Phase Behavior and Superionic Transport Characteristics of (MxRb1- x)3H(SeO4)2 (M = K or Cs) Solid Solutions

Dezhi Yi, Sheel Sanghvi, Chatr Panithipongwut Kowalski, Sossina M. Haile*

*Corresponding author for this work

Research output: Contribution to journalArticle

2 Scopus citations

Abstract

Solid acid selenates in the class M3H(SeO4)2, in which M is an alkali ion, have garnered attention as a result of the dramatic increase in conductivity that occurs upon transition from a monoclinic to a superprotonic trigonal phase with a dynamically disordered hydrogen-bond network. The significant similarities between three specific members of this class, K3H(SeO4)2, Rb3H(SeO4)2, and Cs3H(SeO4)2, render the pseudobinary systems between end-members ideal for an exploration of the crystal chemical features that drive the superprotonic transition and control the magnitude of the conductivity. Accordingly, a systematic study of the (KxRb1-x)3H(SeO4)2 and (CsxRb1-x)3H(SeO4)2 systems is carried out here using in situ high-temperature diffraction and conductivity measurements by ac impedance spectroscopy. Across the entire composition range, a transition to a superprotonic phase of the space group R3̄ m is observed. With decreasing average size of the alkali cation, the superprotonic transition temperature generally decreases and, for a given temperature, the conductivity monotonically increases. The variation in transition temperature with composition is discussed in terms of possible strain effects on the internal energy of the monoclinic phase. The conductivity in the superprotonic phase is found to obey the phenomenological compensation rule, in which the logarithm of the pre-exponential term is linearly correlated with the activation energy. The decrease in activation energy for proton transport with decreasing average cation size is tentatively ascribed to a contraction in the length of the hydrogen bonds, presumed from the contraction of the cell volume. Subtle changes in structure and conductivity occur at the special composition Cs2RbH(SeO4)2, corresponding to a structure in which Cs and Rb species could be fully unmixed over available cation sites. While a fully ordered arrangement of cations was not observed, the larger cation was found to preferentially occupy the larger of the two cation sites in both high- and low-temperature phases and in both chemical systems.

Original languageEnglish (US)
Pages (from-to)9807-9818
Number of pages12
JournalChemistry of Materials
Volume31
Issue number23
DOIs
StatePublished - Dec 10 2019

ASJC Scopus subject areas

  • Chemistry(all)
  • Chemical Engineering(all)
  • Materials Chemistry

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