Low-Temperature Heat Capacities of Concentrated Lithium–Ammonia Systems

The heat capacities of Li/NH3 systems having the molecular compositions 12.00%, 14.14%, 20.40%, and 22.25% were measured in an adiabatic calorimeter from 60° to 110°K. Measurements on the 20.40% and 22.25% solution extended to about 200°K. First-order transitions were observed at 82.2° and 88.8°K and the associated heats of transition at each transition temperature were measured for the several compositions. Maximum enthalpy change is observed at a composition corresponding to NH3/Li = 4.15 and is consistent with the proposal that a solid compound, lithium tetrammine, is formed at low temperatures. The transition at 82.2°K is associated with a solid–solid phase transformation in Li(NH3)4 [or Li(NH3)4.15]; the transition at 88.8°K is discussed in terms of the equilibria which would have to occur in order to account for the formation of lithium tetrammine.