Structural and photoluminescence features of Pr³⁺-activated different alkaline sodium-phosphate-phosphors

A series of Pr³⁺ activated alkaline-sodium-phosphate (NaMPO₄, where M = Mg, Ca, Sr and Ba) phosphors were prepared by conventional solution combustion method with the fixed doping concentration of Pr³⁺ (1.0 mol%). The crystal phase formation and morphology of the phosphates were analyzed by X-ray powder diffraction (XRD) and field emission scanning electron microscope (FE-SEM), respectively. The optical properties including reflectance, excitation and emission and the color coordinates were investigated. The band gap of the phosphors was calculated from diffuse reflectance spectra data using the Kubelka-Munk function. The photoluminescence (PL) excitation spectra showed the excitation peaks ranging from 420 to 475 nm and emission peaks were observed in the visible region and were found to vary for different hosts. Pr³⁺-doped phosphors can display very distinct dominant red emission due to the ³P₀ → ³H₅, ³P₀ → ³H₆, ¹D₂ → ³H₄ and ¹D₂ → ³H₅ transitions. The Pr³⁺ PL emission spectrum was observed from NaMPO₄ phosphors at 605 nm. This emission under 445 nm excitation makes these materials possible candidates to serve as sources of red light in tricolor system used for solid state lighting. Different results on the luminescence features of Pr³⁺:NaMPO₄ are discussed on the basis of crystal structure. The Pr³⁺ ion has structure-dependent transitions due to the special microstructures present in a given host. The results indicated that among the four different alkaline cations, Ca⁺² not only provides the most intense emission from Pr³⁺ but it also produces electric-dipole dominated transition (³P₀ → ³H₅) resulting in the most intense reddish-orange emission.