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 (MC) 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, we find that Al atoms in neighbouring tetrahedra are avoided. Long-range ordering with Al and Si atom alternating in the network is found for Al fractions higher than 0.44. The resulting atom ordering is compared with that obtained by using an 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 Pm3mBAR for low Al content to Fm3cBAR for Al atomic fraction near 0.5).