HYDROGENATION BEHAVIOR AND STRUCTURE OF R5FE2B6 (R=CE, PR, ND, SM, GDAND TB) BORIDES

The hydrogenation behaviour and structural characteristics of the R5Fe 2B6(R=Ce, Pr, Nd, Sm, Gd and Tb) borides have been studied in the pres ent work. Hydride formation is accompanied by an anisotropic expansion of the initial trigonal unit cells (Pr5Co2B6 type crystal structure) in the [001] direction only. Heating in hydrogen of the Pr and Nd cont aining borides results in their disproportionation and the formation o f the R-hydride and two compounds, namely R1.1Fe4B4 and R2B5. Recombin ation to form the initial (Pr,Nd)(5)Fe2B6 borides is observed on vacuu m heating of this three phase mixture. The disappearance of additional phases present in the cast material, demonstrates another example of the successful use of the hydrogenation-disproportionation-desorption and recombination process for the homogenisation of a rare earth conta ining alloy. Under the same conditions, reversible hydrogen absorption -desorption takes place in the case of the corresponding Sm, Gd and Tb compounds indicating an increase of the thermodynamic stability of th e borides with increasing atomic number of the R-component. Hydrogen a bsorption-desorption properties were characterised by means of tempera ture-pressure analysis and hydrogen differential thermal analysis. An increase in the thermal stability of the R5Fe2B6Hx-hydrides with incre asing atomic number of the R-component is consistent with the insertio n of the hydrogen atoms into the structural slabs consisting of the ra re earth atoms only.