Structure and properties of an amorphous metal--organic framework

TL;DR

This study demonstrates the transformation of a crystalline MOF, ZIF-4, into a glass-like amorphous phase with a continuous random network structure, revealing potential for designing new amorphous materials.

Contribution

It provides the first detailed structural characterization of an amorphous MOF, showing its similarity to a-SiO₂ and its reversible phase transitions.

Findings

a-ZIF is an isotropic glass-like phase capable of plastic flow.

The structure is best described as a continuous random network.

Amorphous ZIF can be recovered or converted back to crystalline form.

Abstract

We show that ZIF-4, a metal-organic framework (MOF) with a zeolitic structure, undergoes a crystal--amorphous transition on heating to 300 $^\circ$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 $^\circ$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 nanoindentation 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.