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

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• 作者：Qianqian Zhu ; Ran Shang ; Sa Chen ; Chunli Liu ; Zheming Wang ; Song Gao
• 刊名：Inorganic Chemistry
• 出版年：2014
• 出版时间：August 18, 2014
• 年：2014
• 卷：53
• 期：16
• 页码：8708-8716
• 全文大小：593K
• ISSN：1520-510X

文摘

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鈥揻ormate wavy sheets and intercalated (C₂H₅)₄N⁺ cations. The sheet consists of pentagonal bipyramidal uranyl cations connected by equatorial anti鈥?i>anti and anti鈥?i>syn HCOO^{鈥?/sup> bridges, and it has a topology of 33路43路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鈥?/sup> is 2-fold disordered, showing a flip motion between the two anti鈥?i>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 鈭?i>b direction, with respect to the anionic sheets, result in an estimated spontaneous polarization of 0.86 渭C cm鈥? in LT. The phase transition is thus para- to ferro-electric, in Aizu notation 4虆2mF2, 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鈥損roperty relationship is established.}