Very large swelling in hybrid frameworks: a combined computational and powder diffraction study.

Area of Science:

• Materials Science
• Nanotechnology
• Crystallography

Background:

• Nanoporous materials offer unique properties for various applications.
• Iron(III) carboxylates, like MIL-88, are known for their structural versatility.
• Understanding framework flexibility is crucial for designing advanced materials.

Purpose of the Study:

• To investigate the large reversible swelling of a three-dimensional nanoporous iron(III) carboxylate (MIL-88).
• To elucidate the structural changes and mechanisms behind the swelling phenomenon.
• To determine if the open-framework topology is retained during volume expansion.

Main Methods:

• Combined use of computational simulations and experimental powder diffraction.
• Crystal structure refinement of the open form of MIL-88.
• X-ray thermodiffractometry to analyze structural transitions.

Main Results:

• Observed an approximately 85% reversible increase in MIL-88's cell volume.
• Atomic displacements exceeding 4 angstroms were detected upon adsorption of water and alcohols.
• The open-framework topology was fully retained throughout the swelling process.
• X-ray thermodiffractometry confirmed a displacive transition, indicating no bond breaking.

Conclusions:

• MIL-88 exhibits exceptional framework flexibility, allowing for significant reversible volume changes.
• The swelling mechanism involves large atomic displacements rather than chemical bond alteration.
• This flexibility preserves the material's nanoporous structure, suggesting potential for responsive material applications.