[(C2H5)4N][U2O4(HCOO)5], an Ammonium Uranyl Formate Framework Showing Para- to Ferro-Electric Transition: Synthesis, Structures, and Properties

We report an ammonium uranyl formate framework of formula [(C2H5)4N]­[U2O4(HCOO)5], prepared by using components of tetraethylammonium, uranyl, and formate. The compound possesses a layered structure of anionic uranyl–formate wavy sheets and intercalated (C2H5)4N+ cations. The sheet consists of pentagonal bipyramidal uranyl cations connected by equatorial anti–anti and anti–syn HCOO– bridges, and it has a topology of 33·43·54 made of edge-sharing square and triangle grids. The high-temperature (HT) phase belongs to the chiral but nonpolar tetragonal space group P4̅21m. In the structure, one HCOO– is 2-fold disordered, showing a flip motion between the two anti–syn orientations. On cooling, this flip motion slowed and finally froze, leading to a phase transition at ∼200 K. The low-temperature (LT) structure is monoclinic and polar in space group P21; the cations shift, and the layers slide. Especially, the concerted and net shifts of the ammonium cations toward the −b direction, with respect to the anionic sheets, result in an estimated spontaneous polarization of 0.86 μC cm–2 in LT. The phase transition is thus para- to ferro-electric, in Aizu notation 4̅2mF2, accompanied by significant, anisotropic dielectric anomalies, with a quite significant thermal hysteresis. Variable-temperature luminescent spectroscopy and differential scanning calorimetry confirmed the transition and provided further information. The structure–property relationship is established.