CERIUM DIBORIDECARBIDE - A NEW RARE-EARTH BOROCARBIDE WITH THE TRIGONAL THB2C STRUCTURE

The new layer-type rare earth borocarbide of synthetic composition CeB2C was prepared by arc-melting cerium, boron and carbon on a water-cooled copper hearth under high purity argon atmosphere. CeB2C crystallizes in the rhombohedral space group R 3 ̄m with a = 6.6138(4) A ̊, c = 11.2594(4) A ̊, c a = 1.702, V = 426.5 A ̊3, z = 9 and thus is not isomorphous with the tetragonal lanthanide borocarbides REB2C reported earlier for the smaller rare earth metals, but presents the same structure as ThB2C. The compound was characterized by X-ray powder, metallographic and scanning electron microscopic techniques. The structure consists of infinite layers which alternate along the threefold axis. Within the planar B-C network, each atom is bonded to three other atoms to form aromatic-like fused six- and nine-membered rings. The cerium atoms form slightly puckered 63 metal layers alternating with the non-metal sheets. A notable feature of the rare earth borocarbides situated on the isopleth REB2 graphite is the laminar structure with covalently bonded B-C networks sandwiching the metal atoms. The structure of CeB2C ((Th, U)B2C) is the fourth representative found among these series. The representatives of the other types as well as metal size effects on their formations are discussed. © 1990.