Pubhcaiion costs assisted by Argonne National LaboratoryEquilibrium hydrogen pressures ( P H J of lithium-lithium hydride mixtures sealed in iron capsules were measured over the following temperature-pressure-composition ranges: 710-903", 0.5-760 Torr, and 0.5-99 mol O/o LiH. The isotherms show the phase diagram to consist of two homogeneous terminal solutions separated by a miscibility gap whose boundaries range from 25.2 and 98.4 mol % LiM a t 730'' to 45.4 and 85.5 mol % LiH at 903". The data were fitted by a consistent thermodynamic treatment rising the Margules forms, for the activity coefficients. For each homogeneous solution region, this treatment yielded analytical expressions for the chemical potentials and activity coefficients of each species. The data show strong positive deviations from ideality of the solute species, especially in the LiH-rich solutions, where they may be related to the associative tendency of the lithium atoms. For dilute solutions of LiN in lithium, the ratio N~l~/ ( P~z ) 1~2= E(' is given by K'(atm-1/2) = exp(-6.498 + 6182T-l), where N L~H is the mol fraction of LiH and iP is the Kelvin temperature. For dilute solutions of Li in liquid LiH, the distribution of lithium between the gas and the solution phases is heavily weighted in favor of the latter, reflecting strong interactions between Li atoms and liquid LiH. The standard free energy of formation of liquid LIP1 IS given by .AGE' (kcal/mol) --13.47 X 10-"T -16.55.

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