The ternary rare-earth boride carbides R 15 B 4 C 14 (R¼ Y, Gd-Lu) were prepared from the elements by arcmelting followed by annealing in silica tubes at 1270 K for 1 month. The crystal structures of Tb 15 B 4 C 14 and Er 15 B 4 C 14 were determined from single crystal X-ray diffraction data. They crystallize in a new structure type in space group P4/mnc (Tb 15 B 4 C 14 : a ¼8.1251(5) Å, c ¼ 15.861(1) Å, Z¼ 2, R 1 ¼0.041 (wR 2 ¼ 0.088) for 1023 reflections with I o 42s(I o ); Er 15 B 4 C 14
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 C 4 À by C 2 4 À . The actual composition has then to be described as R 15 B 4 C 14.2 . The isoelectronic substitution does not change the electron partition of R 15 B 4 C 14 which can be written as (R 3 + ) 15 (C 4 À ) 6 (CBC 5 À ) 4 e À . The electronic structure was studied with the extended H ückel method. The investigated compounds Tb 15 B 4 C 14 , Dy 15 B 4 C 14 and Er 15 B 4 C 14 are hard ferromagnets with Curie temperatures T C ¼145, 120 and 50 K, respectively. The coercive field B C ¼3.15 T for Dy 15 B 4 C 14 is quite remarkable.