Why host to dopant energy transfer is absent in the MgAl2O4:Eu3+ spinel? And exploring Eu3+ site distribution and local symmetry through its photoluminescence: interplay of experiment and theory

An undoped and Eu3+ doped magnesium aluminate spinel (MAS) was synthesized using a citric acid assisted combustion technique. MAS samples were characterized systematically using X-ray diffraction (XRD), time resolved photoluminescence spectroscopy (TRPLS) and ab initio calculations. On irradiating the undoped MAS with UV light; multicolor emission is observed. The blue emission peak was attributed to Mg2+ vacancies whereas the one in the green region was attributed to oxygen vacancies. Based on the emission spectrum it was inferred that the majority of europium ions are localized at the Mg2+ site which was also confirmed using lifetime measurements. DFT based cohesive energy calculations also showed Eu doping in the Mg position is energetically more favorable than doping in the Al position. Photoluminescence (PL) spectroscopy shows that the emission spectrum consists of host as well as Eu3+ emission indicating the absence of complete host-dopant energy transfer. DFT calculated density of states analysis shows that Eu states are solely localized in VB and CB regions and do not contribute in defects states. From the emission spectrum of the undoped MAS sample it was observed that photoluminescence properties of the MgAl2O4 are dominantly governed by the defect states coming from the presence of cation and oxygen vacancies (neutral and charged). As a result photon energy transfer from host MAS to dopant Eu is difficult. The actual site symmetry for europium ions in MAS was also evaluated based on a stark splitting pattern which comes out to be C-2v. Based on Judd-Ofelt analysis it was found that the Omega(2) value is greater than Omega(4); indicating high covalency and low symmetry around europium ions which is also observed in the emission spectrum. The high purity of the red emission coupled with good fluorescence quantum yields highlights the potential of this unexplored MAS as a promising phosphor.