Solid-state proton conduction: An ab initio molecular dynamics investigation
of ammonium perchlorate doped with neutral ammonia
L. Rosso and M. E. Tuckerman
Department of Chemistry, New York University, New
York, NY 10003, USA;
Department of Chemistry and Courant Institute of Mathematical Sciences,
New York University, New York, NY 10003, USA
Abstract:
The charge-transport mechanism in solid ammonium perchlorate crystal
exposed to an ammonia-rich environment is studied using ab initio molecular
dynamics. Ammonium perchlorate is an ionic crystal composed of NH4+
and ClO4- ; units that possesses an orthorhombic
phase at T < 513 K and a cubic phase at T > 513 K. Exposure
to an ammonia-rich atmosphere allows ammonia molecules to be absorbed
into the crystal at interstitial sites. It has been proposed that these
neutral ammonias can form short-lived N2H7+
complexes with the NH4+ ions allowing proton transfer
between them, thereby enhancing the conductivity considerably. To date,
however, there has been no direct evidence of this proposed mechanism.
In this paper, ab initio molecular dynamics techniques are employed
to explore this mechanism. By comparing computed infrared spectra of
the pure and ammonia-doped crystals, we observe a significant broadening
of the NH stretch peak into a lower frequency region, indicating through
an experimentally verifiable observable, the formation of hydrogen bonds
between NH3 and NH4+ units. This suggestion
is confirmed by direct observation of N2H7+
complexes from the trajectory. Comparison of the diffusion constants
of NH4+ in the pure and doped crystals yields
a ratio that is comparable to the experimentally measured conductivity
ratio and clearly shows an enhanced positive charge mobility. Finally,
compelling evidence suggesting the possibility of an ammonia umbrella
inversion following proton transfer from NH4+
and NH3 is obtained.
*Lecture presented at the European Molecular Liquids
Group (EMLG) Annual Meeting on the Physical Chemistry of Liquids: Novel
Approaches to the Structure, Dynamics of Liquids: Experiments, Theories,
and Simulation,Rhodes, Greece, 7-15 September 2002. Other presentations
are published in this issue, pp. 1-261.
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