High luminescent thermal stability and water resistance of K2SiF6:Mn4+@CaF2 red emitting phosphor

K₂SiF₆:Mn⁴⁺ (KSF:Mn⁴⁺), as an efficient red-emitting phosphor, has a promising application in WLEDs (white light-emitting diodes). However, poor moisture resistance performance still hinders its deeper commercialization. Here, KSF:Mn⁴⁺@ CaF₂ with high water resistance and luminescent thermal stability has been prepared though H₂O₂-free hydrothermal method and surface coating process. Both KSF:Mn⁴⁺ and KSF:Mn⁴⁺@CaF₂ all have high luminescent thermal stability, due to negative thermal quenching (NTQ) effect. Mechanism of the NTQ has been discussed and suggested as thermal-light energy conversion mechanism. Compared with KSF:Mn⁴⁺, water resistance of KSF:Mn⁴⁺@CaF₂ is greatly improved by coating of CaF₂, because the outer shell of CaF₂ can effectively prevent the [MnF₆]^2- group on the surface of the phosphor from being hydrolyzed into MnO₂. The results of water resistance test shows that after immersing in water for 360 min (6 h), luminescent intensity of the uncoated product drops to 41.68% of the initial one, while that of the coated product remains to have 88.24% of its initial one. Warm white light with good luminescent performances (CCT = 3956 K and Ra = 89.3) is got from prototype WLEDs assembled by using the optimal coated sample. The results suggest that the optimal coated sample has potential application in blue-based warm WLEDs.     

Hydrophobic surface modification toward highly stable K2SiF6:Mn4+ phosphor for white light-emitting diodes

K₂SiF₆:Mn⁴⁺ phosphor is well known for its excellent red emission performance which is vital for improving the color rendering of white light-emitting diodes. However, the poor moisture resistance limits its application in optical devices. In this paper, K₂SiF₆:Mn⁴⁺ phosphor is coated with an inorganic hydrophobic protective layer to obtain good moisture resistance. Chemical vapor deposition method was used to decompose acetylene at high temperature, and the generated nanoscale carbon layer worked as a hydrophobic protective coating on the surface of the phosphor. Microstructure, compositions and properties of the synthesized K₂SiF₆:Mn⁴⁺@C phosphor were investigated in detail. It is found that most of the deposited carbon is coated on the surface of phosphor crystals in amorphous state. The carbon atoms are bonded with the fluorine element in K₂SiF₆:Mn⁴⁺ phosphor, forming carbon-fluorine (C–F) covalent bonds. The moisture resistance of K₂SiF₆:Mn⁴⁺@C phosphor is improved owing to the protection of the hydrophobic carbon. The relative emission intensity of K₂SiF₆:Mn⁴⁺@C phosphor could maintain 73% of the initial luminous intensity after immersing in the aqueous solution at room temperature for 8 h, whereas K₂SiF₆:Mn⁴⁺ phosphor without carbon coating was only 0.7% remaining of the initial value under the same conditions.     

HF-free molten salt route for synthesis of highly efficient and water-resistant K2SiF6:Mn4+ for warm white LED

It has been one of the hot issues to prepare the red-emitting Mn⁴⁺-doped fluoride phosphors with highly efficient and waterproofness for warm white-light-emitting diodes (WLEDs) by the green and environmentally friendly method. Herein, we design a novel green molten salt route to synthesize K₂SiF₆:Mn⁴⁺ red powder using molten NH₄HF₂ salt instead of HF liquor as the reaction medium. The results show that KMnO₄ and MnF₂ could produce Mn⁴⁺ in NH₄HF₂ molten salt through a reduction reaction, and the resulting Mn⁴⁺-doped K₂SiF₆ exhibited a bright red emission peaked at 632 nm under blue light excitation. The luminescence intensity of the as-obtained product after immersing into water for 24 hours maintain nearly 100% of that before soaking and emission peak shape remains unchanged. The thermal stability of the sample was evaluated by temperature-dependent luminescence spectral intensity during heating and cooling. Furthermore, a warm white-light-emitting diodes (WLEDs) with an excellent color rendering index (Ra = 87.1), lower correlated color temperature (CCT = 3536K), and high luminous efficacy (116.99 lm·W⁻¹) was fabricated based on blue chip and K₂SiF₆:Mn⁴⁺ and commercial yellow phosphor (Y₃Al₅O₁₂:Ce³⁺).     

Key role of Nb5+ in achieving water-resistant red emission in K2Ta1-xNbxF7:Mn4+ phosphors

Mn⁴⁺-activated fluoride is one of the most important red phosphors for white light-emitting diodes (WLEDs) with high color rendering index (CRI). Due to a lack of water resistance, their potential applications are limited. Although surface coating strategies improve the waterproof stability of fluoride red phosphors, they have downsides. It was found that Nb⁵⁺ plays an important role in improving the water resistance of Mn⁴⁺-activated oxyfluorides by preventing the hydrolysis of [MnF₆]^2-. In this work, the influence of Nb⁵⁺ on the waterproof stability of Mn⁴⁺-activated fluorides was explored. A set of synthesized K₂Ta_1-xNb_xF₇:Mn⁴⁺ phosphors exhibit tunable and superior water resistance. The photoluminescence (PL) intensity of the representative sample K₂Ta_0.6Nb_0.4F₇:5%Mn⁴⁺ remains nearly 100% of its initial value even after being immersed in water for 60 min, which is significantly higher than the commercial K₂SiF₆:Mn⁴⁺ red phosphor (8.7%). Our findings open up new possibilities for the development of waterproof fluoride red phosphors.     

Synthesis of K2SiF6:Mn4+ Phosphor from SiO2 Powders via Redox Reaction in HF/KMnO4 Solution and Their Application in Warm‐White LED

In this article, we propose a facile method for synthesis of K₂SiF₆:Mn⁴⁺ phosphor and discuss its promising application in warm‐white light emitting diodes (LED). The K₂SiF₆:Mn⁴⁺ was synthesized from SiO₂ powders through redox reaction in HF/KMnO₄ solution. The optical properties of LEDs containing different ratios of K₂SiF₆:Mn⁴⁺ phosphor and commercial Ce³⁺‐doped garnets (YAG‐40) yellow–green phosphor were studied. A warm‐white LED, with color temperature of 3510 K and color rendering index of 90.9 and efficacy of 81.56 lm/W was demonstrated.     

Solution growth of millimeter-scale Na2SiF6 single crystals for Mn4+-doping as red phosphor

The cation exchange method has been demonstrated to be efficient in doping Mn⁴⁺ ions into various fluorides to synthesize the red-emitting LED phosphors. This paper, however, reports the challenge in using this method to dope Mn⁴⁺ into the Na₂SiF₆ single crystals, to prepare the fluoride phosphor in single-crystal form, a state-of-the-art study in the white LED lighting field. The millimeter-sized Na₂SiF₆ single crystals with a uniform columnar morphology (2–3 mm in length) were successfully grown in solution by a slow cooling process after optimizing the precursors. Then, the crystals were soaked in the HF solution dissolved with K₂MnF₆ to implement Mn⁴⁺-doping via the cation exchange process. Evaluation of the Mn⁴⁺-doping behavior reveals that the Mn⁴⁺ ↔ Si⁴⁺ cation exchange is less efficient in the case of single crystal host compared with the polycrystalline powdery ones and by-reactions also occur which generates new phases. The Na₂SiF₆ single crystals doped with Mn⁴⁺ exhibit a series of discrete sharp peaks with intense zero phonon line emission at 617 nm under 450 nm blue irradiation. This study may trigger the exploration of new single crystal fluoride phosphor.     

Achieving highly efficient far-red emission from luminescence-ignorable Ca2MgTeO6:Mn4+ to Ca2-zLnzMgTe1-yWyO6:Mn4+ (Ln = La,Y, Gd) phosphors via solid solution and impurity doping strategies

The Mn⁴⁺-doped Ca2MgTeO6 (CMTO) far-red emitting phosphors with double perovskite-type structure were successfully synthesized. Upon near-ultraviolet (n-UV, 300 nm) light excitation, the as-prepared phosphors showed far-red light at 700 nm attributed to the ²E_g→⁴A_2g transition of Mn⁴⁺ ion. The doping concentration of the CMTO:xMn⁴⁺ samples was optimized to be 0.8 mol%. The relevant mechanism of concentration quenching was demonstrated as the dipole-dipole interaction. Furthermore, solid solution and impurity doping strategies were adopted to improve the far-red emission of the luminescence-ignorable CMTO:Mn⁴⁺ phosphor. Series of Ca₂MgTe_(1−y)W_yO₆:0.8 mol%Mn⁴⁺ (y = 0–100 mol%) solid solution and Ca_2−zLn_zMgTe_0.6W_0.4O₆:Mn⁴⁺ (Ln = La, Y, and Gd, z = 10 mol%) phosphors were synthesized through the above two strategies. The luminescence intensity of the optimal Ca_1.9Gd_0.1MgTe_0.6W_0.4O₆:Mn⁴⁺ phosphor was 13.7 times that of the CMTO:Mn⁴⁺ phosphor and 2.51 times that of red commercial phosphor K₂SiF₆:Mn⁴⁺. Notably, both CMTO:Mn⁴⁺ and Ca_1.9Gd_0.1MgTe_0.6W_0.4O₆:Mn⁴⁺ phosphors exhibited remarkable thermal stability compared with most Mn⁴⁺-doped phosphors. Finally, the highly efficient Ca_1.9Gd_0.1MgTe_0.6W_0.4O₆:Mn⁴⁺ phosphor was successfully applied in fabricating the warm white light diode (w-LED). This working along both lines strategy exhibited great potential for luminescence optimization of Mn⁴⁺-doped oxide phosphors.     

Mn4+ activated dodec-fluoride red phosphor Na3Li3In2F12:Mn4+ with excellent waterproof stability for WLEDs

Mn⁴⁺ activated fluoride red phosphors, as candidate red materials in white light-emitting diodes (WLEDs), have received widespread attention. However, the poor water stability limits their application. Herein, a novel dodec-fluoride red phosphor Na₃Li₃In₂F₁₂:Mn⁴⁺ with good waterproof stability was successfully synthesized by solvothermal method. The crystal structure, optical property, micro-morphology, element composition, waterproof property and thermal behavior of Na₃Li₃In₂F₁₂:Mn⁴⁺ phosphor were analyzed. Under the 468 nm blue light excitation, the Na₃Li₃In₂F₁₂:Mn⁴⁺ phosphor has narrow emission bands in the area of 590–680 nm. Compared with commercial red phosphor K₂SiF₆:Mn⁴⁺, the Na₃Li₃In₂F₁₂:Mn⁴⁺ phosphor possesses better waterproof stability. When soaked in water for 360 min, the PL intensity of the Na₃Li₃In₂F₁₂:Mn⁴⁺ phosphor remains at initial 80%. Finally, warm WLEDs with CRI of 87 and CCT of 3386 K have been fabricated using blue InGaN chip, YAG:Ce³⁺ yellow phosphor and Na₃Li₃In₂F₁₂:Mn⁴⁺ red phosphor.     

Surface engineered environment-stable red-emitting fluorides for white light emitting diodes

Poor water stability is the main problem of commercialized Mn⁴⁺-doped fluorides for white light emitting diode (WLED) application. This work proposes a surface engineering strategy to rebuild a Mn⁴⁺-free fluoride shell on fluorides to effectively resist the destruction from water molecules. By simple processing using glyoxylic acid (GA) solution, the moisture resistance of the red-emitting fluorides can be significantly improved. The photoluminescence (PL) quantum efficiency (QE) of the surface-engineered K₂SiF₆:Mn⁴⁺ (KSFM-GA) still maintain 98.43% after water immersion for 360 h, in sharp contrary to the untreated one (its PLQE decreases to 59.79%). Additionally, PL intensity of the hydrolyzed KSFM can be recovered to 99.1% through the treatment of the reducing GA solution. By using the high-stability KSFM-GA red phosphor, the as-fabricated high-performance warm-WLED device can still maintain 84.6% in luminous efficacy, higher than that (79.6%) with the untreated KSFM, after 500 h of aging in a high temperature (85 °C) and high humidity (85%) environment.     

A novel Mn4+‐activated red phosphor for use in light emitting diodes,K3SiF7:Mn4+

Phosphors that exhibit a narrow red emission are particularly interesting due to the advantage of providing a more extensive color gamut and better rendering in LED applications such as displays and solid‐state lighting. Although some Eu²⁺‐activated nitridosilicates have been discovered in this regard, K₂SiF₆:Mn⁴⁺ phosphors are the only option in actual LED applications thus far. We discovered a novel phosphor, K₃SiF₇:Mn⁴⁺, with P4/mbm symmetry. The luminescent properties of K₃SiF₇:Mn⁴⁺ are almost identical to those of the K₂SiF₆:Mn⁴⁺ phosphor, but its materials identity is distinct due to a completely different crystallographic structure, which leads to reduced decay time. The fast decay is one of the most serious disadvantages of existing K₂SiF₆:Mn⁴⁺ phosphors. The K₃SiF₇:Mn⁴⁺ phosphor was examined in comparison to the K₂SiF₆:Mn⁴⁺ via density functional theory calculation, Rietveld refinement, X‐ray photoelectron spectroscopy, X‐ray absorption near‐edge structure spectroscopy, and time‐resolved photoluminescence.     

A single‐phase full‐color emitting phosphor Na3Sc2(PO4)3:Eu2+/Tb3+/Mn2+ with near‐zero thermal quenching and high quantum yield for near‐UV converted warm w‐LEDs

A single‐phase full‐color emitting phosphor Na₃Sc₂(PO₄)₃:Eu²⁺/Tb³⁺/Mn²⁺ has been synthesized by high‐temperature solid‐state method. The crystal structure is measured by X‐ray diffraction. The emission can be tuned from blue to green/red/white through reasonable adjustment of doping ratio among Eu²⁺/Tb³⁺/Mn²⁺ ions. The photoluminescence, energy‐transfer efficiency and concentration quenching mechanisms in Eu²⁺‐Tb³⁺/Eu²⁺‐Mn²⁺ co‐doped samples were studied in detail. All as‐obtained samples show high quantum yield and robust resistance to thermal quenching at evaluated temperature from 30 to 200°C. Notably, the wide‐gamut emission covering the full visible range of Na₃Sc₂(PO₄)₃:Eu²⁺/Tb³⁺/Mn²⁺ gives an outstanding thermal quenching behavior near‐zero thermal quenching at 150°C/less than 20% emission intensity loss at 200°C, and high quantum yield‐66.0% at 150°C/56.9% at 200°C. Moreover, the chromaticity coordinates of Na₃Sc₂(PO₄)₃:Eu²⁺/Tb³⁺/Mn²⁺ keep stable through the whole evaluated temperature range. Finally, near‐UV w‐LED devices were fabricated, the white LED device (CCT = 4740.4 K, Ra = 80.9) indicates that Na₃Sc₂(PO₄)₃:Eu²⁺/Tb³⁺/Mn²⁺ may be a promising candidate for phosphor‐converted near‐UV w‐LEDs.     

Comparative analysis on the photoluminescence properties of Cs2BF6:Mn4+ (B = Ge,Si, Zr,Ti) red phosphors for WLEDs

A series of Cs₂BF₆:Mn⁴⁺ (B = Ge, Si, Ti, Zr) red phosphors were synthesized by a precipitation-cation exchange route. The phase purity, morphology, and constituent were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Optical properties were investigated by photoluminescence (PL) spectra and high-resolution PL. Temperature-dependent PL examination at the range of both 273-573 K and 10-300 K was performed to investigate the emission mechanism of Mn⁴⁺ in these fluorides. The intensity for both zero-phonon lines (ZPLs) and vibration coupled emission of Mn⁴⁺ in these four systems with different crystal structures was investigated systematically. These phosphors present bright red emission under blue light (467 nm) illumination, among which Cs₂GeF₆:0.1Mn⁴⁺ shows the highest emission intensity with ultrahigh quantum efficiency of 94%. The white light-emitting diodes (WLEDs) fabricated with this sample, blue InGaN chips and commercial YAG:Ce³⁺ phosphor exhibited high luminous efficacy beyond 100 lm/w with high color rendering index (~88.6) and low color temperature (~3684 K).     

Formation mechanism and optimized luminescence of Mn4+‐doped unequal dual‐alkaline hexafluorosilicate Li0.5Na1.5SiF6

A novel red phosphor Li_0.5Na_1.5SiF₆:Mn⁴⁺ (LNSF:Mn) based on the unequal dual‐alkaline hexafluorosilicate with superior optical performances has been synthesized via ion‐exchange between [MnF₆]^2? and [SiF₆]^2? at room temperature. The composition and the crystal structure of the as‐obtained phosphor LNSF:Mn were determined by energy‐dispersive x‐ray spectroscopy (EDS) and x‐ray diffraction (XRD), respectively. The formation mechanism of the red phosphor LNSF:Mn has been discussed in detail. The phosphor LNSF:Mn exhibits good chromaticity properties and a quantum yield (QY) of 96.1%, which are better than the identified fluorosilicate phosphors Na₂SiF₆:Mn⁴⁺ (NSF:Mn) and K₂SiF₆:Mn⁴⁺ (KSF:Mn). A broad and intense absorption in the blue and a bright emission in red‐shifted wavelengths make the phosphor LNSF:Mn a desired candidate for applications in warm white light‐emitting diodes.     

Efficient and tunable Mn2+ sensitized luminescence via energy transfer of a novel red phosphor Ca19Mn2(PO4)14: Eu2+ for white LED

The exploration of efficient and high-purity red phosphors is an urgent need in LED development. Due to the compact and compositional-tunable structure of whitlockite compound, manganese-based Ca₁₉Mn₂(PO₄)₁₄ is chosen as phosphor host for Eu²⁺ sensitization. Rietveld refinement, steady-state spectra, decay lifetime analysis and temperature-dependent emission spectra were investigated and clearly discussed. Under 360 nm excitation, Ca₁₉Mn₂(PO₄)₁₄: Eu²⁺ shows a strong Mn²⁺ sensitized emission at 655 nm with FWHM of 82 nm, benefiting from the short-distance-induced high-efficient Eu² -Mn²⁺ energy transfer. Emission engineering of Ca₁₉Mn₂(PO₄)₁₄: Eu²⁺ is achieved by Sr²⁺ co-doping, leading to both tunable peak wavelength (ranging from 650 to 610 nm) and improved intensity (130% of original value). Moreover, Ca₁₉Mn₂(PO₄)₁₄: Eu²⁺ exhibits a promising thermal stability where only 40% of emission intensity is lost at 200 °C. Finally, we explored the working performance of the fabricated RGB phosphor-converted white LED. The present work indicates that Ca₁₉Mn₂(PO₄)₁₄: Eu²⁺ phosphor is of great potential as a promising and efficient red phosphor in phosphor-converted white LED.     

Surface modification of K2TiF6:Mn4+ phosphor with SrF2 coating to enhance water resistance

K₂TiF₆:Mn⁴⁺ is an attractive narrow-band red-emitting phosphor for warm white light-emitting diodes (LEDs). Nevertheless, the hexafluoride phosphor is liable to deliquesce in moist environments, which leads to a sharp deterioration performance of luminescence. Surface modification of K₂TiF₆:Mn⁴⁺ phosphor with SrF₂ coating has been introduced, with the aid of KHF₂ transition layer to moderate the lattice mismatch. The reaction mechanism is discussed in detail, as so as the influence of SrF₂ coating on the luminescence intensity. The SrF₂ coating is able to prevent the hydrolysis of internal [MnF₆]²⁻ group; thereby, the luminescence intensity retains over 90% of initial value after being immersed in distilled water for 2 h. The LED devices fabricated with commercial Y₃Al₅O₁₂:Ce³⁺ and as-modified K₂TiF₆:Mn⁴⁺ phosphors exhibit bright white light with tunable chromaticity coordinate, correlated color temperature, and color rendering index. It enlightens a convenient method to enhance the moisture resistance of Mn⁴⁺ doped fluoride phosphors for commercial application in the field of white LEDs.     

Photoluminescence and energy transfer of efficient and thermally stable white-emitting Ca9La(PO4)7:Ce3+, Tb3+, Mn2+ phosphors

The development of novel single-component white-emitting phosphors with high thermal stability is essential for improving the illumination quality of white light-emitting diodes. In this work, we synthesized a series of Ce³⁺, Tb³⁺, Mn²⁺ single- and multiple-doped Ca₉La(PO₄)₇ (CLPO) phosphors with β-Ca₃(PO₄)₂-type structure by the simple high-temperature solid-state reaction. The crystallization behavior, crystal structure, surface morphology, photoluminescence performance, decay lifetime and thermal stability were systematically investigated. The PL spectra and decay curves have evidenced the efficient energy transfer from Ce³⁺ to Tb³⁺ and from Ce³⁺ to Mn²⁺ in the CLPO host, and corresponding energy transfer efficiency reaches 41.8% and 54.1%, respectively. The energy transfer process of Ce³⁺→Tb³⁺ and Ce³⁺→Mn²⁺ can be deduced to the resonant type via dipole-dipole and dipole-quadrupole interaction mechanism, and corresponding critical distance were determined to be 12.23 and 14.4 Å, respectively. Based on the efficient energy transfer, the white light emission can be successfully achieved in the single-component CLPO:0.15Ce³⁺, 0.10Tb³⁺, 0.04Mn²⁺ phosphor, which owns CIE chromaticity coordinates of (0.3245, 0.3347), CCT of 5878 K, internal and external quantum efficiency of 84.51% and 69.32%. Especially, compared with the emission intensity at 25 °C, it still remains 98.5% at 150 °C and 92.0% at 300 °C. Based on these results, the single-component white light emission phosphor CLPO:0.15Ce³⁺, 0.10Tb³⁺, 0.04Mn²⁺ is a potential candidate for UV-converted white LEDs.     

Highly sensitive optical thermometer of Sm3+, Mn4+ activated LaGaO3 phosphor for the regulated thermal behavior

Sm³⁺, Mn⁴⁺ co-activated LaGaO₃ phosphors, giving the characteristic emissions of orange and red emission simultaneously, were prepared by a solid-state reaction. Their luminescence properties, energy transfer behavior, thermal stability, and ratiometric temperature sensing performance were investigated. Thanks to the inhibition of energy transfer between Sm³⁺ and Mn⁴⁺ ions at high temperature and the reconstruction of the traps, the distinct optical behavior of the involved activators dependent on the ambient temperature was evaluated. Anti-thermal quenching performance of Sm³⁺ ions along with the emission declination of Mn⁴⁺ ions was observed. Hence, the optical thermometry characteristics of the resultant phosphor based on the fluorescent intensity ratio (orange/red) realize a recorded temperature sensitivity of 4.19% K⁻¹ and 2.09% K⁻¹. Moreover, the as-explored film combined with the LaGaO₃: Sm³⁺, Mn⁴⁺ phosphor is demonstrated to be a promising multi-color optical thermometer.     

Deep-red emission of Mn4+ and Cr3+ in (Li1−xAx)2MgTiO4 (A=Na and K) phosphor: Potential application as W-LED and compact spectrometer

Mn⁴⁺ doped and Mn⁴⁺/Cr³⁺ co-doped alkali metal titanate phosphors have been prepared by solid state reaction method. A part of Li⁺ ions in the Li₂MgTiO₄: Mn⁴⁺ are substituted with Na⁺ and K⁺ ions and consequently the intensity of Mn⁴⁺ emission at 678 nm is enhanced by 1.7 and 2.5 times, respectively. In the Mn⁴⁺/Cr³⁺ co-doped (Li_0.95K_0.05)₂MgTi_0.999O₄, both emission of Cr³⁺at 726 nm and emission of Mn⁴⁺ at 678 nm of Mn⁴⁺ are observed. It is interesting to find that the intensity ratio of 726–678 nm emissions in the Mn⁴⁺/Cr³⁺ phosphor continually increases with excitation wavelength increasing from 290 nm to 455 nm, which means that the intensity ratio in turn can be used to identify the excitation light wavelength. This refers a possible approach to design novel compact light-wavelength detector or spectrometer based on the phosphor. The mechanism of Na⁺ or K⁺ substitution induced luminescence enhancement in the Mn⁴⁺ phosphor and the competition between the Cr³⁺ and Mn⁴⁺ emissions in the Mn⁴⁺/Cr³⁺ co-doped has been discussed.     

Self-reduction and enhanced luminescence in transparent Mn2+-doped mullite glass-ceramics derived from EMT-type zeolite

Mn²⁺ activated glass-ceramic (GC) has received tremendous attention in the exploration of luminescent materials for solid-state lighting due to the high stability, broad red emission, and low toxicity. However, the doped Mn²⁺ ions still suffer from the oxidation and uncontrollable ions migration during the melting process of conventional preparation techniques, which is detrimental to the luminescence performance. Herein, transparent Mn²⁺-doped mullite GCs have been prepared at low temperature (∼850°C) via the spark plasma sintering of EMT-type zeolite. The GC samples show typical red emission peaking at 620 nm that can be assigned to spin-forbidden ⁴T₁(G)→⁶A₁(S) transition of Mn²⁺ located in the octahedral coordination site of the host. Owing to the charge compensation mechanism and produced oxygen vacancies, the self-reduction of Mn³⁺ to Mn²⁺ ions is realized and the oxidization is inhibited. The mullite nanocrystals acted as additional scattering centers introduce Rayleigh scattering to enhance the emission intensity. Moreover, benefitted from the established mullite nanocrystals network, the Mn²⁺-doped GCs exhibit improved thermal conductivity up to 1.79 W K⁻¹ m⁻¹ and more excellent mechanical properties than conventional GCs, simultaneously.     

Novel efficient SrMgY3(SiO4)3F:Ce3+,Tb3+ phosphor with tunable color and thermal stability through energy transfer

A novel apatite-type SrMgY₃(SiO₄)₃F was synthesized by a high-temperature solid-state reaction. The crystal structure was refined using powder X-ray diffraction data. SrMgY₃(SiO₄)₃F crystallizes in P6₃/m hexagonal space group with lattice parameters of a = b = 9.45270 Å, c = 6.77617 Å, and V = 524.357 Å³. The incorporation of the Ce³⁺ and Tb³⁺ ions into the matrix can generate bright blue and green lights under ultraviolet (UV) light excitation. The codoped Ce³⁺ and Tb³⁺ in SrMgY₃(SiO₄)₃F can effectively improve green emission intensity and thermal stability through the energy transfer from the Ce³⁺ to Tb³⁺ ions. With the increase of Tb³⁺-doping content, the luminescent color of phosphor changes from blue to cyan and finally to green. SrMgY₃(SiO₄)₃F:0.06Ce³⁺,0.90Tb³⁺ phosphor exhibited intense green light emission with a quantum yield of 59.49% and good thermal stability, with an emission intensity at 150°C was 96% of that at 30°C. Finally, the prepared sample was coated on 365 nm UV chips to fabricate white light-emitting diodes with a color rendering index of 82.6 and a correlated color temperature of 2912 K, demonstrating its potential for applications in display and lighting.