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Flat Phonon Band-Based Mechanism of Amorphization of MOF‑5 at Ultra-low Pressures

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journal contribution
posted on 06.07.2021, 17:04 by Meha Bhogra, Umesh V. Waghmare
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.