Dpto. Química Fundamental, Universidad de A Coruña, Spain.
Inorg Chem. 2010 Feb 15;49(4):1510-6. doi: 10.1021/ic901872g.
We have found that the hybrid organic-inorganic perovskite-like formate Mn(HCOO)(3)[(CH(3))(2)NH(2)] shows a dielectric transition around 190 K. According to single crystal X-ray diffraction, the compound shows rhombohedral symmetry at room temperature and monoclinic symmetry at low temperature (100 K), and the main difference between both structures is that the (CH(3))(2)NH(2)(+) (DMA) cations are disordered in the high temperature phase but cooperatively ordered in the low temperature one. The vibrational spectra of this compound reveal that significant changes take place in the vibrations ascribed to the DMA cation (changes in the frequency of certain vibrations, splitting of particular vibrations, and changes in the intensities), while no significant changes have been observed in those attributed to the formate anion. On the basis of all this information, we attribute the origin of the dielectric transition to the dynamics of the DMA cations: above 190 K these cations can rotate inside the cubooctahedral cavity created by the Mn(HCOO)(3) framework, while for lower temperatures such rotation gets frozen, and their cooperative arrangement inside the cavities give rise to the observed dielectric transition.
我们发现，混合有机-无机甲酸盐型锰[(HCOO)(3)[(CH(3))(2)NH(2)]在 190 K 左右表现出介电转变。根据单晶 X 射线衍射，该化合物在室温下呈现三方对称，在低温（100 K）下呈现单斜对称，两种结构的主要区别在于（CH(3))(2)NH(2)(+)（DMA）阳离子在高温相无序，但在低温相协同有序。该化合物的振动谱表明，DMA 阳离子的振动发生了显著变化（某些振动频率的变化、特定振动的分裂以及强度的变化），而与甲酸盐阴离子有关的振动则没有明显变化。基于所有这些信息，我们将介电转变的起源归因于 DMA 阳离子的动力学：在 190 K 以上，这些阳离子可以在[Mn(HCOO)(3)]（-）框架所形成的立方八面体空腔内旋转，而对于较低的温度，这种旋转会被冻结，它们在空腔内的协同排列导致了观察到的介电转变。
