Monovalent Cation-Exchanged Natrolites and Their Behavior under Pressure. A Computational Study

Recently natrolite was shown to be an auxetic material that is able to exchange extra-framework Na+ cations with other mono-, di-, and trivalent cations. Under pressure up to several GPa, these cation-exchanged natrolites undergo superhydration and/or phase transformations in the cation–water arrangement. Using density functional theory we studied in silico the ion exchange in natrolites. First we optimized the structures of Li+-, Na+-, K+-, Rb+-, and Cs+-exchanged natrolites at ambient conditions and compared the resulting lattice energies to that of the hypothetical H-form of natrolite. Of all natrolites, the smallest formal exchange energy was found for Na-NAT, in agreement with the natural occurrence of this material. Then we modeled the effect of pressure on Na-, Rb-, and Cs-natrolites, addressing (i) the incorporation of water ligands into the zeolite framework, accompanied by an increase in volume; and (ii) the changes in the cation–water arrangement within the zeolite pores. The computational models reproduce reasonably well the critical pressure, at which these phenomena occur, and, in the case of Cs-NAT, point toward a cation displacement model for its structural transition under pressure.