Estudo cristalográfico dos compostos Cs2HgBr4 e Re6Se8Br3

Abstract

In the present work Cs2HgBr4 and crystals have been studied by single crystal X-ray diffraction. Crystal structures and crystallographic properties were investigated. Re6 Se8 Br3 is a semiconductor crystal. Its structure is very close to the Chevrel phases presented specially in Mo compounds. These compounds crystallize with well-defined Re6Se8 cluster, in which the Re atoms are placed in an octahedron vertices. In some cases, the Re octahedron lies inside a cube whose corners are occupied by atoms X. In the systems Re6XiYj, several compounds have been synthesized and structures have been determined for different types and number of inter-cluster atoms. Cs2HgBr4 is a compound in the well-known A2BX4 family, which presents five different structural phases between room temperature and 85 K. Its room temperature phase is isomorphous with β-K2SO4 (orthorhombic space group Pnma). At Tc1 =245 K a second order structural phase transition takes place: an incommensurate phase with invariant modulation vector (q = 0.165(2)a⋆1) appears. At Tc2=232 K a Lock-in phase with no superstructure is observed; this structure is better described by a P21/n space group with twinned domains. At Tc3=167 K a second order transition brings the crystal to the P ̄1 space group and at Tc4=85 K, a symmetry.