Structure, optical properties and preferential site substitution of Eu3+ activated Ca8NaBi(PO4)6F2 red emitting phosphors prepared by modified Pechini process
Sami Slimi, Pavel Loiko, Anna Volokitina, Kirill Bogdanov, Rosa Maria Solé, Magdalena Aguiló, Francesc Dı́az, Ezzedine Ben Salem, Xavier Mateos
Abstract
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.