Determination of 4-connected framework crystal structures by simulated annealing

Abstract

THE initial derivation of atomic-scale models of the 4-connected framework crystal structures of zeolites and related materials is generally difficult. Interpretation of powder X-ray diffraction data to yield a unit cell and some symmetry information is often straightforward. Chemical analyses and sorption experiments indicate the number of framework tetrahedra, n_T, present in the unit cell (T represents the framework tetrahedral species, such as Si or Al) and the approximate pore dimensions. However, because synthetic zeolites are almost invariably microcrystalline, the possibilities for structure solution by conventional diffraction methods are limited, and framework structure determinations have traditionally relied heavily on the building of physical models. We describe here an alternative approach, in which approximate T-atom coordinates are derived from the unit-cell size and symmetry, and the value of n_T from computer modelling. An initially arbitrary T-atom configuration is optimized with respect to a 'cost function' based on the T–T distances, T–T–T angles and number of first-neighbour T-atoms, by simulated annealing using Monte Carlo methods. The potential of the method for structural determinations in both two dimensions (for structural projections) and three dimensions are illustrated by results obtained for a hexagonal cell of space group P6/mmm and dimensions a = 18.4 Å and c = 7.5 Å, and by determination of the previously unknown structure of lithium gallosilicate.