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 (pnH(2)(2+)), has a rare three-dimensional binodal (4(12)6(3))(4(9)6(6))(3) Mg-formate framework with elongated cavities accommodating pnH(2)(2+)H(2)OpnH(2)(2+) assemblies. It displays a para-electric to antiferroelectric phase transition at 275K, with a 36-fold multiple unit cell from the high-temperature cell of 1703 angstrom(3) to the low-temperature one of 60980 angstrom(3). The change results from the disorder-order transition of the pnH(2)(2+) 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.