Probing the site occupancy of dopants in deep red-NIR emitting LiAl5O8: Eu3+, Cr3+ and Fe3+ nano phosphors using photoluminescence and X-ray absorption spectroscopy

Eu3+, Cr3+ and Fe3+ doped LiAl5O8 nano phosphors were synthesized through the solution combustion synthesis route in a single step. The powder XRD patterns of the as-prepared samples indicated a single phase and highly crystalline inverse spinel system. Scanning electron microscopy (SEM) image showed a highly compacted morphology of nanoparticles with an average particle size of 60 nm. Optimization of dopant concentration enhanced the photoluminescence (PL) from Eu3+, Cr3+ and Fe3+ with 100% color purity suggesting them as potential deep red-NIR emitting phosphors. Time-resolved emission spectroscopy (TRES) studies indicated the presence of Fe3+ at more than once site. The local coordination geometries of Eu3+ and Fe3+ were investigated using X-ray absorption spectroscopy (XAS) for the first time. The XAS studies confirmed the occupancy of Fe3+ at Al3+-octahedral (Oh), Al3+-tetrahedral (Td) and Li+-Oh sites, and Eu3+ at distorted Li+-Oh site only, corroborating the cause of the unusually broad and deep red-NIR emission of Fe3+ and the deep red emission of Eu3+.