New Insights into the Negative Thermal Expansion: Direct Experimental Evidence for the “Guitar-String” Effect in Cubic ScF3

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• 作者：Lei Hu ; Jun Chen ; Andrea Sanson ; Hui Wu ; Clara Guglieri Rodriguez ; Luca Olivi ; Yang Ren ; Longlong Fan ; Jinxia Deng ; Xianran Xing
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：July 13, 2016
• 年：2016
• 卷：138
• 期：27
• 页码：8320-8323
• 全文大小：283K
• 年卷期：0
• ISSN：1520-5126

文摘

The understanding of the negative thermal expansion (NTE) mechanism remains challenging but critical for the development of NTE materials. This study sheds light on NTE of ScF₃, one of the most outstanding materials with NTE. The local dynamics of ScF₃ has been investigated by a combined analysis of synchrotron-based X-ray total scattering, extended X-ray absorption fine structure, and neutron powder diffraction. Very interestingly, we observe that (i) the Sc–F nearest-neighbor distance strongly expands with increasing temperature, while the Sc–Sc next-nearest-neighbor distance contracts, (ii) the thermal ellipsoids of relative vibrations between Sc–F nearest-neighbors are highly elongated in the direction perpendicular to the Sc–F bond, indicating that the Sc–F bond is much softer to bend than to stretch, and (iii) there is mainly dynamically transverse motion of fluorine atoms, rather than static shifts. These results are direct experimental evidence for the NTE mechanism, in which the rigid unit is not necessary for the occurrence of NTE, and the key role is played by the transverse thermal vibrations of fluorine atoms through the “guitar-string” effect.