Fluorine in Complex Alumino-Boro-Silicate Glasses: Insight into Chemical Environment and Structure

Area of Science:

• Materials Science
• Solid State Chemistry
• Geochemistry

Background:

• Fluorine incorporation into silicate glasses is crucial for applications in geophysics, metallurgy, dentistry, optics, and waste management.
• Understanding fluorine's structural role is key to tailoring glass properties for specific technical demands.

Purpose of the Study:

• To investigate the structural impact of increasing fluorine content in alkaline alumino-borosilicate glass.
• To identify the coordination environments and potential crystallization pathways of fluorine within the glass matrix.

Main Methods:

• Synthesized alkaline alumino-borosilicate glasses with up to 25 mol% fluorine.
• Employed X-ray diffraction (XRD), ²⁷Al and ¹⁹F magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, and electron probe microanalysis for characterization.

Main Results:

• All fluorine was retained in the glass up to a critical concentration.
• Excess fluorine (>12-15 mol%) partitioned into CaF₂, NaF, and Na-Al-F crystalline phases.
• Identified five distinct fluorine sites in the glass, with two persistent glassy sites likely involving ^[4]Al-F-Ca/Na structures.

Conclusions:

• Fluorine incorporation influences the structure of alkaline alumino-borosilicate glasses, leading to phase separation and crystallization at higher concentrations.
• Established the existence of specific fluorine coordination environments within the glass.
• Provided insights into predicting ¹⁹F NMR chemical shifts in related multi-component systems.