PR2REC2 AND OTHER TERNARY RARE-EARTH-METAL RHENIUM CARBIDES WITH FILLED PBCL2 (CO2SI) STRUCTURE

The carbides R2ReC2 (R = Y, CeNd, Sm, GdTm, Lu) were prepared by arc-melting of cold-pressed pellets of the elemental components. They crystallize with the orthorhombic space group Pnma and Z = 4 formula units per cell. The lattice constants for Pr2ReC2 are; a = 655.69(6) pm, b = 509.46(5) pm, c = 984.42(8) pm. The crystal structure was determined from quantitative evaluations of Guinier powder data for Y2ReC2 and Pr2ReC2, which refined to conventional residuals of R = 0.088 and R = 0.072, respectively. The structure of the isotypic carbide Er2ReC2 was determined from single-crystal diffractometer data: R = 0.032 for 842 structure factors and 26 variable parameters. The positions of the metal atoms correspond to those of the Co2Si-branch of the "anti"-PbCl2 structure. The carbon atoms are isolated from each other and occupy two different octahedral voids (4R + 2Re, 5R + 1Re). There are no bonding ReRe interactions in the one-dimensionally infinite [ReC-62]n polyanions. The ReC bond lengths of between 192 pm and 203 pm suggest bond orders of about two and the rhenium atoms seem to obey the 18-electron rule. The metal atom positions of the Pr2ReC2 type carbides correspond to those of a distorted cubic body-centered structure, as is also found for several other complex carbides. The average valence electron number of the metal atoms is usually greater in unfilled (host) structures than in the corresponding filled (interstitial) structures. © 1990.