Effect of ligand environment of rare-earth ions on temperature measurement performance of SrAO4:xEu3+ (A = Mo and W) phosphors

Molybdate and tungstate with scheelite-type structure are excellent self-luminescent materials, which can be used as ideal hosts for the doping of rare-earth ions. In this study, a series of Eu³⁺-activated SrAO₄ (A = Mo and W) phosphors were successfully synthesized, and their crystal structures, photoluminescence properties, and temperature measurement performance were analyzed in detail. These phosphors were excited by UV light (291 nm and 247 nm, respectively), with clear energy transfer (ET) (MoO₄^2?→Eu³⁺ or WO₄^2?→Eu³⁺). According to fluorescence intensity ratio (FIR) and Judd–Ofelt (J–O) theory, compared to SrWO₄:0.01Eu³⁺ phosphor, SrMoO₄:0.01Eu³⁺ phosphor exhibited better thermal stability, with relatively low Sa value (maximum values were 5.082 %K^?1 and 20.74 %K^?1, respectively), and their Sr values were not significantly different (maximum values were 0.864 %K^?1 and 0.83 %K^?1, respectively). Sa value was negatively correlated to central asymmetry of Eu³⁺, but the optimal Sr value tended to be more suitable for central asymmetry of Eu³⁺. In addition, Eu³⁺ exhibited stronger central asymmetry as well as covalency of Eu–O bond in SrMoO₄. Results reveal that SrMoO₄:xEu³⁺ and SrWO₄:xEu³⁺ can be used for luminescent thermometers.     

Dual-mode optical thermometry based on Bi3+/Eu3+ co-activated BaGd2O4 phosphor with high sensitivity and signal discriminability

A series of BaGd₂O₄:Bi³⁺,Eu³⁺ phosphors with dual-emitting centers were prepared by high-temperature solid-state method. X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), fluorescence spectroscopy, lifetime decay curve and variable temperature emission spectroscopy were used to systematically study the structure, luminescence performance and temperature characteristics. Under ultraviolet (UV) excitation, the BaGd₂O₄:Bi³⁺,Eu³⁺ phosphor showed a broad-band emission in the blue region corresponding to transitions of Bi³⁺ ions and the sharp red light emission corresponding to Eu³⁺ ions. The Bi³⁺ and Eu³⁺ ion emission peaks were well-separated, which meets a prerequisite for efficient temperature signal resolution measurement. The fluorescence intensity ratio (FIR) technique was used to measure the different temperature response characteristics between Bi³⁺ blue emission and Eu³⁺ red emission. When the temperature varies from 293 K to 473 K, the relative temperature sensitivity (S_r) of BaGd₂O₄:Bi³⁺,Eu³⁺ phosphors is obtained, was determined as 1.0182%K⁻¹. In addition to calculating the relative sensitivity by FIR technology, we can also obtain the value of S_r through experiments and formulas related to the decay life, and found to be 1.0651%K⁻¹. Therefore, BaGd₂O₄: Bi³⁺,Eu³⁺ phosphor is an excellent non-contact optical temperature measurement material.     

Na2GdMg2V3O12:Sm3+ phosphors for three-mode optical temperature sensing

Multi-mode optical thermometry is emerging as a promising technique for accurate temperature measurement. Here, a series of Na₂GdMg₂V₃O₁₂:Sm³⁺ (NGMVO:xSm³⁺) phosphors with dual-emission centers were successfully elaborated. Irradiated by 349 nm light, NGMVO:Sm³⁺ can generate emissions from VO₄³⁻ and Sm³⁺ simultaneously due to energy transfer between them. Benefitting from difference of thermal quenching performance of two luminescent centers, fluorescence intensity (FI), fluorescence intensity ratio (FIR), and Commission Internationale de L'Eclairage coordinate modes of NGMVO:Sm³⁺ thermometer were designed. Maximum relative sensitivities (S_R) are found to be as high as 2.21 %K⁻¹, 2.12 %K⁻¹, and 0.244 %K⁻¹, respectively. Besides, with temperature increasing, S_R values of FI and FIR modes decrease slowly. Their minimal S_R values between 303 and 513 K maintain above 1.36 %K⁻¹ and 1.08 %K⁻¹, respectively. Our findings reveal that NGMVO:Sm³⁺ phosphors have promising applications for multimode temperature senor.     

High-accuracy dual-mode optical thermometry based on up-conversion luminescence in Er3+/Ho3+-Yb3+ doped LaNbO4 phosphors

Due to the non-contact and high sensitivity, optical thermometry based on rare earth doped phosphors has been paid much attention to. Herein, dual-mode optical thermometers are designed using up-conversion luminescence of Er³⁺/Ho³⁺-Yb³⁺ doped LaNbO₄ phosphors, which were synthesized for the first time by high-temperature solid-state reaction method. The LaNbO₄:1Er³⁺:10Yb³⁺ and LaNbO₄:1Ho³⁺:10Yb³⁺ phosphors exhibit reliable and excellent thermometric performance by combining fluorescence intensity ratio and decay lifetime for self-calibration. Specifically, the maximal relative sensitivities based on fluorescence lifetime were 0.27 %K⁻¹ and 0.33 %K⁻¹ for LaNbO₄:1Er³⁺:10Yb³⁺ and LaNbO₄:1Ho³⁺:10Yb³⁺ phosphors, respectively. The maximal relative sensitivity is 1.12 %K⁻¹ when using intensity ratio between thermal coupling energy levels in LaNbO₄:1Er³⁺:10Yb³⁺ as a detecting signal. Furthermore, the maximal relative sensitivity reaches as high as 0.98 %K⁻¹ when taking advantage of special non-thermal coupling energy levels in LaNbO₄:1Ho³⁺:10Yb³⁺. These results indicate that Er³⁺/Ho³⁺-Yb³⁺ doped LaNbO₄ phosphors possess great potential in self-calibrated optical thermometric techniques.     

Luminescence and self-referenced optical temperature sensing performance in Ca2YZr2Al3O12:Bi3+,Eu3+ phosphors

Ca₂YZr₂Al₃O₁₂:Bi³⁺,Eu³⁺ phosphors were elaborated by a traditional solid-state reaction method. The luminescence of Ca₂YZr₂Al₃O₁₂:Bi³⁺ samples, energy transfer from Bi³⁺ to Eu³⁺, and the temperature sensing properties of Ca₂YZr₂Al₃O₁₂:Bi³⁺,Eu³⁺ samples have been systematically researched. Under the excitation of ultraviolet light, Bi³⁺ single doped phosphors give 313 and 392 nm emission bands, which origin from the substitutions of Bi³⁺ instead of Ca²⁺ and Y³⁺ sites, respectively. And the color-adjustable emission from blue to red were observed by increasing Eu³⁺ content in Ca₂YZr₂Al₃O₁₂:Bi³⁺,Eu³⁺ samples. Relying on different temperature dependent variation tendency, the fluorescence intensity ratio (FIR) values present outstanding temperature sensing properties. The absolute and relative sensitivity can be up to 0.826 %K^-1 and 0.664 %K^-1, respectively. All above results suggest that Ca₂YZr₂Al₃O₁₂:Bi³⁺,Eu³⁺ phosphor is a potential alternative for optical thermometer.     

Effect of Variations in Annealing Temperature and Metallic Cations on Nanostructured Molybdate Thin Films

Crystalline molybdate thin films were prepared by the complex polymerization method. The AMoO₄ (A = Ca, Sr, Ba) films were deposited onto Si wafers by the spinning technique. The Mo–O bond in the AMoO₄ structure was confirmed by FTIR spectra. X-ray diffraction revealed the presence of crystalline scheelite-type phase. The mass, size, and basicity of A²⁺ cations was found to be dependent on the intrinsic characteristics of the materials. The grain size increased in the following order: CaMoO₄ < SrMoO₄ < BaMoO₄. The emission band wavelength was detected at around 576 nm. Our findings suggest that the material’s morphology and photoluminescence were both affected by the variations in cations (Ca, Sr, or Ba) and in the thermal treatment.     

Preparation and luminescent modulation of KGaSiO4:Eu3+ phosphors using for multiple anti-counterfeiting

Anti-counterfeiting technology is of great significance to information security. To obtain high-quality anti-counterfeiting materials, the developments of inorganic materials are crucial. In this paper, a series KGaSiO₄:xEu³⁺ phosphors with persistent luminescence, photoluminescence, and thermochromic have been successfully prepared and the application of quadruple anti-counterfeiting is realized. The X-ray diffraction and Rietveld refinement indicate that the phosphors are pure phase. With Eu³⁺ ions doping, the structure change, site occupancies, and color-tunable phenomenon are carefully investigated. Different from another Eu³⁺ doping phosphor, the emission of KGaSiO₄:0.2% Eu³⁺ phosphor changes with the excitation light in the region of 240 nm–306 nm. The emission color can be modulated with the surrounding temperature. Surprisingly, this phosphor can emit green afterglow light, which is attributed to the different luminescent properties of the matrix and doping of Eu³⁺ ions. The series of phosphors exhibit abundant luminescent properties. Based on their wavelength dependence, concentration quenching, long afterglow, and thermochromic properties, the KGaSiO₄:xEu³⁺ phosphors can be effective materials for quadruple-modal anti-counterfeiting devices.     

Photoluminescence and temperature sensing of lanthanide Eu3+ and transition metal Mn4+ dual-doped antimoniate phosphor through site-beneficial occupation

For a long time, rare-earth ion-doped phosphors have been widely used in temperature sensing because of their excellent light-emitting properties. However, most of the rare earth elements are relatively rare and expensive, so the transition group elements that are economical and easy to obtain have been favored by researchers. This paper presents a new type of phosphor doped with rare earth ion and transition metal for optical temperature measurement. In recent years, Mn⁴⁺-doped phosphors have attracted wide attention because of their strong deep red light-emitting properties. La₂LiSbO₆ provides a good host environment for Mn⁴⁺ and Eu³⁺ due to its unique crystal structure. In this paper, a series of La₂LiSbO₆ phosphors singly doped with Mn⁴⁺ and Eu³⁺, and co-doped with Eu³⁺/Mn⁴⁺ were synthesized. The crystal phases and optical properties of these materials were characterized and analyzed in detail. We specifically studied the temperature dependence of the fluorescence intensity of the optimized La₂LiSbO₆: Eu³⁺, Mn⁴⁺ phosphors at 303K–523K. The experimental results prove that the thermal responses of Mn⁴⁺ and Eu³⁺ are different. With increasing temperature, the thermal quenching of the Mn⁴⁺ fluorescence intensity is much faster than that of Eu³⁺, so the temperature characteristics can be explored by the fluorescence intensity ratio (FIR) of Eu³⁺ to Mn⁴⁺. At 523 K, its maximum relative sensitivity and maximum absolute sensitivity can reach 0.891% K⁻¹ and 0.000264 K^-1, respectively. Our experimental analysis shows that La₂LiSbO₆:Eu³⁺/Mn⁴⁺ phosphors have relatively high temperature sensitivity and have potential application prospects in the field of high temperature sensing.     

A novel Eu2+/Tb3+ co-doped phosphor with pyroxene structure applied for cryogenic thermometric sensing

Pyroxene-type phosphors were widely developed due to the advantages of high chemical stability, luminous efficiency, and low production cost. In this contribution, a series of Eu²⁺/Tb³⁺ co-doped Ca_0.75Sr_0.2Mg_1.05Si₂O₆ (CSMS) phosphors with pyroxene structure were successfully synthesized by the solid-state method. Under the 340 nm excitation, the emission peaks of the phosphor show a redshift with the increase of Eu²⁺ concentration. The emitting color of Eu²⁺/Tb³⁺ co-doped samples shows a redshift attributed to the energy transfer from Eu²⁺ to Tb³⁺. Simultaneously, acquired thermometer exposes superbly temperature-sensitive properties (S_a and S_r having maximum values 4.7% K⁻¹ and 0.6% K⁻¹, respectively) over the cryogenic temperature range (77–280 K). Furthermore, it has good stability and precision at cryogenic temperatures, indicating that CSMS:0.03Eu²⁺/0.03Tb³⁺ phosphor is a very promising fluorescent material suitable for cryogenic temperature sensing.     

Host sensitized photoluminescence in Sr2.9-3x/2LnxAlO4F: 0.1Eu3+ (Ln = Gd,Y) for innovative flexible lighting applications

Tetragonal structured Sr₃AlO₄F is highly strained as reported from its global instability index estimation. Moreover, our results of X-ray photoelectron spectroscopy (XPS) also ascertained that the structure of Sr₃AlO₄F is highly strained with oxygen vacancies. Herein, aliovalent substitutions of divalent Sr ions with trivalent Ln (Ln = Gd/Y) ions were carried out to improve the stability of Sr₃AlO₄F lattice, which subsequently enhanced the photoluminescence in a series of Sr_2.9-3x/2Ln_xAlO₄F: 0.1Eu³⁺ phosphors. All the phosphors showed intense red-orange emission (⁵D₀ → ⁷F_1,2) at excitation with UV and near-UV light. The critical concentrations of Gd³⁺ and Y³⁺ up to which the Eu³⁺ emission intensities increased linearly were observed to be x = 0.09 and x = 0.07, respectively. Nevertheless, further enhancement in the Eu³⁺ luminescence of the optimized phosphors was realized by subsequently annealing in low oxygen atmospheres. The enhancement in oxygen deficiency during the post-annealing in Ar or vacuum led the energy transfer (O^2--Eu³⁺) to a greater extent which afterward increased the Eu³⁺ luminescence. The optimized Sr_2.765Gd_0.09AlO₄F: 0.1Eu³⁺ and Sr_2.795Y_0.07AlO₄F: 0.1Eu³⁺ phosphors showed high red color purity (~99%), as well as CIE coordinates of (0.62, 0.38), indicated that these phosphors could be appropriate red-emitting components for making flexible optical films for many lighting devices. Therefore, flexible polydimethylsiloxane based films were also fabricated using optimized Sr_2.765Gd_0.09AlO₄F: 0.1Eu³⁺ phosphor. The electroluminescence of a flexible PDMS-phosphor composite film showed an intense and pure red color with good thermal stability suggesting its suitability in flexible lighting and display devices.     

Synthesis and luminescent properties of Sr4−xSi3O8Cl4: xEu3+ red phosphor by hydrothermal method

Sr_4‐xSi₃O₈Cl₄:xEu³⁺ (SSOC:Eu³⁺) phosphors were successfully synthesized by hydrothermal method. The crystallization of this phosphor was analyzed by means of X‐ray diffraction patterns. The size and morphology were recorded using SEM patterns of samples. And the PLE and PL spectra were characterized by a PL spectrophotometer. Excited by 394 nm UV light, the intense red emission is recognized in SSOC:Eu³⁺ phosphor and the main emission peak located at 620 nm. The influences of Eu³⁺ concentration, pH value of reaction solution, and charge compensator on PL spectra of SSOC:Eu³⁺ phosphors were investigated. The results revealed that this red phosphor had potential applications for white LEDs.     

Effects of Eu3+ doping on the luminescence properties of novel Sr2CaLa(VO4)3 from partial substitution of Sr2+ by Ca2+ in Sr3La(VO4)3

Eu³⁺-activated Sr_3−xCa_xLa(VO₄)₃ phosphors were fabricated via citric-acid-assisted sol combustion. Characterization of the Sr_3−xCa_xLa(VO₄)₃:Eu³⁺ samples with different concentrations of Ca²⁺ revealed a hexagonal crystal structure belonging to the R-3m space group. The amount of Ca²⁺ added (x) was controlled within 0 ≤ x ≤ 2 to yield high-purity phosphors. Scanning electron microscopy results showed that an increase in Ca²⁺ concentration resulted in a decrease in the particle size of Sr_3−xCa_xLa(VO₄)₃:Eu³⁺, with the shape gradually changing from nearly equiaxed to lath-shaped. The Sr₂CaLa(VO₄)₃:Eu³⁺ phosphor (denoted as SCLVO:Eu³⁺) exhibited the strongest photoluminescence (PL) intensity at 618 nm among the samples under excitation of 394-nm near-UV (NUV) light. The study of Eu³⁺ doping concentration confirmed that Eu³⁺ could enter the lattice of the SCLVO matrix without altering its crystal structure. SCLVO:Eu³⁺ was found to strongly absorb 394 nm NUV light and 464 nm blue light. The optimal concentration of the Eu³⁺ dopant in the SCLVO host was 0.11, which resulted in the phosphor achieving an excellent PL intensity and a color purity of 98.68%. Tunable luminescence from the orange area (0.5280, 0.4522) of Commission Internationale de l'éclairage (CIE) to the red area (0.6313, 0.3650) was achieved by adjusting the concentration of Eu³⁺. Under 394 nm excitation, SCLVO:0.11Eu³⁺ phosphor has a quantum yield (QY) of 28.2% and excellent thermal stability with 0.383 eV activation energy. Consequently, White-light-emitting diode (WLED) based on SCLVO:0.11Eu³⁺ phosphor yielded a high color rendering index (CRI), low correlated color temperature (CCT), and CIE coordinates of 91.8, 5196 K, and (0.3407, 0.3612), respectively, under the 20 mA driven current. These results indicated the tremendous potential of SCLVO:0.11Eu³⁺ phosphors for application in WLEDs excited by NUV or blue light.     

Insight into the evolution mechanism of carbon film and Eu valence in carbon coated BaMgAl10O17: Eu2+ phosphor annealed in air

Performing carbon coating on the surface of phosphors has been proven to be an effective strategy to enhance the oxidation resistance, which is an important factor to achieve stable luminescent devices. Therefore, a good understanding of the protection mechanism favors a continuous improvement of oxidation resistance of phosphors. In present paper, the evolution of the carbon layer, Eu valence (Eu²⁺/Eu³⁺), and luminescent properties for the C coated BaMgAl₁₀O₁₇: Eu²⁺ phosphor when annealed at high temperature is investigated carefully. Decrease of carbon layer promotes the appearance color transition from black to white as the annealing temperature rises to 1000?°C in air. As expected, the decrease of carbon layer will enhance the luminescence intensity, but risk the possible oxidation of Eu²⁺ to Eu³⁺, which inhibits the blue emission ascribed to Eu²⁺. The results indicate that luminescence intensity of phosphor is dependent on the synergistic effect of carbon thickness and Eu²⁺/Eu³⁺ ratio. Additionally, a reduction reaction of Eu³⁺ to Eu²⁺ is observed in C coated BaMgAl₁₀O₁₇: Eu²⁺ phosphor when annealed at high temperature, which also contributes to the higher luminescence intensity.     

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.     

Tunable color emission in LaScO3:Bi3+,Tb3+,Eu3+ phosphor

LaScO₃:xBi³⁺,yTb³⁺,zEu³⁺ (x = 0 − 0.04, y = 0 − 0.05, z = 0 − 0.05) phosphors were prepared via high-temperature solid-state reaction. Phase identification and crystal structures of the LaScO₃:xBi³⁺,yTb³⁺,zEu³⁺ phosphors were investigated by X-ray diffraction (XRD). Crystal structure of phosphors was analyzed by Rietveld refinement and transmission electron microscopy (TEM). The luminescent performance of these trichromatic phosphors is investigated by diffuse reflection spectra and photoluminescence. The phenomenon of energy transfer from Bi³⁺ and Tb³⁺ to Eu³⁺ in LaScO₃:xBi³⁺,yTb³⁺,zEu³⁺ phosphors was investigated. By changing the ratio of x, y, and z, trichromatic can be obtained in the LaScO₃ host, including red, green, and blue emission with peak centered at 613, 544, and 428 nm, respectively. Therefore, two kinds of white light-emitting phosphors were obtained, LaScO₃:0.02Bi³⁺,0.05Tb³⁺,zEu³⁺ and LaScO₃:0.02Bi³⁺,0.03Eu³⁺,yTb³⁺. The energy transfer was characterized by decay times of the LaScO₃:xBi³⁺, yTb³⁺, zEu³⁺ phosphors. Moreover absolute internal QY and CIE chromatic coordinates are shown. The potential optical thermometry application of LaScO₃:Bi³⁺,Eu³⁺ was based on the temperature sensitivity of the fluorescence intensity ratio (FIR). The maximum S_a and S_r are 0.118 K⁻¹ (at 473.15 K) and 0.795% K⁻¹ (at 448.15 K), respectively. Hence, the LaScO₃:Bi³⁺,Eu³⁺ phosphor is a good material for optical temperature sensing.     

A novel scheelite-type LiCaGd(WO4)3:Eu3+ red phosphors with prominent thermal stability and high quantum efficiency

A series of LiCaGd(WO4)3 : xEu³⁺ (0 ≤ x ≤ 1.0) red phosphors with tetragonal scheelite structure were synthesized via the conventional solid-state reaction. Their crystal structure, photoluminescence excitation (PLE), and photoluminescence (PL) spectra, thermal stability and quantum efficiency were investigated. The phosphors exhibit a typical red light upon 395 nm near ultraviolet excitation, and the strongest emission peak at 617 nm is dominated by the ⁵D₀→⁷F₂ transition of Eu³⁺ ions. The PL intensity of the phosphors gradually increases with the increase of Eu³⁺ doping concentration, and the concentration quenching phenomenon is hardly observed. The quantum efficiency and the color purity of the phosphor reach maximum values of about 94.2 and 96.6% at x = 1.0, respectively. More importantly, LiCaGd(WO₄)₃:xEu³⁺ phosphors have prominent thermal stability. The temperature-dependent PL intensity of the phosphors at 423 K is only reduced to 89.1% of the PL intensity at 303 K, which is superior to that of commercial red phosphors Y₂O₃:Eu³⁺. Finally, LiCaGd(WO₄)₃:Eu³⁺ phosphor is packaged with near ultraviolet InGaN chips to fabricate white light emitting diodes, which has a low color temperature (CCT = 4622 K) and a high color rendering index (CRI= 89.6).     

Scalable synthesis of ytterbium and erbium codoped calcium molybdate phosphors as upconversion luminescent thermometer

Luminescent thermometry is a noninvasive method of temperature detection with high sensitivity and response speed. The present study demonstrated the process-intensified synthesis of ytterbium and erbium codoped calcium molybdate phosphors (CaMoO₄:Yb³⁺/Er³⁺). The experiment involved the initial premixing of the precursors using a high-gravity rotating packed bed (RPB) reactor and subsequent calcination processing. The pronounced mass transfer and micromixing of the reactants in the RPB facilitated the scalable and controllable synthesis of CaMoO₄:Yb³⁺/Er³⁺ particles with submicron sizes and regular morphologies. The CaMoO₄:Yb³⁺/Er³⁺ particles exhibited a bright-green emission with temperature-dependent luminescence characteristics under 980 nm laser irradiation. Furthermore, the maximum absolute sensitivity was determined to be 0.02837 K⁻¹. These results indicated that the synthesized product was a suitable candidate for application in upconversion luminescent thermometers capable of temperature sensing at the microscale.     

Preparation and Luminescence Properties of Blue‐emitting Phosphor Ba2Ca(PO4)2:Eu2+

Using the conventional high temperature solid‐state reaction method Ba₂Ca(PO₄)₂:Eu²⁺ phosphors were prepared. The phase structure, photoluminescence (PL) properties, and the PL thermal stability of the samples were investigated, respectively. Under the excitation at 365 nm, the phosphor exhibited an asymmetric broad‐band blue emission with peak at 454 nm, which is ascribed to the 4f–5d transition of Eu²⁺. It was further proved that the dipole–dipole interactions results in the concentration quenching of Eu²⁺ in Ba₂Ca_1?x (PO₄)₂:xEu²⁺ phosphors. When the temperature turned up to 150°C, the emission intensity of Ba₂Ca_0.99(PO₄)₂:0.01Eu²⁺ phosphor was 59.07% of the initial value at room temperature. The activation energy ΔE was calculated to be 0.30 eV, which proved the good thermal stability of the sample. All the properties indicated that the blue‐emitting Ba₂Ca(PO₄)₂:Eu²⁺ phosphor has potential application in white LEDs.     

Synthesis and optical characterization of novel Sr3Ga2O6:Eu phosphor

Alkaline earth metal gallets have been identified as an important ceramic material. The crystal chemistry of many of these gallets is well explored; however, very rare studies regarding optical properties of rare earth (RE) ions doped in such gallets, particularly in Sr₃Ga₂O₆ host, have been carried out. The present study reports on synthesis and characterization of novel Sr₃Ga₂O₆:Eu³⁺ phosphors. The phosphors have been synthesized using a conventional solid state reaction method. Crystal structure, morphology and luminescence properties (excitation, emission and CIE coordinate) of these phosphors have been studied as a function of sintering temperature and Eu³⁺ concentration. X-ray diffraction study reveals that the phosphor sintered at low temperature (900 °C) contains an impurity phase which is removed at higher sintering temperatures and results into cubic crystalline phase of Sr₃Ga₂O₆. Particle size of the phosphor increases with an increase in sintering temperature which results to a red shift in the peak position of excitation band lying in a broad range from 250 to 370 nm. Optimum emission intensity is attained for 0.12 mol% concentration of Eu³⁺ ions; above this concentration, a quenching in emission intensity is observed.     

Ca2Na2La6(SiO4)4(PO4)2O: Eu2+/Eu3+: A visual dual-emitting fluorescent ratiometric temperature sensor

A novel apatite-based UV-excited dual-emitting Ca₂Na₂La₆(SiO₄)₄(PO₄)₂O: Eu²⁺/Eu³⁺ phosphor (CNL: Eu²⁺/Eu³⁺) was designed and successfully synthesized by a solid-state reaction. Compared with previous reports on this family of materials, a structural study based on DFT calculation exhibited a new consequence that the monovalent ions in this system are more inclined to occupy the seven-coordinate cationic sites rather than the nine-coordinate sites. This result was confirmed by the structural refinement and high-resolution transmission electron microscopy (HRTEM) data. Due to the coexistence of Eu²⁺ and Eu³⁺ dopants in the material, under 345 or 392 nm excitation, CNL: 0.02Eu²⁺/Eu³⁺ exhibited a green Eu²⁺ emission band (528 nm) and red Eu³⁺ emission peaks (around 618 nm). The application potential of CNL:0.02Eu²⁺/Eu³⁺ in luminescent thermometry was studied by exploiting the temperature sensitivity of the fluorescent intensity ratio (green/red) at different temperatures. It was found that, under 345 nm excitation, the fluorescent intensity ratio of CNL: 0.02Eu²⁺/Eu³⁺ displayed linear correlation over the temperature range of 298 to 473 K with a high sensitivity of 2.82%K⁻¹. Additionally, the emission color of the CNL: 0.02Eu²⁺/Eu³⁺ sample under UV lamp (254 and 365 nm) excitation showed an obvious change (from green to red) as the temperature increased from 298 to 473 K (from green to red). These results indicated that CNL: Eu²⁺/Eu³⁺ can serve as an excellent visual luminescent ratiometric thermometer. Furthermore, this work provides a novel reference for developing high-performance luminescence temperature-sensing materials.