New ternary rare-earth metal boride carbides R15B4C14 (R=Y, Gd-Lu) containing BC2 units: Crystal and electronic structures, magnetic properties

The ternary rare-earth boride carbides R₁₅B₄C₁₄ (R=Y, Gd-Lu) were prepared from the elements by arc-melting followed by annealing in silica tubes at 1270 K for 1 month. The crystal structures of Tb₁₅B₄C₁₄ and Er₁₅B₄C₁₄ were determined from single crystal X-ray diffraction data. They crystallize in a new structure type in space group P4/mnc (Tb₁₅B₄C₁₄: a=8.1251(5) Å, c=15.861(1) Å, Z=2, R₁=0.041 (wR₂=0.088) for 1023 reflections with I_o>2σ(I_o); Er₁₅B₄C₁₄: a=7.932(1) Å, c=15.685(2) Å, Z=2, R₁=0.037 (wR₂=0.094) for 1022 reflections with I_o>2σ(I_o)). The crystal structure contains discrete carbon atoms and bent CBC units in octahedra and distorted bicapped square antiprisms, respectively. In both structures the same type of disorder exists. One R atom position needs to be refined as split atom position with a ratio 9:1 indicative of a 10% substitution of the neighboring C⁴⁻ by C₂⁴⁻. The actual composition has then to be described as R₁₅B₄C_14.2. The isoelectronic substitution does not change the electron partition of R₁₅B₄C₁₄ which can be written as (R³⁺)₁₅(C⁴⁻)₆(CBC⁵⁻)₄•e⁻. The electronic structure was studied with the extended Hückel method. The investigated compounds Tb₁₅B₄C₁₄, Dy₁₅B₄C₁₄ and Er₁₅B₄C₁₄ are hard ferromagnets with Curie temperatures T_C=145, 120 and 50 K, respectively. The coercive field B_C=3.15 T for Dy₁₅B₄C₁₄ is quite remarkable.