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 (pnH2(2+)), has a rare three-dimensional binodal (4(12)⋅6(3))(4(9)⋅6(6))3 Mg-formate framework with elongated cavities accommodating pnH2(2+)⋅⋅⋅H2O⋅⋅⋅pnH2(2+) 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 Å(3) to the low-temperature one of 60,980 Å(3). The change results from the disorder-order transition of the pnH2(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.