Temperature and impurity concentration effects on Mg{sub (1-x)}Co{sub x}Ga{sub 2}O{sub 4} photoluminescence

Ceramic materials doped with magnetic ions that present emission in the visible and near-infrared spectral regions are very attractive due to their inherent tunability. Possible applications include their utilization as optoelectronic and display devices, as spintronic material, in signal transmission and information storage, in the fabrication of special papers, as dosimetric materials and room temperature solid state lasers. Materials doped with tetrahedrally coordinated Co{sup 2+} present wide bands originated from electronic transitions in the ionic unfilled 3d electronic shell. The Co{sup 2+} 3d electrons are outside of the ion core and therefore their optical properties are directly affected by static and dynamic properties of ligand anions. The magnesium gallate MgGa{sub 2}O{sub 4} is a partially inverted spinel described as a AB{sub 2}O{sub 4} material with two possible positions for A{sup 2+} and B{sup 3+} cations. Polycrystalline MgGa{sub 2}O{sub 4} : Co{sup 2+} samples were produced by solid-state reactions between ultra-pure raw oxides MgO, {beta}-Ga{sub 2}O{sub 3} and the desired CoCO{sub 3} quantities. Photoluminescence data at room temperature and 77 K of MgGa{sub 2}O{sub 4} polycrystalline samples doped with 0.1 and 1.0% of Co{sup 2+} are presented. The visible emission observed is attributed to the {sup 4}T{sub 1}({sup 4}P) {yields}{sup 4}A{sub 2}({sup 4}F) spin-allowed transition of Co{sup 2+} ions tetrahedrally coordinated by O{sup 2-} ions. The photoluminescence intensity decreases with temperature, but 90% of the 77 K emission integrated intensity remains at room temperature. Moreover, from lifetime results we estimate that Co{sup 2+} emission quantum efficiency is about 1.0 at room temperature. We also observe that between 0.1 and 1.0% of Co{sup 2+} the luminescence intensity decreases. For 1.0% of Co{sup 2+} the luminescence intensity is 37% of the obtained for 0.1%. This fact is attributed to non-radiative transfer processes of impurity ion relaxation that become competitive at 1.0% of Co{sup 2+} and show that there is a strongly impurity-concentration luminescence dependence. We also observed that for the higher concentration the band is slightly shifted to the infrared region. Due to low impurity ion concentration it would not be expected an energy transfer between Co{sup 2+} ions, therefore non-radiative decay processes are responsible by temperature luminescence quenching.