Doping-Induced Room Temperature Stabilization of Metastable β-Ag2WO4 and Origin of Visible Emission in α- and β-Ag2WO4: Low Temperature Photoluminescence Studies

A new strategy of synthesizing hexagonal metastable beta-Ag2WO4 at room temperature based on aliovalent Eu3+ doping induced orthorhombic-to-hexagonal phase transition using coprecipitation method is offered. Both alpha-Ag2WO4 and beta-Ag2WO4 phase were characterized systematically using X-ray diffraction (XRD), Raman spectroscopy, Fourier transformed infrared spectroscopy (FTIR), and time-resolved photoluminescence (TRPL). Emission spectra at low temperature (77K) show two clear bands in the case of alpha-Ag2WO4 and beta-Ag2WO4, which are designated as PL1 (low wavelength region) and PL2 (high wavelength region). PL1 (similar to 430-440 nm) is attributed to charge transfer transition within tungsten octahedra in alpha-Ag2WO4 where as it is attributed to a similar transition within the tungsten tetrahedral in beta-Ag2WO4. Temperature dependent studies showed that origin of PL2 emission in alpha-Ag2WO4 and beta-Ag2WO4 might be different. PL2 in the case of alpha-Ag2WO4 is because of presence of F+ center (singly ionized oxygen vacancy) within the band gap which is supported by density function theory measurement (DFT). Doping Eu3+ in alpha-Ag2WO4 diminishes PL2 emission. Lifetime measurement supports that, in the case of beta-Ag2WO4, both PL1 and PL2 might have common origin, i.e., charge transfer transition.