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 Na2.5K5.3[SbW6O24](NO3)0.8·12H2O (1) and Na2K5.35[SbW6O24]Cl0.35·12H2O (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 Na2K5.4[SbW6O24](NO3)0.4·12H2O (1-recry) and Na2K5.25[SbW6O24]Cl0.25·12H2O (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.