Proton Motions in Ammonium Halides by Slow Neutron Cross-Section Measurements

Total neutron cross sections were measured at room temperature for NH/ sub 3/ gas at l15 psia, NH/sub 4/F, NH/sub 4/Cl, NH/sub 4/Br, and NH/sub 4/I, and the scattering cross sections per hydrogen atom plotted vs. neutron wavelength ( lambda n). At short lambda n. the cross sections approach the values for the free atoms, while at long lambda /sub n/n they may be represented bv sigma s = a + b lambda /sub n/. The slopes b of the ammonium halides were determined from the graph and correlated with other properties of the halides, e.g., torsional frequency and potential barrier to rotation. The order-disorder transitions between phases I, II, and III of the halides are discussed. It is concluded that neutron cross section measurements can give evidence of the dynamics and chemical binding of hydrogen in molecules and crystals. (D.L.C.)