A Closer Look at Structural Relaxation in Superionic (NH₄)₄H₂(SeO₄)₃: ¹H NMR Studies

Abstract

Structural relaxation and the role of molecular and proton dynamics in the electrolyte system, (NH 4 ) 4 H 2 (SeO 4 ) 3 (TeAHSe), are studied at ambient pressure, as a function of temperature and time, by using the solidstate 1 H NMR technique. Analysis of 1 H NMR spectra collected in the temperature range from 20 to 400 K yielded information about the dynamics of protons in the ordered and disordered crystalline TeAHSe phases. It has been shown that in the low-temperature phase TeAHSe is a proton conductor. In contrast, the electric charge carriers in the superionic phase are protons and NH 4 + ions. Therefore, the electrical conductivity in the superionic phase of TeAHSe is a combination of the chemical exchange of protons, proton diffusion within hydrogen bonds, and diffusion of ammonia cations in the bulk of the crystal. We also demonstrate that the enhanced charge transport in the superionic phase of TeAHSe mainly results from the NH 4 + cation diffusion process. The unique data from the time evolution of NMR spectra taken at a constant temperature just below the phase transition provide the first direct evidence of ammonium cations involvement in the lowtemperature structure recovery (structural relaxation). We proposed a scenario of the process of structural relaxation in the TeAHSe crystal as well as the origin of the phase transformation mechanism to the superionic stage.

The Journal of Physical Chemistry C Virtual Special Issue on Advanced Characterization by Solid-State NMR and In Situ Technology and in Recognition of Michael Hunger’s 65th Birthday

“…The Hu group reported NMR evidence of the multielectron reaction mechanism of Na 3 V 2 (PO 4 ) 3 in Na-ion batteries . Gradišek and coworkers presented a molecular dynamics study of an ionic conductor, and Zdanowska-Fra̧czek and coworkers investigated the structural relaxation in superionics …”

“…11 The Buntkowsky group used 1D and 2D multinuclear magic-angle spinning (MAS) NMR to monitor the synthesis of the novel Wilkinson's type silica supported polymer catalysts. 12 The guest localization in mesoporous silica particles was mapped by 1 H and 19 F MAS NMR by the Potrzebowski group. 13 The Sun group investigated the structure−property relationship of chitosanbased biopolymers.…”

“…17 Gradisěk and coworkers presented a molecular dynamics study of an ionic conductor, 18 and Zdanowska-Fraçzek and coworkers investigated the structural relaxation in superionics. 19 Solid acid is one of the most popular heterogeneous catalysts utilized in the petrochemical process and biorefining. The VSI reports several articles on the NMR characterization of solid acids.…”