On the Colossal and Highly Anisotropic Thermal Expansion Exhibited by Imidazolium Salts

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• 作者：I. de Pedro ; A. Garc铆a-Saiz ; J. Dupont ; P. Migowski ; O. Vallcorba ; J. Junquera ; J. Rius ; J. Rodr铆guez Fern谩ndez
• 刊名：Crystal Growth & Design
• 出版年：2015
• 出版时间：November 4, 2015
• 年：2015
• 卷：15
• 期：11
• 页码：5207-5212
• 全文大小：418K
• ISSN：1528-7505

文摘

The imidazolium salts 1-ethyl-2,3-dimethylimidazolium chloride, Edimim[Cl], and bromide, Edimim[Br], exhibit negative and positive thermal expansions, as determined by variable-temperature synchrotron powder X-ray diffraction experiments. Both compounds crystallize in the same monoclinic centrosymmetric space group, showing an anisotropic H-bonding network and imidazolium鈥搃midazolium 蟺⁺鈥撓€⁺ interactions, which have been corroborated by density functional theory studies. The chloride derivative displays a highly anisotropic thermal expansion with a colossal positive coefficient along one direction. Replacement of Cl^{鈥?/sup> by Br鈥?/sup> in the same crystal structure produces an increase of the colossal coefficient and induces a biaxial negative thermal expansion. By studying the molecular vibration factors and the H-bonding framework in their crystals as a function of temperature, it was possible to rationalize at the molecular level the mechanism for the observed anomalies in thermal expansion.}