The interfacial and transport phenomena related to multivalent cation migration in solids are of major interest in our understanding of many fundamental mechanisms and for numerous potential applications. The exact identification and quantitative analysis of these phenomena are, however, among the most complex tasks in solidstate electrochemistry. In those systems which comprise several types of ion that may be mobile, a precise separation of the contributions provided by different ionic species is only possible by combining several complementary electrochemical, spectroscopic, and microscopic techniques. These experiments require also careful examination of their reproducibility, an assessment of the relative roles of all factors that influence ionic transport, a thorough characterization of the materials, the elimination of various parasitic effects, and the correct accounting of possible parallel and alternative mechanisms. For multivalent cation conductors, typical experimental problems include the presence of minor amounts of impurities (e.g., protons or alkaline metal cations), interfacial transport (via extended defects, grain boundaries or surfaces), electronic conduction and/or phase decomposition processes induced by the applied electrical field, cation demixing under chemical potential gradients, possible volatilization of components, phase separation at the interfaces, and other complex electrode phenomena. In particular, due to the electrostatic and stereological factors discussed in previous chapters, the mobility of multivalent cations is always lower compared to that of monovalent species. Consequently, even microscopic amounts of ionic impurities, barely distinguishable by standard analytical methods, may have non-negligible effects on the transport properties. In the case of electrochemical measurements involving direct currents, high electrical and polarization resistances may lead to excessively high voltages, the generation of electronic charge carriers, and/or decomposition. These factors are in part responsible for the contradictory conclusions, various different opinions, doubts and questions reported with regards to migration-and intercalation-related phenomena when the cation Solid State Electrochemistry I: Fundamentals, Materials and their Applications. Edited by Vladislav V. Kharton