Hg3AsE4X (E = S, Se; X = Cl, Br, I), a family of isotypic compounds with an acentric, layered structure

Four isotypic compounds Hg3AsS4Cl, Hg3AsS4Br, Hg3AsSe4Br, and Hg3AsSe4I were synthesized in the quaternary system mercury/arsenic/chalcogen/halogen by vapor transport reactions from As, Hg(II) halide, Hg(II) chalcogenide, and elemental chalcogen at temperatures between 250 and 310 degreesC. Their structures were solved by single-crystal X-ray diffraction analyses. The compounds crystallize in the hexagonal system with the acentric space group P6(3)mc and the following lattice parameters: a = 7.431(5) Angstrom, c = 9.003(8) Angstrom for Hg3AsS4Cl, a = 7.430(5) Angstrom, c 9.364(5) Angstrom, for Hg3AsS4Br, a = 7.707(8) Angstrom, c = 9.47(2) Angstrom, for Hg3AsSe4Br, and a = 7.6902(7) Angstrom, c = 9.968(1) Angstrom for Hg3AsSe4I, and Z = 2. All compounds are air-stable and form hexagonal prismatic crystals. The colors vary from yellow (Hg3AsS4Cl) to red (Hg3AsSe4I. The structures consist of (2)(infinity)[He3AsE4](+) polycationic layers built of six-membered Hg3E3 and Hg2AsE3 rings in chair and boat conformations. Alternatively, the structure of the layers can be described as composed of [AsE3](3-) and [Hg3E](4+) ions both forming regular trigonal pyramids. The halide ions are intercalated between the layers and have only weak bonds to the mercury atoms. The acentricity of the structures was confirmed through SHG measurements using IR laser radiation (lambda = 1064 nm).