Crystal Field Strength Research Articles

Pure-red phosphor Ca3Si2O7:Eu2+ obtained by the CO reduction strategy

The discovery of pure-red emitting phosphor is great meaningful for the achievement of high color rendering index white light emitting diodes, but still a challenge due to the strict crystal field limitations. In this work, we demonstrate the preparation of a 630 nm pure-red emitting Ca3Si2O7:Eu2+ phosphor for the first time through a CO reduction strategy. The changes in crystal field strength caused by different methods for the preparation of Ca3Si2O7:Eu2+ are investigated and discussed based on the understanding of the thermodynamic theory and structure resolution. Finally, a white light-emitting diode with a high color rendering index (Ra = 98) is accomplished by combining the as-prepared Ca3Si2O7:Eu2+ pure-red emitting phosphor with a 375 nm chip and the commercial used blue and green phosphors. The results also demonstrate the generalizability of the weak reduction strategy for the exploration of “redder” Eu2+ doped phosphors.

Solid solution structural engineering enhances the luminescence of SrMgAl10O17:Cr3+ for agricultural lighting

Lead-free non-rare earth oxide phosphors have attracted wide attention due to their environmental protection, sustainability, and potential to replace halides and fluorides in the field of plant lighting. Among them, the Cr3+-excited aluminate phosphor exhibits high brightness, high thermal stability, and far red to near-infrared (NIR) emission due to the influence of the crystal field strength (CFS). This property gives rise to a variety of strategies used to modulate the CFS, for example, single ion substitution, chemical unit co-substitution, etc. Here, we chose the substitution of a single ion, with [BaO6] gradually replacing [SrO6] to form a solid solution. Their structural characteristics and the local structure of Cr3+ are studied and discussed. The device is packaged to evaluate the feasibility of the material for practical application. The prepared phosphor had a bright far-red light emission of 693 nm under blue light excitation, and this spectrum strongly matched the absorption of plant phytochrome PFR. This work provides the design principle of far red light emission activated by Cr3+ aluminate solid solution, which can inspire further research on pc-LED lights for plant lighting.

Valence state conversion of Mn ions in Li2O–ZnO-GeO2 glass-ceramics: Spectral, structural, ESR and XRF studies

Lithium-zinc-germanate glass-ceramics doped with Mn ions are synthesized by volume crystallization method. Structural studies show nucleation of different forms of lithium germanate crystals depending on the isothermal treatment regime of the initial glass. With the increase of the heat treatment temperature and the lithium content in the initial glass, the [GeO4] /[GeO6] ratio in the nucleated crystalline phases increases. The initial distribution of Mn2+/Mn3+/Mn4+ is defined by the Li2O/ZnO ratio in the glass composition: the ratio growth increases the contribution of Mn4+ over the Mn2+ and Mn3+. Low-temperature heat treatment of initial glass leads to uprise of intense red emission of Mn4+ ions in the octahedral environment. High-temperature heat treatment leads to occurrence of intense green emission related to Mn2+ ions in a tetrahedral environment. Discussions on all transformations of symmetry, the crystal field strength of Mn ions’ environment and their valence state based on the results of optical spectroscopy, ESR and XRF studies are provided. The maximum quantum yield of red luminescence is 61%, and of green luminescence is 23%. The synthesized glass-ceramics can be used as a luminescent converter of the UV LED radiation with the maximum energy efficiency of 20%.

Modeling the Zigzag Curves of Transition Metal Ions' Charge Transition Levels in Crystals.

Predicting the defect levels of transition metal (TM) dopants in the band gap of crystals is critical in determining the charge states of TM dopants and explaining their electronic and optical properties. By analyzing the calculated charge transition levels and the crystal-field strengths of all the 3d-TM ions in several insulators, we demonstrate that the variation trend of the 3d-TM dopants in a crystal is a scaling of the variation of 3d-electron binding energies (ionization potential) of the free TM ions corrected by adding the contribution of the 3d-orbital's crystal-field splitting. We therefore develop a model to predict the relative location of TM ions' defect levels in the band gap from the defect level and crystal-field splitting of a reference TM ion in the host of concern. The model is applied to predict the defect levels of the series of TM ions in β-Ga2O3 and ZnO, which have moderate to small band gaps, making some of the levels fall into the conduction or valence bands. These results show that the model may serve as a quick reference for related material design and optimization.

Host Dependency of Boundary between Strong and Weak Crystal Field Strength of Cr3+ Luminescence.

Cr3+ doped near-infrared phosphors hold significant applications and generate considerable research interest. The critical parameter for assessing the strength of the crystal field for Cr3+ in the Tanabe-Sugano diagram is the boundary value of Dq/B, representing the ratio of crystal field splitting to the Racah parameter B. Nevertheless, there are conflicting values for this parameter, as reported in various studies, such as 2.1, 2.2, and 2.3 for C/B = 4.5-4.8. Moreover, some Cr3+ doped phosphors with wide-band emissions exhibit a Dq/B value that falls within the region of a contradictory strong field. In this study, we numerically determine the boundary value of Dq/B, which distinguishes between strong and weak fields. The results then demonstrate a dependence on the host material and are correlated with the values of Racah parameters B and C. This work resolves the inconsistency between the boundary values of Dq/B and the emission profile of Cr3+, providing researchers with a more profound comprehension of Cr3+ luminescence.

Unveiling Local Electronic Structures of Manganese‐Doped Double Perovskites Red Emitters

AbstractOver the past few years, light‐emitting diodes (LEDs) have emerged as an efficient replacement for standard incandescent and fluorescent lamps, due to their superior efficiency, energy conservation, service longevity, reliability, and affordability. However, identifying a red emitter for white LEDs has presented significant challenges. This study seeks to synthesize and analyze a novel red emitter, investigating its potential use in white LEDs and indoor plant growth lighting. The Sr2Lu(Nb,Mn)O6 is manufactured employing the traditional high‐temperature solid‐phase method. The internal and external quantum efficiencies (IQE and EQE) of double perovskites reached 32.39% and 21.67%. The g value calculates to 2.146, which corroborates that the magnetic dipolar transitions of Mn4+ ions inhabit symmetric octahedral sites. First‐principles investigations are utilized to unveil local electronic structures. Emission spectra reveal a substantial broad red emission peak at 688 nm. The Tanabe–Sugano diagram is applied to understand the luminescence mechanism. Additionally, the evaluation of parameters such as the internal UV emission ratio, crystal field strength, and Racah parameters is conducted. The luminescent intensity of the Sr2Lu(Nb,Mn)O6 can still be maintained at 60% upon reaching a temperature of 423 K. The findings suggest that Sr2Lu(Nb,Mn)O6 offers a novel alternative for white LEDs and indoor plant growth lighting.

Bandgap engineering—Influence of the evolution of (Zn/Mg)Al2O4:Cr3+ spinel phosphors on the photoluminescence properties

AbstractAs a luminescent center ion in materials for deep red and near‐infrared light, Cr3+ has been extensively studied in octahedral host materials. In this study, using zinc aluminate (ZnAl2O4) and magnesium aluminate (MgAl2O4) spinels with abundant octahedra as substrates, a series of Zn1−xMgxAl2O4:0.5%Cr3+ (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) fluorescent powders were first prepared via a high‐temperature solid‐state method. The influence of different Mg/Zn ratios on (Zn/Mg)Al2O4:0.5%Cr3+ optical properties was thoroughly explored. Experimental results show that an increase in the Mg/Zn ratio reduces the crystal field strength (Dq) and leads to distortion of the [AlO6] octahedra, resulting in broadening of the photoluminescence emission spectrum. Furthermore, the addition of Mg gradually reduces the formation of inverse spinel. An appropriate Mg/Zn ratio can improve luminescent intensity and quantum efficiency. In summary, this paper, through bandgap engineering by adjusting the Mg/Zn ratio, provides a detailed account of the changes in optical properties and the underlying mechanisms during the transition from ZnAl2O4:0.5%Cr3+ to MgAl2O4:0.5%Cr3+ spinels. It offers valuable insights for further research on the practical applications of Cr3+ in areas such as lighting displays and bioimaging.

Defect assisted self‐reduction and post annealing of Mn ions activated garnet transparent ceramics

AbstractTransition‐metal Mn2+ ion‐doped transparent ceramics (TCs) are promising color‐convertors to improve color rendition for solid‐state lighting. However, the stabilization of Mn2+ remains a challenge owing to the changeable atmosphere in the specific preparation of TC. In this work, the variations of valence states of Mn ions in typical garnet lattice host (YAG and LuAG) throughout the TC fabrication were demonstrated. By combined first principles calculation and experimental investigation, an enhanced self‐reduction of Mn ions was clarified in LuAG host, which was originated from the higher concentration of oxygen vacancy defects. Moreover, the air annealing treatment could significantly weaken and enhance the absorption of Mn2+ and Mn3+ respectively, degrading the orange–red emission peaking at 594 nm. On account of the oxidation of Mn2+ ions, a photoluminescence spectra redshift of 9 nm was observed with increasing air annealing temperature and oxygen content, owing to strengthened crystal field strength surrounding Mn2+ ions. Finally, a high color rendering index of 74.5 of resultant white laser diodes device structured by single‐phase LuAG:Ce,Mn TCs was obtained based on an efficient energy transfer process from Ce3+ to Mn2+. This study will deepen the understanding of the crystal defects inside TCs on luminescence properties, as well as inspiring more exploration on defect control to develop high performance ceramic color‐convertor.

Tailoring Ultra‐Wide Visible‐NIR Luminescence by Ce3+/Cr3+/Yb3+‐alloying Sc‐Based Oxides for Multifunctional Optical Applications

AbstractVisible‐to‐near‐infrared (VIS‐NIR) luminescent materials are in great demand in the field of non‐destructive testing such as component determination and hyperspectral imaging. Although Cr3+‐activated phosphors are widely reported, controllable tailoring ultra‐wide VIS‐NIR luminescence excited by blue light is still a challenge. The strategies of cationic substitution and energy transfers are effective for adjusting the luminescence of Cr3+‐activated phosphors. In this work, a series of Cr3+‐doped Sc‐based solid solution phosphors (Ba3‐mSrmSc4O9:Cr3+) are reported. Under the excitation of blue light, these phosphors exhibit broadband emission due to the different luminescence centers induced by Cr3+ occupying different cationic sites. Because of the weaker crystal field strength, Cr3+ realizes a broadband emission with a longer peak position (λem = 820 nm) and broader full width at half maximum (FWHM≈182 nm) in Ba2SrSc4O9. Furthermore, Ce3+/Yb3+ ions are introduced into Ba2SrSc4O9:Cr3+, achieving an ultra‐wide VIS‐NIR luminescence (460–1200 nm) by the Ce3+→Cr3+→Yb3+ multiple energy transfers. Designing energy transfers is beneficial to improve the quantum efficiency and weaken the thermal quenching. Finally, the NIR phosphor‐converted light‐emitting diode (pc‐LED) fabricated by Ba2SrSc4O9:Cr3+ demonstrates great potential in night‐vision and water component detection. This work provides an effective design idea for controllable tailoring ultra‐wide VIS‐NIR luminescence by chemical substitution and energy transfer.

Fabrication, microstructure, spectral properties, and laser performance of Yb:GdxY3−xAl5O12 ceramics

AbstractHighly transparent 10 at.% Yb:GdxY3−xAl5O12 (x = 0.3, 0.5, and 0.7) ceramics were successfully fabricated by vacuum solid‐state reactive sintering at 1800°C for 20 h combined with hot isostatic pressing posttreatment at 1700°C for 3 h under 176 MPa pressure in Ar atmosphere. The in‐line transmittances of these ceramics with the thickness of 2.5 mm are 84.4%, 84.0%, and 83.5% at 1100 nm for x = 0.3, 0.5, and 0.7, respectively. The relationships of the Gd3+ content among the phase, microstructures, spectral, and laser properties of the Yb:GdxY3−xAl5O12 ceramics were elucidated in detail. The introduction of Gd3+ has a little influence on the crystal field strength of the Yb3+ sites in GdxY3−xAl5O12. The absorption, emission spectra, and fluorescence lifetime of the Yb3+ in these ceramics are similar, close to 10 at.% Yb:YAG ceramics. Quasi‐continuous wave laser outputs of the ceramics were also achieved. The maximum laser output powers are 7.8, 7.7, and 6.6 W with the corresponding slope efficiency as 46.2%, 44.7%, and 39.2% for x = 0.3, 0.5, and 0.7, respectively. The 7.8 W laser output power is the highest value achieved so far for the Yb:GdYAG ceramics, to the best of our knowledge. The previous discussions show that not all the mixed garnets could enhance full width at half maximum of absorption and emission peaks for the active ions; however, the Yb:GdYAG ceramics are still excellent laser gain media.

Spectroscopy of thulium ions in solid-solution sesquioxide laser ceramics: Inhomogeneous spectral line broadening, crystal-field engineering and C3i sites

We present a detailed spectroscopic study of Tm3+ ions in stoichiometric and “mixed” (solid-solution) cubic (C-type, sp. gr. Ia3‾) sesquioxides R2O3 (RY, Lu, Sc or their mixture), with the goal of developing broadband-emitting gain media for ultrafast lasers at ∼2 μm. Evidence of a linear variation (increase) of the crystal-field strength when decreasing the ionic radius of the host-forming cation R3+ in the isostructrural R2O3 series is presented. The crystal-field splitting of Tm3+ multiplets is determined for C2 sites and justified using a barycenter plot, and the first evidence of C3i Tm3+ species is presented. A remarkable inhomogeneous spectral broadening for compositionally “mixed” sesquioxides (in particular with Sc3+) with respect to the parent compounds is revealed at low temperatures. It is proven that binary and ternary “mixed” R2O3 compounds form substitutional solid-solutions with a mixing of the host-forming cations at the atomic level. The stimulated-emission and gain cross-sections for the 3F4 → 3H6 Tm3+ transition are determined. Guidelines for material engineering of Tm3+-doped gain media for mode-locked 2-μm lasers are also provided.

Achieving broadband near-infrared emission with superior anti-thermal quenching by optimizing the excited-state population of Cr3+ in Gd3ScGa4O12 garnet phosphors.

Cr3+-activated garnet phosphors with broadband near-infrared (NIR) emission have attracted considerable interest due to their high quantum efficiency (QE) and thermal stability for widespread advanced applications. Nevertheless, how to achieve energy-saving broadband NIR phosphors that possess anti-thermal quenching (anti-TQ) without compromising the high QE has yet to be fully addressed. Herein, we report on site reconstruction within the garnet lattice by strategically positioning Sc and Ga atoms into octahedral B sites with a mole ratio of 1 : 1 to produce Gd3ScGa4O12. A reduction in crystal field strength (CFS) is thus induced, leading to a redshift of Cr3+ broadband NIR emission. The inherent rigidity of the structure and the weak electron-phonon coupling (EPC) effect lay the groundwork for a thermally robust broadband NIR phosphor. The combination of bandgap engineering, finely optimizing the 4T2 excited state population, and precise control over the doping concentration contributes a high-performance broadband NIR emission (IQE = 82.75%) with unprecedented anti-TQ such that the NIR emission of Cr3+ even increases to 198% of its room-temperature intensity at 543 K. A prototype broadband NIR pc-LED is encapsulated to deliver an NIR output power of 125.20 mW@900 mA and a wall-plug efficiency (WPE) of 6.88%@30 mA, enabling night vision, noninvasive imaging, and non-destructive detection applications.

Regulating luminescence thermal quenching based on the synergistic effect of energy transfer and energy gap modulation.

Thermal quenching is the core challenge that hinders the application of luminescent materials. Herein, a synergistic mechanism involving energy transfer and energy gap modulation is proposed based on the local crystal field regulation around sensitizers. The substitution of coordination cation V5+/P5+ weakens the crystal field strength of the sensitizer Bi3+, and the weakening of crystal field splitting causes an increase in the 3P1 energy level, thus increasing its energy gap. Compared with the YVO4:Bi3+,Eu3+ phosphor, the thermal stability of the YV0.25P0.75O4:Bi3+,Eu3+ phosphor is significantly improved, and the relative emission intensity of Eu3+ continuously increases with heating and reaches 1.24 times the original intensity at 523 K and does not show a decreasing trend in the studied temperature range. The anti-thermal quenching performance is mainly attributed to the increasing thermal quenching activation energy (ΔE) of the sensitizer by energy gap modulation, which enhances the energy transfer to compensate thermal quenching. Based on the thermal quenching characteristics of the materials, an optical thermometer is designed. The maximum relative sensitivity (Sr) and absolute sensitivity (Sa) are as high as 1.74% K-1 and 0.59 K-1, respectively, and the minimum temperature resolution reaches 0.288 K. The synergistic effect between energy gap modulation of the sensitizer and energy transfer enables the regulation of thermal quenching. Thus, this study provides a new strategy for exploiting high-performance luminescent materials.

Promoting structural distortion to enhance the crystal field strength of Mn(II) in tetrahedral bromide for near-unity yellow emission.

Two zero-dimensional hybrid manganese bromide polymorphs ((DMAPH)2MnBr4) exhibit single-crystal to single-crystal phase transformation, accompanied by an increase in MnBr4 tetrahedral bond angle variance (σ2). This structural change leads to an emission redshift due to enhanced crystal field strength, achieving yellow emission with near-unity quantum yield, which highlights its potential for solid-state lighting applications.

Towards ultra-sensitive multimodal luminescent thermometers enabled by high crystal field strength of Lu2CaMg2Ge3O12:Yb3+,Nd3+,Er3+ phosphors

An effective full-spectrum thermometer covering from the visible to NIR-II band was designed based on garnet Lu2CaMg2Ge3O12:Yb3+,Er3+/Nd3+ phosphors; meanwhile, Er3+ (4I13/2) split into four Stark sublevels due to high crystal field strength.

Effect of annealing conditions on the luminescence properties and thermometric performance of Sr3Al2O5Cl2:Eu2+ and SrAl2O4:Eu2+ phosphors.

We report on the synthesis, photoluminescence optimization and thermometric properties of Sr3Al2O5Cl2:Eu2+ and SrAl2O4:Eu2+ phosphor powders. The photoluminescence of Sr2.9Al2O5Cl2:0.1Eu2+ phosphors exhibits a blue-shift with an increasing annealing temperature owing to a decrease in the crystal field strength of the host caused by evaporation of Cl from the material. The quenching of the blue band in favour of the red band observed in the luminescence spectra of Sr2.9Al2O5Cl2:0.1Eu2+ with an increased annealing temperature was explained using the mechanism of the Landau-Zener transitions. The quantum yield and the lifetime of the phosphors depend on the annealing temperature. Phosphor samples annealed at 850 °C, 1000 °C, 1200 °C and 1500 °C were found to be potential luminescence thermometers using the luminescence spectral method. For Sr3Al2O5Cl2:Eu2+ annealed at 1000 °C, the temperature-dependent dual-band intensity ratio demonstrated a high-temperature sensitivity of ∼1.47%/°C in the temperature range of 23 °C to 40 °C which is superior to other reported phosphors with a microsecond decay time, suggesting that the material has potential for sensitive thermometry applications at ambient temperatures.

Improved luminescence of CaAl12-xGaxO19:Cr3+ near-infrared phosphor based on Sr2+ and Ga3+ substitution

Cr3+-doped near-infrared phosphors have recently attracted a wide attention. Here, NIR Ca1-ySryAl12-xGaxO19:0.12Cr3+ phosphors are obtained by a high-temperature solid-phase method. First, a series of CaAl7.88Ga4O19:0.12Cr3+ samples is studied and shows that the crystal field strength can be tuned by cation substitutions, until achieving a sharp emission at 692 nm and a broad emission at 780 nm under a 400 nm excitation, and an internal quantum yield of 90.73%. A further substitution by an appropriate amount of Sr2+ is suggested to be beneficial to increase the rigidity of the crystal structure by Debye temperature calculations. Experimentally, it is observed that Sr2+ for Ca2+ substitutions did not affect the luminescence wavelength and resulted in a significant increase of the thermal stability. When 0.7 Sr2+ substitute Ca2+, the thermal stability at 450 K increased by 28% compared to the samples without Sr2+.

Multifunctional applications of novel near-infrared emitting Ca2ScNbO6:Cr3+ double-perovskite phosphors

Novel Ca2ScNbO6 (CSNO): Cr3+ phosphors were synthesized through a solid-phase method. The crystal structures, luminescence performances, and applications were introduced in this work. The optimized doping concentration was found with a Cr3+ concentration of 0.01 mol. Under the 336 nm excitation, the recorded photoluminescence spectrum in the range of 650–1300 nm indicated that the obtained sample can emit a broad near-infrared emission with a dominant peak of 858 nm while the full width at half maximum (FWHM) was determined to be 191 nm. To gain a deeper comprehension of the luminescent properties, the crystal field strength was investigated as Dq/B = 2.03, and the electron-phonon coupling for parameter S was calculated to be about 2.01. As for the potential application, utilizing the variable temperature decay lifetime technique, the relative sensitivity (SR) was approximately 1.16% K−1 at 387 K, which can be promised for utilization in optical temperature measurement. On the other hand, a near-infrared phosphor-converted lighting-emitting diode (Pc-LED) was fabricated for night vision. Eventually, the security ink based on the obtained sample was made for use in the anti-counterfeiting field.

Systematic First-Principles Calculations of Ce3+-Dopant in Garnet-Type Oxide Phosphors

A study of Ce3+-doped oxide phosphors was performed using first-principles calculations based on the relativistic DV-Xα method. In particular, we focused on the Ce3+-doped garnet-type oxides, which have potential applications in solid-state lighting and display devices. The electronic structure and optical properties of the phosphors were analyzed. The energy levels of the Ce3+ ions were affected by the coordination number and the crystal field strength of the surrounding oxygen ions. In addition, the crystal structure of the material had a significant impact on the energy levels of the Ce3+ ions, with different crystal structures leading to different energy levels.The Ce3+ ion is substitutionally incorporated in the garnet lattice, and the 4f electron of Ce3+ plays an important role in the luminescence of the material. The emission wavelengths of the 5d-4f transition energy of Ce3+ can be tailored by varying the crystal structure and the composition. The 5d orbitals of Ce3+ are substantially affected by the crystal field splitting while the amplitude of crystal field splitting (εcfs) is caused by many parameters: coordination number, symmetry of Ce3+ site, and bond lengths between the Ce3+ ion and the oxygen ions (O2-). This study incorporated the following computational conditions: i) lattice relaxation, ii) the choice of the exchange-correlation potential, and iii) Slater’s transition state method. The effects of these conditions were analyzed. In addition, the correlations among physical properties and electronic structure parameters were also investigated in detail.The first-principles calculation results were compared to the experimental data to validate the accuracy of the theoretical models, which showed a good agreement between the experimental and predicted values.

Effect of the local environment on the luminescence of Er3+/Yb3+-doped aluminate upconversion phosphors

In recent years, trivalent rare-earth ion (RE3+) doped upconversion luminescent (UCL) materials have been studied extensively owing to their potential applications. The structure of the material is the main factor affecting the UCL performance. In this study, three structurally identical Er3+/Yb3+ co-doped aluminates CaYAlO4, CaGdAlO4, and CaLaAlO4 (CYA, CGA, and CLA) phosphors were synthesized, and the effects of the local environment on the UCL performance of Er3+ ions were investigated. The morphology and phonon energy of the samples were not the main factors affecting the luminescence performance in the same structural material. The difference in the crystal field strength was the main reason for the difference in the UCL of the three phosphors, which was attributed mainly to the difference in the crystal field strength and the difference in the degree of distortion of the Al–O octahedra that changes the transition probabilities of the parity forbidden intra-4fN–4fN electronic transitions of Er3+. A comparison of the distances between the RE3+ and the lifetimes of the 4I11/2 and 4I13/2 energy levels in the three materials showed that the red–green ratio of the materials was affected by the energy transfer. Finally, an analysis of the changes in the energy band structure and density of states (DOS) distribution before and after Er3+/Yb3+ doping of CYA, CGA, and CLA found that the effect of the bandgap on the UCL can be neglected when the bandgap of the matrix is larger than the energy difference from the 4I15/2 energy level to the 2H11/2 energy level of the Er3+ ion.