Unveiling the multifaceted properties of a 3d covalent-organic framework: Pressure-induced phase transition, negative linear compressibility and auxeticity

Giant Negative Linear Compressibility, Isosymmetric Phase Transition, and Breathing Effect in a 3D Covalent Organic Framework

“…All first-principles calculations were conducted using Quantum Espresso (QE) package, which employs a plane wave-based DFT. The electron–ion interactions were described using projector augmented wave (PAW) potential, chosen for its ability to effectively simulate high-pressure conditions. ,,, The exchange–correlation potential was treated with the generalized gradient approximation (GGA) and the Perdew–Burke–Ernzerhof (PBE) parameterization . To account for dispersive interactions, we introduced Grimme’s semiempirical potential (DFT-D2) within the GGA framework .…”

“…Flexible porous crystals have emerged as highly intriguing materials in the field of materials science, capturing significant interest due to their unique combination of structural flexibility and porosity. − In contrast to rigid porous crystals, these crystals possess flexible pores that can undergo reversible phase transformations in response to external stimuli including host–guest interactions, temperature, and pressure. This diverse class of materials encompasses a wide range of structures, such as metal–organic frameworks (MOFs), , covalent organic frameworks (COFs), , zeolites, , and porous organic polymers. , Their inherent flexibility allows for significant changes in their crystal structures, leading to a wide range of functional properties, including tunable porosity, , selective adsorption, , guest molecule release, , and stimulus-responsive behavior. , …”

“…While the high-pressure behavior and mechanical properties of MOFs have been extensively investigated in numerous studies, the same level of understanding is lacking for COFs. Limited work has been conducted on COFs with few theoretical predictions ,− and only a few experimental investigations reported so far. , One of the main challenges in studying the mechanical properties of COFs is the difficulty in synthesizing them as single crystals. COF synthesis often results in polycrystalline or amorphous materials, which hinders the application of experimental techniques, such as diamond anvil cells (DACs), to determine pressure-dependent mechanical properties.…”

Indentation Depth‐Dependent Hardness of Metal‐Organic Framework Crystals: The Effect of Local Amorphization Induced by Indentation

“…Such large pressure may cause large deformations and eventually the mechanical instability of OFMs. [10][11][12][13][14][15][16] Thus, understanding the mechanical behaviours and properties of MOFs is essential for their technical applications. Among various mechanical property metrics, the hardness of MOFs is an important parameter measuring their resistance to the permanent plastic deformation.…”

“…During the densification process of open framework materials (OFMs) including MOFs and their organic analogues, covalent organic frameworks (COFs), for industrial applications, they are usually subjected pressures with several hundred megapascals. Such large pressure may cause large deformations and eventually the mechanical instability of OFMs [10–16] . Thus, understanding the mechanical behaviours and properties of MOFs is essential for their technical applications.…”