PREPARATION, CRYSTAL-STRUCTURE, AND PROPERTIES OF THE LANTHANOID CARBIDES LN(4)C(7) WITH LN=HO, ER, TM, AND LU

The isotypic carbides Ln(4)C(7) (Ln = Ho, Er, Tm, Lu) were prepared by are-melting of the elemental components, followed by annealing at 130 0 degrees C. The positions of the metal and of some carbon atoms of th e monoclinic crystal structure of Lu4C7 were determined from X-ray pow der data, and the last carbon positions were found and refined from ne utron powder diffraction data: P2(1)/c, a = 360.4(1), b = 1351.4(3), c = 629.0(2) pm, beta = 104.97(2)degrees, Z = 2, R = 0.026 for 429 stru cture factors and 15 positional parameters. The structure contains iso lated carbon atoms with octahedral lutetium coordination and linear C- 3-units, with C-C bond lengths of 132(1) and 135(1) pm. This carbide m ay therefore be considered as derived from methane and propadiene. The hydrolysis of Lu4C7 with distilled water yields mainly methane and pr opine, while the hydrolyses of the corresponding holmium and erbium ca rbides resulted in relatively large amounts of saturated and unsaturat ed C-2-hydrocarbons in addition to the expected products methane and p ropine. The structure comprises two-dimensionally infinite NaCl-type b uilding elements, which are separated by the C-3-units, It may be desc ribed as a stacking variant of a previously reported structure of Ho4C 7, now designated as the alpha-modification. The Lu4C7-type beta-modif ication was obtained at higher temperatures. Its structure was refined by the Rietveld method from X-ray powder data to a residual R = 0.037 for 320 F values and 15 positional parameters. LU(4)C(7) is Pauli par amagnetic; beta-Ho4C7 and Er4C7 show Curie-Weiss behavior with magneti c ordering temperatures of less than 20 K.