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Negative-to-Positive Thermal Conductivity Temperature Coefficient Transition Induced by Dynamic Fluctuations of the Alkyl Chains in the Layered Complex (C4 H9 NH3 )2 CuCl4.
Chemistry - A European Journal ( IF 4.3 ) Pub Date : 2020-01-23 , DOI: 10.1002/chem.201904553
Norihisa Hoshino 1 , Syunsaku Tamura 2 , Tomoyuki Akutagawa 1
Affiliation  

A negative-to-positive transition of the temperature coefficients of thermal conductivity was found in the two-dimensional organic-inorganic layered complex (C4 H9 NH3 )2 CuCl4 (C4CuCl4) over the three structural phase transitions in the range 176-218 K. The coefficients of the low-temperature phases (85-200 K, α and β phases) were negative, as is typical for insulating crystals, whereas those of the high-temperature phases (200-300 K, γ and δ phases) were positive, as is typical for glasses and liquids. Single-crystal X-ray structure analyses revealed that the tilted C4 H9 NH3 + chains in the α and β phases were fully outstretched in the δ phase, and the interlayer distances between the CuCl4 2- planes increased significantly. The γ phase was an intermediate phase that crystallized with an incommensurate structure, in which the CuCl4 2- sheets formed wave-like structures consisting of connected alternating regions of β-like and δ-like moieties. In the γ and δ phases, thermal fluctuations of the C4 H9 NH3 + chains were found in the electron density maps; however, powder X-ray diffraction (PXRD) data indicated that the thermal expansion of the C4 H9 NH3 + layers was restricted by the rigid CuCl4 2- layers. This situation was considered to induce glass-like thermal conducting properties in the material, such as a positive temperature coefficient. The mean free path of the phonons estimated by using the thermal conductivities and heat capacities was a function of T-1 in the range 85-200 K, as would be expected for crystals, whereas it was approximately constant in the range 200-300 K, which is typical of glasses. In addition, the existence of soft vibration modes in the two-dimensional perovskite CuCl4 2- sheets was revealed by analysis of the incommensurate crystal structure of the γ phase. These low-energy vibration modes were believed to induce the cooperative phase transitions, along with the thermal fluctuations and van der Waals interactions in the C4 H9 NH3 + layers.

中文翻译:

层状复合物(C4 H9 NH3)2 CuCl4中烷基链动态波动引起的负至正热导率温度系数跃迁。

在176-218 K范围内的三个结构相变中,在二维有机-无机层状复合物(C4 H9 NH3)2 CuCl4(C4CuCl4)中发现了热导率温度系数的从负到正的转变。低温相(85-200 K,α和β相)的系数为负,这是绝缘晶体的典型系数,而高温相(200-300 K,γ和δ相)的系数为正,这是玻璃和液体的典型代表。X射线单晶结构分析表明,α和β相中倾斜的C4 H9 NH3 +链在δ相中完全伸出,并且CuCl4 2-平面之间的层间距离显着增加。γ相是中间相,结晶不规则,其中,CuCl 4 2-片形成由相连的交替的β-型和δ-型部分组成的波状结构。在γ相和δ相中,在电子密度图中发现了C4 H9 NH3 +链的热涨落。然而,粉末X射线衍射(PXRD)数据表明C4 H9 NH3 +层的热膨胀受到刚性CuCl4 2-层的限制。人们认为这种情况会在材料中引起类似玻璃的导热特性,例如正温度系数。通过热导率和热容量估算的声子平均自由程是T-1在85-200 K范围内的函数,这是晶体所期望的,而在200-300 K范围内则大致恒定。 ,这是典型的眼镜。此外,通过分析γ相的不等价晶体结构,揭示了二维钙钛矿型CuCl4 2-片中存在软振动模式。据信,这些低能振动模式会引起协同相变,以及C4 H9 NH3 +层中的热涨落和范德华相互作用。
更新日期:2020-01-23
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