title: [(C2H5)4N][U2O4(HCOO)5], an ammonium uranyl formate framework showing para- to ferro-electric transition: synthesis, structures, and properties.
abstract: 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 3(3)·4(3)·5(4) made of edge-sharing square and triangle grids. The high-temperature (HT) phase belongs to the chiral but nonpolar tetragonal space group P42(1)m. 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 P2(1); 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 42mF2, 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.
authors: Beijing National Laboratory for Molecular Sciences, Fundamental Science Laboratory on Radiochemistry & Radiation Chemistry, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University , Beijing 100871, China.
authors: Qianqian
authors: Q
authors: Zhu
authors: Ran
authors: R
authors: Shang
authors: Sa
authors: S
authors: Chen
authors: Chunli
authors: C
authors: Liu
authors: Zheming
authors: Z
authors: Wang
authors: Song
authors: S
authors: Gao