Crystal, electronic and magnetic structures of a novel series of intergrowth carbometalates R₄Co₂C₃ (R = Y, Gd, Tb)

A series of new ternary isostructural R₄Co₂C₃ (R = Y, Gd, Tb) carbides was synthesized by annealing of arc-melted stoichiometric samples. The crystal structure of Tb₄Co₂C₃ [space group P2/m, Pearson symbol mP18, a = 12.754(2) Å, b = 3.6251(4) Å, c = 7.0731(9) Å, β = 105.601(6)°] was solved by direct methods from neutron powder diffraction data collected at 100 K. The room temperature unit cell parameters of the new phases were determined by X-ray powder diffraction technique. The crystal structure of Tb₄Co₂C₃ is characterized as an intergrowth structure resulting from the stacking of alternating TbCoC (YCoC-type) and Tb₂C (anti-CdCl₂ type) fragments with a 2 : 1 ratio. Tb₄Co₂C₃ orders ferromagnetically at T_C = 35(1) K, whereas the isostructural Gd₄Co₂C₃ reveals two magnetic transitions at T_C1 = 82(3) K and T_C2 = 13(2) K. Density functional theory (DFT) calculations confirm that the magnetic moments of the R₄Co₂C₃ (R = Gd, Tb) carbides are exclusively due to the rare-earth elements. Y₄Co₂C₃ is shown to be a Pauli-paramagnet by experimental and theoretical studies.