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A closer look at superionic phase transition in (NH 4 ) 4 H 2 (SeO 4 ) 3 : impedance spectroscopy under pressure
Author(s) -
Lindner Łukasz,
Zdanowska-Frączek Maria,
Czapla Zbigniew,
Frączek Ziemowit
Publication year - 2020
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520620003741
Subject(s) - analytical chemistry (journal) , conductivity , hydrostatic pressure , dielectric spectroscopy , phase (matter) , electrical resistivity and conductivity , phase diagram , chemistry , nucleation , phase transition , thermodynamics , materials science , physics , organic chemistry , chromatography , electrode , electrical engineering , electrochemistry , engineering
The proton‐conducting material (NH 4 ) 4 H 2 (SeO 4 ) 3 is examined to check whether its conductivity spectra are sensitive to subtle changes in the crystal structure and proton dynamics caused by external pressure. The AC conductivity was measured using impedance spectroscopy, in the frequency range from 100 Hz to 1 MHz, at temperatures 260 K < T < 400 K and pressures 0.1 MPa < p < 500 MPa. On the basis of the impedance spectra, carefully analyzed at different thermodynamic conditions, the p – T phase diagram of the crystal is constructed. It is found to be linear in the pressure range of the experiment, with the pressure coefficient value d T s /d p = −0.023 K MPa −1 . The hydrostatic pressure effect on proton conductivity is also presented and discussed. Measurements of the electrical conductivity versus time were performed at a selected temperature T = 352.3 K and at pressures 0.1 MPa < p < 360 MPa. At fixed thermodynamic conditions ( p = 302 MPa, T = 352.3 K), the sluggish solid–solid transformation from low conducting to superionic phase was induced. It is established that the kinetics of this transformation can be described by the Avrami model with an effective Avrami index value of about 4, which corresponds to the classical value associated with the homogeneous nucleation and three‐dimensional growth of a new phase.