Thermodynamics of the lithium--lithium deuteride system

Pressure-composition-temperature data were collected for the Li--LiD system in the ranges 0 to 750 Torr, 1 to 99 mol percent LiD, and 705 to 8710C by measuring equilibrium deuterium pressures over encapsulated Li--LiD mixtures. The data yielded a family of five P/sub D2//sup 1/2/ vs. N/sub LiD/ isotherms whose shapes indicate the existence of two homogeneous terminal solutions that are separated by a wide miscibility gap. Beginning at the consolute point (1000 +- 100C, 61 +- 3 mol percent LiD), the gap widens to a range of 21.3 to 99.0 mol percent LiD at the monotectic temperature (6890C). The mole-fraction solubility at the Li-rich gap boundary and plateau pressures above and below the monotectic temperature may be represented by N'/sub LiD/(satd) = exp(2.604 -- 3992T-1), P/sub P1,l/(Torr) = exp (21.21 -- 16940T-1), and P/sub P1,s/(Torr) = exp(28.04 -- 23510T-1), respectively. The data yielded the equilibrium constant [K(atm/sup -1/2/) = exp(-6.630 + 7995T-1)] for the reaction Li(soln) + 1/2D2(g) reversible LiD(soln) and expressions for the chemical potentials and activity coefficients of each species as functions of temperature and composition. Activity coefficients, evaluated at N/sub LiD/ → 0 and N/sub Li/ → 0, were combined with the equilibrium constant to generate temperature-dependence equations for various constants associated with dilute solutions. For the Li-rich limit, the Sieverts' constant [K' = N/sub LiD//(P/sub D2/)/sup 1/2/] is given by K'(mol fraction LiD/atm/sup 1/2/) = exp(-6.138 + 5599T-1). Advantages of the Ostwald coefficient in describing the distribution of the dissolved species between liquid and gas phase are discussed