Structural, Electronic, and Magnetic Properties of Ternary Rare-Earth Metal Borocarbides R5B2C5 (R=Y, Ce–Tm) Containing BC2 “Molecules”

Abstract

The borocarbides R5B2C5 (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 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 Hückel tight-binding and density-functional theory calculations. Results reveal a rather strong covalency between the metallic matrix and the BC2 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, TC, as well as the paramagnetic Curie temperatures, ΘP, 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.