Linking the Molecular Shape with the High Ionic Conduction in MX2-Type Compounds

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.