Abstract
Quasi-elastic neutron scattering has been used to study the reorientation mechanism for ammonium ions in cesium-exchanged zeolite Y. This sample contains ammonium ions preferentially located in the sodalite cages at site 1'. These ions reorientate with 120' or 180' jumps around fixed axes through the nitrogen atom. This corresponds to a bonding geometry in which the ammonium ion interacts via three or two hydrogen atoms with the lattice oxygen atoms. Additionally, a reorientation around a fixed axis through two hydrogen atoms of the ammonium ion may occur. The ammonium ion then jumps between two equilibrium positions: one position involves a 2-fold bonding geometry and the other a 3-fold bonding geometry. At 300 K the effective time interval between two jumps is T = 1.2 f 0.5 ps. Compared to zeolite Rho, the ammonium ions in the cesiumexchanged zeolite Y show a more restricted reorientational behavior, resulting in a larger elastic incoherent structure factor.
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