H. A. Levy, S. W. Peterson
Jun 1, 1952
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Journal
Physical Review
Abstract
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\ifmmode^\circ\else\textdegree\fi{}. The length of the N---H link is 1.03\ifmmode\pm\else\textpm\fi{}0.02 $A$. The second-order transition at -30\ifmmode^\circ\else\textdegree\fi{}C is thus shown to be of the order-disorder type.