A new generation of red phosphors of complex fluoride matrices activated with Mn has gained a broad interest in getting high color quality and low color temperature of solid-state white light-emitting diodes (WLEDs). However, besides their instability toward moisture, the extremely irregular and nonuniform morphologies of these phosphors have limited their practical industry applications. In the present study, a novel type of KScF:Mn red phosphor with highly regular, uniform, and high color purity was obtained successfully through a facile coprecipitation route under mild conditions. The crystal structure was identified with aids of the powder X-ray diffraction, Rietveld refinement, and density functional theory calculations. The prototype crystallizes in the space group Fm3 m with a cubic structure, and the lattice parameters are fitted well to be a = b = c = 8.4859(8) Å and V = 611.074(2) Å. The Mn ions occupy Sc sites and locate at the centers of the distorted ScF octahedrons. A wide band gap of approximately 6.15 eV can provide sufficient space to accommodate impurity energy levels. Unlike most other Mn ion-activated fluoride phosphors, the as-prepared KScF:Mn phosphors demonstrate highly uniform and regular morphologies with shapes transforming from cube to octahedron with increasing Mn ion concentration. Under blue light excitation, the as-prepared KScF:Mn sample exhibits intense sharp-line red fluorescence (the strongest peak located at 631 nm) with high color purity. An excellent recovery in luminescence upon heating and cooling processes implies high stability of KScF:Mn. Furthermore, a warm WLED fabricated with blue GaN chips merged with the mixture of KScF:Mn and the well-known commercial YAG:Ce yellow phosphors exhibits wonderful color quality with lower correlated color temperature (3250 K) and higher color-rendering index ( R = 86.4). These results suggest that the KScF:Mn phosphor possesses stupendous potentiality for practical applications.

Upconversion (UC) luminescence materials doped with rare earth ions are extensively investigated as optical temperature probes by the fluorescence intensity ratio technique. However, most Er 3+ -doped materials are still suffering from low sensing sensitivity. In the present study, we attempt to develop high-sensingsensitivity Er 3+ -doped materials based on the thermally coupled energy levels (TCLs) from Stark sublevels as well as the properties at subzero temperatures, for which there is continuous lack of research. Er 3+ /Yb 3+ codoped Ba 3 Y 4 O 9 (BYO) phosphors were produced via a solid-state reaction. Excited by 980 nm, various output colors, including bright green, yellow, and red, in BYO:Er 3+ /Yb 3+ phosphors as well as the relative emission intensities could be regulated through altering Yb 3+ concentrations. Subsequently, on the basis of all 12 pairs of TCLs especially from Stark sublevels of 2 H 11/2 , 4 S 3/2 , and 4 F 9/2 of Er 3+ ions, multiple temperature-sensing performances are evaluated over a wide range of 73−573 K. The results show that the maximum sensitivity of the 2 H 11/2 and 4 S 3/2(1) levels is approximately 1-fold higher than that of traditional TCLs of 2 H 11/2 / 4 S 3/2 at elevated temperature and the maximum sensitivity based on the 2 H 11/2(1) and 2 H 11/2(2) levels is more than 12 times higher than that of the traditional TCLs of 2 H 11/2 / 4 S 3/2 at subzero temperature. Therefore, it is expected to realize high-sensitivity temperature detection from subzero to elevated temperatures by combining two pairs of different TCLs. In addition, the potential of Er 3+ / Yb 3+ codoped BYO phosphors to be used as an optical heater is studied. The generated temperature can be accurately monitored by BYO:Er 3+ /Yb 3+ phosphors and regulated by adjusting the excitation power, which indicate that BYO:Er 3+ /Yb 3+ phosphors can be used as an optical heating device.

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A series of Ba 3 Lu 4 O 9 :Er 3+ /Yb 3+ (EYBLO) phosphors co-doped with F À ions were synthesized by a simple solid-state reaction method. The results showed that the primary rhombohedral structure was maintained and the crystal lattice began to shrink when F À ions were introduced into the host matrix to occupy the O 2À site. The agglomerations and the impurities (OH À and CO 2 ) with higher phonon energy on the sample surface could be minimized and the sample crystallinity could be improved. Under 980 nm laser diode excitation, the green and red UC emissions of the EYBLO:0.4F À sample show nearly 5-and 7.5-fold enhancements in contrast to F À -free EYBLO. The upconversion luminescence can be finely tuned from yellow to red light to some extent by increasing F À concentration. Based on pumppower dependence and decay lifetime analysis, the energy level diagram was illustrated and the upconversion energy-transfer mechanism was discussed. The green and red emission enhancements are attributed to the modification of the local crystal field of Er 3+ ions and reduction of crystal site symmetry.The cross-relaxation and back-energy-transfer processes play an important role to enhance the red/ green UC emission intensity ratios. The fluorescence intensity ratio technique was employed to investigate the temperature sensing behavior of the synthesized phosphors. The temperature sensing properties can be enhanced by doping of F À ions, and the maximum sensitivity is found to be 44.57 Â 10 À4 K À1 at 523 K. It is promising to provide an alternative approach for enhancing UC luminescence and the temperature sensitivity in oxide matrixes and then obtain high-quality optical temperature-sensing materials by simply co-doping F À ions.

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A series of novel SrLu 2 O 4 : x Ho 3+ , y Yb 3+ phosphors (x=0.005-0.05, y=0.1-0.6) were synthesized by a simple solid-state reaction method. The phase purity, morphology, and upconversion luminescence were measured by X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL) spectroscopy. The doping concentrations and sintering temperature were optimized to be x=0.01, y=0.5 and T=1400°C to obtain the strongest emission intensity. Under 980 nm laser diode excitation, the SrLu 2 O 4 :Ho 3+ , Yb 3+ phosphors exhibit intense green upconversion (UC) emission band centered at 541 nm ( 5 F 4 , 5 S 2 ? 5 I 8 ) and weak red emission peaked at 673 nm ( 5 F 5 ? 5 I 8 ). Under different pump-power excitation, the UC luminescence can be finely tuned from yellow-green to green light region to some extent. Based on energy level diagram, the energy-transfer mechanisms are investigated in detail according to the analysis of pump-power dependence and luminescence decay curves. The energy-transfer mechanisms for green and red UC emissions can be determined to be two-photon absorption processes. Compared with commercial NaYF 4 :Er 3+ , Yb 3+ and common Y 2 O 3 :Ho 3+ , Yb 3+ phosphors, the SrLu 1.49 Ho 0.01 Yb 0.5 O 4 sample shows good color monochromaticity and relatively high UC luminescence intensity. The results imply that SrLu 2 O 4 :Ho 3+ , Yb 3+ can be a good candidate for green UC material in display fields. K E Y W O R D S energy transfer, phosphor, rare earths, SrLu 2 O 4 , upconversion