Estudo cristalográfico dos compostos Cs2HgBr4 e Re6Se8Br3

• Ano de defesa: 1995
• Autor(a) principal: Carlos Basílio Pinheiro
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal de Minas Gerais; Brasil; ICX - DEPARTAMENTO DE FÍSICA; Programa de Pós-Graduação em Física; UFMG
• Palavras-chave em Português: Cristalografia; Difração de raios X; Cristais incomensuráveis
• Link de acesso: http://hdl.handle.net/1843/45564; https://orcid.org/0000-0002-8674-1779

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.