Off-Axis Elastic Properties and the Effect of Extraframework
Species on Structural Flexibility of the NAT-Type Zeolites:
Simulations of Structure and Elastic Properties
posted on 2007-05-15, 00:00authored byJennifer J. Williams, Christopher W. Smith, Kenneth E. Evans, Zoe A. D. Lethbridge, Richard I. Walton
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 Cerius2, 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 νxy and νyx become negative when stress is applied at 45° 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.