A general protocol for determining the structures of molecularly ordered but noncrystalline silicate frameworks

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• Determining the structure of noncrystalline solids with short-range order is challenging.
• Traditional methods like X-ray diffraction (XRD) are limited by the absence of long-range periodicity.
• Layered silicates often exhibit complex structures that resist conventional analysis.

Purpose of the Study:

• To develop and demonstrate a general protocol for elucidating the structures of molecularly ordered, noncrystalline solids.
• To apply this protocol to a surfactant-directed layered silicate lacking three-dimensional (3D) long-range order.
• To identify and refine candidate framework structures compatible with experimental data.

Main Methods:

• Integration of X-ray diffraction (XRD) for unit cell parameters.
• Utilization of one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy for detailed site analysis and connectivity.
• Application of first-principles quantum chemical calculations, including density functional theory (DFT), for structure refinement and validation.

Main Results:

• The study successfully determined structural constraints for a layered silicate without long-range 3D periodicity.
• A combination of XRD and solid-state (29)Si NMR data, alongside DFT calculations, identified a small set of candidate structures.
• Three closely related and topologically equivalent framework configurations were found to be consistent with all experimental and theoretical data.

Conclusions:

• The developed protocol is effective for characterizing complex noncrystalline solids.
• The identified structures highlight the intricate nature of the layered silicate framework.
• The findings suggest the material likely contains coexisting or subtly distributed structural orders.