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 th e pure rare-earth metals, boron and carbon. The structural arrangement of t hese compounds, which crystallize in the tetragonal space group P4/ncc, con sists Of a three-dimensional framework of rare-earth atoms resulting from t he stacking of slightly corrugated two-dimensional squares, which lead to t he formation of octahedral holes and distorted bicapped square antiprismati c cavities filled with isolated carbon atoms and C-B-C chains, respectively . In contrast to the heavy rare-earth metal (Gd-Tm)-containing compounds wh ich melt congruently, those with the early rare-earth elements (Ce-Sm) are formed peritectically, The electronic structure and chemical bonding of Sm5 B2C5 and Gd5B2C5 are analyzed using extended Huckel tight-binding and densi ty-functional theory calculations. Results reveal a rather strong covalency between the metallic matrix and the BC, groups and the isolated carbon ato ms, respectively, similar to what is generally computed in related rare-ear th metal borocarbides. Magnetic susceptibility measurements indicate that a ll samples, which were investigated, undergo ferromagnetic transitions in t he temperature region below T = 130 K, The heavy rare-earth metal (Tb-Tm) b orocarbides exhibit a magnetic behavior typical of narrow-domain-wall ferro magnets, Both the Curie temperatures, T-C, as well as the paramagnetic Curi e temperatures, Theta (P), scale approximately with the de Gennes factor. H ence the indirect exchange interaction via conduction electrons (RKKY-inter acting) is the dominating force of the R-R coupling mechanism, (C) 2000 Aca demic press.