Aluminum fluoride crystals

Anhydrous alpha-AlF3 features a rhombohedral crystal structure that closely resembles a cubic lattice. These specific crystals in the picture were produced by cooling molten fluoride salts (cryolites) during the microwave-assisted Hall-Héroult process for primary aluminum reduction.

Interestingly, AlF3 exhibits a complex, three-dimensional polymeric crystal structure. Within this network, every aluminum atom bonds to six fluorine atoms, forming a six-sided shape (AlF6 octahedron). All six corners of each octahedron link directly to neighboring octahedra, meaning that each fluorine atom at the corner connects two separate aluminum centers.

Each rhombohedral unit cell consists of six AlF6 octahedra, containing a total of 6 aluminum and 18 fluorine atoms. Consequently, unlike the molecular gas SiF4, AlF3 possesses high thermal and chemical stability, along with a relatively high sublimation point of 1,291 °C.