Time-resolved diffraction studies of ion exchange: K⁺ and Na⁺ exchange into (Al, Ge) gismondine (GIS) Na₂₄Al ₂₄Ge₂₄O₉₆·40H₂O and K ₈Al₈Ge₈O₃₂·8H₂O

Time-resolved in situ synchrotron X-ray powder diffraction (XRPD) was used to study ion-exchange mechanisms and pathways in K⁺ and Na ⁺ forms of an aluminogermanate with the zeolite gismondine topology (AlGe-GIS). The Na⁺ and K⁺ forms differ in their distribution of extraframework cations with the Na⁺ ions ordered in the C-centered monoclinic Na-AlGe-GIS of unit-cell dimensions a = 14.490(3) Å, b = 9.3840(2) Å, c = 23.530(5) Å, and β = 105.90(3)° and K⁺ ions disordered in the I-centered monoclinic K-AlGe-GIS of unit-cell dimensions a = 10.311(2) Å, b = 9.749(1) Å, c = 10.238(1) Å, and β = 90.000(2)°. Rietveld structure refinements indicate K⁺ first occupies sites K2, K4, and K6 in the [101] channel of the Na-AlGe-GIS structure. After 10% (±2%) K ⁺ exchange into the [101] channel, the I2/a unit cell forms and subsequent replacement of Na⁺ is consistent with site independent exchange along the [101] and [201] channels. Ion exchange proceeded to approximately 90% (±1%) substitution of K⁺ into Na-AlGe-GIS within the course of the experiment. However, in the reverse exchange of Na ⁺ into the K-AlGe-GIS, an abrupt growth of Na-AlGe-GIS occurred at 4.5% (±3.5%) Na⁺ exchange and ended at 10% (±2.5%) Na⁺ exchange. Bond valence calculations demonstrate that K ⁺ has a stronger interaction with framework O²-(0.423 v.u.) than Na⁺ (0.242 v.u.) and the valence matching principle shows that Na⁺ has a stronger affinity to interstitial H₂O. These results imply that the AlGe-GIS structure has a preference for K ⁺ as the charge-balancing extraframework cation.