THE CRYSTAL STRUCTURE OF VASILYEVITE, (Hg2)102+ O6 I3 (Br,Cl)3 (CO3)

Abstract

The crystal structure of vasilyevite, ideally (Hg 2 ) 2 + 10 O 6 I 3 Br 2 Cl (CO 3 ), triclinic, a 9.344(2), b 10.653(2), c 18.265(4) A, a 93.262(5), β 90.548(4), γ 115.422(4)°, V 1638.3(9) A 3 , space group P1, Z = 2, D(calc) = 9.57 g cm - 3 , has been solved by direct methods and refined to an R 1 index of 8.9% for 3400 unique reflections measured with MoKa X-radiation on a four-circle diffractometer equipped with a CCD detector. There are twenty unique Hg sites that are associated as pairs of sites with pair separations of 2.533 to 2.593 A; this association of Hg atoms is typical of Hg + cations pairing to form (Hg 2 ) 2 + dimers. Each (Hg 2 ) 2 + dimer is extended at each end by bonding to axial O atoms with Hg-O distances in the range 2.00 to 2.32 A, with one dimer showing a terminal I atom with a very short Hg-I distance of 2.75 A. These [Φ-Hg-Hg-Φ] tetramers form near-linear arrangements that are surrounded by several meridional anions, O atoms at distances from 2.44 to 2.85 A, mixed (Cl,Br) sites at distances from 2.89 to 3.78 A, and I atoms at distances from 3.11 to 4.02 A. Bonds to axial anions make angles of <23° with the Hg-Hg axis, involving the Hg atoms to which they are bonded; bonds to meridional anions make angles of <31 ° with the perpendicular to the Hg-Hg axis, involving the Hg atoms to which they are bonded. There is one site, labeled C,S, that is surrounded by a triangle of O atoms at a mean distance of 1.25 A; the presence of S in the chemical composition and the refined scattering at the C,S site indicates that this site is occupied by both (CO 3 ) and S 2 - in the ratio 0.85: 0.15. There are three distinct I sites occupied solely by I - anions that are [8]- or [9]-coordinated by Hg cations. There are three X sites that are occupied by 0.73 Br + 0.27 Cl, 0.69 Br + 0.31 Cl, and 0.22 Br + 0.78 Cl and are [7]- and [8]-coordinated by Hg cations. The fundamental building block of the vasilyevite structure is the strongly bonded [Φ-Hg-Hg-Φ] tetramer. These groups link together at their anion vertices to form a complex interpenetrating framework of two discrete three- and four-connected nets. The halogen atoms and the (CO 3 ) and S 2 - anions occur in the interstices of the two nets and serve to stabilize the nets via additional weak linkages to He atoms.