Neutron Diffraction Study of the Crystal Structure of Ammonium Chloride

The crystal structure of the room-temperature phase of ammonium chloride has been determined by powder and single crystal neutron diffraction methods. The N—H links are directed essentially toward four of the eight surrounding chloride ions, and, contrary to a recent report, the two orientations fulfilling this condition are occupied at random in the crystal. Thermal motion of the hydrogen atoms is nonspherical and suggests a rotatory oscillation of the ammonium ion as a whole with a half-angle of about 10°. The length of the N—H link is 1.03±0.02 $A$. The second-order transition at -30°C is thus shown to be of the order-disorder type.