Crystal Structure, Thermal and Electric Behaviour of [(CH3)2NH2]3[As2Cl9] and [(CH3)2NH2][AsOCl2]
Abstract
Single crystals of the organic-inorganic arsenate(III): [(CH3)2NH2]3[As2Cl9] and [(CH3)2NH2][AsOCl2] have been grown from aqueous hydrochloric acid solution. The crystals [(CH3)2NH2]3[As2Cl9] have been investigated by X-ray diffraction (at 253 K), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dilatometric and dielectric methods. They undergo two structural phase transitions of first order at 228/235 and 298/307 K (on cooling/heating), respectively, which are classified as an "order-disorder" type. The trigonal [(CH3)2NH2]3[As2Cl9] structure (at 253 K, intermediate phase (II)) refined in the space group R3c, consists of isolated [As2Cl9]3- bioctahedral units and dimethylammonium cations hydrogen bonded to the bridging Cl atoms of the anions. The crystals of [(CH3)2NH2][AsOCl2] at 100 K are orthorhombic, space group Cmca. The structure contains one-dimensional chains formed by strong distorted [AsO2Cl4] octahedra. The dimethylammonium cations reveal distinct disorder.
