Synthesis and study of structure and optical properties of RE³⁺(RE = Sm³⁺ and Tb³⁺) activated Ca₈NaBi(PO₄)₆F₂ orange-red and green emitting phosphors prepared by Pechini method

In this paper, we report that Ca₈NaBi(PO₄)₆F₂ fluorapatites doped with (RE³⁺ = Sm³⁺ and Tb³⁺) ions synthesised by the modified Pechini method are promising candidates for orange-red and green-emitting materials for optical applications. The characterization of this synthesized sample was done, and XRD and Rietveld refinement analysis confirm that it crystallised into a hexagonal structure. By analysing the emission of activators, it is shown that RE³⁺ substitutes for the Bi³⁺ ones in the host lattice and is put up in two different sites (M1 and M2). Photoluminescence (PL) analysis shows that Ca₈NaBi(PO₄)₆F₂: Sm³⁺, Tb³⁺, show highly intense peaks at 600 nm (⁴G_5/2 to ⁶H_5/2)_, and 544 nm (⁵D₄ to ⁷F₅) under the excitation of 402 nm and 350 nm, respectively. Ca₈NaBi(PO₄)₆F₂:Sm,Tb apatite has high available doping concentrations with moderate concentration quenching of luminescence (up to 8 mol% for Sm³⁺ and 7 mol% for Tb³⁺, as shown in the present work); also, this phosphor has relatively long lifetimes of the (⁴G_5/2) Sm³⁺ level and (⁵D₄) Tb³⁺ (1.91 ms and 2.21 ms, respectively). The above sample was successfully prepared by the environmentally friendly synthesis technique by the modified Pechini method at low temperatures (at 900 °C). The analysis of the optical characteristics of the Ca₈NaBi(PO₄)₆F₂:Sm³⁺ and Tb³⁺ phosphors indicates that these materials are promising candidates for applications as orange-red and green-emitting phosphors, with the combination of blue phosphors for warm white light-emitting diode applications.