Structural investigations of zeolites ERS-7, mordenite, and sodalite using a combination of powder diffraction and computer simulation techniques

The framework structure of dehydrated template-free ERS-7 was determined by a combination of synchrotron X-ray powder diffraction and the simulated annealing approach of Deem and Newsam. The orthorhombic unit cell ( a = 9.79976(4) Å, b = 12.41163(6) Å, c = 22.86063(11) Å) contains 6 tetrahedral framework atoms (Si/Al = 8.4), all on general positions, and 14 bridging oxygens. It can be constructed as a network of purely face-sharing 17-sided (465 46582) “picnic-basket”-shaped cages that are stacked so as to form a 1D 8-ring channel system (3.5 Å x 4.7 Å) that runs through the handles of the baskets. Three Brønsted acid sites were then identified via a multi-dataset Rietveld refinement against both PXD and PND data.; Previous reports of correlations between diffracted intensities and aluminum content in as-synthesized Na mordenites (Si/Al = 5 to 10) are shown to be explained as a selective suppression of the (h, k,ℓ = 2n + 1) reflections caused by linear faulting along the main channel axis. Computer generated structures with up to 500,000 atoms were used to simulate the diffuse scattering patterns for several different correlated defect models, which were then compared to previously reported experimental results. The regions of faulted material are shown to consist of c-axis chains of shifted framework 4-rings, such that neighboring pairs of chains along the a-axis shift together. The intensity of odd-ℓ reflections is suppressed by a factor of (1 − 2p_o) 2 where p_o is the fraction of fault-shifted material, the intensity of the diffusely scattered sheets in the odd-ℓ planes perpendicular to c* increases in proportion to p_o(1 − p_o), and the in-plane modulations of these sheets are shown to have the form sin 2(πh/2).; Variable-temperature synchrotron PXD data from dehydrated sodium sodalite, Na₆[Al₆Si₆O₂₄], reveals a structural phase transition at 500 K that involves both a commensurate ordering of the extra-framework Na cations and a one-dimensional incommensurate modulation of the framework. Peak splittings and superlattice reflections implicate an orthorhombically distorted supercell which is related to the original 9.1 Å cubic cell by a 45° rotation about a 4-fold axis and a doubling of the volume (√2 a x √2a x a). The incommensurate modulation vector is then shown to be [211]/8.9 on the commensurate supercell basis. The 88 unique orderings of the 12 Na cations of the supercell among its 16 available sites were enumerated, and each configuration was subjected to lattice energy minimization using an empirical interatomic potential. A unique solution was identified that minimizes the structural energy and also matches the experimental diffraction data. The high-temperature phase (Pm3¯n) is well described by dynamical disorder among the cation sites and also between opposite framework “partial collapse” angles.