Multivalent Cationic Conduction in Crystalline Solids

The issue “Do trivalent cations conduct in crystalline solids or not?” appears in the basic research of solid-state chemistry and deals with the characteristics and properties of a multivalent cationic current transport through a solid material. Cations in an oxidation state higher than two are not unanimously accepted as mobile species. Due to the high charge density and the resulting strong Coulombic interactions with the rigid anionic host lattice, they are expected to be rather fixed within their crystallographic sites and thus not available for the current transport. Nevertheless, some compounds are already mentioned in the literature to conduct multivalent cations. But it appears that the comprehensive demonstration and the direct verification of this phenomenon is a demanding challenge which is not completed for many of these reported multivalent conductors. On the other hand, observations and experimental data are available which unambiguously identify cations in a trivalent oxidation state as mobile charge carrying species. Such a doubtless and reliable demonstration of the migrating species in an electrochemical potential gradient can be achieved, for example, by a mutual connection of dc electrolysis experiments, electromotive force, and polarization measurements. After a careful evaluation of the literature data, it turns out that the above-mentioned question concerning the possibility of mobile multivalent cationic charge carriers in solid materials must be answered in the affirmative, even in the case of trivalent cations.