The role of extra-framework cations on the structure of dehydrated Li-ABW. A computer simulation study

The structural changes and the dynamical behavior of Li cations in dehydrated zeolite Li-ABW are studied by means of a Car-Parrinello molecular dynamics simulation. Two trajectories were performed at two different temperatures, 25 and 650 degreesC. The crystallographic coordinates are calculated for both framework atoms and extra-framework cations at the two temperatures, with the confirmation of the hypothesized space group Pna2(1). A distortion in the framework structure allows Li cations to recover tetrahedral coordination. Distortions modify especially the orientations of the tetrahedral units, but not single tetrahedra. At high temperatures the framework maintains its structure and the mobility of the Li cations increases, but without diffusion along the channels. The movement of the Li cations is hypothesized to be responsible for the experimentally observed transition to 7-eucryptite. (C) 2002 Elsevier Science Inc. All rights reserved.