Molecular-based polar structures are one of most intriguing candidates to generate mechanical function such as ferroelectric. These structures undergo a symmetry switching on heating and generally yields anisotropic lattice variation. Volumetric negative thermal expansion (NTE) remains challenging to observed, and most of which exhibit relatively small magnitude as well. Herein, we realize on a colossal volumetric NTE in the metal-organic frameworks via novel polar-nonpolar structure transition. Intriguingly, a colossal volumetric NTE (α_v = −302.45 × 10⁻⁶ K⁻¹) was achieved from 320–380 K in CAU-10-CH₃ due to polar-nonpolar transition (space group from I4₁md to I4₁/amd), much larger than most NTE MOFs. Variable-temperature crystallographic analysis reveals that the aluminum-oxygen octahedra turns from low symmetry to high symmetry on heating and thus causes a “spring” compression of the octahedral helical chain, thus leading to the colossal NTE. Functional groups modifications further identify the excellent tunability of such transition. The strongest uniaxial NTE, to date, was simultaneously found in CAU-10-H along c direction (α_c = −577.22 × 10⁻⁶ K⁻¹), whereas the CAU-10-NH₂ contributes to a sluggish phase transition. This work provides the first example of tailoring a colossal NTE via polar-nonpolar transition in frameworks.

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中文翻译：

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金属有机框架中极性-非极性转变导致的巨大负热膨胀

基于分子的极性结构是产生铁电等机械功能的最有趣的候选结构之一。这些结构在加热时经历对称转变，并且通常产生各向异性晶格变化。体积负热膨胀（NTE）仍然难以观察到，而且其中大多数也表现出相对较小的幅度。在这里，我们通过新颖的极性-非极性结构转变在金属有机框架中实现了巨大的体积NTE。有趣的是，^由于_极性-非极性转变（^空间_群 从I 4 ₁ md 到I 4 ₁ /a md )，比大多数 NTE MOF 大得多。变温晶体学分析表明，铝氧八面体在加热时从低对称性转变为高对称性，从而引起八面体螺旋链的“弹簧”压缩，从而产生巨大的NTE。官能团修饰进一步确定了这种转变的出色可调性。迄今为止，在CAU-10-H中沿c方向同时发现了最强的单轴NTE（α _c  = -577.22 × 10 ^-6  K ^-1），而CAU-10-NH ₂导致缓慢的相变。这项工作提供了第一个通过框架中的极性-非极性转变定制巨大 NTE 的例子。

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