Cation sites and framework deformations in dehydrated chabazites. Crystal structure of a fully silver-exchanged chabazite

A sample of silver-chabazite, Ag3.7Al3.8Si8.3O24.10H2O, was completely dehydrated at suitable conditions of vacuum and temperature (2 × 10−6 Torr, 60°C). After dehydration, the symmetry of the Ag-CHA crystals was reduced from the usual rhombohedral space group R3m to the monoclinic C2m, with the following cell dimensions: a = 19.240(6)A, b = 13.771(4)A, c = 11.868(4)A, β = 113.28(3)A, Full matrix least-squares refinement reduced R index to 0.093 on the basis of 2305 reflections. The framework of d-Ag-CHA can be described as a tridimensional linkage of D6R units in alternate layers rotated about the monoclinic b-axis. The silver ions occupy eight independent positions, related to sites I, III and IV of the rhombohedral cell, near the centre of the 6-ring, at the centre (0, 12 12) of the D6R cage and near the 8-ring aperture, respectively. The positions and the populations of the cation sites of d-Ag-CHA are compared with those of h-Ag-CHA and related with the relevant variation of the framework.