HYDROGEN-INDUCED METAMAGNETISM IN R(6)FE(13)X COMPOUNDS

The ternary intermetallics R(6)Fe(13)X (R = Pr, Nd; X = Cu, Sb, Bi) al l crystallize in the tetragonal Nd6Fe13Si structure. Precise atom para meters and interatomic distances have been derived from a single cryst al X-ray refinement of Nd6Fe13Sb (R(F)= 3.6%). These compounds order m agnetically in the range of 450-550 K with compensated multisublattice structures having a net moment <1 mu(B)/formula unit. They absorb app roximately 13-20 hydrogen atoms/formula unit in an irreversible reacti on at 120 degrees C under a gas pressure of 1 bar, expanding their uni t cell volume by 10-16% without any change in the crystal symmetry and structure type. The increase of volume per hydrogen atom is similar t o 2.9 x 10(-3) nm(3)/H. The hydrogenated compounds are ferromagnetic w ith a moment of 31-33 mu(B)/formula unit. Mossbauer spectra df the par ent compounds and the hydrides are analysed in term of four hyperfine sextets corresponding to the four iron sites. There is a large increas e of average Fe-57 hyperfine field for the hydrides, of up to approxim ately 20% at 15 K (from 29.3 to 34.8 T) and 13% at 293 K (from 23.8 to 27.0 T). The change in magnetic structure with hydrogen is attributed to a change of sign of the weak antiferromagnetic coupling of the iro n sublattices.