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A series of garnet-structured Li 6 Ca(La 0.99- x Yb x Er 0.01) 2 Nb 2 O 12 (0≤ x ≤0.05) phosphors were originally synthesized and examined for upconversion (UC) luminescence and optical temperature sensing under 978 nm laser excitation. It was identified that the optimal content of Yb3+ sensitizer is x =0.04 for Er3+ emission and the non-thermally coupled 2H 11/2 /4F 9/2 energy levels of Er3+ possess a superior capability of optical thermometry, whose maximum absolute sensitivity (~15.20 × 10−3 K−1 at 523 K) is generally ~1.7-6.1 times the values reported to date for most of other UC systems. The optimal Yb3+ content of Li 6 Ca(La 0.99- x Yb x Er 0.01) 2 Nb 2 O 12 is x =0.04 for Er3+ emission and the non-thermally coupled 2H 11/2 /4F 9/2 Er3+ levels were identified to be superior for optical thermometry, whose maximum absolute sensitivity is generally ~1.7-6.1 times those reported for most of other Yb3+/Er3+ codoped systems. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Garnet‐type Li6Ca(La0.97Yb0.02RE0.01)2Nb2O12 (RE = Ho, Er, Tm) new phosphors were successfully synthesized via solid reaction at 900°C for 5 hours, whose course of phase evolution, macroscopic/local crystal structure and up‐/down‐conversion (UC/DC) photoluminescence were clarified. Mechanistic study and materials characterization were attained via XRD, Rietveld refinement, DTA/TG, electron microscopy (FE‐SEM/TEM), and Raman/reflectance/fluorescence spectroscopies. The phosphors were shown to exhibit UC luminescence dominated by a ~ 553 nm green band (5F4/5S2 → 5I8 transition) for Ho3+, a ~ 568 nm green band (4S3/2 → 4I15/2 transition) for Er3+ and a ~ 806 nm near‐infrared band (3H4 → 3H6 transition) for Tm3+ under 978 nm laser excitation, with CIE chromaticity coordinates of around (0.31, 0.68), (0.38, 0.60) and (0.17, 0.24), respectively. Analysis of the pump‐power dependence of UC intensity indicated that all the emissions involve a two‐photon mechanism except for the ~ 486 nm blue emission of Tm3+ (1G4 → 3H6), which requires a three‐photon process. The DC luminescence of these phosphors is featured by dominant bands at ~ 553 nm for Ho3+ (green, 5F4/5S2 → 5I8 transition), ~568 nm for Er3+ (green, 4S3/2 → 4I15/2 transition) and ~ 464 nm for Tm3+ (blue, 1D2 → 3F4 transition). The UC and DC properties were also comparatively discussed. [ABSTRACT FROM AUTHOR]

A series of garnet-type Li 6 CaLa 2(1- x) Nb 2 O 12 : x RE phosphors (LCLN: x RE, RE = Pr, Sm, Tb and Dy) were prepared via sol-gel processing and subsequent calcination for the first time, with the course of phase evolution and the characteristics of the LCLN host and RE3+ luminescence being investigated in detail. It was found that ∼900–1000 °C is a suitable temperature for precursor calcination to produce the garnet phase and LCLN has a relatively low phonon energy (∼725 cm−1) and a relatively wide bandgap (∼4.53 eV) for luminescence. The LCLN:0.012Pr, LCLN:0.04Sm, LCLN:0.06 Tb and LCLN:0.06Dy typical phosphors were analyzed to have dominant excitation/emission bands at ∼278/489, 265/614, 268/549 and 264/581 nm, emission colors of white, orange, green and yellowish white, and fluorescence lifetimes of ∼0.43, 0.74, 0.90 and 0.44 ms, respectively. Temperature-dependent photoluminescence found that the four phosphors maintained ∼28, 37, 45 and 56% of their room-temperature intensities at 423 K, with activation energies for thermal quenching of ∼0.37, 0.29, 0.34 and 0.25 eV, respectively. The results demonstrated that the Li-stuffed LCLN garnet could be a suitable host for UV excitable multicolor phosphors. • ∼900–1000 °C is an appropriate temperature for precursor calcination to derive Li 6 CaLa 2 Nb 2 O 12 (LCLN) garnet. • The LCLN host is suitable for luminescence purposes. • The photoluminescence of Pr3+, Sm3+, Tb3+ and Dy3+ activators was comprehensively elaborated in LCLN. • The concentration quenching of Sm3+ luminescence in LCLN is dominantly resulted from electric dipole-dipole interaction. [ABSTRACT FROM AUTHOR]

• Li 6 CaLa 2 Nb 2 O 12 (GCN) garnet was identified to be a favorable host for efficient red luminescence of Eu3+. • The optimal Eu3+ content is ~35 at.% relative to La3+. • The GCN:Eu phosphor is efficiently excitable with either blue or NUV light. • The GCN:0.35Eu optimal phosphor has an excellent thermal stability and favorably high quantum yield. Li 6 CaLa 2 Nb 2 O 12 garnet is known as an electrolyte for Li+-batteries. The sol–gel processing of this work yielded a series of phase pure Li 6 CaLa 2(1− x) Nb 2 O 12 : x Eu3+ garnet phosphors (LCLNG: x Eu; x = 0–0.40) that are efficiently excitable with either near-ultraviolet (NUV, ~350–400 nm) or blue (~450–475 nm) light to produce dominantly strong 610 nm red emission. Increasing Eu3+ substitution for La3+ (larger x) led to smaller cell constant and lower symmetry of the (La/Ca)O 8 dodecahedron, as revealed via Rietveld structure refinement, and meanwhile narrower bandgap. The excitation/emission behaviors and high quenching concentration of Eu3+ (x = 0.35) were interpreted for the phosphor series by considering bandgap energy, (La/Ca)O 8 distortion and local arrangement of atoms. The LCLNG:0.35Eu optimal phosphor was analyzed to have absolute quantum yields of ~51% (λ ex = 393 nm) and 65% (λ ex = 463 nm), fluorescence lifetime of ~0.70 ms and color coordinates of around (0.65, 0.35) at room temperature and to be able to retain as high as ~85% of its room-temperature intensity at 423 K for the 610 nm emission. Application of LCLNG:0.35Eu further indicated that this phosphor has the potential to be used in NUV-excited LEDs. [ABSTRACT FROM AUTHOR]

• Uniform NaLn(WO 4) 2 micro-disks were successfully converted from a novel layered precursor. • No calcination or organic additive is needed to produce phase-pure NaLn(WO 4) 2. • The temperature/time course of phase/morphology evolution was elucidated. • Down/upconversion luminescence was elaborated in terms of spectral feature, performance and photon process. The two-dimensional (2D) crystallite morphology and low OH−/Ln3+ molar ratio of Ln 2 (OH) 4 SO 4 ·2H 2 O (Ln = lanthanide) make it an ideal precursor for materials synthesis via phase conversion, which was manifested in this work by the direct generation of well-defined NaLn(WO 4) 2 phosphor particles via hydrothermal reaction with Na 2 WO 4. Kinetics study showed that pure NaLn(WO 4) 2 can be produced by reaction at 180 °C for ~24 h or at 200 °C for ~6 h under WO 4 2−/Ln3+ = 10 M ratio. Morphology analysis revealed that, though NaLn(WO 4) 2 evolved via re-precipitation, the layered crystal structure and 2D crystallite morphology of the precursor could have templated the nucleation/growth of NaLn(WO 4) 2 , leading to uniform particles (~4–5 μm) of a unique microdisc-like morphology. Under 394 nm excitation, the Ln = La 0.95 Eu 0.05 phosphor showed down-conversion luminescence having an absolute quantum yield of ~35.4%, a fluorescence lifetime of ~1.13 ms for its 616 nm main emission, and color coordinates of around (0.63, 0.37). Under 978 nm laser excitation, the Ln = La 0.97 Yb 0.02 Ho 0.01 and Ln = La 0.97 Yb 0.02 Er 0.01 phosphors exhibited the strongest up-conversion (UC) luminescence at ~660 nm (the 5F 5 → 5I 8 transition of Ho3+) and 551 nm (the 4S 3/2 → 4I 15/2 transition of Er3+), average fluorescence lifetimes of ~178.3 and 82.3 µs for the above emissions, and chromaticity coordinates of around (0.62, 0.38) and (0.25, 0.72), respectively. The two UC phosphors were also analyzed to exhibit UC luminescence through a two-photon mechanism. [ABSTRACT FROM AUTHOR]