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.