Synthesis and Crystal Structures of Hg6Sb5Br7, Hg6As4BiCl7, and Hg6Sb4BiBr7, Built of a Polycationic Mercury-Pnictide Framework with Trapped Anions

Hg6Sb5Br7, Hg6As4BiCl7, and Hg6Sb4BiBr7 were prepared from stoichiometric mixtures of Hg2X2, HgX2 (X=Cl, Br), As, Sb, and Bi in sealed, evacuated glass ampoules in temperature gradients 260->240°C for Hg6Sb5Br7, 340->320°C for Hg6As4BiCl7, and 290->270°C for Hg6Sb4BiBr7. All compounds crystallize in the cubic space group Pa with Z=4 and the lattice constants a=13.003(1) ? for Hg6Sb5Br7, a=12.178(2) ? for Hg6As4BiCl7, and a= 12.998(4) ? for Hg6Sb4BiBr7. The structures have been solved based on single-crystal Xray diffraction data and refined to R(F)=0.0431, 666 Fo for Hg6Sb5Br7, R(F)=0.0478, 690 Fo for Hg6As4BiCl7, and R(F)=0.0444, 840 Fo for Hg6Sb4BiBr7 with 30 parameters for each refinement. The structures are characterized by a three-dimensional polycationic framework of pnictide dumb-bells (As-As distance 2.43 ?, Sb-Sb distance 2.78 ?), each connected by six mercury atoms to six neighbored As2/Sb2 groups. There are two different cages in the framework; one type is occupied by nearly regular MX6 octahedra (M=Sb,Bi; X=Cl, Br), the other by halide ions. The three compounds crystallize closely related to Cd7P4Cl6, which contains a similar polycationic framework of P2 dumb-bells connected by Cd, but with only one type of cage occupied by octahedral [CdCl6]4- ions. The interactions between the atoms of the polycationic framework and the anions are very weak. The observed diamagnetism of all three compounds is in agreement with the ionic formulas (Hg6Sb4)4+[SbBr6]3-Br-, (Hg6As4)4+[BiCl6]3-Cl-, and (Hg6Sb4)4+[BiBr6]3-Br-.