Structural, electronic, and magnetic properties of ternary rare-earth metal borocarbides R5B2C5 (R = Y, Ce-Tm) containing BC2 "molecules"

The borocarbides R5B2C5 (R = Y, Ce-Tm) were prepared by are melting from the pure rare-earth metals, boron and carbon. The structural arrangement of these compounds, which crystallize in the tetragonal space group P4/ncc, consists Of a three-dimensional framework of rare-earth atoms resulting from the stacking of slightly corrugated two-dimensional squares, which lead to the formation of octahedral holes and distorted bicapped square antiprismatic cavities filled with isolated carbon atoms and C-B-C chains, respectively. In contrast to the heavy rare-earth metal (Gd-Tm)-containing compounds which melt congruently, those with the early rare-earth elements (Ce-Sm) are formed peritectically, The electronic structure and chemical bonding of Sm5B2C5 and Gd5B2C5 are analyzed using extended Huckel tight-binding and density-functional theory calculations. Results reveal a rather strong covalency between the metallic matrix and the BC, groups and the isolated carbon atoms, respectively, similar to what is generally computed in related rare-earth metal borocarbides. Magnetic susceptibility measurements indicate that all samples, which were investigated, undergo ferromagnetic transitions in the temperature region below T = 130 K, The heavy rare-earth metal (Tb-Tm) borocarbides exhibit a magnetic behavior typical of narrow-domain-wall ferromagnets, Both the Curie temperatures, T-C, as well as the paramagnetic Curie temperatures, Theta (P), scale approximately with the de Gennes factor. Hence the indirect exchange interaction via conduction electrons (RKKY-interacting) is the dominating force of the R-R coupling mechanism, (C) 2000 Academic press.