Cation-vacancy ordering in dehydrated Na-6[AlSiO4](6)

The low-temperature cation-ordered superstructure of anhydrous sodium sodal ite, a zeolite with composition Na-6[AlSiO4](6), has been determined throug h the use of both density functional theory (DFT) and classical force-field lattice energy minimizations. The charge-balancing Na+ cations are assumed to occupy their characteristic locations within the cubic alumino-silicate framework near the centers of the 6-ring windows. Within the constraints o f the volume-doubled pseudotetragonal supercell reported in a previous x-ra y diffraction study [B. Campbell, S. R. Shannon, H. Metiu, and N. P. Blake (submitted)], all possible arrangements of cations and vacancies amongst th e 6-ring window sites were considered. Force-field calculations employing t he ab initio based potential energy function derived by Blake, Weakliem, an d Metiu [J. Phys. Chem. B 102, 67 (1998)] and the empirical shell-model pot ential of Catlow [J. Chem. Soc. Commun. 1984, 1271; Mol. Simul. 1, 207 (198 8)], were used to perform full lattice-energy minimizations of each configu ration, and to assess their relative stabilities both before and after mini mization. The most stable configurations were then examined in more detail via ab initio density functional calculations in the generalized gradient a pproximation. The lowest-energy supercell ordering proved more stable than the lowest-energy parent cell ordering, and also yielded a pseudotetragonal distortion (space group Pnc2) and a calculated diffraction pattern that qu alitatively match experimental results. The structural influences that cont ribute to the low energy of the correct vacancy ordering are described in d etail. (C) 2000 American Institute of Physics. [S0021-9606(00)00744-3].