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

Maria Zdanowska-Fra̧Czek,Krystyna Hołderna-Natkaniec,Łukasz Lindner

doi:10.1021/acs.jpcc.1c01317

Abstract

Structural relaxation and the role of molecular and proton dynamics in the electrolyte system, (NH4)4H2(SeO4)3 (TeAHSe), are studied at ambient pressure, as a function of temperature and time, by using the solid-state 1H NMR technique. Analysis of 1H 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 NH4+ 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 NH4+ 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 low-temperature 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.

Highlights

One of the enigmatic phenomena occurring in the crystals of proton conductors, belonging to the family of acid salts, is structural relaxation associated with some local ordering of the structure under the influence of a disturbing factor
On the basis of our results, we proposed a scenario for the process of structural relaxation in the TeAHSe crystal
The 1H NMR spectrum of TeAHSe recorded below 60 K is typical of NH4 groups and consists of a robust central line and two weak satellites.[12]

Summary

Introduction

One of the enigmatic phenomena occurring in the crystals of proton conductors, belonging to the family of acid salts, is structural relaxation associated with some local ordering of the structure under the influence of a disturbing factor. As the crystals of this family, it shows some features that are characteristic of all solid acids salts: proton conductivity, ordered low-temperature phase stabilized by hydrogen bonds, transformation to the superprotonic phase with a disordered lattice of hydrogen bonds, a jump in conductivity by several orders of magnitude at the phase transition temperature (Ts), and low activation energy in the superprotonic stage. At room temperature (low-temperature phase), the TeAHSe crystal is triclinic with the space group P1̅.2−5 The crystal is built of NH4+ cations and the dihydrogen triselenate anions, in which short hydrogen bonds link three SeO4 tetrahedra (2.56 and 2.46 Å).[6] The strong O−H···O hydrogen bonds of the dihydrogen triselenate anions form a zero-dimensional network. The trimers lie lengthwise along the [021] crystallographic direction (Figure 1).[4]

Methods

Results

Conclusion