arXiv:1001.1332v1 [cond-mat.mtrl-sci] 8 Jan 2010 Structure and properties of an amorphous metal–organic framework Thomas D. Bennett, 1 Andrew L. Goodwin, 2, 3 Martin T. Dove, 2 David A. Keen, 4, 5 Matthew G. Tucker, 4 Emma R. Barney, 4 Alan K. Soper, 4 Erica G. Bithell, 1 Jin-Chong Tan, 1 and Anthony K. Cheetham 1, ∗ $^{1}$Department of Materials Science, University of Cambridge, Cambridge CB2 3QZ, U.K. $^{2}$Department of Earth Sciences, Cambridge University, Downing Street, Cambridge CB2 3EQ, U.K. $^{3}$Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, U.K. $^{4}$ISIS Facility, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0QX, U.K. $^{5}$Department of Physics, Oxford University, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, U.K. (Dated: October 23, 2018) We show that ZIF-4, a metal-organic framework (MOF) with a zeolitic structure, undergoes a crystal–amorphous transition on heating to 300 $^{◦}$C. The amorphous form, which we term a-ZIF, is recoverable to ambient conditions or may be converted to a dense crystalline phase of the same composition by heating to 400 $^{◦}$C. Neutron and X-ray total scattering data collected during the amorphization process are used as a basis for reverse Monte Carlo refinement of an atomistic model of the structure of a-ZIF. We show that the structure is best understood in terms of a continuous random network analogous to that of a-SiO$_{2}$. Optical microscopy, electron diffraction and nanoin- dentation measurements reveal a-ZIF to be an isotropic glass-like phase capable of plastic flow on its formation. Our results suggest an avenue for designing broad new families of amorphous and glass-like materials that exploit the chemical and structural diversity of MOFs. PACS numbers: 61.43.Er,64.70.K-,62.20.-x,81.07.Pr Inorganic zeolites can transform to amorphous struc- tures under the application of either pressure or tem- perature [13]. In the present work, we have discov- ered that the zinc imidazolate ZIF-4 [14] will amor- phize at ambient pressure, simply on heating. Bulk sam- ples of ZIF-4 were prepared via solvothermal reaction of zinc(II) nitrate hexahydrate and imidazole in dimethyl- formamide (DMF) [14]. Thermogravimetric analysis and variable-temperature X-ray and neutron diffraction data all show that the as-prepared phase, which contains DMF molecules within its framework pore structure, loses this DMF on heating above 200 $^{◦}$C, giving a solvent-free nanoporous phase of composition Zn(im)$_{2}$ (im − = im- idazolate anion) while maintaining the ZIF-4 topology. Further heating to 300 $^{◦}$C causes an irreversible trans- formation to an amorphous phase (referred to hereafter as a-ZIF), indicated by the loss of all Bragg reflections in the diffraction pattern. This phase is recoverable to ambient temperature with no change in its diffraction pattern. On further heating to 400 $^{◦}$C the densest of the known crystalline Zn(im)$_{2}$ phases (ZIF-zni) [10] evolves from the amorphous solid and is stable to around 500 $^{◦}$C, whereupon thermal decomposition takes place. The over- all transformation of ZIF-4 to ZIF-zni is consistent with the greater thermodynamic stability of the latter, pre- dicted by recent density functional theory calculations [15]. While the existence of other amorphous transition- metal imidazolates has been documented [8, 9, 16, 17], the unique advantage of having discovered this particu- Amorphous materials are found extensively in the fields of intermetallic [1], inorganic [2], and organic ma- terials [3], and have a wide range of important appli- cations. Throughout the last 5–10 years the materials science community has studied extensively a fourth class of materials: namely, the so-called metal-organic frame- works (MOFs) [4, 5]. These are network structures in which metal atoms are combined with organic ligands in a crystalline array. Their structures can be dense or porous and, by virtue of their combined structural and chemical versatility, they have a wide range of applica- tions, including gas sorption and separation, magnetism and photoluminescence. While amorphous products have been identified in this emerging area [6–8] (including a very recent report of pressure-induced amorphization [9]) there remain no well-characterized examples. The ze- olitic imidazolate frameworks (ZIFs) [10–12] are inter- esting candidates for amorphization because their struc- tures are closely related to those of certain zeolitic sil- ica polymorphs by virtue of their analogous tetrahedral connectivites [Fig. 1]. Just as silica-based materials can form network glasses, we show here that ZIFs can also be prepared in silica-glass-like forms. Our results demon- strate how the chemical versatility of MOFs might be coupled with the attractive mechanical, optical and elec- tronic properties of the amorphous state to yield a new generation of advanced functional materials. FIG. 1: Representations of the Zn–im–Zn and Si–O–Si link- ages in tetrahedral ZIF and silicate networks, respectively.