Phase Separation and Nanocrystallization in KF-ZnF₂-SiO₂ Glasses: Lessons from Solid-State NMR.

Area of Science:

• Materials Science
• Solid-State Chemistry
• Glass Science

Background:

• Fluoride-silicate glasses are investigated for their unique properties.
• Understanding their structural evolution upon thermal treatment is crucial for material design.

Purpose of the Study:

• To investigate the structural evolution and crystalline phase formation in KF-ZnF₂-SiO₂ glasses after thermal treatment.
• To characterize the local structure of silicon and fluoride environments.

Main Methods:

• Transmission electron microscopy (TEM)
• X-ray diffraction (XRD)
• Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy, including ²⁹Si-MAS, ²⁹Si{¹⁹F}CPMAS, and ²⁹Si-{¹⁹F}-REDOR

Main Results:

• Phase separation into SiO₂-rich and fluoride-rich phases occurred.
• Crystalline phases K₂SiF₆ and KZnF₃ formed initially, followed by ZnF₂.
• Significant amounts of SiF₆^2- units were detected in the amorphous phase by NMR, despite amorphous status by XRD.
• A core-shell structure was proposed with a fluoride core and SiO₂-enriched shell.

Conclusions:

• Thermal treatment induces phase separation and crystallization in these glasses.
• NMR reveals unexpected SiF₆^2- units in the amorphous matrix, challenging initial structural assumptions.
• The findings suggest a complex structural evolution and a core-shell morphology.