Polyborides with Th2NiB10-type structure: Synthesis, crystal structure, and magnetic and electrical properties

The new rare-earth nickel polyborides R2NiB10 (R = Y, Ce-Nd, Sm, Gd-Ho) wer e prepared by reacting cold-pressed pellets of the elemental components in an are-melting furnace, followed by annealing in evacuated silica tubes. Th ey crystallize with the orthorhombic Th2NiB10-type structure, which was ref ined from X-ray powder diffractometer data of Ce2NiB10 (a = 565.4(1) pm, b = 1125.8(2) pm, c = 419.6(1) pm) and Nd2NiB10 (a = 561.4(1) pm, b = 1119.2( 2) pm, c = 417.7(1) pm). The magnetic properties of these polyborides and a lso of Th2CoB10 and Th2NiB10 were determined with a superconducting quantum interference magnetometer between 2 and 400 K with magnetic flux densities up to 5.5 T. Y2NiB10, Th2CoB10, and Th2NiB10 are Pauli paramagnetic. The o thers show Curie-Weiss behavior with magnetic moments corresponding to thos e of the free R3+ ions, with the exception of Ce2NiB10, where cerium has a mixed valence and Sm2NiB10 with the van Vleck paramagnetic Sm3+ ion. Antife rromagnetic order was observed for the compounds R2NiB10 (R = Pr, Nd, Sm, G d-Ho) with Neel temperatures ranging between 8 It for Pr2NiB10 and 33 K for Gd2NiB10. Tb2NiB10, Dy2NiB10, and Ho2NiB10 are metamagnetic. Electrical re sistivity measurements reflect the magnetic ordering temperatures and indic ate metallic behavior for all compounds. (C) 2000 Academic Press.