Luminescence features of multicomponent cubic fluoride pyrochlores doped with europium ions

Ceramic samples of multi-component fluorides Na₃CaMg₃AlF₁₄ and NaCaMg₂F₇ doped with europium ions have been obtained by solid-phase synthesis. A comparison of the XRD patterns for the obtained compounds confirmed that a new polymorphic modification with a cubic pyrochlore structure has been obtained for Na₃CaMg₃AlF₁₄ fluoride, which differs from the polymorph of this compound with a rhombohedral structure 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. Eu²⁺ ions in the synthesized fluoride matrices emit luminescence with a band peak at about 395 nm due to 4f ⁶5d–4f ⁷ interconfiguration transitions. In these matrices the Eu²⁺ ions form predominantly one type of optical centers, which provides a fairly narrow band-width of 4f ⁶5d–4f ⁷ luminescence ~30 nm. Also, in the synthesized ceramics Eu²⁺ ions emit narrow-line luminescence with a dominant line at ~362 nm, associated with 4f⁷–4f⁷ intraconfiguration transitions from the lowest excited state ⁶P_7/2 to the ground state ⁸S_7/2. The synthesized ceramics demonstrate good temperature stability of Eu²⁺ 4f ⁶5d–4f ⁷ luminescence with 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 ceramics in an argon atmosphere with the addition of NaHF₂ instead of NH₄(HF₂) led to a partial conversion of europium ions from a divalent to a trivalent state. As a result, a series of narrow luminescence lines appears in the red region of the spectrum due to intraconfiguration transitions 4f ⁶–4f ⁶ (⁵D₀ → ⁷F_J) in Eu³⁺ ions.