THE GADOLINIUM CARBIDE HALIDES, GD(4)C(2)X(3) (X=CL,BR)

The compounds Gd(4)C(2)X(3) (X = Cl, Br) and Tb4C2Br3 have been prepar ed by reaction of the metals (RE), REX(3), and C in sealed Ta capsules at 1100 degrees and 1300 degrees C, respectively. Monophasic samples of Gd4C2Br3 and Tb4C2Br3 were obtained by reacting stoichiometric mixt ures of the starting materials for five days. The needle shaped crysta ls are bronze-coloured and sensitive to air and moisture. Gd(4)C(2)X(3 ) crystallizes in the space group Pnma (No. 62) with lattice constants a = 1059.6(4), b = 368.4(1), c = 1962.7(8)pm (Gd4C2Cl3), a = 1084.4(1 ), b = 373.0(1), c = 2036.1(1)pm (Gd4C2Br3). According to Guinier phot ographs, Tb4C2Br3 is isotypic (a = 1074.3(2), b = 370.6(1), c = 2019.4 (1) pm). In the crystal structure C is octahedrally coordinated by Gd. The Gd-6 octahedra are linked via common edges to form corrugated lay ers. The X-anions coordinate all free edges and corners of these layer s and connect them via X(i)-X(i) contacts parallel [001]. Gd4C2Br3 sho ws metallic conductivity. The magnetic susceptibility follows at high temperatures a Curie Weiss law with an effective moment of 7.95 mu(B). At temperatures below 50 K antiferromagnetic order is observed.