Preparation and crystal structures of (Hg-3)(PO4)Cl, (Hg-3)(AsO4)Cl and (Hg-3)(AsO4)Br - Mercury compounds with the triangular Hg-3(4+) cluster

The compounds (Hg-3)(PO4)Cl, (Hg-3)(AsO4)Cl and (Hg-3)(AsO4)Br have been pr epared under hydrothermal conditions at 200 degreesC, starting from stoichi ometric mixtures of the corresponding mercury halides and mercury phosphate s or arsenates, respectively. The formula (Hg-3)(AsO4)Cl is identical with that of the mineral kuznetsovite. All compounds are isotypic and crystalliz e with four formula units in the space group P2(1)3 [(Hg-3)(PO4)Cl: a = 8.2 912(5) Angstrom; (Hg-3)(AsO4)Cl: a = 8.3983(6) Angstrom; (Hg-3)(AsO4)Br: a = 8.4611(5) Angstrom]. All crystal structures have been refined from single crystal diffractometer data sets under consideration of merohedral twinnin g according to a diagonal mirror plane as twin element. The structures comp rise of equilateral mercury triangles with mean distances (d) over bar (Hg- Hg) = 2.659 Angstrom, which are bonded to halogen and oxygen atoms of nearl y perfect PO4 or AsO4 tetrahedra, respectively.