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 T₅O₁₀secondary building units (T = Si, Al) linking two Al-and three Si-based TO₄tetrahedra 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^-1per 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.