Phase transitions in Ln_x⁽¹⁾ Ln_1-x⁽²⁾ H₂-H₃ alloy-hydrogen systems

Thermal h.c.p.-f.c.c. phase transitions for some Ln_x⁽¹⁾Ln_1-x⁽²⁾ H₂-H₃ systems were studied using combined TG-DTA techniques. Transition temperatures, transition enthalpy changes and composition limits were evaluated as a function of alloy composition, i.e. of x. These quantities were found to be influenced not only by the mean atomic number of the alloys but also by the nature of the elements. Irreversible h.c.p. → f .c.c. transitions occurred when the corresponding enthalpy changes were less than 15 kcal (mol H₂)^-1. A linear relation was obtained between differences in composition limits and reciprocal transition temperatures. This relation has been accounted for by a simplified statistical model based on the Bragg-Williams approximation.