FIG. 2: (a) Low-angle ( Q < 5 . 5 ˚ A − $^{1}$) region of the neutron total scattering patterns of (top) the desolvated ZIF-4 frame- work at 300 $^{◦}$C, (middle) the in-situ amorphized a-ZIF prod- uct at 320 $^{◦}$C, and (bottom) subsequently-recrystallised ZIF- zni at 400 $^{◦}$C. (b) Neutron differential correlation functions D ( r ) [18]; the inset illustrates the practically-identical local structure in all phases. (c) D ( r ) functions compared for a-ZIF and a-SiO$_{2}$ [19], where the latter data have been stretched in r by a factor of two to reflect approximately the difference in Zn . . . Zn and Si . . . Si distances in the two materials. 2 PDFs begin to differ at larger distances with evidence for different medium-range order (MRO) in all three phases over distances of 10–20 ˚ A. Peaks characteristic of crys- talline order persist in the PDFs of ZIF-4 and ZIF-zni for r > 20 ˚ A, whereas the PDF of a-ZIF is essentially featureless after r ≃ 25 ˚ A. A very similar comparison can be drawn between crystalline silica polymorphs and amorphous silica (a-SiO$_{2}$) [22], where the lower critical lengthscales reflect the shorter distance between tetra- hedral centres ( d$_{Si}$$_{...}$$_{Si}$ ≃ 3 . 1 ˚ A; cf . d$_{Zn}$$_{...}$$_{Zn}$ ≃ 5 . 9 ˚ A for Zn–im–Zn). Indeed there are strong qualitative similari- ties in the PDFs of a-SiO$_{2}$ and a-ZIF at longer distances [Fig. 2(c)] that add further weight to the analogy often drawn between ZIFs and silicate analogues [14]. Detailed insight into the structure of a-ZIF was ob- tained via Reverse Monte Carlo (RMC) refinement against both neutron and X-ray total scattering data us- ing the program RMCProfile [23]. RMC modelling of crystalline–amorphous transitions in zeolites [24] and the open framework material ZrW$_{2}$O$_{8}$ [25] has shown frame- work connectivity is commonly preserved during frame- work amorphization. Consequently our initial starting models were based on the crystalline topologies of either ZIF-4 or ZIF-zni frameworks. We found RMC refine- ments that used these models were not capable of re- producing both neutron and X-ray scattering data, even after incorporation of substantial disorder of the imidazo- late linkages during refinement (see SI for further details). The key implication of this result is that there must be a series of changes in connectivity within the Zn(im)$_{2}$ framework during both the conversion from ZIF-4 to a- ZIF and then again from a-ZIF to ZIF-zni. Indeed this is consistent with the changes in MRO evident in the PDF data themselves [Fig. 2(b)] and also that ZIFs are known to undergo reconstructive rather than displacive transi- tions under application of modest pressures [26, 27]. Motivated by the correspondence between features in the a-SiO$_{2}$ and a-ZIF PDFs shown in Fig. 2(c), we tested a model based on an established continuous random net- work (CRN) topology of a-SiO$_{2}$ itself [19, 28], modified to reflect the replacement of Si by Zn and oxygen by imi- dazolate. RMC refinement then gave markedly improved fits to data, with the final RMC configuration capable of reproducing all key aspects of the experimental to- tal scattering patterns. The configuration itself is illus- trated in Fig. 3(a), with the corresponding fits shown in Fig. 3(b). This is strong evidence that the structure of a-ZIF is itself well described by a CRN of tetrahedral Zn centres connected via imidazolate linkages. While the CRN of a-SiO$_{2}$ probably has different “ring statis- tics” to that of a-ZIF (we note that both ZIF-4 and ZIF- zni networks contain a relatively large number of four- membered rings, while a-SiO$_{2}$ is thought to contain very few), so far our RMC modelling + total scattering ap- proach is not sensitive to these subtleties. What we can state with confidence is that the network topology in a- The most striking feature of the PDFs — both neutron and X-ray — is that they are virtually identical for each of ZIF-4, a-ZIF and ZIF-zni up to approximately 6 ˚ A (see inset to Fig. 1(b) and SI), confirming that the tetrahedral Zn coordination environment and the bridging coordina- tion motif of the imidazolate ions [Fig. 1] are common to all three phases. We note that there is some slight broad- ening of the peaks in the PDFs with increasing tempera- ture as is consistent with increased thermal motion. The In order to probe the detailed structure of a-ZIF, we prepared a perdeuterated sample of ZIF-4 and collected neutron total scattering data at a number of temper- atures using the GEM diffractometer at the ISIS spal- lation source [20]. Neutron powder diffraction patterns of the desolvated (300 $^{◦}$C) ZIF-4 phase, the as-prepared amorphous phase (320 $^{◦}$C), and the recrystallized ZIF-zni phase (400 $^{◦}$C) are shown in Fig. 2(a). Following appro- priate normalization (see SI), the data were converted to the corresponding pair distribution functions (PDFs) [Fig. 2(b)]. Further structural data were collected for the a-ZIF and ZIF-zni phases using Ag K α X-ray radi- ation ( λ = 0 . 561 ˚ A) [21]; the complete set of X-ray to- tal scattering patterns and their corresponding Fourier transforms ( i.e. effective PDFs [18]) are given as SI. lar system is that the thermal stability field of a-ZIF is bounded by crystalline phases of the same composition Zn(im)$_{2}$, removing any ambiguities of composition dur- ing structure determination.