Thermodynamic and magnetic properties of RCo5 hydrides

Abstract The intermetallic compounds RCo 5 (R = rare‐earth metal) form ferromagnetic hydrides. For some of the RCo 5 ‐H systems (R = La. Pr. Nd, Sm, Gd) room‐temperature isotherms have been measured. These isotherms show two‐phase regions. At 300 K the hexagonal α‐phase has a limited hydrogen solubility (<0.3 H/RCo 5 ). In the LaCo 5 ‐H system two hydrides, β I and γ, are observed. In the PrCo 5 ‐H and NdCo 5 ‐H systems two structurally closely related hydrides, β I and β II , are found. β I has a C‐centered orthorhombic structure with stoichiometric composition RCo 5 H 4 . At lower hydrogen concentration the superstructure β II is formed which has a doubled c ‐axis compared with β I . A neutron‐diffraction study showed the hydrogen positions in the lattice of the β I phase to be (1/4, 1/4, 0) and (x, 0, 1/2). The magnetization of the RCo 5 compounds is influenced by the absorption of hydrogen. The Co moments become smaller and the coupling of the R moments to the Co moments is weakened. In all cases, except GdCo 5 , this leads to a decrease of the magnetization. The absorption of hydrogen by PrCo 5 and NdCo 5 decreases the temperature at which the easy direction of the magnetization leaves the c ‐axis.