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 SiF62- 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 SiF62- 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.