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.