In each of a set of experiments, 300 g of bismuth containing the equivalent of about 2000 ppm of rare earth (either La or Nd) and about 4000 ppm of thorium was equilibrated for at least 8 hr at several temperatures between 750 and 300°C. At the end of each equilibration period, a filtered sample of the liquid phase was removed for analysis. The experiments were initiated by heating the system to about 750"C, where both the thorium and the rareearth metal were soluble. The temperature of the system was then progressively lowered to about 300°C; finally, the temperature was returned, in steps, to 750°C. The results from one of our recent experiments (LA-3), along with those from a previous6 experiment (LA-2), are shown in Fig. 16.2. Agreement among the results of these two experiments is obvious. As the temperature of the system was lowered, the thorium concentration in the liquid remained at its original value until about 650°C was ieached. Below 650"C, the thorium concentration in the liquid decreased regularly with decreasing temperature (Fig. 16.2). The equilibrium thorium concen-
trations in each experiment are in excellent agreement with the published7 values for the solubility of thorium in liquid bismuth. Similarly, as the temperature was lowered, the lanthanum concentration in the liquid remained at its original value until some temperature below 550°C w& reached; then it decreased regularly with decreasing temperature (Fig. 16.2). The lanthanum behavior in each experiment was identical. The equilibrium lanthanum concentrations in the liquid were much lower than the reported solubility values.899On the basis of the lanthanum results, we postulate the formation of a La- and Thcontaining solid phase that has a very low solubility in liquid bismuth. The results obtained for thorium indicate that there was sufficient thorium present in the syitem to satisfy the solubility product of the compound containing the rare earth and thorium and to saturate the system with thorium bismuthide. In this case, the maximum thorium concentration in solution at a given temperature is determined by the solubility of thorium in bismuth. The variation of thorium solubility in bismuth with temperature has been shown7 to follow the usual relationship
Substitution of Eq. (3) into Eq. (2) yields --INITIAL CONCENTRATIONS 0 La. EXPT LA-2 oLa, EXPT LA-3 rn Th, EXPT LA-2 ATh. EXPT LA-3
x log NLa=A" + B"/T ,
indicating that a plot of the logarithm of the lanthanum concentration in the bismuth solution vs 1/T should be linear. As seen in Fig. 16.2, lanthanum behaved as predicted. It should be noted that a value of x cannot be determined from data obtained in the manner outlined above if thorium bismuthide is present as a second solid phase. An experiment (Nd-6) with neodymium as the rare earth was conducted in the same manner as experiments LA-2 and LA-3, and the results obtained were similar to those obtained with lanthanum. In addition, an approximate value of x = 1 was obtained in an experiment (E-70) conducted entirely at 640°C. In this experiment, neodymium was added to a thorium-bismuth solution, and the equilibrium concentrations in solution were determined after each addition. From Eq.(l), a plot of 42
' 0 . W T (rJK)
Fig. 16.2. Mutual solubilities of lanthanum and thorium in liquid bismuth solutions. Excess solid thorium bismuthide was present, tjxing the thorium concentration in solution.
7. C. E. Schilling and L. M. Ferris, J. Less-Common MetdS 20,155 (1970). 8. V. I. Kober et al., Russ. J. Phys: Chem. 42,360 (1968). 9. D. G. Schweitzer and J. R. Weeks, Trans. ASM 54, 185 (1961).