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 (Cij’s) of a micron-size ZIF-8 crystal, 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, 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 .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 . It is proposed that connections exist between the anisotropic elastic properties of the open porous framework and their collective vibrational motions (rotor dynamics and linker buckling) to attain anomalous elastic response such as negative Poisson’s ratio (auxeticity). 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].
 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
 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
 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
 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
 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.
 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
 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