TY - JOUR
T1 - Direct evidence of an anomalous charge transport mechanism in ammonium perchlorate crystal in an ammonia-rich atmosphere from first-principles molecular dynamics
AU - Rosso, Lula
AU - Tuckerman, Mark E.
N1 - Funding Information:
Most of the calculations performed here were performed on the Lemieux system at the Pittsburgh Supercomputing Center and at the Hewlett-Packard high-performance computing facilities in Richardson, TX. This work has been supported by NSF CHE-9875824, NSF CHE-0121375, and a Research Corporation Research Innovations Award RI0218.
PY - 2003/8
Y1 - 2003/8
N2 - The charge transport mechanism in solid ammonium perchlorate (AP) crystal exposed to an ammonia-rich environment is studied using ab initio molecular dynamics (AIMD). Ammonium perchlorate is an ionic crystal composed of NH 4+ 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 observation, the formation of hydrogen bonds between NH3 and NH4+ units. This suggestion is confirmed by direction observation of N2H 7+ 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 NH 3 is obtained.
AB - The charge transport mechanism in solid ammonium perchlorate (AP) crystal exposed to an ammonia-rich environment is studied using ab initio molecular dynamics (AIMD). Ammonium perchlorate is an ionic crystal composed of NH 4+ 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 observation, the formation of hydrogen bonds between NH3 and NH4+ units. This suggestion is confirmed by direction observation of N2H 7+ 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 NH 3 is obtained.
KW - Ammonium perchlorate
KW - Grotthuss mechanism
KW - Solid state proton conductor
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U2 - 10.1016/S0167-2738(03)00273-X
DO - 10.1016/S0167-2738(03)00273-X
M3 - Article
AN - SCOPUS:0042832398
VL - 161
SP - 219
EP - 229
JO - Solid State Ionics
JF - Solid State Ionics
SN - 0167-2738
IS - 3-4
ER -