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The crystal structure of phases in the pseudo-binary system CeCo1–хNiхC2 (x = 0, 0.33, 0.5, 0.67, 0.79, 0.80, 0.83, 1) was investigated by means of X-ray powder diffraction. Co richer solid solutions CeCo1–хNiхC2 (0≤ x ≤0.5) crystallize in the monoclinic CeCoC2-type structure; a = 5.3968(2) Å, b = 5.4013(3) Å, c = 7.4762(3) Å, β = 102.136(3)°, V = 213.06(3) Å3 for x = 0.5. Ni-rich CeNi1–yCoyC2 (0≤ y ≤0.2) are isotypic with the orthorhombic CeNiC2-type structure, a = 3.8486(2) Å, b = 4.5479(2) Å, c = 6.1531(3) Å, V = 107.70(1) Å3 for y = 0.2. In the intermediate region (0.5< x 0.21Ni0.79C2 and CeCo0.5Ni0.5C2, coexist. The non-isoelectronic substitution of Ni by Co in solid solutions CeNi1–yCoyC2 causes a continuous reduction of the Néel temperature and finally, for CeCoC2, results in a paramagnetic Kondo-lattice ground state.

New ternary phase Tb2Ni5C3 was gained in the three-phases sample of nominal composition after 90 d annealing at 800 °C. Crystal structure of Tb2Ni5C3 was determined using synchrotron powder diffraction: structure type La2Ni5C3, space group P4/mbm, a = 8.2372(1) A, c = 3.83402(6) A, RB = 5.0%; Rp = 1.8%. Crystallographic analysis and electronic structure calculations confirm the La2Ni5C3-isotypes including Tb2Ni5C3 refer to carbometalates. Metallic bonding is predominant in the R2Ni5C3 compounds. Novel La2Ni5C3-isotypic compounds were synthesized with rare earths Sm and Gd. Their unit cell parameters have been determined from X-ray powder diffraction data.

Magnetic properties of single crystalline HoCoC 2 and the evolution of magnetic and structural features in a series of polycrystalline solid solutions HoCo 1− x Ni x C 2 ( 0 ⩽ x ⩽ 1 ) are investigated by means of X-ray diffraction, magnetization, magnetic susceptibility and specific heat measurements. The crystal structures of all investigated samples refers to the CeNiC 2 -type structure (space group Amm 2 and Pearson symbol oS8). Non-isoelectronic substitution of Co by Ni causes a non-linear increase of the unit cell volume and especially a non-monotonous variation of the a and c lattice parameters as well as a pronounced reduction of the C–C bond length of carbon dimers. Temperature dependent magnetization and specific heat measurements reveal a crossover from a ferromagnetic for HoCoC 2 with T C = 10.6 (1) K to an antiferromagnetic ground state for HoNiC 2 with T N = 2.78 (6) K and a non-monotonous variation of the magnetic ordering temperature with a minimum at intermediate compositions. Crystalline electric field effects of HoCoC 2 and HoNiC 2 are analysed using combined thermodynamic and magnetic susceptibility data. The electrical resistivity of HoNiC 2 displays a distinct anomaly near room temperature which indicates the formation of a charge density wave (CDW) state as earlier reported for several other rare earth nickel dicarbides.