The effects of Eu3+ concentration on the photoluminescence of Na4Ca (PO3)6 phosphors prepared by a solid-state reaction method

A series of Na₄Ca (PO₃)₆: xmol% Eu³⁺ (x = 1, 2, 3, 4 and 5) was synthesized using a solid-state method and characterized using various techniques. The X-ray powder diffraction data confirmed that the phosphors crystalized in the monoclinic phase. The Fourier-transform infrared spectroscopy confirmed the presence of phosphate groups and an HO-H peak. The optical bandgaps got smaller as the Eu³⁺ concentration went up as seen in the diffuse reflectance data. The photoluminescence excitation spectra obtained when monitoring the 613 nm emission line showed peaks assigned to 4f→4f Eu³⁺ transitions. When the excitation was monitored at 394 nm, the emission exhibited strong narrow bands at 591, 613, 651 and 698 nm owing to the ⁵D₀ → ⁷F_{J (J = 0 - 4)} transitions of Eu³⁺. The main emission peak increased with Eu³⁺ concentration from 1 mol% to 4 mol% and decreased significantly as the Eu³⁺ concentration reached 5 mol%. The concentration quenching effect is responsible for the drop in PL intensity at concentrations higher than the optimum Eu³⁺ concentration. The International Commission on Illumination (CIE) chromaticity coordinates of the phosphors displayed colour response in the orange- red region, suggesting that they have potential application in the orange-red colour-emitting light displays.