The Charge Transfer Band as a Key to Study the Site Selection Preference of Eu³⁺ in Inorganic Crystals

Abstract

On account of the strong oxidizing property of the europium(III) ion, its charge transfer band (CTB) can be easily formed in many inorganic compounds. In this work, the Eu³⁺ ions were singly doped into the K₃LuSi₂O₇ compound with a hexagonal structure, and two kinds of Eu³⁺-O^2- CTBs were detected by monitoring at specific wavelengths. The qualitative analysis of Eu³⁺ ion site occupation was illuminated by combining Eu³⁺-O^2- CTBs with the corresponding cell volume. Furthermore, the two kinds of Eu³⁺ sites are eventually assigned to the K(2) and Lu sites, which means that Eu³⁺ ions selectively occupy the site with a low coordination number, according to the calculated CT energy by the dielectric theory of complex crystals and the magnitude of CT energy in the excitation spectra. Meanwhile, at high temperatures, the CTB does not show the traditional thermal quenching like f-f transitions but demonstrates thermal enhancement; thus, by using this opposite variation in excitation spectra, a noninvasive optical thermometer is presented, and this opposite variation tendency is thought to be the difference of thermal stability of disparate excited energy states. When new luminescent phosphors are designed with interesting spectral properties, this work will give us an alternative approach to determine the site occupation preference of Eu³⁺, especially when there are more than two different sites in the compound.