A 36‐Fold Multiple Unit Cell and Switchable Anisotropic Dielectric Responses in an Ammonium Magnesium Formate Framework

An ammonium Mg formate framework, prepared by using di‐protonated 1,3‐propanediamine (pnH₂²⁺), has a rare three‐dimensional binodal (4¹²?6³)(4⁹?6⁶)₃ Mg‐formate framework with elongated cavities accommodating pnH₂²⁺???H₂O???pnH₂²⁺ assemblies. It displays a para‐electric to antiferroelectric phase transition at 275 K, with a 36‐fold multiple unit cell from the high‐temperature cell of 1703 ų to the low‐temperature one of 60 980 ų. The change results from the disorder–order transition of the pnH₂²⁺ cations and H₂O molecules. The motions of these components freeze in a stepwise fashion on going from the high‐temperature disorder state to the low‐temperature ordered state, triggering the switch from high to low dielectric constants, and the spatial limitation of such motions contributes the strong dielectric anisotropy.