On the rotational motion of the ammonium ion in the CsClhyphen;type phase of NH4Br: Results from quasielastic neutron scattering

摘要

The rotational motion of the ammonium ion in polycrystalline NH4Br has been investigated by quasielastic incoherent neutron scattering at 12 different temperatures between 297 and 409 K. The measured spectra were corrected for multiple scattering and compared to the C4hyphen;rotational jump model. The elastic incoherent structure factor was determined experimentally within the range 0.15les;Qles;1.72 Aring;minus;1of elastic wave vector transfer. It yields aNhyphen;Hbond length of 1.028plusmn;0.005 Aring;. The rotational jump rate was found to follow an Arrhenius law, 1 tgr;C4lsqb;sminus;1rsqb; plusmn;0.16)times;1013explcub;(minus;1623plusmn;16)/Tlsqb;Krsqb;rcub;, with an activation energy ofEa=13.49plusmn;0.13 kJ/mole. Within the studied temperature interval the mean square hydrogen vibration amplitude can be described by the expression lang;u2rang; lsqb;Aring;2minus;rsqb;plusmn;1.6)times;10minus;4Tlsqb;Krsqb; minus;(16.4plusmn;6)times;10minus;2.