Molecular simulations of anhydrous Na₆[Al₆Si ₆O₂₄] sodalite

An empirical energy force field, one that combines previously published force field parameters with a flexible SPC water model, is used to examine the structures and dynamical properties of the hydrosodalite family of zeolitic materials. In this paper, we present the results for Na₆[Al ₆Si₆O₂₄], one of two anhydrous end-members in this family. Experimentally derived unit cell volumes, T-O bond lengths, and T-O-T and O-T-O bond angles are used to validate the force field. Supplemental plane-wave pseudopotential density functional calculations fully support the potential-based models. Although sodalite materials usually possess P43n symmetry, the direct modeling results from both simulation techniques employed in this study suggest the formation of volume-doubled C2/c supercells. The loss of symmetry is due to the presence of only six monovalent ions in the unit cell, rather than the usual eight. This result stands in partial agreement with recent simulation and experimental studies that report volume-doubled Pcn2 and orthorhombic supercell structures, respectively. More specifically, Le Bail profile and Rietveld refinements-when compared to the synchrotron-based XRD data from the recent experimental study-favor P2/c symmetry, a sub-group of C2/c. Le Bail profile agreement indices of R_P = 6.18% and R _WP = 7.92% for P2/c are lower than those reported for the orthorhombic cases, and Rietveld refinement indices of R_P = 7.91% and R_WP = 10.50% are lower than those reported for Pcn2. We further propose a new S6R ring structure that can be explained in terms of two crystallographically and energetically distinct oxygen sites. These structural and energetic results offer explicit evidence to support the hypothesis that "vacant ring avoidance" drives supercell formation. The new S6R structure leads to the formation of two enantiomeric β-cages, which, in turn, leads to the volume-doubled supercell. Finally, the periodic DFT calculations suggest that the sodium ions occupy the centers of the six-membered rings, in full agreement with the force field results. This arrangement, however, stands in contrast to the results of a recent DFT study, where a significant offset of the sodium ions out of the planes of the S6Rs is observed.