Solid electrolytes are unique functional materials in which only single ion species can generally migrate in the solid as a charge carrier, and some have already been commercialized as components of batteries and chemical sensing devices. Although a large number of solid electrolytes have been found to obtain high ion conductivity that enters into the practical application range, such high ion conductivity is realized only for solids whose conducting species are mono-, di-, and trivalent ions. The high-valence tetravalent cation has been regarded as an extremely poor migrating species in solids because of strong electrostatic interaction between the tetravalent cation and surrounding anions, which prevents smooth ionic conduction in the solid lattice. To realize tetravalent cation conduction in solids, it is necessary to strictly select not only the crystal structure but also the constituent ions, in order to reduce such strong interactions. One of the most effective ways is the introduction of the cations with higher valence than the tetravalent ion into the structure, because such higher valence cations may attract anionic species more strongly than the tetravalent cation, enabling tetravalent cation migration in the solid lattice.
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