1868
STANLEY CANTOR AND WILFRED T. WARD
gives it a practical utility and provides a provocative glimpse into the topological foundation of the whole problem. Finally, it serves as a warning that refinements of r-electron treatments based on physical models must be interpreted n i t h caution. The numerical success of a refinement by no means establishes the true physical significance of the improvement. Acknowledgment.-The author wishes to acknowl-
Vol. 67
edge that the possible relex-ance of relating the bond energy of aromatic molecules to some kind of bond order via a series expansion was originally suggested by Professor C. J. McGinn. It is a pleasure to acknowledge the assistance of Mr. R. Gavin in the calculations. The author is also indebted to Professor K. Ruedenberg for permission t o use his wave functions for polyacenes before publication.
FREEZING POINT DEPRESSIONS I N SODIUM FLUORIDE. 111. EFFECTS OF 3d TRANSITION METAL DIFLUORIDES AND CADMIUM FLUORIDE BY STANLEY CAKTOR AND WILFREDT. WARD Oak Ridge National Laboratoryll Reactor Chemistry Division, Oak Ridge, Tennessee Receiued March 19,1963 As part of a program of investigating the effects of structural parameters on molten fluoride solution behavior, measurements were made of the freezing point depressions of NaF caused by the addition of up to 0.20 mole fraction MnFz, FeF2, CoF2, SiFz, CuF2, ZnF2, or CdF2. The excess partial molal free energies of solution of S a F , ( F - F o ) E s aevaluated ~, from the measurements, were all negative and approached zero mymptotically as the mole fraction of NaF approached unity. .4t fixed concentration, the trend in ( P- F o ) E ~with , + the atomic number of the ?vlnFs,FeFL,CoFI, XFs, and ZnFnsolutes was consistent with ligand field theory. This trend also suggested that these five 3d solute cations were octahedrally coordinated in XaF solution. The other 3d difluoride, CuF2, decomposed a t high temperature, thereby preventing experimental study of the effect of this solute on ( P - F o ) E ~ ,The ~ . effects of ZnF2in contrast t o MgFz, and of CdFz in contrast to CaF2, were attributed to the differing polarizing powers of the solute cations.
Introduction This study is part of a comprehensive investigation which seeks to relate the structural parameters (such as radius, charge, and polarizability) of the components to the thermodynamic properties of fused fluoride solutions. The approach used in the investigation has been to determine a thermodynamic property of a solvent, in which type and concentration of solute can be systematically altered, and then to relate variations in this property to structural parameters of the solutes. The object of this study was to measure the depressions of freezing points in the solvent, NaF, and correlate the excess partial molal free energies of solution of NaF, ( F - F o ) E ~with a ~the , structuresof several divalent fluoride solutes. h previous study2 which dealt with the effects of alkaline earth difluoride solutes established that, at fixed concentration, the smaller the solute cation the more negative was ( F - FO)ENaB-. I n the present study, we seek to observe and explain differences in (F - F o ) E caused ~ a ~ by a group of difluoride solutes (MnF2, FeF2, CoF2, NiF2, CuF2, ZnF2) whose cation sizes do not differ very greatly. Since N g 2 +is about the same in size as the 3d lIz+ cations, it was also of interest to contrast the values of ( F - Fo)Eixa~ caused by the 3d difluoride solutes with those caused by lIgFa. Similarly, the comparative effects of CdF, and CaF2 are interesting because Cd2+and Ca2+cation radii are approximately equal. Experimental Apparatus and Procedures.-Temperature measurements and manipulations were the same as those discussed in a previous publication.2 The containing vessel was also substantially the same except for modifications which permitted the melt to be stirred mechanically. The stirrer shaft entered the vessel (1) Operated for the United States Atomic Energy Commission by Union Carbide Corporation. (2) 8. Cantor, J . Phys. Chem., 66, 2208 (19Gl).
through a tightly fitting Teflon gland. An inert atmosphere was maintained in the vessel by passing helium in through a side arm and permitting the gas to escape only through the very small annular space between the stirrer shaft and the Teflon gland. I n melts containing CdFz, cadmium metal was produced by the reaction of CdFi with the nickel vessel. To prevent this reaction, the basic vessel was altered so as to contain a graphite cup, a graphite stirring rod, and a copper thermowell. These substitutions in materials eliminated contact of the melt with the nickel surfaces of the vessel. Because CuF2 reacts with nickel, melts with CuFz solutes were also contained in these altered vessels. Chemicals.-KaF was purified in the manner previously reported.* MnFz (City Chemical Corp., “Electronic” grade) was vacuum dried overnight a t 230’ before use. Analyses3 of the dried material showed the following impurities in weight per cent: Al, 0.5; Si, 0.2; Fe, 0 . 0 i ; Ca, 0.05; 0, 0.44. CoF2 was prepared by the method described by M a t a r r e ~ e . ~ CoCli.GH20 (Baker “Analyzed”) was the starting material. The final CoFz product contained 0,033y0 oxygen and less than 0.01% chlorine. NiF2 and FeFs were obtained from batches previously prepared a t Oak Ridge Kational Laboratory by passing anhydrous H F over the respective chlorides a t 400”. Analysis of the XiFi showed 0.22% C1, 0.126% 0, and 0.058% Fe. The FeF2 was further purified by mixing with iron filings and SHdHFs in a graphite-lihed nickel vessel and heating to 1050”. Slow cooling of this product yielded large clear crystals which were handseparated from the iron metal. These crystals, which served as samples for the freezing point determinations, contained the following impurities in weight per cent: Al, 0.05; Cr, 0,30; Mg, 0.05; Mn, 0.20; S a , 0.01; Xi, 0.03; V, 0.05; 0, 0.04. X-Ray diffraction analysis confirmed the absence of FeFa in the FeF2 samples. CuFl was prepared by hydrofluorinating CuF2.2HiO (City Chemical Co.) a t 400”. Impurities in weight per cent were: A1,O.l; Ca, 0.1; Fe,0.5; Na,0.02; Pb,0.1. ZnFJ was prepared by the elimination of impurities from commercial ZnFz.2H20(City Chemical Co.). The dihydrate, held in a platinum crucible, was heated overnight a t 110” and then (3) All spectrochemical, Iron, oxygen, and chlorine analyses were carried o u t b y the Analytical Chemistry Division of Oak Ridge National Laboratory. (4) L. XI. JIatarrese, “The Magnetic Anieotropy of Iron Group I‘luorides,” P1i.D. Dissertation, Unlverslty of Chicago, 1954.
FREEZING POl[XT DEPRESSIONS IN SODIUM FLUORIDE
Sept., 1963
hydrofluorinated a t 950" for 3 hr. By slowly cooling from t h k temperature, large clear crystals were formed. Analyses indicated that these crystals contained 0.16570 oxygen and only traces of metallic impurities. CdF, was purified by treating the commercial material (Baker and Adamson, Code 2983) with NH4HFn in a graphite crucible a t 250' for 2 hr. and then heating the mixture to 600°, simultaneously flushing with a helium gas stream. Analysee of this product showed 0.28Y0 oxygen and an absence of metallic impurities except for a trace of magnesium (lens than 0.01%).
YNaFNNaF
=
LM- (a'- a:) Tnr
4
SOLUTE CONCENTRATION (mole %), 6 8 to 12 14
20
18
2 -200 b g
'
-300
l i
-400
-500
+ -2b TnlZ+ I 14
I
where Y N ~ Fis the activity coefficient of NaF at mole fraction N N ~ FL;M is the heat of fusion a t the melting point Tb1; T is the liquidus temperature; a' is the molar heat capacity of liquid NaF; a, b, and c are constants in the heat capacity-temperature equation (C, = a bT - cT-~) for solid NaF. For eq. 1 the following constants were used: LM = 8017, Tbf = 1268°K. (ref. 2) ; a' = 16.40, u = 10.40, b = 3.88 X c = 0.33 X lo5 (ref. 5 ) . The excess partial molal free energy of solution of NaF, ( F F o ) E ~was a ~calculated , from 111 Y N ~ I 'by the equation
+
-
( F - F o ) E N=a RT ~ In Y N ~ F These values of ( F - F o ) E h a are ~ plotted vs. composition in Fig. 1 along with values previously obtained2 for MgFz and CaF2. With melts containing CuFz, constant liquidus temperatures were not observed. I n post-experi~ment~al examinations, specks of copper meta,l were found to be dispersed in the solidified salt mixtures. I n every case, the copper thermomell appeared not to have been affected. von Wartenberg has reported6 that pure CuFz decomposes to CuF on melting (at 950") and that CuF disproportionates to CuFz and Cu on cooling. Presumably, CuF2, dissolved in NaF, followed the same sequence of chemical reactions. Discussion Comparative Effects of MnFz, FeFz, CoFz, NiF2, and ZnFz.--A greater understanding of the physical and chemical behavior of the cations of these solutes has resulted from the application of ligand field theory. Hence, in the present study we were very interested in determining whether ligand field theory could serve as a basis for explaining the effects of these solutes on the thermodynamic behavior of these solutions. At fixed concentration, entropies of solution involving these solute metal ions in combination with a common S.Bur. Mines Bull. ,584 (1960),p. 171. (F) H. yon Wartenberg, Z. anorg. allgem. Chem., Z41, 381 (1939).
( 5 ) K. K Kelley. U.
46
-100
--
Results The temperatures a t which NaF began precipitating from solution are given in Table I. The absence of solid solubility in NaF was established by X-ray and petrographic examination of the solidified samples. From these liquidus temperatures activity coefficients of NaF were calculated by means of eq. 1
- R In
2
0
1869
I
15 16 17 I8 19 SOLUTE CONCENTRATION ( m o l e 7- I .
20
I 1 1 I 1 partiarl molal free energy of solution of NaF us. mole % of solute.
Fig. 1.-Excess
TABLE I OBSERVED FREEZING P O I N T SOF~ NaF CONTAINING MF2 SOLUTES Mole fraction NaF
1.000 0.981 ,980 ,975
F.P. of NaF, "C. I
FeFz
MnFa
995.0
995.0
... ...
...
...
...
984.5
...
... 6
984.5
...
...
971.8
971.8
.
.
I
...
...
...
974.0
...
941.4
939.4
...
...
,850
906.2
..,
,8369 ,8256
884.4
,8185 ,8051
864.9
,8005
.,.
... ...
...
928.6
... 971.7
955.3 941.6
...
CdFz
995.0 987.2
972.4
...
...
ZnFz
995.0 987.0
984.9
955.8 ,8961 .880 .879 ,875
a
995.0
987.0
0 ,949 ,925
99.5.0
Solute CoFg NiFz
958.5
...
...
958.4 941.5
... ...
...
934.2
898.6
913.5
...
874.7
922.2
..,
.,.
870.5
,
.
,
These temperature measurements have a precision of 0.3".
anion or polar gmup appear to be essentially indepencjent of the m.eta1 i0n.I By analogy with this general observation and because, for any fixed concentration, the liquidus temperatures did not differ very much, the term of temperature multiplied by excess partial molal entropy of solution of NaF [Le., T ( S -S o ) E ~isalikely ~ ] tlo be nearly independent of solute a t constant solute concentration. Therefore, differences in ( F - FO)%F, a t fixed solute concentration, may be assumed to reflect differences in the partial
STANLEY CAXTOR AND WILFRED T. WARD
1870 500 I
I
I
I
1
Vol. 67
omitted because CuFz decomposed in the melt) are octahedrally coordinated by fluoride ions. The plausibility of the inference is strengthened by the results of a recent spectroscopic studya which showed that Xiz+ was octahedrally coordinated when KiFz was dissolved in molten alkali fluoride. Although the evidence is not conclusive, it is likely that these five transition metal cations exhibit primarily octahedral coordination when dissolved in melts containing only sodium and fluoride ions. Comparative Effects of ZnF2 and MgFz.-Crystalline MgFz and ZnFz are isomorphous and have virtually identical metal-fluorine distances. Therefore, Mg2+ and 2 n 2 + have approximately equal ionic radii in an environment of fluoride ions. If tm7o solute cations have the same charge and size, the cation with the greater polarizing power would be expected to have the greater fluoride affinity, thereby causing a more negative value of ( F - F o ) E ~ , The ~ . significant structural difference between Zn2+and Mg2+is the greater polarizing power of the d'O electronic configuration (Zn2+) over that in the inert gas configuration (Mgz+). As shown in Fig. 1, ZnFz does indeed cause significantly lower values of ( P - F o ) E ~ a F . The values of ( F F o ) E ~with a ~MgF2 solutes were obtained from ref. 2. Comparative Effects of CdFz and CaFz.-Like the &IgFz and ZnFz pair, CdFz and CaFz are isomorphous with nearly equal metal-fluorine interatomic distances.l' Kote that for CdFz the curve of ( F - Fo)Ehap us. concentration lies below the corresponding curve for CaF2 solutes (see Fig. 1). This difference in the effect B ~explained by the hypothesis of on (F - F o ) E is~ also differing polarizing powers. I n this case, the Cd2+ ion has the dl" electronic configuration (and therefore the greater fluoride affinity) and Ca2+has the inert gas electronic core. Acknowledgments.-The authors wish to thank Dr. John H. Burns for helping to determine the purity of some of our chemicals by X-ray diffraction techniques, and Mr. B. J. Sturm for preparing the pure CuFz Q710
'
300 MnG
I
1
I
FeF2
co5
NiFZ
I CuFz
I ZnFz
SOLUTE,
Fig. 2.-Excess
partial molal free energy of solution of NaF a t 20 mole % solute.
molal enthalpy of solution of NaF, and it is the differing enthalpies for some process (hydration, crystallization) involving these cations that ligand-field theorists find instructive in inferring certain structural features which affect the process.' Figure 2 shows a plot of (P- Fo)EyVaF at 20 mole % solute us. atomic number of the solute. Qualitatively similar plots were obtained a t lower solute concentrations where there were significant differences among the five ( F - Fo)EN,F values. The dotted line joins atomic numbers of solutes whose metal ions (with d5 and d10 electronic configurations) have no ligand-field stabilization energy in a weak field. Xote that values of ( F - P o ) E ~ for, ~FeF2, CoF2, and XFz solutes are more negative than values given by the dashed line. This extra excess free energy is consistent with ligand field theory, Le., those metal ions with d6paelectronic Configurations have an extra stabilization because of the surrounding field of fluoride ions. The relationship of the solid to dashed lines is very similar to that exhibited by corresponding curves of enthalpy ts. atomic iiuniber in cases where the 3d5-'" metal ions are known to be octahedrally ~oordinated.~Hence, the results depicted in Fig. 2 suggest that these metal ions with 3d5-8,1° electronic configurations (3dg is (7) P. George and D. S. AIcClure, P r o p . Inorg. Chem., 1, 381 (1959).
(8) J. P. Young and G. P. Smith, Oak Ridge National Laboratory, personal communication. (9) W. H. Baur, Acta Cryst.. 9 , 519 (1956). (10) W. H. Baur, %bid., 11, 488 (1958). (11) R. G. Wyckoff, "Crystal Structures," Vol. 1, Interscience Publishers, New York, N.Y., 1948, Chapter 4, Table p. 13.
