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.