Approximate model for the crystal structure of precipitates in NaF doped with AlF3
The crystal structure of precipitates in a mixed crystal of NaF and 1 mol% AlF3 has been studied at room temperature using both stationary-crystal and rotating-crystal X-ray photographic methods. It has been found that almost all the reflections can be assigned to a face-centered-cubic (f.c.c.) lattice with unit-cell parameter 7.77 Å. The main feature of the diffraction pattern is that the 311 reflection is very strong while the 222 reflection is practically zero, in contrast to the case of high-form cryolite, i.e. cubic Na3AlF6. These features are explained by assuming an f.c.c. arrangement of AlF6 octahedra that are rotated around the 〈111〉 axes by about 47° from the highest-symmetry orientation. It has also been shown that the F ions in each octahedron make large overlaps with the Al ion at the center of the octahedron. A random distribution of rotation axes is also proposed to conform to the cubic symmetry of the lattice structure of the precipitate.