Luminescence properties of complex fluoride Na3CaMg3AlF14 phosphors doped with cerium and europium ions

Abstract

Ceramic samples of two multi-cation fluoride compositions, namely Na3CaMg3AlF14 doped with Ce3+ or Eu2+ and KCaAl2F9 doped with Eu2+, have been synthesized using a conventional solid-state reaction method. The XRD analysis shows that the new polymorphic modification of Na3CaMg3AlF14 with the cubic A2B2X7 pyrochlore crystal structure (space group Fd3‾m, lattice parameter 10.172 Å) has been obtained. This conclusion is confirmed with the simulation of the XRD pattern for the cubic pyrochlore structure by introducing random occupation of Na+ and Ca2+ ions at the A-type sites as well as Mg2+ and Al3+ ions at the B-type sites in the structure of rhombohedral Na3CaMg3AlF14. It has been shown that doping of the Na3CaMg3AlF14 pyrochlore matrix with Ce3+ ions creates several Ce3+ optical centers with different spectral and decay properties of 5d-4f luminescence. In contrast to that, doping with Eu2+ ions creates a single optical center by the substitution for the Na/Ca A-type site in the cubic Na3CaMg3AlF14 pyrochlore structure. In Eu2+-doped Na3CaMg3AlF14, the Eu2+ emission band caused by the interconfigurational 4f65d – 4f7 transition has a rather narrow width (FWHM = 29 nm) which is considerably less than that observed in Eu2+-doped KCaAl2F9 (FWHM = 44 nm) in which the emission band is broadened due to the existence of two types of sites for doping Eu2+ ions. The Eu2+ ions doped into Na3CaMg3AlF14 and KCaAl2F9 hosts emit not only broad-band 4f65d – 4f7 luminescence but also a narrow-line 4f7 – 4f7 (6P7/2 - 8S7/2) emission which is not observed under excitation to the lowest-energy 4f65d state. It has been shown that both Na3CaMg3AlF14:Eu2+ and KCaAl2F9:Eu2+ demonstrate a rather good thermal stability of luminescence with thermal quenching temperature T1/2 equal to 504 and 573 K, respectively.