The phase diagram of (lithium + ammonia) in the vicinity of the compound Li(NH3)4

Abstract

Further examination of the transitions in {xLi + (1 − x)NH3} by means of warming curves and adiabatic calorimetry has been undertaken to determine the dependence of the transition temperatures on composition. For x ⩽ 0.2 the temperature of the upper transition previously observed at 88.79 K was verified. However, for x > 0.2 the upper transition was found to be at (88.91±0.01) K. The lower transition (solid-to-solid) was found at (82.23±0.03) K near the temperature previously reported. The occurrence of two transitions slightly below 89 K is regarded as further indication of the existence of the compound Li(NH3)4 which presumably forms (1) a eutectic at 88.80 K also involving NH3(s) and a saturated solution for x < 0.2, and (2) a second eutectic at 88.91 K which involves Li(s) and a saturated solution, as well as Li(NH3)4(s), for x > 0.2. Alternatively, the 88.91 K transition may be regarded as a peritectic transition, although results requiring or eliminating this interpretation have not been acquired. It is proposed that the solid-to-solid transition at 82.23 K is an order-disorder transition, involving NH3 molecules within each Li(NH3)4 cluster, which accompanies the hexagonal-to-f.c.c. phase change of Li(NH3)4(s). If such is the case, the absence of a corresponding solid-to-solid transition close to the eutectic temperatures of (lithium + fully deuterated ammonia) may be explained as an isotope effect which results in the elevation of the temperature at which disordering occurs to above, or to coincidence with, the eutectic temperatures.