Flat Phonon Band-Based Mechanism of Amorphization of MOF-5 at Ultra-low Pressures,The Journal of Physical Chemistry C

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Flat Phonon Band-Based Mechanism of Amorphization of MOF-5 at Ultra-low Pressures
The Journal of Physical Chemistry C ( IF 3.3 ) Pub Date : 2021-07-06 , DOI: 10.1021/acs.jpcc.1c02598
Meha Bhogra ₁ , Umesh V. Waghmare ₁

Affiliation

MOF-5 is a crystalline metal–organic framework (MOF) with large pore volume and exceptional thermal stability. However, it undergoes irreversible amorphization at surprisingly low pressures of about 10 MPa. While such disruption of framework-topology was attributed to the rupture of −C–O– bonds of the carboxylate groups in its rigid secondary building units (SBUs), these energy-intensive bond-breaking events are unlikely to occur at minuscule pressures of a few MPa. Using first-principles theoretical phonon-spectral analysis, we demonstrate that thermally stable MOF-5 crystal cannot sustain hydrostatic compression, primarily because of pressure-induced symmetry-lowering torsional forces that destabilize its octahedral SBUs. Group-theoretical analysis of phonons of MOF-5 unravels the role of normal modes in mid-frequency range (ω ∼ 1.6–3.2 THz), which become unstable and form dispersionless phonon bands at very small compressive strains (∼−0.3%), leading to an order-to-disorder structural phase transition. At slightly larger strains, structures distorted with random combinations of localized modes in the flat bands of these unstable phonons and associated instabilities of the transverse acoustic branches relax to lower-energy states that exhibit structural shearing at the nanoscale. This results in the loss of long-range order and irreversible amorphization of the MOF-5 crystal, while preserving the local structural coordination environment in this topologically disordered state. Our work will stimulate exploration of this microscopic mechanism of amorphization in other MOFs that consist of high-symmetry directionally constrained rigid building units in their network structure.

中文翻译：

MOF-5在超低压下非晶化的扁平声子带机制

MOF-5 是一种结晶金属有机骨架 (MOF)，具有大孔体积和出色的热稳定性。然而，它在约 10 MPa 的令人惊讶的低压力下发生不可逆的非晶化。虽然框架拓扑的这种破坏归因于羧酸基团在其刚性中的 -C-O- 键的断裂对于二级建筑单元 (SBU)，这些能源密集型断键事件不太可能在几兆帕的微小压力下发生。使用第一性原理理论声子光谱分析，我们证明热稳定的 MOF-5 晶体不能承受流体静力压缩，主要是因为压力引起的对称性降低扭转力会破坏其八面体 SBU 的稳定性。MOF-5 声子的群论分析揭示了正常模式在中频范围 (ω ∼ 1.6–3.2 THz) 中的作用，在非常小的压缩应变 (∼-0.3%) 下，它们变得不稳定并形成无色散声子带，导致有序到无序的结构相变。在稍大的应变下，结构因局部模式的随机组合而扭曲这些不稳定声子的平坦带和横向声学分支的相关不稳定性松弛到低能状态，在纳米尺度上表现出结构剪切。这导致 MOF-5 晶体失去长程有序性和不可逆非晶化，同时在这种拓扑无序状态下保持局部结构配位环境。我们的工作将激发对其他 MOF 中这种非晶化微观机制的探索，这些 MOF 由网络结构中的高对称性定向约束刚性构建单元组成。

更新日期：2021-07-15

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