Simulation Study of Structural Changes in Zeolite RHO

Flexibility in zeolite RHO manifests itself in its ability to adopt one of two structures, depending on composition and applied external conditions. In this work, structural changes of dehydrated zeolite RHO of composition vertical bar Na6Cs3 vertical bar[Al9Si39O96] and pure silica were investigated by molecular dynamics (MD) and Monte Carlo methods using the force field by Nicholas et al. (J. Am. Chem. Soc. 1991, 113, 4792). The structure was found, depending on temperature, to be stable in the acentric form (space group I (4) over bar 3m) below similar to 300 K and in the centric form (Im (3) over barm) above, in qualitative agreement with experimental findings from the literature. Additionally, the structure may remain in the centric form in metastable conditions at all temperatures and exhibits a negative thermal expansion (NTE) of -(13 +/- 3) X 10(-6) K-1. Calculated X-ray diffraction patterns match the experimental ones closely, confirming the assignments to space groups made on the basis of lattice sizes. Modifications on the force field were investigated and other established force fields probed. A detailed knowledge and control over the behavior of this structure should lead to more reliable predictions on applications like separation of gases.