Decomposition pressures in the (α + β) fields of the Li-LiH, Li-LiD, and Li-LiT systems

Decomposition (“plateau”) pressures of H 2, D 2, and T 2 over Li-LiH, Li-LiD, and Li-LiT alloys were determined between 600 and 850°C, for pressures ranging from 3 to 460 Torr, and for alloy compositions falling within the (α + β) miscibility gaps. The measurements were carried out separately for each hydrogen isotope using the same lithium sample and experimental procedures. For each system the ln P vs. 1/ T data form a pair of linear segments, the intersection of which represents the monotectic temperature (694°C for Li-LiH, 690°C for Li-LiD, and 688°C for Li-LiT). For a given temperature plateau pressures were in the order P T 2 > P D 2 > P H 2 . The D H and T H isotope effects in pressures varied from 1.48 and 1.74 at 600°C to 1.34 and 1.45 at 850°C. The results were used to calculate the standard free energies of formation of solid LiH, LiD, and LiT. The tritium gas used in this study had significant amounts of hydrogen and deuterium. A method for correcting the plateau pressures of this mixture to those of pure tritium is presented.