Hydrogen absorption and desorption in Nd2Fe17 and Sm2Fe17

The uptake of hydrogen by Nd₂ Fe₁₇ and Sm₂Fe₁₇ has been monitored in a thermopiezic analyser as a function of temperature at an initial pressure of 1 bar (10⁵ Pa). The first stage of hydrogen absorption around 250° C yields R₂Fe₁₇H_y (R = Nd, Sm) withy 2.2; this compound retains the Th₂Zn₁₇ structure of the starting alloy but the cell volume is increased by about 3%. The Curie temperature increases from 57 to 175° C for R = Nd and from 115 to 253° C for R = Sm. A second stage of hydrogen absorption at about 600° C corresponds to disproportionation of the alloy into -Fe + RH_2–.