Sieradzki A, Pawlus S, Tripathy S N, Gągor A, Ciupa A, Mączka M, Paluch M
Department of Experimental Physics, Wrocław University of Technology, WybrzeżeWyspiańskiego 27, 50-370 Wrocław, Poland.
Institute of Physics, University of Silesia, ul. Uniwersytecka 4, PL-40-007 Katowice, Poland.
Phys Chem Chem Phys. 2016 Mar 28;18(12):8462-7. doi: 10.1039/c6cp00064a.
The fundamental aspects of the relaxation dynamics in niccolite-type, mixed valence metal-organic framework, multiferroic [(CH3)2NH2][Fe(3+)Fe(2+)(HCOO)6] single crystals have been reported using dielectric relaxation spectroscopy covering eight decades in frequency (10(-2) ≤ f ≤ 10(6)) in the temperature range 120 K ≤ T ≤ 250 K. The compound shows antiferroelectric to paraelectric phase transition near T = 154 K with the relaxor nature of electric ordering. The temperature dependent dielectric response in modulus representation indicates three relaxation processes within the experimental window. The variable range hopping model of small polarons explains the bulk non-Debye type conductivity relaxation. The fastest relaxation with activation energy Ea = 0.17 eV is related to progressive freezing of the reorientation motions of DMA(+) cations. X-ray diffraction data revealed that complete freezing of orientational and translational motions of DMA(+) cations occurs well below phase transition temperature. These experimental observations are fundamentally important for the theoretical explanation of relaxation dynamics in niccolite-type metal-organic frameworks.
利用介电弛豫谱在120 K≤T≤250 K的温度范围内、频率跨越八个数量级（10⁻²≤f≤10⁶）报道了镍黄铁矿型混合价金属有机框架多铁性单晶[(CH₃)₂NH₂][Fe³⁺Fe²⁺(HCOO)₆]中弛豫动力学的基本方面。该化合物在T = 154 K附近呈现反铁电到顺电的相变，具有电有序的弛豫铁电体性质。模量表示下的温度依赖介电响应表明在实验窗口内存在三个弛豫过程。小极化子的变程跳跃模型解释了体相非德拜型电导率弛豫。激活能Ea = 0.17 eV的最快弛豫与DMA⁺阳离子重取向运动的逐步冻结有关。X射线衍射数据表明，DMA⁺阳离子的取向和平动运动在远低于相变温度时完全冻结。这些实验观察结果对于镍黄铁矿型金属有机框架中弛豫动力学的理论解释具有根本重要性。
