Structural Studies of NaPO₃–AlF₃ Glasses by High-Resolution Double-Resonance Nuclear Magnetic Resonance Spectroscopy C

The local structure of the model glasses (NaPO₃)₁–ₓ-(AlF₃)ₓ (0 ≤ x ≤ 0.4), prepared by standard melt-cooling, was extensively investigated by high-resolution solid-state nuclear magnetic resonance (NMR) including advanced double-resonance techniques. This glass system offers the opportunity of studying five different heteronuclear distance correlations (Na–F, Na–P, P–F, Al–F, and P–Al) by 10 distinct double-resonance experiments, involving all of the constituent elements present. ²⁷Al MAS-NMR data indicate that aluminum is predominantly six-coordinated. According to ²⁷Al{³¹P} and ²⁷Al{¹⁹F} rotational-echo double-resonance (REDOR) spectroscopic results, two to three Al–F and three to four Al–O–P linkages occur in these glasses, independent of composition x. ¹⁹F MAS-NMR spectra show the presence of terminal P-bound and Al-bound fluorine species. A small amount of fluorine bridging to two aluminum octahedra, which could be assigned based on ¹⁹F{²⁷Al} and ¹⁹F{³¹P} REDOR experiments, was also detected. ¹⁹F{²³Na} REDOR experiments indicate that the Al-bound terminal F atoms interact significantly more strongly with sodium ions than the P-bonded terminal F atoms, which is consistent with local charge considerations. On the basis of the detailed quantitative dipole–dipole coupling information obtained, a comprehensive structural model for these glasses is presented.