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Fluoride phosphors as red components for warm white LEDs have attracted a tremendous amount of research attention. But these phosphors are extremely sensitive to moisture, which seriously limits their practical industrial applications. To tackle this problem, unlike all the straightforward preventive strategies, a reverse strategy "Good comes from bad" was successfully developed to treat the degraded K2SiF6 : Mn4+ (D‐KSFM) phosphor in the present study, which not only completely restores the luminescence properties, but also significantly enhances the moisture resistance at the same time. After treatment with an oxalic acid solution as restoration modifier, the emission intensity of the D‐KSFM phosphor can be restored to 103.7% of the original K2SiF6 : Mn4+ red phosphor (O‐KSFM), and the moisture resistance is remarkably improved. The restored K2SiF6 : Mn4+ (R‐KSFM) maintains approximately 62.3% of its initial relative emission intensity after immersing in deionized water for 300 min, while the reference commercial K2SiF6 : Mn4+ with a protective coating (C‐KSFM) is only 33.2%. As a proof of general applicability, this strategy was also conducted to K2TiF6 : Mn4+ phosphor, which is less moisture‐stable than K2SiF6 : Mn4+. The luminescence intensity of the degraded K2TiF6 : Mn4+ (D‐KTFM) phosphor can be restored to 162.6% of original level of the K2TiF6 : Mn4+ synthesized through a cation exchange approach without any treatment (O‐KTFM). The emission intensity of the restored K2TiF6 : Mn4+ (R‐KTFM) phosphor retains 62.8% of its initial emission intensity after soaking in deionized water for 300 min. Finally, the R‐KSFM phosphors were packaged into white light‐emitting diodes with blue InGaN chips and Y3Al5O12 : Ce3+ yellow phosphors. The WLEDs display excellent color rendition with higher color rendering index, lower color temperature (WLED‐II: Ra=83.6, R9=57.3, 3743 K, ηl=199.68 lm/W; WLED‐III: Ra=90.4, R9=94.2, 2892 K, ηl=183.3 1 m/W). The above results show that the reverse strategy can be applied in those phosphor materials with poor moisture resistance to restore luminescence properties and improve moisture resistance without excessively care about the deterioration during the production, storage and transportation. [ABSTRACT FROM AUTHOR]

An oxyfluoride red phosphor Cs2MoO2F4:Mn4+ was synthesized via a facile co-precipitation route with a certain molecular ratio of CsF and MoO3. X-ray diffraction analysis and its Rietveld refinement reveal that Cs2MoO2F4:Mn4+ crystallized in an orthorhombic structure with the Amam (63) space group. Upon blue light excitation, Cs2MoO2F4:Mn4+ exhibits a series of sharp red emission lines around ∼634 nm and the zero-phonon line (ZPL) is visible at 619 nm. The optimal doping amount of Mn4+ in Cs2MoO2F4 is 1.12%, and the decay curves show a good fit with the single exponential decay model. The fluorescence lifetime of the synthesized phosphors is relatively short and calculated as 3.18 to 2.46 ms, the Mn4+ ions in Cs2MoO2F4 experience a strong crystal field strength with a Dq/B of ∼4.87, and the distinct nephelauxetic ratio β1 is determined to be ∼1.0226. The thermal quenching mechanism of Mn4+ was also studied. Furthermore, by using the as-synthesized Cs2MoO2F4:Mn4+ phosphor as a red component and β-SiALON as a green light component, a WLED was fabricated with a high luminous efficacy of 114.70 lm·W−1 and wide color gamut of 109.1% of the National Television Standard Committee (NTSC) value. Hence, the Cs2MoO2F4:Mn4+ phosphor with a short fluorescence lifetime could potentially be an efficient red compensator for application in rapid-response backlight displays. [ABSTRACT FROM AUTHOR]

Mn4+-Activated narrow-band red-emitting fluoride phosphors are an efficient red compensator able to widen the color gamut and improve the color quality of pc-WLEDs for backlight display applications. However, the low synthetic yields and extremely non-uniform, irregular morphologies of these fluorides seriously restrict their practical industrial applications. Herein, a novel Mn4+-doped narrow-band red-emitting phosphor K2NaScF6:Mn4+ was prepared through a simple co-precipitation route. High-resolution synchrotron XRD and its Rietveld refinement reveal that K2NaScF6:Mn4+ crystallized in the space group Fm3̅m with a cubic structure and that the octahedral Sc3+ sites were occupied by Mn4+, resulting in a very small amount of alkali metal cation vacancies for charge compensation. The electronic band gap of K2NaScF6 was calculated to be 6.46 eV by DFT calculations, which ensured the location of Mn4+ energy levels in the host band gap and was verified by DRS results. Unlike previously reported Mn4+-activated fluorides, K2NaScF6:Mn4+ has an ultra-high synthetic yield (∼100%) and a uniform spherical morphology with extremely narrow size distribution (∼1.17 μm). Excess KHF2 and the extremely low solubility of K2NaScF6 were shown to be responsible for the high synthetic yield of K2NaScF6:Mn4+. Under blue light illumination, K2NaScF6:Mn4+ exhibited an intense sharp-line red fluorescence peak at 630 nm. Meanwhile, the ZPL intensity could be tailored through the control of Mn4+ amount in the K2NaScF6 matrix. By employing the efficient and stable K2NaScF6:Mn4+ as an efficient red emitter, two pc-WLEDs with ultra-wide color gamut (Rec. 2020 value of 95.1% and NTSC value of 127.3% for LED-1, Rec. 2020 value of 96.7% and NTSC value of 129.5% for LED-2) were successfully obtained. These results suggest the great potential of K2NaScF6:Mn4+ as a red-emitting phosphor for applications in high-quality backlight displays, and this work may open a perspective for researchers to design new luminescence materials with high synthetic yield and uniform spherical morphology. [ABSTRACT FROM AUTHOR]

Fluoride phosphors as red components for warm white LEDs have attracted a tremendous amount of research attention. But these phosphors are extremely sensitive to moisture, which seriously limits their practical industrial applications. To tackle this problem, unlike all the straightforward preventive strategies, a reverse strategy "Good comes from bad" was successfully developed to treat the degraded K 2 SiF 6  : Mn 4+ (D-KSFM) phosphor in the present study, which not only completely restores the luminescence properties, but also significantly enhances the moisture resistance at the same time. After treatment with an oxalic acid solution as restoration modifier, the emission intensity of the D-KSFM phosphor can be restored to 103.7% of the original K 2 SiF 6  : Mn 4+ red phosphor (O-KSFM), and the moisture resistance is remarkably improved. The restored K 2 SiF 6  : Mn 4+ (R-KSFM) maintains approximately 62.3% of its initial relative emission intensity after immersing in deionized water for 300 min, while the reference commercial K 2 SiF 6  : Mn 4+ with a protective coating (C-KSFM) is only 33.2%. As a proof of general applicability, this strategy was also conducted to K 2 TiF 6  : Mn 4+ phosphor, which is less moisture-stable than K 2 SiF 6  : Mn 4+ . The luminescence intensity of the degraded K 2 TiF 6  : Mn 4+ (D-KTFM) phosphor can be restored to 162.6% of original level of the K 2 TiF 6  : Mn 4+ synthesized through a cation exchange approach without any treatment (O-KTFM). The emission intensity of the restored K 2 TiF 6  : Mn 4+ (R-KTFM) phosphor retains 62.8% of its initial emission intensity after soaking in deionized water for 300 min. Finally, the R-KSFM phosphors were packaged into white light-emitting diodes with blue InGaN chips and Y 3 Al 5 O 12  : Ce 3+ yellow phosphors. The WLEDs display excellent color rendition with higher color rendering index, lower color temperature (WLED-II: R a =83.6, R 9 =57.3, 3743 K, η l =199.68 lm/W; WLED-III: R a =90.4, R 9 =94.2, 2892 K, η l =183.3 1 m/W). The above results show that the reverse strategy can be applied in those phosphor materials with poor moisture resistance to restore luminescence properties and improve moisture resistance without excessively care about the deterioration during the production, storage and transportation., (© 2020 Wiley-VCH GmbH.)

An oxyfluoride red phosphor Cs 2 MoO 2 F 4 :Mn 4+ was synthesized via a facile co-precipitation route with a certain molecular ratio of CsF and MoO 3 . X-ray diffraction analysis and its Rietveld refinement reveal that Cs 2 MoO 2 F 4 :Mn 4+ crystallized in an orthorhombic structure with the Amam (63) space group. Upon blue light excitation, Cs 2 MoO 2 F 4 :Mn 4+ exhibits a series of sharp red emission lines around ∼634 nm and the zero-phonon line (ZPL) is visible at 619 nm. The optimal doping amount of Mn 4+ in Cs 2 MoO 2 F 4 is 1.12%, and the decay curves show a good fit with the single exponential decay model. The fluorescence lifetime of the synthesized phosphors is relatively short and calculated as 3.18 to 2.46 ms, the Mn 4+ ions in Cs 2 MoO 2 F 4 experience a strong crystal field strength with a Dq/B of ∼4.87, and the distinct nephelauxetic ratio β 1 is determined to be ∼1.0226. The thermal quenching mechanism of Mn 4+ was also studied. Furthermore, by using the as-synthesized Cs 2 MoO 2 F 4 :Mn 4+ phosphor as a red component and β-SiALON as a green light component, a WLED was fabricated with a high luminous efficacy of 114.70 lm·W -1 and wide color gamut of 109.1% of the National Television Standard Committee (NTSC) value. Hence, the Cs 2 MoO 2 F 4 :Mn 4+ phosphor with a short fluorescence lifetime could potentially be an efficient red compensator for application in rapid-response backlight displays.

To improve absorption efficiency (AE) and subsequently improve external quantum efficiency (EQE) remains one of the significant challenges for Mn 4+ -doped red-emitting fluoride phosphors. In this study, we propose to use Mn 4+ as a part of matrix to enhance the AE of fluoride phosphors. Red-emission phosphors Cs 2 MnF 6 , Cs 2 MnF 6 :Sc 3+ , and Cs 2 MnF 6 :Si 4+ were synthesized successfully by a coprecipitation method. The Rietveld refinement of X-ray diffraction reveals that this red phosphor exhibits a cubic structure in Fm 3̅ m space group. Owing to Mn 4+ being a part of matrix, this kind of red phosphor possesses an extremely high AE, which can be promoted to 88%. The doping of Sc 3+ and Si 4+ ions into Cs 2 MnF 6 can effectively increase the luminescence intensity to 253 and 232%, respectively, relative to that of Cs 2 MnF 6 . The relative emission intensity of Cs 2 MnF 6 :5%Si 4+ red phosphor preserves about 115% when temperature rises to 175 °C. By employing Cs 2 MnF 6 :5%Si 4+ as a red-emitting component, high-performance LED-1 with Ra = 86.2, R9 = 82.1 and CCT = 3297 K, and LED-2 with an ultrawide color gamut (NTSC value of 122.3% and rec. 2020 value of 91.3%) are obtained. This work may provide a new idea to explore a new type of fluoride phosphor with high EQE for high-performance white-light-emitting diodes.