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 ⁶P_7/2 to the ground state ⁸S_7/2. The prepared ceramics exhibit a good thermal stability of 4f ⁶5d–4f ⁷ luminescence of Eu²⁺ with the thermal quenching temperature T_1/2 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 → ⁷F_J) intraconfiguration transitions in Eu³⁺.