Tuning the crystal structure, optical band gap and persistent luminescence performance of a Cr3+-doped LiGa5O8 spinel by adding aluminium and indium

Abstract

The possibility of tuning the optical band gap, crystal structure and persistent luminescence performance of a Cr³⁺-doped LiGa₅O₈ spinel by partially replacing Ga with Al and/or In has been studied extensively. For this purpose, a series of Cr³⁺-doped Li(Ga_1−x−yAl_xIn_y)₅O₈ (x = 0…0.5; y = 0…0.1) microcrystalline phosphors were synthesised using a conventional solid-state reaction method and characterised using powder X-ray diffraction, SEM-EDX and luminescence techniques. DFT-based electronic structure calculations were carried out for the same Li(Ga_1−x−yAl_xIn_y)₅O₈ compositions, and the results were compared with the experimental ones. Based on the studies performed, the mechanism of Al and In incorporation into the LiGa₅O₈ spinel structure as well as the tuning of the crystal lattice parameters, the local structure of M³⁺ (M = Ga, Al, and In) cations and the optical band gap of the material have been established. The multicentre structure and the broadening of the local structural disorder of the octahedrally coordinated Cr³⁺ centres observed in this case have been confirmed by high-resolution, low-temperature photoluminescence measurements. Band gap engineering through alterations in the chemical composition of the LiGa₅O₈ spinel, as well as the depth of the native point defects responsible for charge trapping, allows for the efficient tuning of the thermoluminescence and persistent luminescence properties of Li(Ga_1−x−yAl_xIn_y)₅O₈:Cr³⁺ phosphors. Thus, the room-temperature persistent luminescence performance of the phosphors modified by the addition of Al and annealing under an oxygen-free atmosphere was increased threefold compared to the pristine LiGa₅O₈:Cr³⁺ phosphor synthesised under the same conditions.