Uranyl Speciation and Bright Green Luminescence in Novel K2MgGeO4 Phosphor under NUV and X-ray Excitation

Abstract

The fascinating photochemistry of uranium and its increasing role in various technological applications have sparked renewed interest in exploring uranium-doped materials. In this study, we present the first detailed photophysical spectroscopy investigation of uranium-doped K2MgGeO4 (KMGO) phosphors, a promising material for optoelectronic and radiation detection applications. Our experimental results, which are well-supported by density functional theory (DFT)-calculated defect energy formation and density of states calculations, provide crucial insights into the behavior of uranium in this matrix. X-ray absorption near-edge spectroscopy (XANES) confirmed the oxidation state of uranium as +6, while extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that uranium stabilizes as the uranyl ion (UO22+). DFT calculations further suggested that uranium substitution at the Ge4+ site is the most energetically favorable. Time-resolved photoluminescence measurements showed a distinct emission band around 530 nm, characteristic of the uranyl ion, and the excited-state lifetime demonstrated multiple decay channels, likely due to defects formed on aliovalent substitution of U6+ at the Ge4+ site. X-ray excited radioluminescence (RL) depicted green band located at 530 nm typical of hexavalent uranium ions emission further highlighting the ability of KMGO:U to convert energetic X-ray photon into visible light and its potential as X-ray phosphors. These findings not only enhance our understanding of uranium's behavior in phosphor matrices but also open up exciting possibilities for their application in advanced technologies, including radiation sensing and imaging.