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

Hg 6 Sb 5 Br 7 , Hg 6 As 4 BiCl 7 , and Hg 6 Sb 4 BiBr 7 were prepared from stoichiometric mixtures of Hg 2 X 2 , Hg X 2 ( X =Cl, Br), As, Sb, and Bi in sealed, evacuated glass ampoules in temperature gradients 260→240°C for Hg 6 Sb 5 Br 7 , 340→320°C for Hg 6 As 4 BiCl 7 , and 290→270°C for Hg 6 Sb 4 BiBr 7 . All compounds crystallize in the cubic space group Pa 3 with Z =4 and the lattice constants a =13.003(1) A for Hg 6 Sb 5 Br 7 , a =12.178(2) A for Hg 6 As 4 BiCl 7 , and a =12.998(4) A for Hg 6 Sb 4 BiBr 7 . The structures have been solved based on single-crystal X-ray diffraction data and refined to R ( F )=0.0431, 666 F o for Hg 6 Sb 5 Br 7 , R ( F )=0.0478, 690 F o for Hg 6 As 4 BiCl 7 , and R ( F )=0.0444, 840 F o for Hg 6 Sb 4 BiBr 7 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 A, Sb–Sb distance 2.78 A), each connected by six mercury atoms to six neighbored As 2 /Sb 2 groups. There are two different cages in the framework; one type is occupied by nearly regular MX 6 octahedra ( M =Sb,Bi; X =Cl, Br), the other by halide ions. The three compounds crystallize closely related to Cd 7 P 4 Cl 6 , which contains a similar polycationic framework of P 2 dumb-bells connected by Cd, but with only one type of cage occupied by octahedral [CdCl 6 ] 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 (Hg 6 Sb 4 ) 4+ [SbBr 6 ] 3− Br − , (Hg 6 As 4 ) 4+ [BiCl 6 ] 3− Cl − , and (Hg 6 Sb 4 ) 4+ [BiBr 6 ] 3− Br − .