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.

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. Tb15B4C14 is a new member of the rare-earth metal boride carbide series in which the finite quasi-molecular CBC entities as well as isolated C atoms are embedded in the voids of the metal atom matrix. The structure of Tb15B4C14 contains two types of slabs: one slab contains finite bent CBC units and isolated carbon atoms whereas another is formed only from octahedral coordinated single carbon atoms. The electronic structure for the idealized composition corresponds to an electron partitioning according to (Tb3+)15(C4−)6(CBC5−)4
• e− giving rise to a single electron per formula for Tb–Tb framework bonding. The magnetism of the ternary rare earth boride carbides R15B4C14 (R=Tb, Dy, Er) is characterized by the onset of ferromagnetic order below T<150 K.Figure optionsDownload as PowerPoint slide