DYNAMICS OF THE AMMONIUM ION IN $NH_{4}ClO_{4}$
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Date
1973
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Ohio State University
Abstract
Crystal structure studies of $NH_{4}ClO_{4}$ by neutron diffraction techniques suggested that the ammonium ion at room temperature is undergoing free or nearly free rotation. Similar conclusions were reached by total neutron cross section measurements and infra-red absorption studies, and NMR measurements indicated that even at $20^{\circ} K$ the ion was undergoing reorientation at a rapid rate ( $>10^{5}$ cps). The neutron total cross section measurements yielded a value for the barrier to reorientation of about 0.2 kcal/mole, the lowest barrier of any ammonium compound known. (The barrier for $NH_{4}Cl$ is 4.4 kcal/mole at room temperature.) Specific heat measurements did not indicate any cooperative phenomena taking place as the crystal warmed from $5^{\circ} K$ to room temperature, suggesting that any change to free rotation was a gradual one. Inelastic neutron scattering measurements at ORNL on a polycrystalline sample at $78^{\circ} K$ did not show any pronounced peaks that could be attributed to a well defined torsional mode of oscillation of the ammonium ion. However, measurements made at $4.6^{\circ} K$ revealed a broad intense band with some structure centered at about $200 cm^{-1}$ (width $\sim$ $80 cm^{-1}$), which is believed due to the torsional mode of the ammonium ion. Sharper but smaller peaks at 103, 23, and $47 cm^{-1}$ are probably due to optic, acoustic, and $ClO_{4}$ torsional modes, respectively. These assignments have been partially confirmed by similar measurements on $ND_{4}ClO_{4}$. Further measurements are underway on isomorphous $RbClO_{4}$ and $RbClO_{4}: (NH_{4})$.
Description
M. Nielsen is a guest scientist from Atomic Energy Research Establishment Ris\""{o}, Roskilde, Denmark. The Oak Ridge National Laboratory is operated by Union Carbide Corporation for the USAEC.
Author Institution: Solid State Division, Oak Ridge National Laboratory
Author Institution: Solid State Division, Oak Ridge National Laboratory