NOTES
3828
Miscibility of Liquid Metals with Salts.
I
0.2
0.4
0.6 I
0.8 I
1.0
IX. The Pseudobinary Alkali Metal-Metal
4100
Halide Systems : Cesium Iodide-Sodium, Cesium Iodide-Lithium, and Lithium Fluoride-Potassium by A. S. Dworkin and M. A. Bredig Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 9’7880 (Received M a y 14, 29’70)
We have extended our studies of the miscibility of molten salts with metals2 to the pseudobinary sections CsI-Na, CsI-Li, and LiF-K of the corresponding ternary systems. Each of the preceding forms the stable pair by more than 10 kcal/mol of a corresponding recipB = BX A. rocal system, AX The purified alkali metals contained less than 0.1% impurity. The CsI and LiF were Harshaw optical grade crystals. The results shown in Figure 1 were determined by thermal analysis (cooling curves). The salts and metals (total charge 50 to 200 mmol) were loaded into tantalum capsules (3 in. long, 0.5in. i.d.) in a drybox under a helium atmosphere where the capsules were then sealed by welding. Temperatures were measured with a Pt-Pt-lO% R h thermocouple placed in a well extending about 1/2 in. into the capsule from the bottom. Rocking the furnace with the capsules in a horizontal position permitted mixing of the components before each cooling curve was run. The apparatus and experimental procedure are described elsewhere3 in more detail. The decantation technique with which a sample of the liquid metal phase in the LiF-K system was separated has also been described previously. Figure 1 shows that the temperature vs. concentration phase diagram of the pseudobinary section CsI-Na resembles that of the truly binary system NaI-Na except for a slightly higher consolute temperature (1115 vs. 1036’) and a considerably lower solubility of the salt in the sodium metal (NcsI(critical) = 22 vs. NNeI(crit) = 41 mol %). The resemblance is greater when volume fraction, N’, rather than mole fraction is plotted, with molar volumes 100 cm3/mol for CsI, 63 for NaI, and 33 for Na metal: N’csI(crit) = 46 and N ’ N ~ I (crit) = 5 6 ~ 0 1 % . For the pseudobinary section CsI-Li, a wider miscibility gap is indicated. We were unable to find discrete points on the cooling curves, although we cooled from temperatures as high as 1300”. This -implies a very Steep rise in the equilibrium temperatures on both sides of a wide miscibility gap and a high consolute temperature. The situation is similar to that in the binary LiILi systeme6
+
,
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I
+
1
Cs(No)I
I
0.2
I
0.4
I
0.6
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I
No
MOLE FRACTION OF No
Figure 1. Phase diagram of the pseudobinary system CsI-Na compared with NaI-Na.
Of particular interest is the comparison of the four systems, binary and ternary, NaI-Na and CsI-Na, or LiI-Li and CsI-Li, with the binary CsI-Cs system exhibiting no immiscibility a t all. This comparison appears to support Pitzer’s notion6 that the nature of the metal, or, more specifically, the energy of converting the electronic structure of a real metal into an “ionic” structure, ( W + e - ” ) , i.e. one with localized electrons, is the determining factor in metal-salt miscibility. Conversion energies were given as 19 and 18 kcal/mol for lithium and sodium, as against 9 for cesium. Significantly, the results for the third ternary system we studied, LiF-K, do not appear to fall in line with this theory. Because of the low conversion energy of potassium metal, 10 kcal/mol,e the theory would predict a high degree of miscibility similar to that in KF-K2 where the consolute temperature is only 904” and the solubility of the salt in the metal is very high (NKdcrit) = 80 mol %). I n contrast, thermal analysis, as with CsI-Li, above, gave a very wide miscibility gap in LiF-I
