Systematic study of the h.c.p. ⇌ f.c.c. transitions in the heavier LnH₂-LnH₃ systems

Thermal h.c.p. ⇌ f.c.c. phase transitions of all the trivalent, heavier LnH₂-LnH₃ systems (Nd-Lu with the exception of Eu and Yb) were systematically studied using combined TG-DTA techniques. Transition temperatures, transition enthalpy changes and composition limits were evaluated. It has been shown that the trend in the lanthanide series is towards increasing stabilities of the h.c.p. trihydrides' structures with increasing atomic number of the lanthanide. The composition limits of the f.c.c. structures decrease with increasing atomic number, whereas the composition limits of the h.c.p. structures are almost independent of their position in the series. Some pseudo-binary Ln⁽¹⁾_0.5Ln⁽²⁾_0.5H₂-Ln⁽¹⁾_0.5Ln⁽²⁾_0.5Ha₃(Ln⁽¹⁾ = Pr, Nd; Ln⁽²⁾ = Tb, Er) systems were also analysed and exhibited intermediate properties with respect to the above-mentioned phase transitions.