Rigid unit modes and the phase transition and structural distortions of zeolite rho

The rigid unit mode model (RUM) was applied to the framework structure of zeolite rho. In conjunction with the ideas of soft-mode phase transition theory much of the complex behaviour that have been observed experimentally for this zeolite can be explained. In particular, all possible distortions of the S6R, S8R and D8R are determined and how these distortions influence the positioning of the cations and evoke the observed negative thermal expansion of this material. The large volume reduction observed through the low to high temperature phase transition of zeolite Sr-rho, the large distance relocation of Cd2+-ions that accompanies the I (4) over bar 3m to Im (3) over bar m phase transition of zeolite Cd-rho, and the dehydration behaviour of zeolite rho, beryllophosphate and -arsenate zeolite rho could be explained. Also the possibility of an Im (3) over bar phase of zeolite rho was predicted and its framework structure was described. (C) 1998 Elsevier Science B.V. All rights reserved.