These compounds crystallize in the hexagonal structure (space group P6 3 /m) related to the Gd 6 Co 1.67 Si 3 -type. The magnetization measurements reveal that: (i) the cerium exhibits a trivalent state in Ce 6 Co 1.67 Si 3 but no magnetic ordering is evidenced above 1.8 K; (ii) on the contrary, two ferro(ferri)magnetic transitions are detected for Nd 6 Co 1.67 Si 3 (84 and 35 K) and Tb 6 Co 1.67 Si 3 (183 and 39 K); in both case, a remanence phenomena appears for temperatures below the higher magnetic transition; (iii) similar behavior does not exist for Gd 6 Co 1.67 Si 3 which shows only one ferromagnetic transition below T C = 294 K. The occurrence of a complex magnetic phase diagram for Nd 6 Co 1.67 Si 3 and Tb 6 Co 1.67 Si 3 can be correlated to the existence of two crystallographic sites for rare earth atoms in this new family of compounds.

International audienceThe hydride CeRuSiH1.0 with space group P4/nmm was synthesized by exposure at 523 K of the heavy-fermion ternary silicide CeRuSi under 4 MPa of hydrogen gas. The investigation of the hydride by x-ray powder diffraction reveals that the hydrogenation induces a pronounced anisotropic expansion of the unit cell. Moreover, CeRuSiH1.0 presents two antiferromagnetic transitions at TN1=7.5(2) K and TN2=3.1(2) K evidenced by magnetization and specific heat measurements. Hydrogenation changes the moderate heavy-fermion compound CeRuSi, which has a gamma=220 mJ/mol K2, to an antiferromagnet, which has a smaller electronic coefficient gamma=26 mJ/mol K2. In other words, the hydrogen insertion diminishes the influence of the Kondo effect. The transition heavy-fermion behavior-->antiferromagnet can be well understood in terms of the classical Doniach diagram where the hydrogenation plays a role opposite to the pressure. The expansion of the lattice induced by hydrogen insertion is here much more important than the role of Ce-H bonding observed in other hydrogenated compounds CeCoSiH1.0 or CeCoGeH1.0, where an opposite transition (antiferromagnetic-->spin fluctuation) was evidenced