Structure cristalline de γ Cu2 (OH)2 Cr4

Abstract

The crystal structure of γ Cu₂ (OH)₂ CrO₄, the monoclinic form of copper hydroxy chromate, has been investigated by the « heavy atom » method. The refinement carried out by the least-squares method, leads to good agreement between calculated and observed structure factors (R = 0,10). In this structure the coordination polyhedron of copper is a deformed octahedron. The oxygen and OH form a distorted hexagonal close packing array in which Cu⁺⁺ and Cr[vi] partially fill the octehedral and tetrahedral sites. There are two cationic layers : in the first one the octahedral sites are half filled by Cu⁺⁺ ions, in the second one 1/4 of the octahedral sites are filled by Cu⁺⁺ and 1/8 of the tetrahedral sites by Cr[vi]. A study of the bonding shows perfect cohesion of the structure, which explains the great stability of this form, as opposed to the other metastable β form in which cohesion is very poor.