Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 (China), Fax: (+86) 10-62751708.
Chemistry. 2014 Jan 20;20(4):1146-58. doi: 10.1002/chem.201303425. Epub 2013 Dec 16.
We present three Mg-formate frameworks, incorporating three different ammoniums: [NH4][Mg(HCOO)3] (1), [CH3CH2NH3][Mg(HCOO)3] (2) and [NH3(CH2)4NH3][Mg2(HCOO)6] (3). They display structural phase transitions accompanied by prominent dielectric anomalies and anisotropic and negative thermal expansion. The temperature-dependent structures, covering the whole temperature region in which the phase transitions occur, reveal detailed structural changes, and structure-property relationships are established. Compound 1 is a chiral Mg-formate framework with the NH4(+) cations located in the channels. Above 255 K, the NH4(+) cation vibrates quickly between two positions of shallow energy minima. Below 255 K, the cations undergo two steps of freezing of their vibrations, caused by the different inner profiles of the channels, producing non-compensated antipolarization. These lead to significant negative thermal expansion and a relaxor-like dielectric response. In perovskite 2, the orthorhombic phase below 374 K possesses ordered CH3CH2NH3(+) cations in the cubic cavities of the Mg-formate framework. Above 374 K, the structure becomes trigonal, with trigonally disordered cations, and above 426 K, another phase transition occurs and the cation changes to a two-fold disordered state. The two transitions are accompanied by prominent dielectric anomalies and negative and positive thermal expansion, contributing to the large regulation of the framework coupled the order-disorder transition of CH3CH2NH3(+). For niccolite 3, the gradually enhanced flipping movement of the middle ethylene of NH3(CH2)4NH3 in the elongated framework cavity finally leads to the phase transition with a critical temperature of 412 K, and the trigonally disordered cations and relevant framework change, providing the basis for the very strong dielectric dispersion, high dielectric constant (comparable to inorganic oxides), and large negative thermal expansion. The spontaneous polarizations for the low-temperature polar phases are 1.15, 3.43 and 1.51 μC cm(-2) for 1, 2 and 3, respectively, as estimated by the shifts of the cations related to the anionic frameworks. Thermal and variable-temperature powder X-ray diffraction studies confirm the phase transitions, and the materials are all found to be thermally stable up to 470 K.
[NH4][Mg(HCOO)3]（1）、[CH3CH2NH3][Mg(HCOO)3]（2）和[NH3(CH2)4NH3][Mg2(HCOO)6]（3）。它们显示出结构相变，伴随着显著的介电异常和各向异性的负热膨胀。在整个发生相变的温度范围内，温度依赖的结构揭示了详细的结构变化，并建立了结构-性质关系。化合物 1 是一个手性的镁甲酸盐骨架，NH4(+)阳离子位于通道中。在 255 K 以上，NH4(+)阳离子在两个浅能量势阱位置之间快速振动。在 255 K 以下，由于通道的内部轮廓不同，阳离子经历了两次振动冻结，产生非补偿的反极化。这导致了显著的负热膨胀和类弛豫介电响应。在钙钛矿 2 中，低于 374 K 的正交相具有有序的 CH3CH2NH3(+)阳离子位于镁甲酸盐骨架的立方腔中。在 374 K 以上，结构变为三角，具有三角无序的阳离子，在 426 K 以上，发生另一个相变，阳离子变为双重无序状态。这两个转变伴随着显著的介电异常和负的和正的热膨胀，这有助于 CH3CH2NH3(+)的有序-无序转变与骨架的耦合。对于镍铁矿 3，NH3(CH2)4NH3的中间乙烯在拉长的骨架腔中的翻转运动逐渐增强，最终导致相变，临界温度为 412 K，三角无序的阳离子和相关的骨架发生变化，为非常强的介电色散、高介电常数（可与无机氧化物相媲美）和大的负热膨胀提供了基础。通过与阴离子骨架相关的阳离子的位移，估计出 1、2 和 3 的低温极性相的自发极化分别为 1.15、3.43 和 1.51μC cm(-2)。热和变温粉末 X 射线衍射研究证实了相变，并且这些材料都被发现在高达 470 K 的温度下是热稳定的。
