Insertion-release of guest species and ionic conduction in polyoxometalate solids with a layer-like Anderson structure

The precipitation of Na⁺ and K⁺ mixed salts of Anderson type SbW₆O₂₄]⁷⁻ by addition of excess of NaNO₃ and NaCl yielded polycrystalline powders of Na_2.5K_5.3SbW₆O₂₄](NO₃)_0.8·12H₂O (1) and Na₂K_5.35SbW₆O₂₄]Cl_0.35·12H₂O (2), respectively. The two compounds are isomorphous and exhibit a layer-like Anderson (LLA) type structure, which consists of SbW₆O₂₄]⁷⁻-containing layers and interstitial Na⁺, K⁺, NO₃⁻ or Cl⁻, and water O atoms. Recrystallization of 1 and 2 from hot water yielded Na₂K_5.4SbW₆O₂₄](NO₃)_0.4·12H₂O (1-recry) and Na₂K_5.25SbW₆O₂₄]Cl_0.25·12H₂O (2-recry) as a result of partial release of NO₃⁻ and Cl⁻ (and Na⁺ and K⁺ for charge compensation). Dehydration of 1 and 2 at 400 and 500 °C (1-dehyd400 and 2-dehyd500) caused a shrinkage of lattice, but their the LLA structures retained. Simulation of X-ray diffraction (XRD) patterns for the dehydrated forms allowed to presume that the each SbW₆O₂₄]⁷⁻ anion had been 30°-rotated within its molecular plane in order to avoid intermolecular repulsion. A compressed powder of 1-dehyd400 exhibited fast alkaline-ion conduction with a bulk conductivity of 1.2×10⁻² Ω⁻¹ cm⁻¹ at 400 °C. The hosting of a sufficient amount of NO₃⁻ together with Na⁺ for charge compensation into the lattice is crucial for high conduction.