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 Io>2σ(Io); Er15B4C14: a=7.932(1) Å, c=15.685(2) Å, Z=2, R1=0.037 (wR2=0.094) for 1022 reflections with Io>2σ(Io)). 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 R15B4C14.2. 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 TC=145, 120 and 50 K, respectively. The coercive field BC=3.15 T for Dy15B4C14 is quite remarkable.