Time-Resolved Diffraction Studies of Ion Exchange: K<sup>+</sup> and Na<sup>+</sup> Exchange into (Al, Ge) Gismondine (GIS) Na<sub>24</sub>Al<sub>24</sub>Ge<sub>24</sub>O<sub>96</sub>·40H<sub>2</sub>O and K<sub>8</sub>Al<sub>8</sub>Ge<sub>8</sub>O<sub>32</sub>·8H<sub>2</sub>O CelestianAaron J. PariseJohn B. GoodellCarmen TripathiAkhilesh HansonJonathan 2004 Time-resolved in situ synchrotron X-ray powder diffraction (XRPD) was used to study ion-exchange mechanisms and pathways in K<sup>+</sup> and Na<sup>+</sup> forms of an aluminogermanate with the zeolite gismondine topology (AlGe−GIS). The Na<sup>+</sup> and K<sup>+</sup> forms differ in their distribution of extraframework cations with the Na<sup>+</sup> ions ordered in the C-centered monoclinic Na−AlGe−GIS of unit-cell dimensions <i>a </i>= 14.490(3) Å, <i>b </i>= 9.3840(2) Å, <i>c </i>= 23.530(5) Å, and β = 105.90(3)° and K<sup>+</sup> ions disordered in the I-centered monoclinic K−AlGe−GIS of unit-cell dimensions <i>a </i>= 10.311(2) Å, <i>b </i>= 9.749(1) Å, <i>c </i>= 10.238(1) Å, and β = 90.000(2)°. Rietveld structure refinements indicate K<sup>+</sup> first occupies sites K2, K4, and K6 in the [1̄01] channel of the Na−AlGe−GIS structure. After 10% (±2%) K<sup>+</sup> exchange into the [1̄01] channel, the <i>I</i>2/<i>a</i> unit cell forms and subsequent replacement of Na<sup>+</sup> is consistent with site independent exchange along the [1̄01] and [201] channels. Ion exchange proceeded to approximately 90% (±1%) substitution of K<sup>+</sup> into Na−AlGe−GIS within the course of the experiment. However, in the reverse exchange of Na<sup>+</sup> into the K−AlGe−GIS, an abrupt growth of Na−AlGe−GIS occurred at 4.5% (±3.5%) Na<sup>+</sup> exchange and ended at 10% (±2.5%) Na<sup>+</sup> exchange. Bond valence calculations demonstrate that K<sup>+</sup> has a stronger interaction with framework O<sup>2-</sup> (0.423 v.u.) than Na<sup>+</sup> (0.242 v.u.) and the valence matching principle shows that Na<sup>+</sup> has a stronger affinity to interstitial H<sub>2</sub>O. These results imply that the AlGe−GIS structure has a preference for K<sup>+</sup> as the charge-balancing extraframework cation.