Luminescence Features of Multicomponent Cubic Fluoride Pyrochlores Doped with Europium Ions

Ceramic samples of europium-doped multi-component fluorides Na3CaMg3AlF14 and NaCaMg2F7 were prepared by high-temperature solid-phase reactions. The prepared compounds both have cubic pyrochlore structures. A comparison of the X-ray diffraction patterns for the prepared compounds confirms that Na3CaMg3AlF14 was prepared as a new polymorph with a cubic pyrochlore structure, which differs from its rhombohedral phase described in the literature. The addition of NH4(HF2) before the last annealing provided reducing conditions for the stabilization of europium ions exclusively in the 2+ charge state. The Eu2+ ions in the synthesized fluoride matrices luminesce with a band peaking at about 395 nm due to 4f 65d–4f 7 interconfiguration transitions. The Eu2+ ions doped into these matrices form predominantly a single type of optical center, which provides for a fairly narrow width (FWHM ~ 30 nm) of the 4f 65d–4f 7 emission band. Also, the Eu2+ ions in the prepared ceramics emit narrow-line luminescence with the major line at about 362 nm, associated with 4f 7–4f 7 intraconfiguration transitions from the lowest excited state 6P7/2 to the ground state 8S7/2. The prepared ceramics exhibit a good thermal stability of 4f 65d–4f 7 luminescence of Eu2+ with the thermal quenching temperature T1/2 equal to 504 and 543 K for Na3CaMg3AlF14:Eu2+ (1.0 at %) and NaCaMg2F7:Eu2+ (0.5 at %), respectively. This property may be of interest for the practical application of these phosphors. Additional annealing of the ceramics under an argon atmosphere with the addition of NaHF2 instead of NH4(HF2) led to a partial conversion of europium ions from the divalent to trivalent state. As a result, a series of narrow luminescence lines appeared in the red region of the spectrum due to 4f 6–4f 6 (5D0 → 7FJ) intraconfiguration transitions in Eu3+.