Refinement of framework disorder in dehydrated CaA zeolite from single-crystal synchrotron data.

An accurate knowledge of zeolite structure is required for understanding their selective sorption capacities and their catalytic properties. In particular, the positions of the exchangeable cations and their interactions with the framework are essential. The present study deals with the accurate crystal structure determination of a fully exchanged and fully dehydrated CaA zeolite (Ca(48)Al(96)Si(96)O(384), Fm3c, a = 24.47 A) using single-crystal high-resolution synchrotron X-ray diffraction [(sin straight theta/lambda)(max) = 1.4 A(-1)]. It is shown that cation exchange severely distorts the skeleton, especially around the O2 atom. The high-resolution synchrotron data reveal that this latter O atom is disordered and lies out of the mirror plane it occupies in other A-type zeolites. This feature is related to that observed for Ca(2+) cations.