Metal organic framework materials form as three-dimensional network structures with metal cations linked via organic ligands. One example is zinc imadazolate, in which the zinc cations are tetrahedrally coordinated to the imadazolate ligands to form structures that are analogous to phases of silicates, including zeolite structures. The elusive structure is the amorphous analogue of silica, which cannot be formed by freezing from the melt. We recently found that an amorphous phase can be formed by heating the structure of one crystalline phase, ZIF-4, prior to its transformation to a higher-density phase.
We produced a phase-pure sample by heating and monitoring a large sample by neutron scattering at the ISIS pulsed neutron source. We cooled the sample to room temperature and collected neutron total scattering data. We also removed a small part to perform an x-ray total scattering measurement.
We generated a trial configuration of the amorphous phase of zinc imadazolate based on a model of amorphous silica we have, and adjusted this model using a Monte Carlo technique until it's predicted total scattering function gave best agreement with the experimental data.
The work has been published in "Structure and properties of an amorphous metal-organic framework. TD Bennett, AL Goodwin, MT Dove, DA Keen, MG. Tucker, ER Barney, AK Soper, EG Bithell, J-C Tan and AK Cheetham. Physical Review Letters 104, art no 115503 (4 pp), 2010 (DOI: 10.1103/PhysRevLett.104.115503)