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October, 2019

Mixing processes are fundamental to many steps in materials processing. By combining more than one component, the properties can be altered or augmented relative to the parent materials to address performance needs. In glasses quenched from a melt, mixing can take place in a number of different ways including diffusion of one component into the other – or the components may stay nearly completely separate like oil and water. Metal-organic frameworks have recently been introduced as a fourth type of glass, and a number of multi-component systems have shown different types of mixing. 
Documenting how mixing takes place at a fundamental level requires a probe that can look at the nanoscale, the length scale where it starts to become possible to directly measure the number of atoms in a given volume of material. By using quantitative spectroscopy in an electron microscope, we have now done three-dimensional measurements recording the number of zinc and the number of cobalt atoms in microscopic particles of two metal-organic framework glasses. These measurements now document exactly what the proportions are of zinc and cobalt during mixing in these new glasses.

Figure caption: A three-dimensional reconstruction of the zinc (blue) and cobalt (red) components of a new type of glass made from metal-organic frameworks. By quantitative analysis at the nanoscale, how the components are mixed together can be mapped in terms of the number density (atoms per unit volume) and the zinc fraction (zinc and cobalt sum to one). This analysis now provides a precise description of the mixture.

S. M. Collins, K. E. MacArthur, L. Longley, R. Tobey, M. Benning, C.-B. Schönlieb, T. D. Bennett, and P. A. Midgley, "Phase diagrams of liquid-phase mixing in multi-component metal-organic framework glasses constructed by quantitative nano-tomography", APL Materials 7, 091111 (2019)