STAXLEY CASTORAND WILFREDT. WARD
2766
Vol. 67
FREEZING POINT DEPRESSIONS IN SODIUX FLUORIDE. IV. EFFECTS OF TRIVALENT FLUORIDES BY STANLEY CANTOR AND WILFREDT. WARD Oak Ridge National Laboratory, Reactor Cheinistry Division, Oak Ridge, Tennessee Received July 9, 19/38 Freezing point depressions in NaF caused by the addition of up to 0.15 mole fraction of AlF,, ScF,, InF3, YF3, LaF3, and eight rare earth trifluorides were measured as part of a program of investigating how thermodynamic behavior of fluoride melts is related to structural parameters of the components. The excess partial molal free energies of solution of XaF, ( E FO)’~,r, evaluated from the measurements, were all negative. For all the solutes except AlF8 and InF3, ( F - FO)’X,F was found to be inversely proportional to the interionic distance of the solute. T_he different solution behavior of AlFs and InF3 was attributed to steric and polarization effects. The values of ( F - F o ) % * p with trivalent fluoride solutes were generally more negative than corresponding values with tetravalent solutes but less negative than the values with divalent solutes.
-
NHIHFz vaporized. Unchanged N H ~ H F and z sorbed HF were Introduction then driven off a t 700’ under continuous helium flush. This The objectives of these studies and the usefulness of treatment lowered the oxygen content of all three trifluorides to N a F as a high temperature cryoscopic medium were less than 0.05% The NdF3 was found to contain the following spectrographically determined impurities: Ca, 0.3; Pr, 0.1; discussed in previous publications. 2 , 3 Sm, 1.0; Gd, 0.57,. This particular investigation had a twofold purpose : (1) to determine which structural parameters of the TABLE I trivalent fluorides appear to affect the excess partial SPECTROGRAPHIC AXALYSES OF RAREEARTH OXIDES molal free energy of mixing of SaF, ( F - F o ) E ~ a ~ ; (2) t o compare the value of ( F - F o ) E ~ icaused a ~ by Tb Dy Lu Others Ca Na Si Eu trivalent fluoride solutes with the corresponding values . . . . . . . . . . . . . . . . . . . . . . . . caused by divalentz,a and tetravalent f l ~ o r i d e s . ~ D
Experimental Apparatus and Procedures.-The apparatus and methods used to obtain the liquidus temperatures were the same as those described in previous publications,2v4 For the melts containing InF3, it was found necessary to use graphite-lined vessels, graphite stirring rods, and copper thermocouple wells. All-nickel equipment was used for the other salt mixtures. Chemicals.-The NaF was purified in the manner previously reported.2 A1F3-containingmixtures were prepared by mixing K a F and cryolite, Na3A1Fs. The cryolite, supplied by Reynolds Metals Co., Sheffield, illabama, was hand-picked natural mineral from Greenland and was used as received. rlnalysis by the Reynolds Co. showed Na, 33.3; Al, 13.02; F, 54.3%; (theoretical for NaaAlFB: Na, 32.85; Al, 12.85; F, 54.3%). Spectrochemical and oxygen analyses6 showed Ca, 0.01; Li, 0.005; 0, 0.075%. InF3 was prepared by a method similar to that discussed by Kwasnik.6 Sheets of indium metal were dissolved in concentrated HCl; concentrated NHaOH was then added, precipitating In(OH)8. This mixture was heated and then centrifuged. After being washed with dilute NH40H and recentrifuging, the In(OH)3 was redissolved with concentrated (48%) aqueous HF. The solution was then evaporated until considerable crystallization occurred. Distilled water was added in quantities just sufficient to redissolve the crystals; a saturated solution of NH4F was then used to precipitate InF,. After being centrifuged, the precipitate was hydrofluorinated in a graphite-lined nickel pot at 600” for about 2 hr. Analysis of this product showed: In, 66.9 (theoretical for In in InF3: 66.83); oxygen, 0.078; -41, 0.05; Ca,0.1; Fe,0.05; Xa, 0.05; Si, 0.027,. LaFa, CeF, (Lindsay Chemical Co.), and NdF, (previously prepared at ORNL by treating Nd2(C03)3with aqueous HF) were treated with NH4.HFz to reduce their oxygen contents. A mixture of approximately 2 parts of MF3 to 1 part of NH4HF2 was heated a t 250” in a nickel vessel until most of the excess (1) Operated for the U. S. Atomic Energy Commission by Union Carbide Corp. (2) S. Cantor, J. Phys. Chem., 65, 2208 (1961). (3) S. Cantor a n d W. T. Ward, ibid., 67, 1868 (1963). (4) S. Cantor and T. S. Carlton, ibid., 66, 2711 (1962). (5) All chemical analyses were performed b y the Analytical Chemistry Division of Oak Ridge National Laboratory unless otherwise indicated. ( 8 ) W. Kwasnik, “Fluorverbindungen,” “Handhuch der Priparativen Anorganischen Chemie,” Val. I, G. Brauer, Ed., Ferdinand Enke Verlag, Stuttgart, Germany, 1960, p. 215.
0.1
......
0.5 . . . . . . . . . . . . . . . . . .
0.3
0.5
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.1 . . . 0.1 0.1 ......
. . . . . . . . . . . . . . . . . . 1.0 . . .
a Only the impurities present to the extent of listed.
O.lycor more are
The other rare earth fluorides were prepared from the oxides (SmzOa,Dy2O3,Erz03,and Yb& from Michigan Chemical Corp., Luz03 from Research Chemicals, Inc., Gd203 from ORNL). Spectrochemical analyses of these rare earth oxides showed no major impurities (see Table I). Each oxide was mixed with an approximately equal weight of NH4HFzand treated in the manner described in the preceding paragraph. The oxygen contents of the resulting fluorides were in all cases less than 0.05%. ScF, and YF3 were also prepared from oxides (SCZOSfrom American Scandium Corp., Y z O ~from ORNL) by the X H ~ H F Z procedure. YF3 had the following impurities in wt. %: Ba, 0.02; Ca, 0.02; Fe, 0.02; Ta, 0.1; 0, 0.089. ScF3 had: 0, 0.088; Ca, Fe, Ni, Yb, and Lu, each 0.05; Y and Gd each 0.1; La, 0.02%.
Results The temperatures at which NaF began precipitating from solution are given in Tables I1 and 111. The absence of solid solubility in K a F was established by X-ray and petrographic examination of the solidified mixtures. From these liquidus temperatures, activity coefficients of NaF were calculated by means of eq. 1
-R In Y R ‘ ~ F N= N ~[Lv F - (a’
b 2 -
-
+
a)T&r
+ c [-l
(Tu- T)
-
2
-1
- 1
(1)
T2;\2 T Z
where Y N ~ Fis the activity coefficient of NaF a t mole fraction N N ~ FLM ; is the heat of fusion a t the melting point T ~ I ;T is the liquidus temperature; a’ is the molar heat capacity of liquid S a F ; a, b, and c are constants in the heat capacity-temperature equation
FREEZING POINT DEPRESSIONS IN SODIUM FLUORIDE
Dec., 1963
2767
+
(C, = a. ZIT - cZ'-~) for solid KaF. For eq. 1 the following constants were used: &I = 8017 cal., Till = 12f~8~112. (ref. 2), a' = 16.40, a = 10.40, b = 3.88 X lod3, and c == 0.33 X 106 (ref. 7). The excess partial molal free energy of solution of XaF, ( F - Fo)E~,F, was calculated from In Y N ~ Fby the equation
( F - FO) ' N ~ F
=
RT In
-400
--
-150
0
(2)
YN~F
The ( F - F o ) E ~us. , ~concentration curves for the rare earth trifluorjdes are given in Fig. 1. At concentra-
-200
L
I.." 0
Y
'4.
TABLE I1
--
OBSERVED FREEZING POINTS" OF N a F CONTAINING MFa SOLUTES Mole fraction KaF
---
Freezing point of NaF, "C. Solute ScF3 InFa SF3
,--.--
AlFs
1,000 995.0 995.0 995.0 985.8 985.7 ... 0,980 975.3 975.2 ... ,960 ... 968.8 967.7 ,950 962.4 ... ... ,9396 ... 958.3 ... ,9351 946.8 945.5 942.7 ,920 937.7 ... ... ,9097 ... ... ... .go5 ... 925.9 922.2 .goo 927.6 ... ... ,8995 916.8 ... ... ,8896 ... 902.0 896.5 ,880 ... ... ... .8756 ,875 898.8 ... ... ... ... ... ,8682 ... 857.0 848.4 .850 These tempemture measurements have a
...
...
7
... 970.7
...
...
... ...
947.0
...
...
...
,.. 929.5
939.6 935.6
... ...
1.000 0.975 ,960 ,9508 ,9477 ,941.5 ,940 ,9377 ,9372 ,9261 ,920 .915 ,910 ,900 ,8989 ,8972 ,895 ,890 ,885 ,8825
,8811 ,880 .8753 ,870 ,8676 ,8651 ,8639 ,855 ,8533 ,850 ,8481 ,8476 a
995.0
995.0
...
I
J
45
46
- 150 -2 0 0
--g
-250
Y
LL
yg
-300
0
995.0
1 7
LuFa
k I
-350
'k
995.0
-400
982.9 976.1
. . . . . . . . . . . . . . . . . . . . . 890.7 . . . . . . . . . . . . 894.1 . . . . . . . . . . . . . . . . . . 894.9 . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 886.2 . . . . . . . . . . . . . . . 887.0 8 8 5 . 5 . . . 879.7 876.7 871.8 868.4 866.9
892.7
877.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . These temperature measurements have a precision of 0.3".
. . . . . . . . .
884.8
(7) C . J. O'Brien and IC
42
-100
89j.9 ... 873.9 891.0 precision of 0.3'.
995.0
14
914.8
Freezing point of KaF, ' C . - - SoluteSinFa GdFs DyFa ErF3 YbF3 99LO
40
-5 0
...
. . . . . . . . . . . . . . . . . . ... 975.0 974.5 . . . . . . . . . . . . . . . . . . . . . . . . 968.5 ... 968.1 ... . . . 068.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 962.3 . . . . . . . . . . . . . 961.9 962.5 . . . . . . . . . . . . 962.9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 960.2 . . . . . . . . . . . . . . . ... 953.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . 947.5 . . . 947.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . 940.2 942.3 942.4 . . . . . . 939.5 ... 936.1 . . . . . . Y33.7 . . . . . . . . . . . . . . . . . . . . . . . . . . . 929.7 . . . . . . . . . . . . 931.8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 924.1 . . . 921.7 . . . 9 2 4 . 2 . . . . . . . . . . . . 916.3 . . . . . . . . . . . . . . . 9 1 6 . 1 . . . . . . 910.8 . . . . . . . . . . . . 914.4 . . . . . . . . . . . . . . . 914 1 . . . . . . . . . . . . . . . 916.8 914.7 . . . . . . . . . . . . . . . . . . 911.8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 896.3 893.7 . . . ...
I 13 44 SOLUTE CONCENTRATION (mole %I 9
Fig. 1.-Excess partial molal free energy of solution of NaF vs. solute concentration of rare earth fluorides.
... ...
... ... ...
8
0
EARTH 'btIFLUORIUE SOLUTES
.NdFs il95.0 995.0
1
...
968.8
---CeFs
- 400
995.0
TABLE I11 O n m n v m FREEZING Porwrs" OF PTaF CONTAIXIXL: RARE Mole fraction NaF
-300 -350
LaFa
995.0
-250
I U n d i xh fur tliJLill.Si"II "f l l l e i,,lerio,li. dirlimcc.8
Dec., 1963
ELECTRON SCATTERING FROM MOLECULES
2769
sider a probable consequence of the high electric field for fluoride ions; the greater the affinity for fluoride strength of the A13+ion. If this high field caused six or ions, the greater the stability of the solution. A measure of relative electric field strength that is often ademore fluorides t o be coordinated t o each A13+ ion in S a F solution, then the relative sixes of AI3+ and Fquate for correlating values of (F F a ) E N a F is Z/dM--F ions do not permit simultaneous contact of all six F(where Z is the charge of the solute cation). But structural factors other than charge and interionic ions with the Ala+ion (all the other 12 trivalent cations are large enough to permit contact with six fluoride distance also appear to be important for evaluating nearest neighbors). Those F- ions out of contact even relative field strengths. These are d10 electronic configuration, ligand field effectj3 and cation t o anion would then be more “available” to the sodium ion for crystallization. I n short, a steric effect would operate radius ratio. Unfortunately, methods for calculating field strengths from the important structural factors in a direction opposite to the expected coulombic effect. The coordinalion number of six is considered probable are not presently available. because AI3+ in crystalline AIF3 has that coordination Acknowledgments.-We wish to thank Messrs. number.1° A similar reversal in trend of interionic R. E. Thoma and C. F. Weaver for carrying out the a ~ exhibited by the tetradistance on ( F - F o ) E ~was X-ray and petrographic examinations. valent f l u o r i d ~in s ~NaF where the order of (F - F a ) E ~ a ~ Appendix A a t fixed concentration was ZrFd > UF4 > ThF4. For The correlation of ( F - F o ) E with ~ a ~interionic disthat series of solutes each tetravalent cation was tance required a consistent calculation of these disassumed to have a t least eight fluoride nearest neighbors (as Zr4+,C4+,and Th4+ have in crystalline ZrF4,11 tances. All eleven interionic distances 2vere the sum of UF4,12and ThF4), with the result that Zr4+ and C4+ the trivalent cation radius and 1.330 A., the constant fluoride ion radius. For the radii of La3+and the eight were too smsill to have eight fluorides contact each rare earth ions, the values published by Templeton and cation simultaneously. Dauben13were used. Comparative Effects of Di-, Tri-, and Tetravalent The radii of Y 3 +and Sc3+ were obtained in the Fluorides.-At all solute concentrations greater than following manner: the cell lengths (measured by Ternabout 2 mole % every polyvalent fluoride studied thus pleton and Dauben13) of the cubic rare earth oxides, far caused negative deviations from ideality (Le., AIz03, were plotted 11s. the cation radii (values of ref. 13) ; (F - , F o ) E ~ a 1 4was ~ always negative). The ranges of this plot was linear; literature values of the cell lengths the negative deviations, plotted according to the charge of cubic Yz03I4and s c ~ 0 were 3 ~ ~then plotted on the line; of the solute cation, are given in Fig. 4. Qualitatively, the Y3+radius, obtained by interpolation, equaled 0.894 the difference in these ranges may be explained by the A.;the Sc3+radius obtained by extrapolation, equaled followina chain of reasoning: the greater the electric 0.732 A. field strength of the solute cation, the greater the affinity
-
(10) A. F. Wells, “Structural Inorganic Chemistry,” 3rd Ed., Oxford University Press, Oxford, England, 1962, p. 341. (11) R. D. Burbank and F. N. Bensey, Jr., “The Crystal Structure of Zrl74,” Report No. [I-1280, Union Carbide Kuclear Co., Oct. 31, 1956. (12) R. D. Burbank, “The Crystal Structure of UF4,” Report No. I
