Solid-state proton conduction: An ab initio molecular dynamics investigation of ammonium perchlorate doped with neutral ammonia

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 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 ammoniarich 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 N ₂H₇⁺ 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.