Off-axis elastic properties and the effect of extraframework species on structural flexibility of the NAT-type zeolites: Simulations of structure and elastic properties

Simulations of the atomic structure and elastic constants of five zeolites with the NAT-type structure, namely, natrolite, mesolite, scolecite, metanatrolite, and ammonium-exchanged natrolite, using the CVFF force field implemented within Cerius(2), are presented. The validity of the simulation method is shown by the excellent agreement between simulated and experimental crystal data, including location of extraframework ions and water molecules, and for natrolite, the only zeolite studied here for which experimental studies of elasticity have been reported, the good agreement between the simulated and experimental elastic constants. For all materials, an off-axis analysis of the elastic constants reveals that the Poisson's ratios nu(xy) and nu(yx) become negative when stress is applied at 45 degrees to the crystallographic axes. Further simulations of the elastic behavior of the materials under stress, using the molecular dynamics method of Parrinello and Rahman, reveal that the elastic behavior may be described by a "modified rotating squares" model. Here three-dimensional structural distortions are reduced to a two-dimensional model where square cross-section units of structure both rotate about their hinges and undergo change of dimension: the balance of these competing processes dictates the resulting Poisson's ratio and is highly dependent upon the direction along which stress is applied. We discuss the effect that various concentrations of extraframework cations and water have on the elastic properties of the NAT-type zeolites.