Localized Symmetry Breaking for Tuning Thermal Expansion in ScF3 Nanoscale Frameworks
摘要:
The local symmetry, beyond the averaged crystallographic structure, tends to bring unusual performances. Negative thermal expansion is a peculiar physical property of solids. Here, we report the delicate design of the localized symmetry breaking to achieve controllable thermal expansion in ScF3 nanoscale frameworks. Intriguingly, an isotropic zero thermal expansion is concurrently engineered by localized symmetry breaking, with a remarkably low coefficient of thermal expansion of about +4.0 X 10(-8)/K up to 675 K. This mechanism is investigated by the joint analysis of atomic pair distribution function of synchrotron X-ray total scattering and extended X-ray absorption fine structure spectra. A localized rhombohedral distortion presumably plays a critical role in stiffening ScF3 nanoscale frameworks and concomitantly suppressing transverse thermal vibrations of fluorine atoms. This physical scenario is also theoretically corroborated by the extinction of phonon modes with negative Gruneisen parameters in rhombohedral ScF3. The present work opens an untraditional chemical modification route to achieve controllable thermal expansion by breaking local symmetries in materials.
我们报告新cryptophane衍生物的合成1 - 4轴承9(1,2)和12(3,4)甲氧基取代基。这些化合物代表在苯环上带有六个以上取代基的隐色烯衍生物的第一个例子。由于在催化量的三氟甲磺酸ScSc(OTf)3存在下从受保护的丁香醇反应得到的新的环三丁烯基衍生物的合成,实现了这些高度取代的隐烯基的制备。该反应还以低收率(7%)提供了受保护的环四环丁烯基衍生物,其特征在于11 H NMR,IR和X射线晶体学。与此相反,为cryptophane-A同类物所观察到的,这些高度取代cryptophanes的合成产生了两个抗-和顺式-diastereomers。这些化合物已得到充分表征,并已获得其X射线结构以确定这些新的隐烷衍生物的立体化学。