Topotactic ion exchange in β-pyrochlore oxide using 18-crown-6: structural incorporation of confined water in (H3O)Os2O6.

We report a crown-ether-assisted topotactic ion-exchange reaction in the β-pyrochlore oxide KOs2O6, enabling complete extraction of K+ ions under mild aqueous conditions without degradation of the rigid Os-O cage framework. Treatment with 18-crown-6 yields a hydrated protonated phase formally described as (H3O)Os2O6, which is inaccessible by conventional acid-mediated ion-exchange routes. Combined synchrotron X-ray and neutron diffraction analyses show that the cubic Fd3̄m symmetry is preserved after ion exchange. The exchanged phase is characterized by protons bonded to cage oxygen atoms and neutral H2O molecules confined within the Os-O cages with orientational disorder, rather than isolated hydronium ions. Partial dehydration induces a large lattice contraction while maintaining the integrity of the Os-O framework. Heat-capacity analysis reveals the complete suppression of low-energy rattling modes associated with K+ ions, accompanied by a marked reduction of the electronic specific heat coefficient. These results demonstrate crown-ether-assisted ion exchange as an effective soft-chemical strategy for selectively modifying guest species in acid-sensitive inorganic cage oxides.