Structure of Ammonium Fluoride. IV

X-ray powder diffraction patterns of ammonium fluoride IV have been obtained at about 450°K and 4 kbar in a pressure vessel made of single-crystal diamond. The unit cell is cubic with a = 5.77 ± 0.04 Å, and has a sodium-chloride-like structure with the space group Fm3m. Since ammonium fluoride IV appears to have a configurational entropy of about 3.8 cal deg−1·mole−1 and the hydrogen atoms of the ammonium ion cannot fit into an ordered sodium-chloride-like structure, the ammonium-ion orientations are probably disordered. The thermodynamic evidence does not favor free rotation. Ammonium fluoride IV is very plastic, in contrast to phases I of ammonium chloride, bromide, and iodide, which have the same structure.