Universal Method for Polyvalent Ions’ Redox State Quantification in Inorganic Materials Down to Trace Concentrations

The oxidation state control of polyvalent cations within materials is a pivotal determinant of their macroscopic properties and practical functionalities, from persistent and stress-stimulated luminescence, to energy storage and photocatalytic activity. Nonetheless, the resulting redox state is highly sensitive to synthetic conditions, and quantifying techniques aimed at its determination – while of the highest importance – remain constrained by practical challenges. In the present work, two simple and accessible wet-chemical methods, employing basic and easy-to-handle glassware and reagents, are introduced to address this issue. After validation on analytical standards encompassing diverse chemistries, they are applied to concrete off-stoichiometry materials of relevance – whether amorphous or crystalline – involving various redox couples and species, from highly reductive (Eu2+, Ti3+) to highly oxidizing (Ce4+, Tb4+). The roles of the host composition and processing conditions in altering the redox equilibria are discussed, and the obtained results are reinforced by the use of complementary analytical techniques. Multiple perspectives are emphasized, including the identification and resolution of the local environments and distributions of polyvalent cations, the isolation of high-valence species, and the provision of key insights into multifaceted phenomena for the design of advanced materials.