Chemical Bonding of Mobile Cations in Superionic Conductors

Abstract Electronic states of monovalent ions in superionic conductors such as Li 3 N, as well as the lithium sulfide superionic conducting glasses, were calculated by the DV-X α cluster method. The movements of the cations were simulated by several model clusters with different positions of the moving cation. The net charge of the moving cation and the total bond overlap population (TBOP) between the moving cation and other ions were used for discussion of chemical bonding of the moving cation. In the Li 3 N crystal, the TBOP of the moving cation along the conduction path changed less than those of the other paths. On the other hand, the changes of the net charges of the moving cations were similar in all paths. Furthermore, the relationship between ionic conductivity and the differential total bond overlap population (DBOP) was discussed in the lithium sulfide superionic conducting glasses. The cluster models were constructed by the coordination number reported by experimental methods and the bond length estimated from the ionic radii of each ion. Especially the relationship between ionic conductivity and the differential bond overlap population was discussed for the sulfide-based lithium ion conducting glasses in the systems Li 2 S–SiS 2 –Al 2 S 3 and Li 2 S–SiS 2 –P 2 S 5 . In these glasses, the DBOP with the movement of the lithium ion had good negative correlations with the ionic conductivity and positive correlations with the activation energy obtained by the experimental measurements. In any case, the smaller change of the TBOP of the moving cations played an important role in the fast ion movement in the superionic conducting glasses, rather than the change of the net charge of the moving cations. This bonding state of the moving cations is one of the characteristics of the electronic state in superionic conductors.