Synthesis and investigation of energy transfer mechanism in Sm³⁺ and Eu³⁺ doped Na₆Mg(SO₄)₄ nanophosphors via solution combustion technique

Singly doped Sm³⁺(1.0 mol%), Eu³⁺(1.0 mol%) and co-doped Na₆Mg(SO₄)₄:Sm³⁺/Eu³⁺ nanophosphors with a fixed Sm³⁺(1 mol%) concentration and different Eu³⁺ (x = 0.5%, 1.0 and 1.5 mol%) concentration were synthesized using the chemical combustion method at a temperature of 580 ⁰C. Structural, morphological and optical properties were studied using different characterization techniques. X-ray diffraction confirmed the crystal structure of the Na₆Mg(SO₄)₄ nanophosphors to be monoclinic with different space groupings. The optical bandgap of the Na₆Mg(SO₄)₄ nanophosphors were calculated from the Kubelka-Munk function using optical data and a decrease in the bandgap after adding Sm³⁺ and Eu³⁺ dopants was obtained. The photoluminescence was measured with two different 404 and 395 nm excitation wavelengths, both from the Sm³⁺ and Eu³⁺ spectra, respectively. An improvement of the color tunability was observed in the co-doped Na₆Mg(SO₄)₄ nanophosphors, with an energy transfer from Sm³⁺ to Eu³⁺. The color purity of all the nanophosphors was analyzed using the CIE-1931 color chromaticity, which all confirmed red color emission. These luminescence results proved that the prepared nanophosphors are suitable to be used in display and light emitting diodes applications.