THE RARE-EARTH CARBIDES R4C5 WITH R = Y, GD, TB, DY, AND HO

The five carbides R4C5 (R = Y, Gd-Ho) have been prepared by are-meltin g cold-pressed pellets of the elemental components and subsequent anne aling at 1050 degrees C, They crystallize with a new orthorhombic stru cture type (Pbam, Z = 2), which has been determined from X-ray powder diffractometer data of Y4C5 (a = 657.35(9) pm, b = 1191.8(1) pm, c = 3 66.92(5) pm, R-F = 0.035 for 179 structure factors) and also refined f rom powder data of Tb4C5 (a = 660.8(1) pm, b = 1197.3(2) pm, c = 368.7 1(6) pm, R-F = 0.035 for 181 F values) and Ho4C5 (a = 653.00(8) pm, b = 1184.6(1) pm, c = 363.80(4) pm, R-F = 0.036 for 171 F values and 10 positional parameters each). The structure contains building blocks, w hich were also found in the two closely related structures of alpha- a nd beta-Ho4C7. One-fifth of the carbon atoms are isolated from each ot her and coordinated octahedrally by rare earth atoms, The other carbon atoms form pairs with C-C bond distances of 133.5(15), 136(3), and 12 9(2) pm in Y4C5, Tb4C5, and Ho4C5, respectively, corresponding to C-C double bonds. Hence, all valence electrons can be accommodated in bond ing R-C and C-C states according to the formula (R+3)(4)(C-2(-4))(2)C- 4, where the superscripts represent oxidation numbers, Nevertheless, t he hydrolyses of Y4C5 and Ho4C5 with distilled water resulted in up to 41 wt% ethane, up to 16 wt% propane and propene, and up to 18 wt% hig her hydrocarbons in addition to the expected products methane and ethy lene. (C) 1997 Academic Press.