Near-infrared luminescence shift in Ni2+-doped double perovskite phosphors

For applications such as near-infrared (NIR) spectroscopy requiring energy-efficient and compact broadband NIR light sources, a variety of Ni ²⁺-doped phosphors emitting in the 1100–2000 nm range has been developed. However, the commercialization of Ni ²⁺-doped NIR phosphors has not yet been achieved, and material exploration is ongoing. To gain insight into the luminescence properties of Ni ²⁺, including emission wavelength and efficiency influenced by compositional and structural variations, Ni ²⁺-doped double-perovskite AELaMgTaO ₆:Ni ²⁺ ( AE = Ba, Sr, or Ca) ceramic samples were synthesized, and their optical properties were examined. Because the energy of the Ni ²⁺ 3d-3d transition is sensitive to structural variations in the local environment around the Ni ²⁺ ions, we investigated the variation in optical properties by indirect ligand field engineering of NiO ₆ octahedra following the substitution of alkaline earth ions AE ²⁺. The prepared AELaMgTaO ₆:Ni ²⁺ samples were Ba-Sr or Ba-Ca complete solid solution systems with cubic-to-monoclinic phase transitions. The broad NIR luminescence band at ∼1350–1700 nm was blue-shifted by ∼100 nm, accompanied by Sr ²⁺ or Ca ²⁺ substitution. The local symmetry of the NiO ₆ octahedra was reduced from O _h to C _i owing to the phase transition, resulting in an enhancement of the NIR luminescence intensity and an increase in the quenching temperature. This study demonstrated that Ni ²⁺ phosphors with the desired optical properties for NIR spectroscopy applications can be designed by controlling the composition ratio of the host compounds and the resultant variation in the local environment of the solid solution compounds.