Single phases of Nd6Ni1.67Si3 and Nd6Co1.67Si3 were prepared by arc melting of a stoichiometric mixture of the elements followed by annealing at 1073 K. The structure of Nd6Co1.67Si3 was determined from single-crystal X-ray data with the space group P63/m and the lattice parameters a = 11.950(4) Å and c = 4.232(2) Å. Its structure derives from the structure of the aristotype compound Ce6Ni1.67Si3. A strong disorder of cobalt atoms in the chains of face-shared octahedra of neodymium is observed. Magnetization measurements reveal the existence of two ferro(ferri)magnetic transitions at 84(1) K and 38(1) K for Nd6Ni1.67Si3. For Nd6Co1.67Si3 it has been previously shown that also two ferro(ferri)magnetic transitions occur at 84 K and 35 K. The comparison between the magnetic properties of Nd6Ni1.67Si3 and Nd6Co1.67Si3 shows a significant difference of the behaviour of the magnetization M versus temperature and field μ0H below 84 K. The curves M = f(T) obtained by field-cooled (FC) process and under μ0H = 0.1 T show an increase of M below 35 K for Nd6Co1.67Si3 and on the contrary, a decrease of M below 38 K for Nd6Ni1.67Si3. This difference suggests the occurrence at low temperature of different magnetic structures for these two ternary silicides.< Leer menos