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

Abstract

Hydrothermally reacting Lu(NO) and NaWO·2HO at 200 °C and pH = 8 produced the new compound NaLuWO·2HO, 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) Å. The crystal structure presents -(NaO)-(NaO)- and -(LuO(HO)WO)-(LuO(HO)WO)- alternating layers linked together by the O ion common to NaO octahedron and WO triangle bipyramid. Tetragonal structured and phase-pure Na(LuLnYb)(WO) phosphors (Ln = Ho, Er, and Tm) were directly produced by calcining their NaLuWO·2HO 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 Ho (F → I transition), green bands at ∼500-575 nm for Er (H/S → I transitions) and a blue band at ∼476 nm for Tm (G → H transition), all via a three-photon process. DC luminescence of the phosphors is characterized by a ∼545 nm green emission for Ho (F/S → I transition, λ = 453 nm), ∼500-575 nm green emissions for Er (H/S → I transitions, λ = 380 nm), and a ∼455 nm blue emission for Tm (D → F transition, λ = 360 nm), with CIE chromaticity coordinates of around (0.27, 0.71), (0.26, 0.72), and (0.15, 0.04), respectively.

Upconversion luminescence and favorable temperature sensing performance of eulytite-type Sr3Y(PO4)3:Yb3+/Ln3+ phosphors (Ln=Ho, Er, Tm)

“…The host lattice for UC should assure not only a satisfactory luminescence efficiency, but also excellent physicochemical stability, safety, and low toxicity. A handful of inorganic compounds have been developed as UC host so far, typically including fluorides [5,6], oxides [7,8], oxysulfides [9], phosphates [10,11] and other oxygenates [12][13][14][15], and new hosts are also under active exploration and/or perfection.…”

Phase evolution, structure, and up‐/down‐conversion luminescence of Li6CaLa2Nb2O12:Yb3+/RE3+ phosphors (RE = Ho, Er, Tm)

“…24 It is also seen from Figure 7A that the green band well splits into four sharp peaks centered at ~ 524, 540, 551 and 568 nm, respectively, which indicates that the 2 H 11/2 and 4 S 3/2 energy levels of Er 3+ are subjected to substantial splitting by the dodecahedral coordination environment in Li 6 CaLa 2 Nb 2 O 12 . Such a phenomenon was found for Gd 2 O 3 :Yb/Er, 13 Y 2 O 3 :Yb/Er, 48 LiYF 4 :Yb/Er, 49 and NaLu(WO 4 ) 2 :Yb/Er, 50 55 In addition, the broad and diffuse red luminescence extending from ~ 640 to 700 nm ( Figure 7A) was similarly observed for Ba 5 Gd 8 Zn 4 O 21 :Yb/ Er UC phosphor in the ~ 640-680 nm spectral region, which was ascribed to widening of the 4 F 9/2 energy level by electronic interactions and spin-orbit coupling. 56 Analysis of the emission intensity against excitation power ( Figure 7B) found n values of ~ 2.15/2.13 for the 524/540 nm emission peaks ( 2 H 11/2 → 4 I 15/2 transition) and ~ 1.82/1.77 for the 551/568 nm ones ( 4 S 3/2 → 4 I 15/2 transition), which suggest a two-photon process is responsible for the observed green emissions.…”

“…It is also seen from Figure A that the green band well splits into four sharp peaks centered at ~ 524, 540, 551 and 568 nm, respectively, which indicates that the 2 H 11/2 and 4 S 3/2 energy levels of Er 3+ are subjected to substantial splitting by the dodecahedral coordination environment in Li 6 CaLa 2 Nb 2 O 12 . Such a phenomenon was found for Gd 2 O 3 :Yb/Er, Y 2 O 3 :Yb/Er, LiYF 4 :Yb/Er, and NaLu(WO 4 ) 2 :Yb/Er, but hardly for YbPO 4 :Er, YPO 4 :Yb/Er and Sr 3 Y(PO 4 ) 3 :Yb/Er orthophosphates, La 2 O 2 S:Yb/Er oxysulfide, and La 2 O 2 SO 4 :Yb/Er oxysulfate . In addition, the broad and diffuse red luminescence extending from ~ 640 to 700 nm (Figure A) was similarly observed for Ba 5 Gd 8 Zn 4 O 21 :Yb/Er UC phosphor in the ~ 640‐680 nm spectral region, which was ascribed to widening of the 4 F 9/2 energy level by electronic interactions and spin‐orbit coupling .…”

“…Under 359 nm excitation, the Tm 3+ activator shows a strong 1 D 2 → 3 F 4 (~464 nm) blue emission and weak 1 G 4 → 3 F 4 (653 nm) and 3 F 2 → 3 H 6 (683 nm) red emissions . Besides, the 1 G 4 → 3 H 6 (~486 nm) UC emission did not appear in the DC PL spectrum for Tm 3+ , and this can be explained by considering the different excitation mechanisms of UC and DC, as follows: (a) the ground state electrons were excited to the 1 G 4 level by three successive laser photons for UC luminescence while they were raised to the 1 D 2 level by 359 nm UV light for DC luminescence, and (b) in the latter case, the electrons excited to 1 D 2 can hardly relax to the 1 G 4 level for 486 nm emission, since the energy gap between the two levels is quite large (~7.3 × 10 3 cm −1 in this work).…”

“…Under 359 nm excitation, the Tm 3+ activator shows a strong 1 D 2 → 3 F 4 (~464 nm) blue emission and weak 1 G 4 → 3 F 4 (653 nm) and 3 F 2 → 3 H 6 (683 nm) red emissions. 64 Besides, the 1 G 4 → 3 H 6 (~486 nm) UC emission did not appear in the DC PL spectrum for Tm 3+ , and this can be explained by considering the different excitation mechanisms of UC and DC, 50 as follows: (a) the ground state electrons were excited to the 1 G 4 level by three successive laser…”