Crystal Structure of NaLuW₂O₈·2H₂O and Down/Upconversion Luminescence of the Derived NaLu(WO₄)₂:Yb/Ln Phosphors (Ln = Ho, Er, Tm)

Hydrothermally reacting Lu­(NO)₃ and Na₂WO₄·2H₂O at 200 °C and pH = 8 produced the new compound NaLuW₂O₈·2H₂O, which was analyzed via the Rietveld technique to crystallize in the orthorhombic system (space group: Cmmm) with cell parameters a = 21.655(1), b = 5.1352(3), and c = 3.6320(2) Å and cell volume V = 403.87(4) ų. The crystal structure presents −(NaO₆)-(NaO₆)– and −(LuO₄(H₂O)₂WO₅)-(LuO₄(H₂O)₂WO₅)– alternating layers linked together by the O^2– ion common to NaO₆ octahedron and WO₅ triangle bipyramid. Tetragonal structured and phase-pure Na­(Lu_0.87Ln_0.03Yb_0.1)­(WO₄)₂ phosphors (Ln = Ho, Er, and Tm) were directly produced by calcining their NaLuW₂O₈·2H₂O analogous precursors at 600 °C for 2 h, followed by a detailed study of their downconversion/upconversion (DC/UC) photoluminescence. It was shown that the UC luminescence is dominated by a red band at ∼650 nm for Ho³⁺ (⁵F₅ → ⁵I₈ transition), green bands at ∼500–575 nm for Er³⁺ (²H_11/2/⁴S_3/2 → ⁴I_15/2 transitions) and a blue band at ∼476 nm for Tm³⁺ (¹G₄ → ³H₆ transition), all via a three-photon process. DC luminescence of the phosphors is characterized by a ∼545 nm green emission for Ho³⁺ (⁵F₄/⁵S₂ → ⁵I₈ transition, λ_ex = 453 nm), ∼500–575 nm green emissions for Er³⁺ (²H_11/2/⁴S_3/2 → ⁴I_15/2 transitions, λ_ex = 380 nm), and a ∼455 nm blue emission for Tm³⁺ (¹D₂ → ³F₄ transition, λ_ex = 360 nm), with CIE chromaticity coordinates of around (0.27, 0.71), (0.26, 0.72), and (0.15, 0.04), respectively.