Flux synthesis, crystal structures, and luminescence properties of salt-inclusion lanthanide silicates: [K₉F₂][Ln ₃Si₁₂O₃₂] (Ln = Sm, Eu, Gd)

New salt-inclusion lanthanide silicates, [K₉F ₂][Ln₃Si₁₂O₃₂] (Ln = Sm, Eu, Gd), have been synthesized using a KF-MoO₃ flux, and structurally characterized by single-crystal and powder X-ray diffraction. The structures of these three isostructural compounds consist of open-branched fünfer silicate single layers with six-, eight-, and twelve-membered rings, which are connected via LnO₆ octahedra to form a 3-D framework. The F ^- and K⁺ ions are located in the structural channels and form a F₂K₇ dimer with a structure similar to that of Cl₂O₇. The photoluminescence properties of the Eu compound have also been studied. The sharp peaks in the room-temperature emission spectrum are assigned and the relative intensities of the ⁵D ₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions are consistent with the crystallography results. Crystal data for the Eu compound: triclinic, space group P1 (No. 2), a = 6.8989(2) Å, b = 11.3834(4) Å, c = 11.4955(4) Å, α = 87.620(2)°, β = 89.532(2)°, γ = 80.221(2)°, and Z = 2. Crystal data for the Sm compound: The same as those for the Eu compound except a = 6.9152(6) Å, b = 11.400(1) Å, c = 11.531(1) Å, α = 87.610(1)°, β = 89.445(1)°, and γ = 80.081(1)°.