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

In the present paper, we studied the structural evolution of the glass matrix and formation of crystalline fluoride phases for 15KF-15ZnF₂-70SiO₂ (glass A) and 25KF-25ZnF₂-50SiO₂ (glass B) glasses upon thermal treatment employing transmission electron microscopy, X-ray diffraction (XRD), and solid-state NMR. The first step marks a phase separation into an SiO₂ rich phase and a phase enriched with Zn, K, and fluoride. As shown by XRD, depending on the nominal compositions, K₂SiF₆ (for 15KF-15ZnF₂-70SiO₂) and KZnF₃ (for 25KF-25ZnF₂-50SiO₂) are found as the first crystalline phases being formed. Upon longer heat treatment, ZnF₂ is additionally formed in both cases. Surprisingly, a significant amount of SiF₆^2- units is detected employing ²⁹Si-MAS and ²⁹Si{¹⁹F}CPMAS-NMR spectroscopy even in the base glass, which is completely amorphous according to the X-ray results. The vast majority of Si, however, is found in an exclusive SiO₄ environment as Q⁴, Q³, or Q² units. The large fraction of nonbridging oxygens per SiO₄ tetrahedron (ca. 0.7 for glass A and 0.65 for glass B) indicates quite large fluorine loss during glass synthesis (approx. 60-80%). Employing dipolar NMR techniques, i.e., ²⁹Si-{¹⁹F}-REDOR (rotational echo double resonance) and ²⁹Si{¹⁹F}CPMAS-NMR (cross polarization magic angle spinning), the presence of Si-F connectivity (in excess of the identified SiF₆^2- units) could be ruled out. The observed differences in the ²⁹Si-¹⁹F heteronuclear dipole couplings between the Si-Q⁴ and Si Q³ units-as determined by REDOR NMR-are compatible with the assumption of a core-shell structure with a mixed cation fluoride as the core and an SiO₂-enriched shell.