Thermodynamics of the (Sm2Fe17-xGax+H-2) system

Sm2Fe17-xGax samples (with x = 0, 0.5, 1, and 2) were disproportionated int o SmH2 and alpha-(Fe,Ga) by heating the samples in 4 MPa hydrogen up to 650 degrees C. It was possible to disproportionate even the more stable compou nds with a higher Ga content under these conditions. The recombination into the original 2:17 phase was can-led out by annealing in low hydrogen press ures of 2.5, 12 and 60 kPa. The values of the hydride formation enthalpy, D elta H, and of the change of entropy, Delta S, for the Sm2Fe17-xGax compoun ds were obtained by using the Van't Hoff relation. It was found that the ne gative Delta H increases monotonically from 141 to 229 kJ/mol H-2 and the n egative Delta S increases monotonically from 112 to 258 J/Kmol H-2 for x = 0 to x = 2, respectively. These results show that the higher stability for the compounds with increasing Ga content is mainly attributed to the increa sing negative change of entropy - Delta S and not, as expected before, to a decrease of - Delta H. (C) 2000 Elsevier Science S.A. All rights reserved.