Rare earth contribution in solid state electrolytes, especially in the chemical sensor field

In solid state electrolytes, rare earths are commonly used, and a number of their properties are exploited in these applications. Firstly, the rare earth can be utilized as a dopant to produce the mobile ion vacancies in an electrolyte and to enhance the ionic conducting characteristics. Secondly, they can be used as the principal constituent of the electrolytes. In both these case, the characteristics that the rare earths hold a stable trivalent state even though the main ionic mobility is via mono- or trivalent ions. Thirdly, the rare earth trivalent ion alone can migrate in some solids. This unexpected feature has been recently clarified by selecting the appropriate structure for the trivalent ion conduction in order to reduce the electrostatic interaction between the mobile ions and the framework of the solid. In solid electrolytes, only one ionic species can migrate. This feature is very advantageous for chemical sensor applications, especially from the viewpoint of selective and quantitative detection of specific gas species, The rare earth contribution to solid state electrolytes has been rapidly increasing, and has been expanding further the realms of application not only in the industrial field but also in the whole science field. Here, the rare earths contribution to the solid state electrolyte field is briefly reviewed.