New quaternary carbon and nitrogen stabilized polyborides: REB15.5CN (RE: Sc, Y, Ho, Er, Tm, Lu), crystal structure and compound formation

A new family of quaternary carbon and nitrogen containing Rare Earth (RE: Sc, Y, Ho, Er, Tm and Lu) borides: REB_15.5CN, has been synthesized and structurally characterized by powder X-ray diffraction data. They are all isotypic with Sc_1−xB_15.5CN whose structure was solved based on single-crystal X-ray data and HRTEM investigations. The structure refinement converged at a R(F²) value of 0.044 for 364 reflections. The new structure type of Sc_1−xB_15.5CN is composed of a three-dimensional network based on interconnected slabs of boron (B₁₂)^ico icosahedra and (B₆)^oct octahedra. A linear CBC] chain and nitrogen tightly bridges icosahedra. Sc partially occupies voids in the sheets of boron octahedra. It crystallizes with the trigonal space group P³m1, with Z=2. Lattice parameters (nm) are as follows: for RE: Sc, a,b=0.5568(4), c=1.0756(2); Y, a,b=0.55919(6), c=1.0873(2); Ho, a,b=0.55883(7), c=1.0878(6); Er, a,b=0.55889(5), c=1.0880(6); Tm, a,b=0.5580(1), c=1.0850(6); Lu, a,b=0.55771(9), c=1.0839(4). Magnetic characterization of ErB₁₇C_1.3N_0.6 has been performed.