A closer look at superionic phase transition in (NH4)4H2(SeO4)3: impedance spectroscopy under pressure.

The proton-conducting material (NH4)4H2(SeO4)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 dTs/dp = -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.

Uncommon first universality of conductivity in superprotonic (NH4)3H(SeO4)2 single crystals.

The proton conducting crystal (NH4)3H(SeO4)2 is examined to check whether the first universality of conductivity spectra is sensitive to subtle changes in the crystal structure and proton dynamics caused by external pressure. The ac conductivity was measured along the trigonal c axis by means of impedance spectroscopy, in the frequency range from 100 Hz to 1 MHz, at temperatures 250 K < T < 330 K and pressures 0.1 < p < 380 MPa. The ac conductivity characteristics were analyzed using the Summerfield scaling procedure. In the temperature range of the experiment the master curve is strongly disturbed by the structural phase transition at Tc1 = 273 K but the scaled spectra superimpose within the temperature range of each individual phase (below and above Tc1). The effect of pressure on the scaled conductivity spectra considered separately for each of the studied phases is similar to that caused by temperature. This means that both stimuli give rise to an acceleration of the dynamics of protons and consequently to an increase in conductivity. The evolution of the scaled conductivity spectra with pressure close to the phase boundary between the triclinic, ferroelastic phase III (P1[combining macron]) and the trigonal, superionic phase II (R3[combining macron]) points to the mixing of phase III with inclusions of phase II.