Cation disorder in NaW2O6+δ·nH2−zO post-ion exchange with K, Rb, Sr, and Cs

The structure of the defect pyrochlore NaW2O6+δ·nH2−zO after ion exchange with K, Rb, Sr or Cs for Na has been investigated using thermal analysis, solid-state nuclear magnetic resonance, laboratory X-ray and neutron diffraction methods. Neutron diffraction studies show that both the A-type cations (Na+, K+, Rb+, and/or Cs+) and the water molecules reside within the channels that form in the 111 direction of the W2O6 framework and that these strongly interact. The analytical results suggest that the water and A-type cations compete for space in the tunnels within the W2O6 pyrochlore framework, with the total number of water molecules and cations being approximately constant in the six samples investigated. The interplay between the cations and water explains the non-linear dependence of the a lattice parameter on the choice of cation. It appears that the ion-exchange capacity of the material will be controlled by the amount of water initially present in the sample.

Research highlights
► In this study we examine the structure of the defect pyrochlore NaW2O6+δ·nH2−zO after ion exchange with K, Rb, Sr or Cs for Na.
► We employed thermal analysis, solid-state nuclear magnetic resonance, laboratory X-ray and neutron diffraction methods.
► In this study we conclude that the water and A-type cations compete for space in the tunnels within the W2O6 pyrochlore framework, with the total number of water molecules and cations being approximately constant.