Department of Physical Chemistry, University of Science and Technology Beijing, Beijing, 100083, China.
Chem Commun (Camb). 2018 May 31;54(45):5712-5715. doi: 10.1039/c8cc03219j.
Negative thermal expansion (NTE), lattice contraction upon heating, has been found in some metal organic frameworks (MOFs). To date, MOF compounds have been found to present either isotropic NTE in the cubic phase or 1D or 2D NTE in a low symmetry structure. Herein, we report anisotropic NTE in orthorhombic MIL-68(In) with the 3D NTE of αa = -5.6 × 10-6 K-1, αb = -2.7 × 10-6 K-1 and αc = -4.0 × 10-6 K-1. A synchrotron X-ray diffraction study shows that the transverse vibration of the phenyl ring leads to the NTE in the ab plane. Further high-energy synchrotron X-ray total scattering investigation reveals that the rotation of rigid octahedrons gives rise to the NTE along the c-axis. Such abnormal 3D NTE in orthorhombic MIL-68(In) is accounted for by the combination of transverse vibration and rigid unit modes. This is the first case to show 3D NTE in a low symmetry MOF compound and it will be beneficial to tailor the thermal expansion of functional MOF materials.
负热膨胀（NTE），即加热时晶格收缩，已在一些金属有机骨架（MOFs）中发现。迄今为止，已经发现 MOF 化合物在立方相呈现各向同性 NTE，或者在低对称结构中呈现 1D 或 2D NTE。在此，我们报告了具有三维 NTE 的正交 MIL-68(In)的各向异性 NTE，其 αa = -5.6 × 10-6 K-1，αb = -2.7 × 10-6 K-1 和 αc = -4.0 × 10-6 K-1。同步辐射 X 射线衍射研究表明，苯环的横向振动导致 ab 平面的 NTE。进一步的高能同步辐射全散射研究表明，刚性八面体的旋转导致沿 c 轴的 NTE。这种正交 MIL-68(In)中的异常三维 NTE 归因于横向振动和刚性单元模式的组合。这是首例在低对称 MOF 化合物中显示三维 NTE 的情况，这将有利于调整功能 MOF 材料的热膨胀。
