B5C5 anionic molecules trapped in a solid matrix:: The crystal and electronic structures of LaBC

The crystal structure of LaBC, prepared by arc melting of mixtures of the pure elements and subsequent annealing at 1000 degrees C, has been determined from single crystal X-ray diffraction data. It crystallizes in a new structure type with the orthorhombic space group P2(1)2(1)2(1) (a = 8.646(1), b = 8.691(1), c = 12.479(1) angstrom, R1 = 0.021 (wR2 = 0.057) for 4437 reflections with I-o q > 2 sigma(I-o)). Its structural arrangement consists of a three-dimensional framework of lanthanum atoms resulting from the stacking of slightly corrugated two-dimensional square nets, leading to voids filled with B5C5 finite chains. Isostructural compounds are formed with Ce, Pr, Nd and Sm. Structural and theoretical analysis suggests a formulation as (La-5)(8.3+)(B5C5)(8.3-). Density functional theory calculations reveal significant covalent bonding between the metal atom framework and the boron carbon chains.