An efficient far-red emission Sr2InSbO6:Mn4+, M (M = Li+, Na+, and K+) phosphors for plant cultivation LEDs

High-efficiency and far-red light phosphors based on Mn⁴⁺-doped inorganic luminescence materials are beneficial to plant cultivation. However, Mn⁴⁺-doped oxide phosphors have a common problem of low quantum efficiency. Alkali metal ion codoping can effectively improve the luminescence properties of Mn⁴⁺-activated oxide phosphors. Herein, a series of Sr₂InSbO₆:Mn⁴⁺, M (SISO:Mn⁴⁺, M) (M = Li⁺, Na⁺, and K⁺) far-red-emitting phosphors codoped alkali metal ions were first synthesized. Density functional theory calculation indicated that SISO is a kind of indirect bandgap material with a bandgap of ∼1.60 eV. The SISO:Mn⁴⁺ samples showed a far-red light at 698 nm upon 365 nm, which perfectly matched the absorption spectrum of the far-red-phytochrome (Pfr) of plants. The doping concentration of the SISO:Mn⁴⁺ samples was optimized to be 0.006 mol. The concentration quenching mechanism was defined as dipole–dipole interaction by combining the Dexter theory and the Inokuti–Hirayama model. Optimizing the sintering temperature and codoped with alkali metal ions (Li⁺, Na⁺, and K⁺) could improve the luminescent intensity of SISO:Mn⁴⁺. The optimum sintering temperature was 1300°C. The internal quantum efficiencies of SISO:0.006Mn⁴⁺ and SISO:0.006Mn⁴⁺, 0.006Li⁺ phosphors are 22.67% and 60.56%, respectively. SISO:Mn⁴⁺, Li⁺ phosphors-based plant growth light-emitting diodes (LEDs) demonstrate excellent optical stability and long lifetime. Thus, these phosphors are promising candidates for plant cultivation LEDs.     

Broadband deep-red-to-near-infrared emission from Mn2+ in strong crystal-field of nitride MgAlSiN3

Broadband near-infrared (NIR) phosphors have received increasing attention for fabricating phosphor-converted light-emitting diodes (pc-LEDs) as NIR light source. Most of the reported broadband NIR phosphors originate from Cr³⁺ in weak crystal field environments. Herein, we report a luminescent material, MgAlSiN₃:Mn²⁺ with CaAlSiN₃-type structure, demonstrating that broadband deep-red-to-NIR emission can be achieved via doping Mn²⁺ into crystallographic sites with strong crystal field in inorganic solids. This phosphor is synthesized via easy-handle solid-state reaction, and the optimized sample, (Mg_0.93Mn_0.07) AlSiN₃ shows an emission band with peak at ~754 nm, FWHM of 150 nm, and internal quantum efficiency of 70.1%. The photoluminescence intensity can further be enhanced by co-doping Eu²⁺ as sensitizer. This work provides a new strategy for discovering new broadband NIR phosphors using Mn²⁺ in strong crystal field as luminescence center.     

Enhancing emission intensity and thermal stability by charge compensation in Sr2Mg3P4O15:Eu3+

Charge compensation was the effective methods to enhance the luminescence properties of phosphors. In this paper, novel single‐phased orange light emitting Sr₂Mg₃P₄O₁₅:Eu³⁺ phosphors were prepared by solid state method. The phase purity and luminous characteristics were examined in detail. Meanwhile, three kinds of charge compensation methods (co‐doping the alkali metal R⁺ (R⁺ = Li, Na, and K), substituting Si⁴⁺ for P⁵⁺ and self‐compensation) were employed to solve the charge imbalance problem between Sr²⁺ and Eu³⁺. The results showed that emission intensity of Eu³⁺ was improved by 1.43 (Li⁺), 1.58 (Na⁺), 1.53 (K⁺), 1.61 (Si⁴⁺), and 1.30 (self) times than that of Sr_1.6Mg₃P₄O₁₅:0.40Eu³⁺, respectively, and there was no change in the emitting color simultaneously. Furthermore, as the temperature reached at 423 K, the emission intensity increased from 41.67% of Sr_1.6Mg₃P₄O₁₅:0.40Eu³⁺ to 55.69% (Li⁺), 61.62% (Na⁺), 58.98% (K⁺), 71.15% (Si⁴⁺), and 80.59% (self) of that at room temperature. The reasons of those phenomena were the reduction in ion vacancies caused by charge imbalance through the charge compensation process. The specific mechanisms were elaborated in detail. Overall, this research validated that the charge compensation strategies could be severed as the key method to improve the luminescence properties, especially the thermal stability of phosphor.     

Tunable Color of Eu2+/Mn2+ Coactivated Na5Ca2Al(PO4)4 via Energy Transfer

Eu²⁺ and Eu²⁺/Mn²⁺‐activated Na₅Ca₂Al(PO₄)₄ phosphors have been synthesized by the combustion method. X‐ray powder diffraction profiles, luminescence spectra, chromaticity variation, and energy transfer of Na₅Ca₂Al(PO₄)₄:Eu²⁺, Mn²⁺ were investigated as a function of the Eu²⁺ and Mn²⁺ concentrations in Na₅Ca₂Al(PO₄)₄. The Na₅Ca₂Al(PO₄)₄:Eu²⁺,Mn²⁺ phosphors can be effectively excited at wavelength ranging from 300 to 430 nm, which matches well with that for near‐ultraviolet (UV) light‐emitting diode (LED) chips. Under excitation at 354 nm, Na₅Ca₂Al(PO₄)₄:Eu²⁺,Mn²⁺ not only exhibits blue‐green emission band attributed to 4f⁶5d¹→4f⁷ of Eu²⁺ but also gives an orange emission band attributed to ⁴T₁→⁶A₁ of Mn²⁺. The emission color of the phosphor can be systematically tuned from blue‐green through white and eventually to orange by adjusting the relative content of Eu²⁺ and Mn²⁺ through the principle of energy transfer. The results indicated that Na₅Ca₂Al(PO₄)₄:Eu²⁺, Mn²⁺ may serve as a potential color‐tunable phosphor for near UV white‐light LED.     

Sr3GdLi(PO4)3F:Eu2+, Mn2+: A tunable blue-white color emitting phosphor via energy transfer for near-UV white LEDs

A series of Eu²⁺ and Mn²⁺ co-doping Sr₃GdLi(PO₄)₃F phosphors have been synthesized through high temperature solid state reaction. Eu²⁺ single doped Sr₃GdLi(PO₄)₃F phosphors have an efficient excitation in the range of 230–430 nm, which is in good agreement with the commercial near-ultraviolet (n-UV) LED chips, and gives intense blue emission centering at 445 nm. The critical distance of the Eu²⁺ ions in Sr₃GdLi(PO₄)₃F is computed and demonstrated that the concentration quenching mechanism of Eu²⁺ is mostly caused by the dipole-dipole interaction. By co-doping Eu²⁺ and Mn²⁺ ions in the Sr₃GdLi(PO₄)₃F host, the energy transfer from Eu²⁺ to Mn²⁺ that can be discovered. With the increase of Mn²⁺ content, emission color can be adjusted from blue to white under excitation of 380 nm, corresponding to chromatic coordinates change from (0.189, 0.108) to (0.319, 0.277). The energy transfer from Eu²⁺ to Mn²⁺ ions is proven to be a dipole-dipole mechanism on the basis of the experimental results and analysis of photoluminescence spectra and decay curves. This study infers that the obtained Sr₃GdLi(PO₄)₃F:Eu²⁺, Mn²⁺ phosphors may be a potential candidate for n-UV LEDs.     

Synthesis and luminescence enhancement of red-emitting Sr9Y(PO4)7:Eu3+ phosphor through Gd3+ co-doping for white light-emitting diodes

A series of Sr₉Y(PO₄)₇:Eu³⁺ and Sr₉Y(PO₄)₇:Eu³⁺, Gd³⁺ red-emitting phosphors were prepared via a high-temperature solid-state method, Gd³⁺ ion was co-doped in Sr₉Y(PO₄)₇:Eu³⁺ as sensitizer to enhance the luminescence property. The X-ray diffraction results verify that the structure of the as-prepared samples is consistent with the standard Sr₉Y(PO₄)₇ phase. All the Sr₉Y(PO₄)₇:Eu³⁺ samples show both characteristic emission peaks at 594 nm and 614 nm under near-ultraviolet excitation of 394 nm. The co-doping of Gd³⁺ significantly improves the luminescence intensity of the Sr₉Y(PO₄)₇:Eu³⁺ phosphors due to the crystal field environment effect and energy transfer of Gd³⁺→Eu³⁺ caused by the introduction of Gd³⁺, especially Sr₉Y(PO₄)₇:0.11Eu³⁺, 0.05Gd³⁺, which emission intensity is higher than that of Sr₉Y(PO₄)₇:0.11Eu³⁺ by 1.21 times. The color purity and lifetime of Sr₉Y(PO₄)₇:0.11Eu³⁺, 0.05Gd³⁺ phosphor are 88.26% and 3.7615 ms, respectively. A w-LED device was packaged via coating the as-prepared phosphor on n-UV chip of 395 nm with commercial phosphors. These results exhibit that the Sr₉Y(PO₄)₇:Eu³⁺, Gd³⁺ red-emitting phosphor can be used as a red component in the w-LEDs application.     

The role of co-dopants on the luminescent properties of α-Al2O3:Mn4+ and BaMgAl10O17:Mn4+

Mn⁴⁺ doped aluminate materials with efficient red emission are promising components for warmer white light-emitting diodes. However, it still remains as a challenge on increasing its luminous efficiency. For Mn⁴⁺ doped aluminate phosphors, co-dopants such as Li⁺, Mg²⁺, Na⁺, Si⁴⁺, or Ge⁴⁺ ions are often added to tailor the photoluminescence properties of phosphors during preparing process. However, the role of the ions is still in debate. In this work we took BaMgAl₁₀O₁₇:Mn (BMA:Mn) and α-Al₂O₃:Mn as examples to study the effects of Li⁺, Mg²⁺, Na⁺, and Si⁴⁺ on their luminescent properties. The energy levels induced by the co-dopants and some possible intrinsic defects of hosts (Al₂O₃) were calculated using the first-principles method. It is found that the Mg²⁺ and Na⁺ ions, compared with Li⁺ and Si⁴⁺, can prefer to form hole-type defects which enhance the valence stability of Mn⁴⁺ and thus enhance the emission intensity of the as-prepared phosphors.     

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.     

Structure and photoluminescence properties of Ca0.99−xSrxAlSiN3:0.01Ce3+ solid solutions

Nitride phosphors of Ca_0.99−xSr_xAl_1.01Si_0.99N₃:0.01Ce³⁺ (0 ≤ x ≤ 0.9) were synthesized by conventional solid-state method. XRD data analysis shows that all samples are single phase with CaAlSiN₃-type structure. Under blue or near-ultraviolet (~400 nm) light excitation, the emission peak can be tuned largely from 615 to 568 nm by increasing Sr content, and the emission intensity is maximal at x = 0.8. With the Sr content increase, the emission band blue-shifts due to the decreased crystal field splitting and the reduced centroid shift; while the thermal luminescence quenching resistance is almost unchanged. The quenching temperature (T₅₀) is well above 500 K for all samples, which satisfies the requirement of commercial application. The quenching process is mainly attributed to the radiationless transitions by thermally activated crossover from the 5d excited state to 4f ground state in the configurational coordinate diagram. The luminescence properties show that the (Ca,Sr)AlSiN₃:Ce³⁺ phosphors are very promising for use in blue and near-ultraviolet light excited white-light-emitting diodes.     

Up-conversion luminescence and temperature-sensing properties of Li+, K+ doped Na2Zn3Si2O8: Er3+ phosphors

In this work we detail the preparation of new luminescent Li⁺ and K⁺ doped Na₂Zn₃Si₂O₈: Er³⁺ up-conversion phosphors using the high-temperature solid-phase method. We investigate the phosphors phase structure, elemental distribution, up-conversion luminescence characteristics and temperature sensing properties. Our fabricated samples were found to be homogeneous and when excited using 980 nm light, they emitted wavelengths in the green and red visible wavelength bands, which correspond to two major emission bands of Er³⁺. Doping with Li⁺ and K⁺ increased the luminescence intensity of the Na₂Zn₃Si₂O₈: Er³⁺ phosphor at 661 nm by 36 and 21 times respectively. The highest relative temperature sensitivity (Sa) of the fabricated phosphor reached a value of 19.69% K^?1 and the highest absolute temperature sensitivity (Sr) reached 1.20% K^?1. These values are superior to other materials which utilize up-conversion by Er³⁺ ions as a tool for temperature sensing. We anticipate that these new phosphors will find significant application as components in optical temperature measurement systems.     

Color tuning in (Ca1−xSrx)8MgLu(PO4)7:Eu2+,Mn2+ phosphor via host composition design and energy transfer

In this work, a series of Eu²⁺-doped (Ca_1−xSr_x)₈MgLu(PO₄)₇ and Eu²⁺/Mn²⁺-codoped Ca_6.5Sr_1.5MgLu(PO₄)₇ phosphors were prepared via the combustion-assisted solid-state reaction process. XRD patterns and Rietveld refinements were used to verify the incorporations of Sr into Ca₈MgLu(PO₄)₇:Eu²⁺. Upon the same excitation wavelength of 380 nm, the emission peaks of Eu²⁺-doped (Ca_1−xSr_x)₈MgLu(PO₄)₇ (0≤x≤1) phosphors red-shifted from 453 to 519 nm with increasing Sr/Ca ratio. The red-shift of the Eu²⁺ emission with increasing Sr/Ca ratio was ascribed to the change of Eu²⁺ emission at different lattice sites. With variation of the Mn²⁺ content, the emission color of Eu²⁺/Mn²⁺ codoped Ca_6.5Sr_1.5MgLu(PO₄)₇ phosphors exhibited the luminescence tunable from greenish blue to white and eventually to red. The energy transfer from Eu²⁺ to Mn²⁺ in Ca_6.5Sr_1.5MgLu(PO₄)₇ host matrix was demonstrated to be of a resonant type via a dipole- dipole mechanism with the critical distance of ∼16.7 Å. By the Sr substitution for Ca and properly tuning by the relative composition change of Eu²⁺/Mn²⁺, chromaticity coordinates of (0.329, 0.326) can be reached at near UV light excitation. The combination of host composition design and energy transfer may provide a novel strategy to obtain white light and tunable luminescence.     

Luminescence properties of Eu2+ and Mn2+ doped Sr1.7Mg0.3SiO4 phosphors

Eu²⁺, Mn²⁺ doped Sr_1.7Mg_0.3SiO₄ phosphors were prepared by high temperature solid-state reaction method. Their luminescence properties were studied. The emission spectra of Eu²⁺ singly doped Sr_1.7Mg_0.3SiO₄ consist of a blue band (455 nm) and a green band (550 nm). The relative intensities of two emissions varied with Eu²⁺ concentration. Eu²⁺ and Mn²⁺ co-doped Sr_1.7Mg_0.3SiO₄ phosphors emit three color lights and present whitish color. The blue (455 nm) and green (550 nm) emissions are attributed to the transitions of Eu²⁺, while the red (670 nm) emission is originated from the transition of Mn²⁺ ion. The results indicate the energy transfer from Eu²⁺ to Mn²⁺. The mechanism of the energy transfer is resonance-type energy transfer due to the spectral overlap between the emission of Eu²⁺and the absorption of Mn²⁺.     

Broadband emission phosphor Sr3Al2O5Cl2:Bi3+: Luminescence modulation and application for a white-light-emitting diode

Bi³⁺ ions can regulate and control the fluorescence of a phosphor by transferring energy to the activating agent or occupying different luminescent centers, which is important for modifying phosphors and revealing fluorescence mechanisms. As a base material, Sr₃Al₂O₅Cl₂ has three types of Sr sites (Sr 1, Sr 2, and Sr 3) that may be occupied by Bi³⁺ ions (Sr²⁺ has a similar radius to Bi³⁺). Herein, we successfully synthesized a series of Sr₃Al₂O₅Cl₂:x%Bi³⁺ phosphors using the high-temperature solid-state method and determined a two-site-occupying emission mechanism. X-ray diffraction patterns indicated that the samples were synthesized well, and Rietveld refinement results provided their structural information. Photoluminescence spectra showed 490 nm (λ_ex = 345 nm) and 556 nm (λ_ex = 376 nm) emission peaks, which might arise from different luminescent centers. The concentration quenching study, peak separation analysis, fluorescence lifetime spectra, and diffuse reflection spectra indicated that the Bi³⁺ ions occupied two of the three Sr sites. Calculations of relative system energies and distortion index proved that the occupation only occurred in the Sr 1 and Sr 3 sites, and crystal splitting analysis determined that Sr 1 site generated 490 nm emission light and Sr 3 site generated 556 nm emission light. The charge compensator and flux were added to enhance the fluorescence intensity of the phosphor, and 5% K⁺ along with 1% BaF₂ is the optimal dosage. Finally, the SrAlSiN₃:Eu²⁺, BaMgAl₁₀O₁₇:Eu²⁺, and optimized Sr₃Al₂O₅Cl₂:5%Bi³⁺ phosphors were combined as a luminous layer and a warm-white light-emitting diode was realized; the color rendering indices were 84.3, 85.8, 86.4, and 86.2 under working currents of 20, 30, 40, and 50 mA, respectively.     

A building-block strategy for dynamic anti-counterfeiting by using (Ba,Sr)Ga2O4:Sm3+ new red persistent luminescent phosphor as an important component

Long persistent luminescence materials developed to commercial standards are primarily concentrated in the blue and green regions, with only a few in the red region. Red, as one of the three basic colors, can be mixed in various proportions with blue and green to yield various colors. The development of red persistent phosphors has a broader application potential but remains a challenge. A solid-state reaction method was used to synthesize new red persistent luminescent materials of Ba_1-xSr_xGa₂O₄:Sm³⁺ (x = 0–0.09). In BaGa₂O₄, both Sr²⁺ and Sm³⁺ preferentially occupy the Ba²⁺ site rather than the Ga³⁺ site. When exposed to UV light at 254 nm, the phosphors emit the characteristic red emission of Sm³⁺ at wavelengths ranging from 500 nm to 750 nm. After removing the UV light source, an intense red afterglow that lasted more than 1400 s was observed. The red afterglow signal reappears after a heating process. Doping Sr²⁺ reduces the trap depth and improves the red persistent luminescence significantly. Because the escaped electrons from traps compensate for the emission loss of Sm³⁺ during the heating process, the red phosphors have unimaginably luminescent thermal stability. Thus, the emission intensity at 200 ^°C is 1.6 times that at room temperature. The prepared red persistent phosphors show multimode luminescence, with the output signal being time and temperature sensitive, indicating that they are potential luminescent materials for anti-counterfeiting applications. Finally, a building-block strategy for advanced anti-counterfeiting applications of dynamic display information is proposed, with red persistent phosphors serving as an important component combined with upconversion phosphors of NaYF₄:Yb³⁺, Tm³⁺, and green persistent phosphors of SrAl₂O₄:Eu²⁺, Dy³⁺.     

A single‐phase,thermally stable and color-tunable white light emitting Na2Ca1-x-yCexMnyP2O7 phosphors for white light emitting diodes via energy transfer

The solid-state reaction method was used to develop a series of Na₂Ca_1-x-yCe_xMn_yP₂O₇ phosphors in an H₂–N₂ environment. The crystal structure of the pyrophosphate host, valence state of dopants (Ce, Mn), emission behavior of dopants, energy transfer mechanism, and thermal quenching behavior were thoroughly examined. Doping with Ce³⁺ and Mn²⁺ ions enhanced the photoluminescence characteristics of Na₂Ca_1-x-yCe_xMn_yP₂O₇ while having negligible effect on the host's phase purity. Under 365 nm UV light irradiation, the addition of Ce³⁺ ion in the Na₂CaP₂O₇ host revealed an asymmetric band with the typical blue emission around 415 nm and a shoulder around 455 nm. To obtain white light, Mn²⁺ ion was supplementarily substituted to the present system. When the Mn²⁺ ions concentration was elevated in the Na₂CaP₂O₇ host, the emission intensity of 560 nm peak corresponding to Mn²⁺ transition enhanced significantly at the cost of Ce³⁺ emission of 415 nm. The systematic decrease of Ce³⁺ emission intensity and corresponding increase in the Mn²⁺ intensity with the increase in Mn²⁺ concentration indicated the possibility of effective energy transfer from Ce³⁺ to Mn²⁺ ions. The obtained results indicated that energy transfer from the Ce³⁺ to Mn²⁺ ions governed by dipole-quadrupole interaction. Because of the efficient energy transfer, the blue emission from Ce³⁺ and the orange red emission of Mn²⁺ provide white light from a single host along with high value of activation energy and low thermal quenching behaviour make the present phosphors to be suitable for high-power LEDs.     

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.     

Preparation,Design, and Characterization of the Novel Long Persistent Phosphors: Na2ZnGeO4 and Na2ZnGeO4:Mn2+

A novel blue‐emitting phosphor Na₂ZnGeO₄ and a novel green‐emitting phosphor Na₂ZnGeO₄:Mn²⁺ have been newly developed via high‐temperature solid‐state reaction. The crystal structure of Na₂ZnGeO₄ has been identified. Energy transfer from Na₂ZnGeO₄ host to Mn²⁺ ions was affirmed. Undoped and Mn²⁺‐doped Na₂ZnGeO₄ phosphors exhibit blue and green long persistent luminescence (LPL) with persistent duration more than 40 min and 4 h, respectively. The traps created in host lattice were clarified. The LPL mechanism in Na₂ZnGeO₄ and Na₂ZnGeO_4: Mn²⁺ was discussed briefly. This investigation provides two new and efficient long persistent phosphors (LPPs).     

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.     

Luminescence of MgAl2O4 and ZnAl2O4 spinel ceramics containing some 3d ions

The luminescence properties of ceramic phosphors based on two spinel hosts MgAl₂O₄ and ZnAl₂O₄ doped with manganese ions have been studied. It has been found that the spectral properties of these phosphors can be strongly varied by changing synthesis conditions. Both types of doped ceramic spinel can serve as efficient Mn²⁺ green-emitting phosphors having peak emissions at 525 and 510 nm, respectively. Mn-doped MgAl₂O₄ spinel can also be prepared as an efficient Mn⁴⁺ red-emitting phosphor having peak emission at ~651 nm by using specific temperatures of heat treatment in air. It has also been shown that the conversion of Mn²⁺ to Mn⁴⁺ and viсe versa, as well as the coexistence of Mn²⁺ green and Mn⁴⁺ red emissions, can be accomplished by properly chosen annealing conditions of the same initially synthesized MgAl₂O₄:Mn sample. Manganese doped MgAl₂O₄ spinel with an optimal intensity ratio of green and red emissions can be a promising single-phase bicolor phosphor suitable for the development of warm white phosphor-converted LED lamps. On the other hand, it has been determined that perfectly normal ZnAl₂O₄ spinel cannot be doped with Mn⁴⁺ ions in contrast to partially inverse MgAl₂O₄ spinel. However, ZnAl₂O₄ samples unintentionally doped with impurity Cr³⁺ ions show emission spectra in the far-red region with well pronounced R, N and vibronic lines of Cr³⁺ luminescence due to the perfect normal spinel structure of synthesized ZnAl₂O₄ ceramics. Also, by partially substituting Al³⁺ cations for Mg²⁺ in ZnAl₂O₄ there is an opportunity to obtain Mn⁴⁺ doped or Mn⁴⁺/Cr³⁺ codoped far-red emitting phosphors which can be suitable for indoor plant growth lighting sources.     

Cation vacancy repair towards a new yellow Ca7Sr3Na(PO4)7:Eu2+ phosphor

Cationic substitution is a prevalent strategy to tune the luminescence spectra of phosphors. In this work, we reported a series of Eu²⁺-activated whitlockite type Ca₇Sr_3.5-0.5xA_x(PO₄)₇ (CSPA; A =Li, Na, K) (x = 0–1.00) phosphors. The substitution by Na⁺ for both half occupied/vacant M(4) site was verified via Raman spectra, Reitveld refinement and HR-TEM, whereas a similar accommodation of K⁺ into the Ca₂Sr(PO₄)₂ (CSP) host cannot be realized due to the significant size mismatch. A continuous increase of Na⁺ contents led to the progressively structural contraction, promoting the migration of Eu²⁺ activator from looser M(4) to other sites, and regulating the luminescence behaviors. Consequently, the gradual red-shift of emission band terminated at a new yellow phosphor Ca₇Sr₃Na(PO₄)₇:0.04Eu²⁺. The cation vacancy repair developed in this work can not only migrate the Eu²⁺ activator among different cation sites, but also serves as a new strategy for tuning the luminescence properties of phosphor.