SESSION 3: MECHANICAL, THERMAL, AND CHEMICAL STABILITY OF MOFS

Mechanics of MOF Crystals: Anisotropic Elasticity & Co-operative Dynamics

Multifunctional Materials & Composites (MMC) Lab, Department of Engineering Science, University of Oxford, United Kingdom

Keywords: elastic anisotropy, nanoindentation, vibrational spectroscopy, density functional theory, terahertz modes

Understanding the mechanical properties of MOF crystals and the structure-property relations of new compounds is central to every practical applications. This talk will discuss exemplars derived from our studies of this fast-expanding topic area. I shall begin by discussing the quasistatic mechanical properties of single crystals of MOF. Example will be drawn from the accurate determination of the elastic constants (C_ij’s) of a micron-size ZIF-8 crystal^[1], measured using a combination of Brillouin spectroscopy and nanoindentation methods, and further validated using ab initiodensity functional theory (DFT) calculations via CRYSTAL14. Recently, we made progress in the nanoindentation measurements of sub-micron crystals of ZIF-8 employing an atomic force microscope (AFM) approach^[2], which provides the unique opportunity to measurethe mechanical behaviour of a MOF crystal (500 nm – 1 µm) which is around 500 times smaller than those employed in [1].In the second part of the talk, I will address the dynamic mechanical response of MOF structures, focussing on the study of co-operative THz vibrations using a combination of synchrotron far-IR spectroscopy, inelastic neutron scattering and DFT techniques ^[3]. It is proposed that connections exist between the anisotropic elastic properties of the open porous framework and their collective vibrational motions (rotor dynamics^[4] and linker buckling) to attain anomalous elastic response such as negative Poisson’s ratio (auxeticity)^[5]. Finally, I shall discuss the liquid intrusion phenomenon into ZIFs subject to a moderate mechanical pressure of tens of MPa, where reversible liquid intrusion-extrusion response has been detected accompanied by rate-dependent deformation effects—indicative of a creep/stress relaxation type mechanism^[6,7].

[1] Tan, J.C., Civalleri, B., Lin, C.C., Valenzano, L., Galvelis, R., Chen, P.F., Bennett, T.D., Mellot-Draznieks, C., Zicovich-Wilson, C.M. and Cheetham, A.K., Exceptionally Low Shear Modulus in a Prototypical Imidazole-Based Metal-Organic Framework, Phys. Rev. Lett., 108, 095502, 2012
[2] Zeng, Z. and Tan, J.C., AFM Nanoindentation To Quantify Mechanical Properties of Nano-and Micron-Sized Crystals of a Metal-Organic Framework Material, ACS Appl. Mater. Interfaces, 9, 39839, 2017
[3] Ryder, M.R., Civalleri, B., Bennett, T.D., Henke, S., Rudić, S., Cinque, G., Fernandez-Alonso, F. and Tan, J.C., Identifying the Role of Terahertz Vibrations in Metal-Organic Frameworks: From Gate-Opening Phenomenon to Shear-Driven Structural Destabilization, Phys. Rev. Lett., 113, 215502, 2014
[4] Ryder, M.R., Van de Voorde, B., Civalleri, B., Bennett, T.D., Mukhopadhyay, S., Cinque, G., Fernandez-Alonso, F., De Vos, D., Rudic, S. and Tan, J.C., Detecting Molecular Rotational Dynamics Complementing the Low-Frequency Terahertz Vibrations in a Zirconium-Based Metal-Organic Framework, Phys. Rev. Lett., 118, 255502, 2017
[5] Tan, J.C., Civalleri, B., Erba, A. and Albanese, E., Quantum mechanical predictions to elucidate the anisotropic elastic properties of zeolitic imidazolate frameworks: ZIF-4 vs. ZIF-zni, CrystEngComm, 17, 375, 2015.
[6] Sun, Y., Li, Y. and Tan, J.C., Liquid Intrusion into Zeolitic Imidazolate Framework-7 Nanocrystals: Exposing the Roles of Phase Transition and Gate Opening to Enable Energy Absorption Applications, ACS Appl. Mater. Interfaces, 10, 41831, 2018
[7] Sun, Y., Li, Y. and Tan, J.C., Framework flexibility of ZIF-8 under liquid intrusion: discovering time-dependent mechanical response and structural relaxation, Phys. Chem. Chem. Phys., 20, 10108, 2018