Tb16Br23B4: Tetrameric terbium clusters with endohedral boron atoms

The new cluster compound Tb16Br23B4 was prepared from a stoichiometric mixt ure of Tb-metal. TbBr3 and B-powder under Ar-atmosphere in scaled Ta ampoul es at 920-950 degreesC. It crystallizes monoclinic in the space group C2/m with a = 17.523(4) Angstrom, b = 12.008(2) Angstrom, c = 11.901(2) Angstrom und beta = 103.95(3)degrees. In the crystal structure B-centered Tb-6 octa hedra are connected via common edges to form tetrameric units. The Br atoms connect the Tb16B4-Clusters 3-dimensionally coordinating the unoccupied ed ges and corners of the octahedra. Tb16Br23B4 is a semiconductor with an ele ctrical band gap of E-g = 0.4 eV. The magnetic susceptibility follows a Cur ie-Weiss law corresponding to an effective magnetic moment mu (eff) = 9.55 mu (B) at high temperatures with an antiferromagnetic ordering below 20 K.