Potassium-Exchanged Natrolite Under Pressure. Computational Study vs Experiment

Using
density functional theory we modeled the effects of pressure on K-exchanged
natrolite, K-NAT, including superhydration and the experimentally
observed structural phase transition. Natrolites are composed of T5O10 secondary building units (T = Si, Al) linking
two Al- and three Si-based TO4 tetrahedra which in projection
have an average chain rotation angle ψ with respect to the crystallographic a- and b-axes. Besides an isomer with pore
axes orientations characterized by a negative chain rotation angle,
found experimentally at moderate pressure, we also examined a superhydrated
isomer with pore axes orientations resulting from positive chain rotation
angles in the pressure range 1–2.5 GPa. We estimated the critical
pressure for possible transformations between various isomers, but
we were unable to identify any specific energetic preference for a
superhydrated structure with a negative chain rotation angle. Therefore,
our computational results suggest that both isomers coexist in the
same pressure range and transform into a more compact structure near
4 GPa. We also modeled the pathways for this latter phase transition
and found rather similar barrier heights, 43–44 kJ mol–1 per K+ ion for both isomers, but distinct
energy profiles. Thus, based on the modeling results, the isomers
of superhydrated K-NAT, with either positive or negative chain rotation
angles, may coexist at moderate pressures, calling for new experiments.