Temperature dependence of anomalous protonic and superprotonic transport properties in mixed salts based on CsH₂PO₄

Abstract

The transport properties of mixed salts based on CsH2PO4 are influenced by the interactions among charge carriers.

Entropic restrictions control the electric conductance of superprotonic ionic solids

“…Figure 2 The solid lines are the fits using eqns (17) and (19) of the real part of the conductivityσ ν (T ) as a function of temperature (including a superprotonic transition) for the experimental data (symbols) of doped cesium phosphates studied in Ref. 8 . The dotted and dashed lines represent the Arrhenius behaviors, eqn (4), far from the transition region.…”

“…It is well known that the thermal behavior of the conductivity of some solid acids such as RbH2PO4, CsHSO4 and CsH2PO4 have a superprotonic phase transition (SPT) in which a drastic increase in conductivity of several orders of magnitude is observed. [4][5][6][7][8] That is, the experiments determine the conductivity of the solid electrolytes in terms of temperature for ranges in which the superprotonic transition takes place. Recently, for instance, broadband dielectric and DSC thermal analysis were used to correlate the superprotonic transition of different solid compounds of cesium phosphate and other solid electrolytes with the structural transition between monoclinic (T < T c with T c the transition temperature) and cubic symmetries (T > T c ).…”

“…Recently, for instance, broadband dielectric and DSC thermal analysis were used to correlate the superprotonic transition of different solid compounds of cesium phosphate and other solid electrolytes with the structural transition between monoclinic (T < T c with T c the transition temperature) and cubic symmetries (T > T c ). 8,24 In this section, we will use eqn ( 14) in order to cope with exper-imental measurements of the electric conductivity as a function of the temperature and show that the superprotonic transition can be adequately explained in terms of the thermodynamic excess entropy associated with the macroscopic structural changes among different crystallographic symmetries of the material due to a phase or structural transition. In doing this, we first note that the structure factor of a system, expressed in terms of its phonon density of states may be indicative of the characteristic length of the corresponding crystallographic phase and, therefore, from a thermodynamic perspective, it may be associated with its density or, more usefully, with the porosity φ of the material.…”

“…Here, we include recent results of the referred phase transitions between monoclinic and cubic phases of CsP and a mixture based on the partial substitution of Cs by Rb. 8 These phase transitions induce a sudden change of three orders of magnitude of the ion conductance 8 . In what follows we propose a simple and direct macro-transport theory from which we deduce a general formula accounting for the complex non-isothermal conductance G ν (T ) of solid and aqueous fuel cells in the temperature range including the superprotonic transition 9,10 .…”

“…Dielectric spectroscopy experiments 8 measure the homogeneous frequency (ν) dependent conductance G ν (T ) at different temperatures (T ). In contrast, the mathematical description of charge carriers transport is provided in terms of a generalized Fick's law involving an ion current density i(r,t) that depends on position r and time t. With the aim to deduce an homogeneous macro-transport coefficient we will perform the implicit volume average present in experiments along the lines of a recently proposed theoretical description of mass transport across confined spaces.…”