Fundamental questions relating to ion conduction in disordered solids

Abstract

A number of basic scientific questions relating to ion conduction in homogeneously disordered solids are discussed. The questions deal with how to define the mobile ion density, what can be learned from electrode effects, what is the ion transport mechanism, the role of dimensionality, and what are the origins of the mixed-alkali effect, of time-temperature superposition, and of the nearly-constant loss. Answers are suggested to some of these questions, but the main purpose of the paper is to draw attention to the fact that this field of research still presents several fundamental challenges.

Ion migration in crystalline and amorphous HfOX

“…One may think of defining defects based on under-or over-coordinated species, 10 but as we will discuss later, this is problematic for amorphous hafnia. This problem is well-known to researchers in the glass community [11][12][13][14][15][16][17] but seems to be ignored by those whose experience is mainly crystalline systems.…”

Maxwell displacement current and nature of Jonsher’s “universal” dynamic response in nanoionics

“…The sign -minus‖ in eq. (2) shows that at y k < 0 (deficit of cations on {X k } planes) the F j,j+1 vector and the positive direction along the x axis coincide. The vector of electric induction in the layer between X j and X j+1 planes is given by the equation…”

“…A new field of investigations appeared. Its subject is the dynamics of ion transport in solids [2,4,35]. The discovered power law of the real part of frequency dependent conductivity Re*()  n (n < 1)…”

“…However, the variation of  with a space coordinate x can be considerable in objects of solid state ionics. For example, local regions with  variation about 1 eV/nm exist in disordered solid electrolytes (SE) [2][3][4] and in crystals of superionic conductors (SIC) [5]. Various degrees of ordering occur near surfaces and in the region of interphase and intercrystalline boundaries [6] where specific ion dynamics can arise on the nanoscale [7] due to a non-uniform potential relief.…”

“…In [19] the attention is drawn to the fact that the Maxwell displacement current should be taken into account for the process of EDL charging. However, this fundamental current has not been even mentioned in the reviews [2][3][4] devoted to multiple-aspect discussions on dynamic properties of solid ionic conductors.…”