The bond fluctuation model of superionic conductors proposed by one of the authors (MA) predicts that there must be a close relationship between the electronic polarizability and the ionic conduction in solids. Guided by this model, the relation between the nonlinear optical constants and the ion transport properties in crystalline and glassy ion conducting materials were studied previously. Those studies revealed that the nonlinear optical constant increases with the increase of the ionic conductivity. It was also noted that the optical properties could be also related with the structural relaxation. All these notions suggest the possibility that the optical properties of ionic conductors are interrelated with the mechanical properties of the materials. In the present study, such a possibility is investigated and corroborated using an anharmonic interatomic potential based theoretical model. Furthermore, it is shown that the previously found relation between the optical and the ionic transport properties can be understood analytically from the same interatomic potential point of view. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2025.

The Stokes-Einstein (SE) law has been widely used in the analysis of transport properties in the liquid state. However, recent studies have revealed that not all the systems obey this law. Particularly, large deviations from the SE law have been reported for systems that exhibit high ionic conduction in the glassy state. Although this observation has strong implication in the development of electrolyte materials, few fundamental studies on the subject have been done. In a recent study, based on the Bond Strength-Coordination Number Fluctuation model of structural relaxation developed in our group, it was pointed out that the correlated ionic motion starts at a temperature much higher than the glass transition temperature. With the objective to gain a further understanding on that behavior, in the present study we analyzed the temperature dependence of the degree of deviation from the SE law in the melts of AgI–AgPO3 and M–PO3 (M=Mg, Ca, Sr, Ba). The result reveals clearly that the deviation from the SE law increases with the decrease of temperature of the melt, which contrast with the behavior found in other systems. It is also noted that the behavior of the above mentioned two systems differ, and that such difference reflects the degree of ion transport. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2025.

Noble metal chalcogenides are attracting considerable interest as thermoelectric materials. These materials exhibit superionic conduction at high temperatures. This fact is certainly affecting the peculiar thermoelectric properties of these materials. However, few attentions have been paid to this fact. The Seebeck coefficient which gives the magnitude of the induced voltage in response to a temperature gradient is a fundamental quantity that characterizes the thermoelectric material. In the present report, a theoretical expression for the electronic Seebeck coefficient of superionic conductors is derived from the point of view of the Bond Fluctuation Model of ionic conduction proposed by the author. According to this model, the transport of ions is accompanied by a fluctuation in the electronic cloud surrounding the moving ion. Such a process affects the electronic properties of the materials. It is shown that the model captures the peculiar temperature dependence of the Seebeck coefficient observed experimentally. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024.

Among the various MX2-type compounds, only a limited number of them exhibit high ionic conduction. Is there any index that demarcates such compounds? The present study focuses on the molecular shape of these type of compounds having the nominal number of valence electrons N = 16. Interestingly, we note that compounds having a bent shape in the molecular state exhibit high ionic conduction in the solid phase. What is the origin of this correlation? In the field of quantum chemistry, the molecular shape has been understood based on the Walsh rule. On the other hand, with the objective to understand the ionic transport mechanism from the electronic theory point of view, the author proposed many years ago, the bond fluctuation model of ionic conduction. In the present study, the origin of the correlation mentioned above is discussed by exploiting the physical background of the Walsh rule in connection with the bond fluctuation model. It is concluded that the hybridization between the electronic orbitals not accounted in the nominal counting of valence electrons and the degree of charge transfer between the constituent atoms controls the behavior. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024.