NMR investigation of the crystallization mechanism of LaF3 and NaLaF4 phases in aluminosilicate glasses

The structure of transparent F-containing nano-crystalline glass-ceramics has been investigated by means of Nuclear Magnetic Resonance as well as X-ray Diffraction as a function of the processing conditions. LaF3 or NaLaF4 are crystallized depending on the glass composition: low silica content and low modifier to alumina ratios lead to crystallization of LaF3, while increasing SiO2 and decreasing Al2O3 results in NaLaF4 nano-crystals. NMR results show that fluorine in glasses is involved in Al–F–Na or La–F environments, the later constituting pre-nucleation sites. After annealing, crystallization of either LaF3 or NaLaF4 crystals takes place, which is correlated with an increase in the intensity of fluorine resonance of La–F environment and a decrease in the Al–F–Na one. The description of the local environment around F, Al and Na is in accordance with a crystallization model of these materials. In low silica containing glasses, it starts from La–F sites and progresses thanks to fluorine diffusion from Al–F–Na bonds until crystal growth is limited through the increase in the viscosity of the glass matrix around crystals. In high silica glasses, long annealing times give rise to crystallization of NaF after NaLaF4 crystals have formed and a silica-enriched region around them inhibits further crystal growth.