Compounds and phase relationships in the lithium-rhodium-hydrogen system

The compounds and phase relationships in the lithium-rhodium-hydrogen system have been characterized. In addition to the well-known saline hydride LiH, the compounds that exist in this system are the intermetallic compound LiRh and the two ternary hydrides Li4RhH4 and Li4RhH5. No evidence was found, at hydrogen pressures up to 1 atm and temperatures up to 6000C, for reaction between hydrogen and rhodium. Pressure-composition isotherms were determined over the temperature range 6000C - 8000C and at pressures up to 1 atm. The gram atom fraction of hydrogen is the condensed phase ranged from 0 to 0.5; the latter limit corresponds to the ternary hydride Li4RhH5. No plateau (invariant) pressure ranges were observed at these temperatures. The integral heat of solution to form a liquid of composition Li4RhH4 was determined as -12.5 kcal (g atom H)-1. Pressure-composition isotherms were also determined by hydriding and dehydriding the intermetallic compound LiRh over the temperature range from room temperature to 7000C and at pressures up to 1 atm. The isotherms determined in these experiments are presented and discussed in the text. (Auth.)