New Ternary Boride Carbides RE10B9+xC10–x (RE = Gd,Tb; x ≈︁ 0.2): Infinite Boron Carbon Branched Chains

New ternary rare earth metal boride carbides with compositions close to RE₁₀B₉C₁₀ (RE = Gd, Tb) were prepared from the elements by melting around 1800 K followed by annealing in silica tubes at 1270 K for one month. The crystal structure of the terbium compound was solved by single‐crystal X‐ray diffraction. It crystallizes in a new structure type in the monoclinic space group P2₁/c, a = 7.937(1), b = 23.786(2), c = 11.172(1) Å, β = 133.74(1)°, Z = 4, R1 = 0.045 (wR2 = 0.11) for 5713 reflections with I_o > 2σ(I_o). In the structure BC₂ units and single carbon atoms are attached to a zigzag boron chain forming the unprecedented B₁₈C₁₈ branching unit with a B–B distance of 2.42(2) Å between these units. In addition isolated carbon atoms occupy the centres of elongated octahedra formed by rare earth metal atoms. Disorder in the terbium position together with anomalous displacement ellipsoids for carbon atoms except of those in the BC₂ fragments can be rationalized in terms of a slight deviation in stoichiometry, Tb₁₀B_9+xC_10–x (x ≈? 0.2). The terbium compound is ferromagnetic below T_C ≈? 45 K. Due to the presence of moderately narrow domain walls the magneto‐crystalline energy is small.