Near-infrared photo-responsive Er3+-K+ co-doped Y2O3 phosphors with enhanced UC luminescence

Y₂O₃: x% Er³⁺ (x=5, 7, 10, 12, 15) and Y₂O₃: 10% Er³⁺，x% K⁺ (x=0, 1, 3, 5, 7, 10, 15) phosphors were successfully prepared by a low-temperature combustion method. The structure as well as the absorption/emission spectra of phosphors were investigated. The effect of doping concentration of K⁺ ions on the upconversion (UC) luminescence of Y₂O₃: 10% Er³⁺ phosphor was examined and the possible optical transitions were discussed. The results showed that K⁺ ion doping not only changed the microstructure and crystallinity of the phosphors, but also enhanced its UC luminescence intensity. The Y₂O₃: 10% Er³⁺, 7% K⁺ phosphor exhibit the strongest UC emission intensity. Compared with the Y₂O₃: 10% Er³⁺ phosphor, the UC luminescence intensity at 563 nm and 661 nm was enhanced by 67.8 and 27.3 times for the K-codoped samples, respectively. The phosphor with the optimal doping concentration was mixed with a polymer to form a composite film, which was employed for the fabrication of near-infrared (NIR) photo-responsive detection devices. The device exhibited strong photo-current response to NIR light at 980 nm, implying that our work could inspire new design strategy for the development of NIR photo-detection devices.     

Enhancement of dual-mode emission and temperature sensing performance in Y2Zr2O7: Er3+ nano phosphor by incorporation of lithium ions

The present work has been planned with the primary objective to study the effect of Li⁺ doping on photoluminescence (PL) emission intensity and temperature sensing performance of Y₂Zr₂O₇(YZO): Er³⁺ phosphors. The hydrothermal method was employed to synthesize the YZO: 4Er³⁺, xLi⁺ (x = 0, 3, 5, 10 mol%) phosphors. The formation of the phase and the crystallinity of the prepared samples were examined from the XRD results. The cell parameters were estimated from Rietveld refinement. The surface morphology and elemental analysis were studied using the FESEM and EDX techniques. UV–Vis–NIR diffuse reflectance spectroscopy was utilized to find the optical band gap of the prepared samples. The Li⁺ doped sample exhibits better optical absorption than the sample without Li⁺ ions. The FTIR spectroscopy confirms the presence of the desired functional groups within the samples. XPS measurements were performed to find the bonding state of the compositions. Photoluminescence down-conversion and up-conversion measurements were carried out under 378 nm and 976 nm excitation, respectively. The optical thermometry of the prepared phosphors was investigated within the temperature range 303K–630K. The reported phosphor shows a significant amount of intensity enhancement after Li⁺ doping in both the down conversion and up conversion processes. In general, the charge compensation effect is used to explain this type of result. As the phosphor is already charge balanced, the phenomenon mentioned above cannot be considered. We have explained the various contributing factors responsible for the changes in intensity and correlated them with the different experimental results collected by characterizing the prepared samples. Overall, the obtained results suggest that the reported phosphor may act as multifunctional material.     

Alkali metal ion substitution induced luminescence enhancement of NaLaMgWO6:Eu3+ red phosphor for white light-emitting diodes

Alkali metal ion substitution is an effective strategy to improve the luminescence properties of phosphors. In this work, a series of red-emitting phosphors Na_1-xLi_x/2K_x/2La_0.6Eu_0.4MgWO₆ were prepared by a traditional high-temperature solid-state reaction. Their phase structure, microstructure, luminescence properties and potential application in phosphor-converted white light-emitting diodes (pc-WLEDs) were investigated in detail. X-ray diffraction (XRD) result revealed the formation of a solid solution when x?≤?0.3, which kept monoclinic structure of NaLaMgWO₆. Photoluminescence investigation indicated that the partial substitution of Li⁺/K⁺ ions for Na⁺ ions improved largely the red emission of Eu³⁺. Based on the optimized Na_0.7Li_0.15K_0.15La_0.6Eu_0.4MgWO₆ sample with relatively good thermal stability, a WLED device was fabricated by combining a near-ultraviolet (NUV) chip (~400?nm) with the phosphor mixture of commercial green/blue phosphors and the optimized red phosphor. The results indicated that the optimized red phosphor in this work could be a potential candidate for WLEDs pumped by NUV chips.     

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.     

Visible green upconversion luminescence of Er3+/Yb3+/Li+ co-doped CaWO4 particles

The upconversion (UC) luminescence of Li⁺/Er³⁺/Yb³⁺ co-doped CaWO₄ phosphors is investigated in detail. Single crystallized CaWO₄:Li⁺/Er³⁺/Yb³⁺ phosphor can be obtained, co-doped up to 25.0/5.0/20.0 mol% (Li⁺/Er³⁺/Yb³⁺) by solid-state reaction. Under 980 nm excitation, CaWO₄:Li⁺/Er³⁺/Yb³⁺ phosphor exhibited strong green UC emissions visible to the naked eye at 530 and 550 nm induced by the intra-4f transitions of Er³⁺ (²H_11/2,⁴S_3/2→⁴I_15/2). The optimum doping concentrations of Yb³⁺/Li⁺ for the highest UC luminescence were verified to be 10/15 mol%, and a possible UC mechanism that depends on the pumping power is discussed in detail.     

Enhanced narrow green emission and thermal stability in γ-AlON: Mn2+, Mg2+ phosphor via charge compensation

A series of novel green emitting γ-AlON: Mn²⁺, Mg²⁺ and γ-AlON: Mn²⁺, Mg²⁺, M (M?=?Li⁺, Na⁺, K⁺, Si⁴⁺) phosphors were fabricated with the gas-pressure sintering reaction process. The phase structure, morphology and photoluminescence properties of the phosphors were characterized via X-ray powder diffraction, scanning electron microscopy, and photoluminescence spectroscopy. Meanwhile, the charge compensation methods were utilized to eliminate the disadvantage of charge imbalance between Al³⁺ and Mn²⁺. The results show that the luminescence intensity of Mn²⁺ was maximally enhanced by the introduction of Si⁴⁺ ions, which was 2.36 times that of the sample without charge compensator. Moreover, as the temperature reached at 150?°C, thermal stability of the samples contained charge compensator Li⁺ and Si⁴⁺ were improved to 93% and 90% of that the room temperature, respectively, while the original sample was 85%. These luminescence properties were enhanced due to the introduction of charge compensators which reduce defects caused by charge imbalance. In addition, the specific mechanisms were discussed in detail. In general, the charge compensation could be used as an effective strategy to strengthen thermal stability and luminescence performances of phosphors.     

Enhancing upconversion emission and temperature sensing modulation of the La2(MoO4)3: Er3+, Yb3+ phosphor by adding alkali metal ions

Trivalent Er³⁺-doped La₂(MoO₄)₃ upconversion phosphors with intense green emmision were synthesized at 800 °C by the solid-state reaction route, promoting the development of novel optical thermometry. The color emitted from the samples was minorly affected by the excitation power and doping concentration. Yb³⁺ is a better sensitizer for the La₂(MoO₄)₃: Er³⁺ phosphor and it can enhance the emission intensity when a certain amount is co-doping in the system. The up-conversion luminescent mechanism was investigated using the pump power-dependent UC emission spectra. Alkali metal doping increased the up-conversion emission intensities drastically, and Li⁺ ions can enhance the luminous intensity by more than 20 times. The fluorescence intensity ratio of the transition emission ²H_11/2-⁴I_15/2 and ⁴S_3/2-⁴I_15/2 was used to study upconversion optical temperature sensing. The sensitivity changes from doping with diverse alkali metal ions and their effects on the optimal temperature range are discussed in detail. Alkali metal ions doping extended the temperature range, indicating that this phosphor is a potential candidate for temperature-sensing probes.     

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.     

Simultaneous enhancement of quantum cutting luminescence in Er-doped NaBaPO4 phosphors by crystal field control and plasmonic modulation

It is of great significance to enhance the quantum-cutting (QC) luminescence for practical applications due to the narrow absorption cross-section and low luminescence efficiency of rare earth ions. In this work, NaBaPO₄:Er³⁺ phosphors doped with Li⁺ were synthesized through a solid-state reaction. The QC luminescence of NaBaPO₄:Er³⁺ phosphor was enhanced 5.71 times by doping Li⁺. XRD patterns and Judd-Ofelt calculations demonstrated the crystal field distortion when introduced Li⁺, which would increase the transition probability of Er³⁺. Furthermore, NaBaPO₄:Er³⁺, Li⁺ phosphors were decorated with silver nanoparticles (Ag NPs). The effect of Ag NPs on QC luminescence was studied, and the results showed that QC luminescence was further enhanced up to 1.95 times by Ag NPs. FDTD simulations revealed that Ag NPs generated substantial surface plasmons, which would boost the excitation rate of Er³⁺. Our studies would provide a useful strategy to enhance QC luminescence, which has potential application in germanium-based solar cells.     

Dual-mode optical thermometry and multicolor anti-counterfeiting based on Bi3+/Er3+ co-activated BaGd2O4 phosphor

Here, Bi³⁺, Er³⁺ co-activated gadolinium phosphors with multimode emission properties are prepared, which can emits blue, green, and orange light under the excitation of ultraviolet, 980 and 1550 nm, respectively. Moreover, BaGd₂O₄:Bi³⁺, Er³⁺ can show multicolor luminescence under different excitation conditions, such as pump light source, ambient temperature, working current, and other factors. Based on the dynamic luminescence characteristics, the dynamic anti-counterfeiting experiments are designed based on the phosphor. At the same time, the material also shows multimode temperature sensing characteristics. Under the excitation of 980 nm laser, three strong up-conversion signals Er³⁺ ions are generated at 528 nm (²H_11/2), 555 nm (⁴S_3/2), and 668 nm (⁴F_9/2), which have different temperature dependences. Based on the fluorescence intensity ratio between thermal-coupled energy levels (²H_11/2/⁴S_3/2) and nonthermal-coupled energy levels (²H_11/2/⁴F_9/2) of Er³⁺ ions, respectively, the dual-mode temperature thermometer was constructed with high-temperature sensitivity. In addition, the fluorescence lifetime of Bi³⁺ ions also has a strong temperature dependence, which can be used as another temperature detection signal, greatly improving the stability of thermometers under harsh conditions. Therefore, the material has a bright prospect in the field of anti-counterfeiting and temperature sensing.     

Optical temperature sensor based on upconversion luminescence of Er3+ doped GdTaO4 phosphors

For the development of optical temperature sensor, a series of GdTaO₄ phosphors with various Er³⁺-doping concentrations (0, 1, 5, 10, 25, 35, 50 mol%) were synthesized by a solid-state reaction method. The monoclinic crystalline structure of the prepared samples was determined by X-ray diffraction (XRD). Under excitations of 980 and 1550 nm lasers, the multi-photon-excited green and red upconversion (UC) luminescence emissions of Er³⁺ were studied, and the critical quenching concentration of Er³⁺-doped GdTaO₄ phosphor was derived to be 25 mol%. By changing the pump power of laser, it was found that the two-photon and three-photon population processes happened for the UC emissions of Er³⁺-doped GdTaO₄ phosphors excited by 980 and 1550 nm lasers, respectively. Furthermore, based on the change of thermo-responsive green UC luminescence intensity corresponding to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺ with temperature, the optical temperature sensing properties of Er³⁺-doped GdTaO₄ phosphor were investigated under excitations of 980 and 1550 nm lasers by using the fluorescence intensity ratio (FIR) technique. It was obtained that the maximum absolute sensitivity (S_A) and relative sensitivity (S_R) of Er³⁺-doped GdTaO₄ phosphors are as high as 0.0041 K⁻¹ at 475 K and 0.0112 K⁻¹ at 293 K, respectively. These significant results suggest that the Er³⁺-doped GdTaO₄ phosphors are a promising candidate for optical temperature sensor.     

A Novel Multifunctional Upconversion Phosphor: Yb3+/Er3+ Codoped La2S3

Yb³⁺/Er³⁺ codoped La₂S₃ upconversion (UC) phosphors have been synthesized using high‐temperature solid‐state method. Under 971‐nm excitation, the maximum luminescence power can reach 0.64 mW at the excitation power density of 16 W/cm² and an absolute power yield of 0.36% was determined by an absolute method at the excitation power density of 3 W/cm², and the quantum yield of La₂S₃:Yb³⁺, Er³⁺ (green ~0.18%, red ~0.03%, integration ~0.21) was comparable to that of NaYF₄:Yb³⁺, Er³⁺ nanocrystals (integration ~0.005–0.30). Frequency upconverted emissions from two thermally coupled excited states of Er³⁺ were recorded in the temperature range 100–900 K. The maximum sensitivity of temperature sensing is 0.0075 K^?1. As the excitation power density increases, the temperature of host materials rapidly rises and the top temperature can reach to 600 K. Given the intense UC emission, high sensitivity, as well as good photothermal stability, La₂S₃:Yb³⁺/Er³⁺ phosphor can become a promising composite material for photothermal ablation of cancer cells possessing the functions of temperature sensing and in vivo imaging.     

Effects of Er3+ concentration on down-/up-conversion luminescence and temperature sensing properties in NaGdTiO4: Er3+/Yb3+ phosphors

Usually, Er³⁺ doping concentration effect on the temperature sensing properties of Er³⁺ containing materials is ignored. In this work, we demonstrated the influence of Er³⁺ concentration and excitation path on the spectral and temperature sensing properties in Er³⁺, Yb³⁺ co-doped NaGdTiO₄ system. The NaGdTiO₄: Er³⁺/Yb³⁺ phosphors were prepared by a high temperature solid state reaction method. Different spectral patterns for down- and up-conversion processes were observed and ascribed to the different excitation and population routes. The concentration quenching behaviors for down- and up-conversion processes were explained via cross relaxations between Er³⁺ ions. Most importantly, the Er³⁺ concentration dependent optical temperature sensing performance was observed and experimentally explained as a fact that the optical transition rate of Er³⁺ in different samples was changed with various Er³⁺ doping concentration.     

Broad-scope dual-mode modulation thermometry based on up-conversion phosphor GaNbO4: Yb3+/Er3+

Fluorescence temperature measurement technology has set off another upsurge in non-contact temperature measurement, but still suffers from the large error for single-mode thermometry. Herein, in a broad temperature range of 93–633 K, a dual-mode modulation thermometry based on up-conversion phosphor of GaNbO₄:Yb³⁺/Er³⁺ is realized with the maximum relative sensitivity (S_r) of 11.7% K⁻¹ (93 K) and 13.1% K⁻¹ (123 K), respectively. GaNbO₄:Yb³⁺/Er³⁺ phosphors were synthesized by high temperature solid-state method. The structure, surface morphology and the optical properties were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL). The fluorescence intensity ratio (FIR) readout method based on Er³⁺ thermal-coupled energy level (TCL) and non-thermal-coupled energy level (NTCL) was used to achieve the dual-mode temperature measurement with high temperature resolution and good repeatability in GaNbO₄:5 mol% Yb³⁺ and 5 mol% Er³⁺ phosphors. All the results show that GaNbO₄:Yb³⁺/Er³⁺ phosphors have great application potential in high sensitivity broadband thermometry.     

Design of a europium-activated lithium-silicate cyan phosphor via cation substitution

Lithium-containing silicates have been considered as a considerable alternative for luminescent materials. In this study, a novel cyan-emitting phosphor, Na₃LiHf₂Si₃O₁₂: Eu²⁺, was successfully synthesized via cationic substitution with Na₄Hf₂Si₃O₁₂: Eu²⁺ as the initial model. The crystal structure, morphology, and luminescence performance of Na_4-xLi_xHf₂Si₃O₁₂: Eu²⁺ were investigated in detail. The substitution of Li⁺ for Na⁺ site causes a significant blue-shift of the emission band in the range of 550–500 nm and a smaller full width at half maximum. As a result, a cyan phosphor Na₃LiHf₂Si₃O₁₂: Eu²⁺ that can be effectively excited by near-ultraviolet and high-energy beams is obtained. The mechanism of emission regulation was proposed based on the transformation of crystal structure and luminescence performance. In addition, the thermal quenching and cathodoluminescence behaviors were also studied. The results show that cation substitution is an effective method to design new lithium-containing silicate phosphors.     

Manipulation of the ratio of Li+/Zn2+ on the structure and persistent luminescence property of Li2Zn0.9992Ge3O8:0.08%Cr3+

Long persistent luminescence (LPL) materials have been widely applied and investigated in the fields of night-safe, bio-fluorescent labeling, and optical anti-counterfeiting because of their unique properties of delayed luminescence. In this work, the focus of this research is to significantly improve the LPL properties of Li₂Zn_0.9992Ge₃O₈:0.08%Cr³⁺ by adjusting the ratio of Li⁺/Zn²⁺. On the one hand, when Li⁺:Zn²⁺ < 2:1 (Li_1.97Zn_1.0292Ge₃O₈:0.08%Cr³⁺), a deep-red LPL is produced in the 650–900 nm band for more than 50 h, which is 2.5 times longer than that without regulation. From another perspective, when Li⁺:Zn²⁺ > 2:1 (Li_2.01Zn_0.9892Ge₃O₈:0.08%Cr³⁺), the intensity of LPL is enhanced about four times compared to unregulated. The relationship between traps and LPL was comprehensively investigated by analyzing thermoluminescence spectra. Biological tissue penetration experiments were performed and a set of anti-counterfeit labels was designed. This research will provide guidance for the design of a novel persistent phosphor for the desired trap depth.     

Enhanced green upconversion luminescence properties of Er3+/Yb3+ co-doped strontium gadolinium silicate oxyapatite phosphor

Upconversion Sr₂(Gd_.98-xEr_.02Yb_x)₈Si₆O₂₆ (SGSO:2Er³⁺/xYb³⁺) phosphor materials were synthesized using a citrate sol-gel process. X-ray diffraction patterns confirmed their hexagonal structure. Field emission scanning electron microscopy images of SGSO:2Er³⁺/xYb³⁺ phosphors depicted submicron particles. The enhanced upconversion luminescence properties of SGSO:2Er³⁺/xYb³⁺ phosphors were analysed as a function of Yb³⁺ ion concentration and laser power. The energy transfer induced enhanced emission of the Er³⁺/ Yb³⁺ ions co-doped SGSO phosphors was ascribed to multi-phonon relaxation. The calculated chromaticity coordinates of the SGSO:2Er³⁺/xYb³⁺ phosphors showed emissions could be tuned by changing Yb³⁺ ion concentration. Optimized sample exhibited the chromaticity coordinate values near to the ultra-high definition television standard green emission coordinates.     

Tuning the luminescence properties of Mn4+-activated CaYAlO4 phosphor by co-doping cations for indoor plant cultivation

To fulfil the demands of high-power plant growth lamps, cation co-doping is an effective way to tune the photoluminescence properties of manganese (Ⅳ)-activated aluminate phosphors. Therefore, we managed to synthesize a series of cations co-doped CaYAlO₄:xMn⁴⁺, mSr²⁺, M⁺ (M⁺ = Li⁺, Na⁺, and K⁺) (CYAO:Mn, Sr, M) far-red-emitting phosphors. The excitation spectrum of these phosphors contained two excitation bands, and the opposite effects of these two bands on the luminescence intensity have been observed with the increase of Mn⁴⁺ concentration. By adding 0.1 mol Sr²⁺ ions to replace Ca²⁺ site, the emission intensity and thermal stability of CYAO:Mn phosphors can be enhanced. Furthermore, the luminescence properties of CSYAO:Mn can be further improved by co-doping monovalent alkali metal ions to serve as charge compensators, the increased number of Mn⁴⁺ luminescence centers. Moreover, 0.6 mol% Na⁺ can increase the initial emission intensity of the phosphors by 117% as the best ratio. The characteristic emission spectrum of the phosphors CYAO:Mn,Sr,M correspond to the phytochrome P_FR of plants. These experiments and characterization results have certified that these phosphors have a potential application in indoor plants cultivation.     

Preparation,luminescence properties,and application of temperature sensing and anti-counterfeiting of La2MgTiO6:Yb3+/Ln3+/Mn4+

The doping of transition metal ions in the up-conversion (UC) luminescent material doped with Yb³⁺/Ln³⁺ is a facile way to increase their UC luminescence intensities and alter their colors. In this study, La₂MgTiO₆:Yb³⁺/Mn⁴⁺/Ln³⁺ (Ln³⁺ = Er³⁺, Ho³⁺, and Tm³⁺) phosphors showing excellent luminescence properties were prepared by a solid-state method. The sensitivity of the La₂MgTiO₆:Yb³⁺/Ln³⁺/Mn⁴⁺ phosphor was double that without Mn⁴⁺, because Mn⁴⁺ affects the UC emissions of Ln³⁺ via energy transfer between these ions. Moreover, Mn⁴⁺ also acts as a down-conversion activator, which can combine with UC ions to achieve multi-mode luminescence at different wavelengths. Under 980 nm excitation, these samples emit green light (from Er³⁺ and Ho³⁺) and blue light (from Tm³⁺). In contrast, under 365 nm excitation, they emit red light (from Mn⁴⁺). Further testing revealed that the La₂MgTiO₆:Yb³⁺/Mn⁴⁺/Ln³⁺ phosphors have potential applications in temperature sensing and anti-counterfeiting.     

The effect of Li+ incorporation in Yb3+-Nd3+ co-doped CaF2 phosphors over the NIR photoluminescence emission excited under visible light

The structural and optical characteristics of Nd³⁺-Yb³⁺ doped CaF₂ phosphors with and without the addition of Li⁺ ions are described in this work. The phosphors synthetized by hydrothermal and co-precipitation methods showed near-infrared (NIR) luminescence emission associated with inter-electronic transition of the Yb³⁺ ion in the range of 900–1050 nm via energy transfer process from Nd³⁺ ions under visible light excitation. The addition of Li⁺ to these phosphors resulted in an improvement of the NIR luminescence intensity by a factor up to 5. The effect of the incorporation of Li⁺ ions into the CaF₂ crystallite structure, the reduction of luminescence quenching states, as well as the energy transfer mechanism involved are discussed.