The borocarbides R 5 B 2 C 5 (R ؍ Y, Ce+Tm) were prepared by arc 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 threedimensional 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 5lled with isolated carbon atoms and C+B+C chains, respectively. In contrast to the heavy rareearth 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 Sm 5 B 2 C 5 and Gd 5 B 2 C 5 are analyzed using extended Hu4 ckel tight-binding and density-functional theory calculations. Results reveal a rather strong covalency between the metallic matrix and the BC 2 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, 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.
2000 Academic press