Atom ordering in the tetrahedral framework of zeolite A

The short-range and long-range order of the distribution of Si and Al atoms in the tetrahedral framework of zeolite A is analysed by the Monte Carlo method in a composition range from 0.25 to 0.5 Al mole fraction. An interatomic potential model previously applied to aluminosilicates, including long-range Coulomb and atomic polarization energies, is employed. The short-range site correlations found from the MC simulations are in good agreement with those derived from earlier nuclear magnetic resonance data. In particular, the author finds that Al atoms in neighbouring tetrahedra are avoided. Long-range ordering with Al and Si atoms alternating in the network is found for Al frictions higher than 0.44. The resulting atom ordering is compared with that obtained by using a Ising-like model, in which only nearest-neighbour interactions between tetrahedral atoms are included. The effect of the non-ergodicity of the MC simulations for compositions showing sub-lattice ordering is discussed and related to a change of space group of the material (from Pm3m for low Al content to Fm3c for Al atomic fraction near 0.5).