Studies of P -C isotherms in RNi5-H (R: La, Pr, Nd, Sm, Gd, Tb and Dy) systems

We have investigated the hydrogenation and dehydrogenation properties of binary RNi5 (R: La, Pr, Nd, Sm, Gd, Tb and Dy) intermetallic compounds in the pressure range of 0.1-35 MPa and temperature range of 223-298 K. Pressure-composition isotherms demonstrated that as the atomic number of R increased in RNi5-H systems, the single pressure plateau (LaNi5) split into two plateaux (PrNi5, NdNi5, SmNi5 and GdNi5), and then into three plateaux (TbNi5 and DyNi5), indicating the presence of two hydrides (β2 and γ2 phases) and then three hydrides (β3, γ3 and δ3 phases). The β3-γ3 and γ3-δ3 phase transitions in the TbNi5-H system corresponded, respectively, to the α2-β2 and β2-γ2 transitions in the RNi5-H (R: Pr, Nd, Sm and Gd) systems having two plateaux.