Theoretical and Experimental Investigation on Structural, Elastic, Electronic, and Optical Properties of Ni ²⁺ -Activated Double Perovskite Sr ₂ MgMoO ₆

In this study, a comprehensive investigation of the properties of Ni²⁺-doped Sr₂MgMoO₆ was carried out through a combination of theoretical calculations and experimental characterization. The dynamical and mechanical stabilities of the Sr₂MgMoO₆ host were accessed by calculating a set of elastic constants and Debye temperatures, which confirms its structural stability and rigidity for the incorporation of the Ni²⁺-emitting center. It was found from DFT formation energy calculations that Mg²⁺ is the most energetically favorable for Ni²⁺ incorporation among the available cation sites. The calculated geometric results of Ni²⁺ ground and excited levels agree with experimental excitation and emission bands, which are mainly assigned from the 3d-3d transitions of Ni²⁺ located in the octahedral site. The as-obtained Sr₂MgMoO₆:Ni²⁺ exhibits a strong NIR-II broadband emission centered around 1415 nm, resulting from the Ni²⁺³<i>T</i>₂→³<i>A</i>₂ transition. An NIR-II LED prototype by combining the Sr₂MgMoO₆:Ni²⁺ phosphor with a blue chip as the lighting source for noninvasive decryption and detection is demonstrated. The yellow body colors of Sr₂Mg_1-<i>x</i>Ni_<i>x</i>MoO₆ (0 ≤ <i>x</i> ≤ 0.4) compounds deepen as the Ni content increases. The plastic composites with desirable color are successfully fabricated using Sr₂MgMoO₆:Ni²⁺ and PMMA. This work offers a perspective on the structure-property relationship in Ni²⁺-doped double perovskite.