Mn²⁺ doped CdAl₂O₄ phosphors with new structure and special fluorescence properties: experimental and theoretical analysis

Mn²⁺-activated CdAl₂O₄ phosphors with the new structure of space group R3 and (no. 148) have been prepared by a high-temperature solid-state reaction and their luminescence properties have been investigated in detail. The reduction of Mn⁴⁺ to Mn²⁺ in air atmosphere has been observed in CdAl₂O₄ powders for the first time. The structural properties including the phase purity and structural parameters were analyzed through Rietveld analysis. The typical transitions of Mn²⁺ ions in emission and excitation spectra were observed both in MnCO₃ and MnO₂ prepared CdAl₂O₄:0.01Mn²⁺ phosphors, which means that the luminescent centers of Mn²⁺ ions were from the Mn⁴⁺ ions which were reduced. Meanwhile, the energy band structures of CdAl₂O₄ and CdAl₂O₄:Mn²⁺ were measured with an ultraviolet-visible diffuse reflection spectroscopy (UV-vis DRS), the electronic structures were calculated using the plane-wave density functional theory (DFT). The Mn²⁺ activated CdAl₂O₄:Mn²⁺ phosphor prepared in air atmosphere is a potential blue-green emitting phosphor.