K ₄Fe ₄P ₅O ₂₀: A new mixed valence microporous compound with elliptical eight-ring channels

A new small-pore compound K ₄Fe ₄P ₅O ₂₀ was obtained by conventional solid-state reaction in a closed crucible. The crystal structure is constructed by Fe ₄P ₅O ₂₀ units forming chains along the c axis and elliptical eight-ring channels on the a-b plane in which K cations locate inside. Such structural characteristics seem to be quite similar to those seen in the natrolite family. However, Fe ions in K ₄Fe ₄P ₅O ₂₀ have trigonal-bipyramidal instead of common tetrahedral coordination. Furthermore, our experimental results combined from magnetic susceptibility and ⁵⁷Fe Mössbauer spectrum measurements show mixed valence Fe ³⁺/Fe ²⁺ in the titled material. To the best of our knowledge, this is the first example that contains mixed valence iron ions in a so-called natrolite framework.