The relationship between 27Al quadrupolar parameters and AlF63- octahedron connectivity in crystalline and glassy fluoroaluminates.
Résumé
27Al SATRAS and MQ-MAS spectra were recorded for eight crystalline compounds, from the CaF2–AlF3 and BaF2–AlF3 binary and BaF2–CaF2–AlF3 ternary systems, and four glass compositions with different CaF2/BaF2/AlF3 contents. For the crystalline phases, the reconstruction of the spectra leads to the precise determination of the NMR parameters. For the glassy phases, the 27Al SATRAS spectra have been reconstructed using quadrupolar parameter distributions. The main finding of this study is the dependence of the quadrupolar frequency on the type of AlF63– octahedron connectivity. For crystalline phases the experimental νQ values range between 75 kHz and 510 kHz for structures built up from isolated octahedra, and are between 560 kHz and 1250 kHz for structures built up from isolated chains of cis-connected octahedra, and are between 1530 kHz and 1580 kHz for structures built up from isolated chains of trans-connected octahedra. In the glassy phases the maximum of the quadrupolar frequency distribution shifts toward larger values and its width increases with increasing AlF3 content, and subsequently with the number of connected octahedra. The range of the νQ values seems to indicate that when the octahedra are connected, cis connections occur most frequently.