Long-range and short-range orderings in K₄Fe₄P₅O₂₀ with a natrolite-like framework

K₄Fe₄P₅O₂₀ shows an interesting natrolite-like structure with a spin–tetrahedron lattice built by mixed valence Fe ions. Single crystals of the title compound are successfully grown by the flux method using KF as flux. Magnetic results combined from magnetic, heat capacity, and⁵⁷Fe Mössbauer spectra measurements show that K₄Fe₄P₅O₂₀ possesses a short-range magnetic ordering at ∼13 K and a long-range ordering at ∼7 K. Magnetic anisotropy of K₄Fe₄P₅O₂₀ is observed between Hc and Hc, suggesting that the c-axis is the magnetic easy-axis. The spin arrangements in the system are suggested to be ferrimagnetic along the natrolite chains.