FIG. 3: (a) A region of the final RMC model of a-ZIF (cen- tre) shown relative to comparable regions of the ZIF-4 (top) and ZIF-zni (bottom) framework structures; the correspond- ing unit cells are shown in red outline. Deuterium atoms have been omitted for clarity. (b) RMC fits (red lines) to experi- mental total scattering data (black lines), calculated using the single atomistic configuration illustrated in the centre panel of (a): (top) Neutron differential correlation function D ( r ), (centre) neutron total scattering structure factor F$_{N}$ ( Q ), and (bottom) X-ray total scattering structure function F$_{X}$ ( Q ) [18]. be isotropic [Fig. 4(f)], with no residual texture from the anisotropic ZIF-4 parent phase [Fig. 4(g)]. Subse- quent recrystallisation of ZIF-zni during in-situ heating yields polycrystalline monoliths with sub-micron grains of random orientation [Fig. 4(h)], suggesting there is no “memory” of the original ZIF-4 orientation. In light of these findings, one also expects a-ZIF to exhibit mechan- ical isotropy rather than the anisotropy of the two crys- talline phases. We tested this by performing a series of nanoindentation studies [29] on single crystals/monoliths of the ZIF-4, a-ZIF and ZIF-zni samples. Elastic moduli and hardness values confirm the loss of anisotropy in the amorphous phase and show the mechanical properties of a-ZIF to be intermediate to but distinct from those of ZIF-zni and ZIF-4 [Fig. 4(i)]. That a-ZIF is not formed directly from a melt pre- cludes the use of the term “glass” in its traditional sense; specifically, there is no glass transition per se . Never- theless it is clear that a-ZIF is fundamentally different from the crystalline ZIF phases. An important ques- tion to be answered will be why such a phase should form at all. What we do know is that, like their sili- cate analogues, ZIFs are only metastable (as indeed are many open-framework MOFs), and this will provide some driving force for thermally-activated reconstructive tran- sitions. The existence of multiple intermediate structure topologies with similar energies will then favour an amor- phous state until the crystalline minimum (ZIF-zni) be- comes thermally accessible. It appears that ZIF materials follow their silica analogues in this respect, and indeed similar arguments have been put forward to explain the formation of amorphous covalent organic networks [30]. There is every reason to expect similar amorphiza- tion mechanisms in other metal-organic frameworks. In- deed we observe this to be the case in other ZIFs, in- cluding those in which Zn is replaced by magnetically- active Co(II). It is precisely this chemical variability that means access to glass-like MOFs will offer a number of exciting opportunities in the development of functional amorphous materials. The use of chiral bridging ligands, for example, suggests a method of producing optically- active network glasses; likewise the incorporation of lan- thanide metal centres and/or electronically active ligands would be an obvious means of preparing electrolumines- cent glasses for advanced photonics. Our results here show that the major hurdle faced by the community in developing such systems — namely, access to a method of characterising and understanding their atomic-scale structures — is no longer insurmountable. We gratefully acknowledge financial support from the European Research Council to A.K.C., from Trinity Col- lege, Cambridge to A.L.G., from the E.P.S.R.C. (UK) to A.L.G. and T.D.B., from the Isaac Newton Trust to J.C.T. and from HH Sheikh Saud Bin Saqr Al Qasimi to T.D.B. We thank A. Thirumurugan (Cambridge) for useful discussions. ZIF is disordered rather than crystal-like: the material is “truly” amorphous with a CRN topology, and not a highly disordered network solid with a crystal-like topology . Further evidence for glass-like behaviour in a-ZIF comes from optical microscopy. Crystals of ZIF-4 de- form, but do not shatter, on the transition to a-ZIF [Fig. 4(a,b)]. Quite remarkably the monolithic “crys- tals” evolve curved internal cavities (presumably coupled with loss of included DMF) in addition to curved exter- nal morphologies across the transition [Fig. 4(b,d)], in- dicating that a-ZIF appears to exhibit viscous flow on its formation at 300 $^{◦}$C. Optical micrographs of partially transformed “crystals” show that the subsequent tran- sition from a-ZIF to ZIF-zni then occurs via nucleation at surface defects [Fig. 4(d,e)]. Electron diffraction pat- terns collected from the amorphous phase indicate it to 3