Bond Fluctuation Model of Superionic Conductors: Concepts and Applications

Abstract

The bond fluctuation model of superionic conductors provides a framework to understand the characteristic properties of superionic materials. According to the model, the superionic behavior is related to a change of bonding that occurs locally and fluctuates in time. In the present report, after presenting the concept of the model and summarizing the main results obtained till now, some recent applications of the model is reviewed. Topics such as new interpretation of the power law exponent of ac conductivity observed in superionic glasses, model for the anomalous electronic properties observed in liquid silver chalcogenides, and prediction of nonlinear optical constants of ionic conducting glasses are covered.

Ionic Conductors: Effect of Temperature on Conductivity and Mechanical Properties and Their Interrelations

“…Since such distortions occur locally at different sites of the material and at different times, they can be visualized as local fluctuations of the chemical bond. The origin of the correlated ionic motions, which is a characteristic of superionic conductors is explained naturally from this model [40][41][42]. Results of ab-initio molecular dynamics study [59] and other recent studies support the view proposed in the bond fluctuation model [60][61][62][63][64].…”

“…This temperature dependence is opposite to that observed in NaCl. It is worth mentioning at this point that the behavior shown in Figure 3 can be related with the bond fluctuation model of ion transport [40][41][42]. Although AgCl and AgBr have the rock-salt structure, their chemical bonding is not purely ionic as suggested from Figure 3.…”

“…For the promotion of a correlated ionic motion, the presence of certain degree of angular force is indispensable. The bond fluctuation model mentioned above [40][41][42] provides the physical background to the discussion given here.…”

“…For instance, light was used to study the influence of elastic stress on ionic transport in a heterojunction formed by a superionic crystal and an electrode [12]. The result provided a hint to understand the role of electrons in the ionic transport mechanism, a subject of fundamental importance but not thoroughly investigated [40][41][42]. Regarding this point, it will be enlightening to point out that the phenomenon reported in [12] has much in common with the photodoping effect known in the field of amorphous semiconductors [43][44][45].…”

Is Geometric Frustration-Induced Disorder a Recipe for High Ionic Conductivity?

“…The overlap between neighboring ellipsoids indicates facile collective motion of the ions; collective ion transport was reported for several superionic conductors (29)(30)(31)(32)(33) and ascribed to a dynamical change of local (frustrated) bonding. (34)(35)(36) Computer modelling has reached a maturity whereby both synthesis and property measurement can be mirrored directly with computer simulation. Here, we demonstrate and then exploit this capability to elucidate the origin of the 5 orders of magnitude increase in ionic conductivity when nanostructured CaF2 is doped with Ba.…”