Structure, optical properties and preferential site substitution of Eu3+ activated Ca8NaBi(PO4)6F2 red emitting phosphors prepared by modified Pechini process

Eu3+-doped and undoped Ca8NaBi(PO4)6F2:xEu (x = 0.01–0.5) apatite phosphors have been synthesized at 900 °C by the modified Pechini process. Their structural and morphological characteristics were determined by XRD, TEM and Raman spectroscopy. The structure of the host matrix was refined by the Rietveld method: Ca8NaBi(PO4)6F2 crystallizes in hexagonal class, sp. gr. P63/m, a = b = 9.3980 (6) Å, c = 6.9039 (9) Å and γ = 120°). The electronic structure was simulated revealing an indirect bandgap of 3.46 eV Eu3+ ions in the host matrix replace for the Bi3+ ones in two types of sites: M1 (C3 symmetry and IX-fold oxygen coordination) and M2 (CS symmetry and VII-fold coordination by one fluoride and six oxygens). The doping induces a redistribution of Eu3+ ions over the M1 and M2 sites in favor of the former species. The transition probabilities for Eu3+ ions in M2 sites were determined using the Judd-Ofelt theory yielding a radiative lifetime of 2.21 ms. It was found that phosphors exhibit relatively weak luminescence quenching. The CIE 1931 color coordinates for the compound with x = 0.35 Eu are (0.648,0.352) falling in the red region and corresponding to a correlated color temperature of 2919 K and a color purity of >96%. The thermal stability of the phosphor was studied until 400 °C (the determined activation energy is 0.70 eV) indicating a redistribution of ions in favor of the M2 sites at high temperatures. Ca8NaBi(PO4)6F2:xEu apatites are promising as red phosphors for white-light LEDs.