Phase transformations in some rare earth-hydrogen systems

The ranges of stability of Pr, Nd and Sm hydrides were investigated using simultaneous TG-DTA techniques. Measurements were made over a temperature range 25-700 °C, at 700 Torr hydrogen pressure. A new, low-temperature hexagonal phase has been detected in the NdH₂-NdH₃ system. Its structure is similar to the hexagonal, heavier lanthanide hydride phases. However, unlike these structures the NdH_{3 - x} hexagonal phase transforms irreversibly on heating to the f.c.c. structure, which remains metastable at room temperature. The enthalpy changes associated with the hexagonal to f.c.c. transitions in the NdH₂-NdH₃ and SmH₂-SmH₃ systems were estimated to be 1.6 ± 0.3 kcal/mole and 4 ± 0.4 kcal/mole, respectively. No phase transition has been detected in the PrH₂-PrH₃ system.