Direct evidence of an anomalous charge transport mechanism in ammonium perchlorate crystal in an ammonia-rich atmosphere from first-principles molecular dynamics

Lula Rosso, Mark Tuckerman

Research output: Contribution to journalArticle

Abstract

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.

Original languageEnglish (US)
Pages (from-to)219-229
Number of pages11
JournalSolid State Ionics
Volume161
Issue number3-4
DOIs
StatePublished - Aug 2003

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ammonium perchlorates
Ammonia
Molecular dynamics
Charge transfer
ammonia
molecular dynamics
atmospheres
Crystals
Proton transfer
crystals
doped crystals
conductivity
protons
ionic crystals
suggestion
Hydrogen bonds
interstitials
infrared spectra
Trajectories
ammonium perchlorate

Keywords

  • Ammonium perchlorate
  • Grotthuss mechanism
  • Solid state proton conductor

ASJC Scopus subject areas

  • Electrochemistry
  • Physical and Theoretical Chemistry
  • Energy Engineering and Power Technology
  • Materials Chemistry
  • Condensed Matter Physics

Cite this

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title = "Direct evidence of an anomalous charge transport mechanism in ammonium perchlorate crystal in an ammonia-rich atmosphere from first-principles molecular dynamics",
abstract = "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.",
keywords = "Ammonium perchlorate, Grotthuss mechanism, Solid state proton conductor",
author = "Lula Rosso and Mark Tuckerman",
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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

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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