboratories, National Carbon, Company, Division of Union Carbide Corporation, P. 0. Box 6116, Cleveland 1, Ohio Received Nouember 14, 1968
The solubility of cerium in cerium trichloride is 9.3 mole % with a temperature coefficient of nearly zero between 777 and 950'. The solubility of CeCla in cerium is not greater than 0.2 mole % at 950". Above 777" and 9.3 mole % added cerium the system consists of two immiscible liquids in equilibrium and has a consolute temperature that is too high to estimate from these data. Tho data from thermal and chemical analysis reported here are convisteiit with e.m.f. measurements on the system reported previously.
Introduction The phase relationships in the cerium-cerium trichloride system were investigated by Cubicci0tti.l He observed defihite freezing point depressions and concluded that this melt must be a true solution rather than an emulsion or colloidal dispersion as supposed in this type of system by Lorenz. Cubicciotti's results may be summarized as follows: the melting point of pure cerium chloride was 802' and successive additions of cerium metal resulted in a linear depression of the freezing point until at 33 mole yo cerium the melting point was about 720'. This he considered to be the maximum solubility of the metal in the salt; no further depression of the freezing point was observed on the addition of more cerium metal and, probably, two immiscible liquids were formed. At one temperature (810') the compositions of the two liquids in equilibrium were measured; a mixture of salt and metal was equilibrated, quenched and analyzed. The upper (salt-rich) phase was a black friable solid which contained 33% cerium metal and the lower phase (which had the appearance of the origiiial metal) contained an insignificant amount of cerium chloride. In an investigation of the e.m.f. of the cell Mo (or W) ICeCl&) Ce(1 z))ICeC13jClz(graphite) it was found that the measured potential remained constant (at constant temperature) from about 9
to 33 mole yo cerium.a Below 0 mole % cerium the e.m.f. exhibited a variation with concentration as predicted by theory. The slope of the e.m.f.concentration curve corresponded to a 2-electron change and it was concluded that Celf arid Ce3+ were the species present in melt. However, it was puzzliiig that the potential showed a "buffering" effect above 9 mole % cerium; this might be explained by the presence of a reservoir for c e l + in an un-ionized particle or a complex ion or, as suggestcd by Flood and Hill4 for Fez03-FeO systems, in vacancies in the structure. At this stage measurements of density and electric conductance were commenced, since these would yield structural information on the melts; these measurements have now been completed and will be described in a succeeding paper. During density measurements it was noted that compositions containing more than 10 mole yo cerium were viscous and the results irreproducible. This was thought to be owing to imperfect homogenization; however, prolonged holding and stirring a t 1000' failed to improve the situation. On several occasions cerium metal adhered to the density bob after such treatment. Approximate measurements of the freezing points of compositions containing 20, 26 and 31 mole % cerium yielded results differing largely from those previously reported. A redetermination of
(1) D. D. Cubicciotti. J . A n . Chsn. Soc., 71,4119 (1949). (2) R. Lorenr, "Die Electrolyse Geechmolzener fialro," W. Knitpp, Halle A-S (1906).
(1958). (4) H. Flood and S. G. Hill, 2. EEeklrochem., 61, 18 (1957).
-
(3) 6. Senderoff and C. W . Mellors, J . Electrochem. Soc., 106, 224
4
. I
July, 1959
PHASE DIAGRAM OF
THE
CERIUM-CERIUM TRICHLORIDE SYSTEM
the phase diagram of this molten system was then commenced. Experimental Materials.-CeCls and Ce metal were prepared as described in a previous paper - 8 All weighings and transfers were performed in a dry box. Argon was purified by passage through titanium chips at 900”. Furnace and Crucibles.-The furnace was a tensile test type made by Marshall Products Company. The bore was 3.5” and an iron tube 3.25” o.d., I/d” wall thickness served as a baffle between the furnace windings and the“Vycor” tube containing the crucible. The cell envelope was a 51 mm. 0.e. “Vycor” tube, 30” in length, completely closed at the lower end by a 101/2 rubber stopper. Through the upper 101/2 stopper passed a 6 mm. 0.d. molybdenum thermowell, an argon inlet tube and an aperture for a l/8” 0.d. molybdenum stirrer. A deep drawn molybdenum crucible, I6/g” o.d., 26/8“ in height (Fansteel Metallurgical Corporation, North Chicago) contained the melt. The pedestal supporting the crucible was solid alumina, the crucible was enclosed in a graphite crucible, and an alumina tube, whose i.d. was approximately equal to the i.d. of the molybdenum crucible, rested on the top of the latter and extended to within 1/2” of the top of the 51 mm. “Vycor” envelope. In this experimental arrangement it was possible to attain a temperature constancy of better than 1’ over the center 3.5” of t8hefurnace containing the crucible and melt. Temperature.-Chromel-alumel thermocouples were used, protected by 6 mm. 0.d. molybdenum sheaths closed at the lower end. The thermocouple was positioned in the center of the melt, equidistant from the walls of the crucible and the surface of the melt. The temperature of this measuring element was indicated on an L and N Azar Recorder. A reference junction a t 0” was provided. It is possible on this type of recorder to suppress all but a small fraction of the thermocouple output, displaying 10, 5 , 2 or 1 mv. as full scale deflection of the “Speedomax” recorder. In this way a greater accuracy of temperature reading was obtained. Generally the first cooling curve was made with 5 mv., the second with 2 or 1 mv. full scale deflection. I n the latter case the temperature could be read to within 0.25’. The chart speed of the recorder was 0.5 inch per minute and the cooling rate of the sample was 2.5” per minute. With some compositions, cooling arrests as long as 2 inches were observed by this procedure. The temperature of the furnace was controlled by a “Wheelco” controller with an independent thermocouple adjacent to the winding. General Procedure.-The apparatus was charged with an amount of CeCla previously calculated to give a satisfactory depth of melt in the crucible (approximately 120 g.), together with the appropriate amount of Ce metal to give the intended composition. All joints in the apparatus were sealed with “Unichrome” stopoff (Metal and Thermit Corporation) prior to commencement of a run. The apparatus was flushed with azgon for several hours and then heated slowly to about 950 . The melt was stirred and held a t this temperature for several hours to ensure homogenization, the furnace power was then switched off and a cooling curve obtained on the recorder chart. The temperature was then raised and the cooling curve repeated. Unless agreement of inflection temperatures to better than 1’ was obtained between the two runs, the results were discarded since a variation invariably indicated contamination. A further weighed amount of cerium metal then was added and the cooling curves repeated for this new composition. In experiments involving quenching from the high temperatures the entire “Vycor” envelope, with argon atmosphere maintained, was removed from the furnace and immersed in iced water.
11110
t I
900
Temperalure Legend
c.’
f- From
\ ;
820
quench data
bc From E M F data (Ref.
0
-
I
l
10
LO
”.
Y
l
30
31
1 Liquid and ._”
.. . 1 40
770
lOO0
From quench data
I Solid
2 Solidi 60
50
725.
c,
I l l 70 80 90 LOO Cornpodtion (Mole % Cal.
700
(4 Temperature (- C.) Legend
780
0 Iat inflection X 2nd inflectIan
I/
------rv 700
1111
t
J
0 1 s t Innection
I
I
5
10
I 15
Cornposltlon (Mole % Ce).
Ib)
Fig. 1.-Phase
diagram of Ce-CeCls system: (a) complete diagram; (b) low Ce end.
first inflection remained virtually constant, falling no more than 2-3’ up to 44 mole yo added cerium. Between 9 and 44 mole yo Ce a second inflection a t 725’ was obtained, this becoming more pronounced with increasing Ce content. It could be shown readily that between $77 and 725’ the system consisted of a solid and a liquid in equilibrium, since it was possible to stir the contents of the crucible. It was now apparent that above 777’ and between about 9 and possibly up to 99 mole % Ce, the system consist& of t w o immiscible liquids. Two samples containing about 20 mole yo Ce were held, with stirring, a t 950’ for 2 hours, then allowed to stand a t this temperature without stirring for a further four hours arid finally quenched, solidifying within a minute. Visual examination of the quenched material showed the presence of two phases, the lower of which appeared to be cerium metal. Analyses showed the composition (in mole fraction) of the upper phase to be 0.097 Ce, 0.903 CeC4 and the lower to be 0.998 Ce, 0.002 CeCh These results, therefore, give the compositions of the two liquids in equilibrium a t 950’. Figure l a shows the entire diagram for the system. It will Results be seen that a further point on the salt-rich vertical I n Fig. l b is shown the low cerium end of the is obtained from e.m.f. data, viz., the point of diagram plotted from cooling curve data from some inflection of the log Ce1+/Ce3+ versus e.m.f. curve.3 of the 60 compositions examined. Below 5 mole It is apparent that no meaningful estimate of the % Ce it was difficult to obtain the second inflection consolute temperature of this system may be made a t about 777’ as is frequently the case in phase since this line is so nearly vertical as far as it has diagrams of this type. Also it was observed that been determined. ’ Ce the temperature of the The horizontal line at 777O represents the freezabove about 9 mole %
1112
F. J. KENESHEA, JR.,AND DANIEL CUBICCIOTTI
ing point of the left hand ( L e . , CeCl3-rich) liquid, while between this temperature and 725' the system consists of this material, now solid, in equilibrium with the right hand (Le., Ce-rich) liquid, the freezing point of the latter being 725'.
Discussion As described in the introduction to this paper, the e.m.f. of the cell Mo(or W)/CeCl3(z)Ce(l- x)ll CeC131C12(graphite) remained constant from about 9 to 33 mole % cerium. The explanation of this behavior is now apparent from the redetermined phase diagram; addition of cerium metal beyond about 9 mole % does not change the cerium ion activity and the Ce1+/Ce3+ ratio remains constant. It follows also that the e.m.f. remains constant. One may calculate the heat of fusion of CeC13 from the phase diagram by the well-known equation YXRTOT
AHr = ___ To - T = heat of fusion, Y = foreign particles/
where AHf mole of added solute, x = mole fraction of solute, R = gas constant, T o = melting point of CeC13, T = melting point of solution. The value of YZ t o
Vol. 63
be used in the equation depends upon the nature of the solute and solvent. Thus, if AHf were known, this calculation could be used to identify the species present in the solution. Unfortunately the only value for AHf which is available (8 kcal./mole)6 is derived from the phase diagram of Eastman, et aL6 This diagram has now been shown to be in error. It may be noted that the calculated values of AHf for Ceo, Ce+ salts and Ce2+ compounds as the assumed species of the solute are, respectively, 6.0, 9.0 and 18.' Acknowledgments.-The authors gratefully acknowledge informative and enlightening discussions with H. R. Bronstein of Oak Ridge National Laboratory, L. M. Litz and E. R. Van Artsdalen of these laboratories. In addition they wish to thank J. C. Fisher, Jr., for generous help in the preparation of cerium chloride in these laboratories. (5) L. Brewer, ctal., Paper 6,Page 76 et seq., "Chemistry and Metallurgy of Miscellaneous Materials: Thermodynamics," ed. L. L. Quill, National Nuclear Energy Series. (6) E. D. Eastman, D. D. Cubicciotti and C. D. Thurmond, Paper 2, Page 10, "Chemistry and Metallurgy of Miscellaneoue Materials: Thermodynamics,'' ed. L. L. Quill, National Nuclear Energy Series. (7) The solvent, CeCls is assumed to contain CeS+, C1- and all POSsible complexes of Cea+ and C1- in amounts such that they are not appreciably affected by a small addition ( L e . , 10-2 mole fraction) of solute.
VOLUME EFFECTS ON MIXING I N THE LIQUID Bi-BiBr3 SYSTEM' BY F. J. KENESHEA, JR.,AND DANIEL CUBICCIOTTI Stanford Research Institute, Menlo Park, California Received NOVEmbeT 1 6 , 1968
Volume effects on mixing in the.liquid Bi-BiBrs system have been determined by measuring the density as a function of temperature for mixtures varying in composition from pure BiBr3to 0.4 mole fraction of bismuth. It was found that there 1s a decrease in the total volume of the system on mixing. I n the mixtures studied, the partial molar volume of the BiBrl is approximately the same as the molar volume of BiBrs and increases with temperature. The partial molar volume of the bismuth in the solutions is less than the molar volume of pure bismuth and decreases with increasing temperature. These effectsare analogous to the volume changes found previously in the Bi-BiCls system and are interpreted in terms of the same model of an interstitial type of solution, with the added bismuth entering into empty octahedral holes in the liquid quasilattice. Above a temperature of 300' the partial molar volume of bismuth becomes negative .at infipite dilution. It is suggested that this contraction in volume is brought about when added bismuth enters into the interstitial holes in the expanded bromide lattice and pulls the surrounding bromides more closely together.
Introduction In previous reports in our study of metal-salt interactions we have discussed the results obtained in the investigation of vapor pressures in the BiBiC13and Bi-BiBr3 systems2s3and volume effects in the Bi-BiCla system.4 As a continuation of this work we have measured the volume changes in the Bi-BiBr3 system. As has been pointed out previously,a the phase diagram for the bromide system is very similar to that for the chloride. At 300' as much as 40 to 45 mole yo bismuth can be dissolved in BiBra; at higher concentrations and a t 300', two liquid phases coexist.6 In the present
study only salt-rich homogeneous liquid solutions of bismuth in BiBr3were investigated. Experimental
As in the chloride experiments, the densities of liquid BiBiBr3 mixtures were determined by a pycnometric method, the details of which have been reported already.4 The BiBrs was prepared by reaction of Biz03 with aqueous HBr and then evaporation of the water under Nz. Before use, the BiBr, was distilled twice under 5 mm. of dry Nz, the final distillation being made directly into the Pyrex pycnometer containing a weighed amount of bismuth. The pycnometer was sealed under vacuum. The material in the pycnometer was always premelted by heating with a torch before being transferred to the furnace, yhich wa8 already heated to a temperature of about 300 This was (1) This work was made possible by the financial support of the Redone to prevent breaking of the bulb by expansion of the search Division of the United States Atomic Energy Commission. solid salt i f the material were allowed to come to temperature (2) D. Cubicciotti, F. J. Keneshea, Jr.. and C. M. Kelley, THIS in the furnace. JOURNAL, 62, 463 (1958). The BiBrs prepared in the above fashion was found by (3) D. Cubiociotti and F. J. Keneshea, Jr., ibid., 62, 999 (1958). analysis to contain 46.36% Bi (determined gravimetrically (4) F. J. Keneshea, Jr., and D. Cubicciotti, ibid., 62, 843 (1958). ( 5 ) (a) B. G . Eggink, 2. phyaik. C h ~ m . 64, , 449 (1908); (b) L. as BiPOd); the theoretical amount for BiBra is 46.56%. A cooling curve taken on a 100-g. sample of the pure BiBra Marino and R. Becarelli, Atti occad. nax?,Lincei, 24, 625 (1915); 26, 105 (1916); 31, 171 (lele), showed a halt corresponding to the freezing point at 21Sa5",
.
