Thermochromic Pr3+ doped CaMoO4 phosphor with diverse thermal responses for temperature sensing

CaMoO₄:Pr³⁺ thermochromic phosphors with diverse thermal responses for temperature sensing were prepared by the traditional solid-phase reaction method. The typical CaMoO₄:Pr³⁺ had scheelite structure belonging to tetragonal crystal system and space group of I4₁/a (88). Pr³⁺ ions can be easily substituted for Ca²⁺ ions of host CaMoO₄ because of similar ionic radius. CaMoO₄: 1.5% Pr³⁺ have the block structure with mean size of 6.84 μm. The E_g (∼3.93 eV) value of pure CaMoO₄ is bigger than that (∼3.65 eV) of CaMoO₄: 1.5%Pr³⁺, attributing to the existence of intermediate defect energy levels. Appropriate Pr³⁺ doping concentration is 1.5%, and the concentration quenching phenomenon can be explained by the concrete electric multipole type of d-d interaction. The emission peak at ∼605 nm from ¹D₂→³H₄ transition have a good thermal stability of 99.452%@423 K, while the wide band centered at ∼490 nm from ³T_1,2 → ¹A₁ transition in the MoO₄²⁻ complex and ³P₀→³H₄ transition in Pr³⁺ have a poor thermal stability of 27.572%@423 K. Calculated activation energy is 0.239 eV. Temperature-dependent FIR for optical thermometry was constructed due to their diverse thermal responses. CaMoO₄: Pr³⁺ phosphor had good relative sensitivities of 2.216%, 0.969% and 0.932% based on FIR of I_{605 nm}/I_{490 nm} with Boltzmann distribution, modified Boltzmann distribution and exponential equation fitting. Thermochromic behavior and thermal quenching mechanism are investigated. The obtained relative sensitivity is better than that of most phosphors, implying that CaMoO₄: Pr³⁺ has a potential for application in optical thermometry.     

Dual-mode optical thermometry based on intervalence charge transfer excitations in Tb3+/Pr3+ co-doped CaNb2O6 phosphors

Self-calibrated temperature measurements combined with luminescence intensity ratio (LIR) and luminescence lifetime are more accurate. A dual-mode self-calibration optical thermometer was designed based on CaNb₂O₆: Tb³⁺/Pr³⁺ phosphor. The obtained sample has excellent sensitivity, with the maximum values of absolute sensitivity (S_a) and relative sensitivity (S_r) being 0.69 K^-1 at 612 K and 2.50% K^-1 at 532 K for LIR mode, and 0.0059 K^-1 at 475 K and 2.62% K^-1 at 535 K for luminescence lifetime mode, respectively. These results indicate that CaNb₂O₆: Tb³⁺/Pr³⁺ phosphor has valuable potential application for self-calibration optical temperature measurement.     

Luminescence and optical thermometry strategy based on emission and excitation spectra of Pr3+ doped SrMoO4 phosphors

Recent developments of luminescence ratiometric thermometry have attracted much attention owing to its merits of fast response, non-invasiveness and high spatial resolution. In this work, the synthesis, crystal structure and luminescence properties has been carried out for Pr³⁺-activated SrMoO₄ phosphors as optical thermometry. The XRD results show that all the phosphors possess the scheelite type tetragonal structure with space group I4₁/a. The efficient luminescence of Pr³⁺ can be observed under intra-configurational (4f-4f) and charge transfer band (Mo–O) excitations, respectively. Upon different excitations, the quenching concentration of Pr³⁺ is diverse due to the multi-phonon relaxation and cross-relaxation processes occurring in different excited states of Pr³⁺ ions. The fluorescence intensity ratio (FIR) techniques based on emissions of ³P₀ and ¹D₂ excited states of Pr³⁺, and the FIR in the excitations of the charge transfer band (Mo–O) and 4f-4f transitions of Pr³⁺ were employed for the thermometric characterizations in the 298–498 K range. Both results show remarkable performance in temperature sensing with the maximum relative sensitivity of 0.45%K^?1@489 K and 0.98 %K^?1@298 K, respectively. Our study demonstrates that Pr³⁺-activated SrMoO₄ phosphors have a promising potential application in non-contact optical thermometry.     

Synthesis and luminescence characterization of Pr3+, Gd3+ co-doped SrF2 transparent ceramics

Pr³⁺, Gd³⁺ co-doped SrF₂ transparent ceramic, as the potential material for visible luminescent applications, was prepared by hot-pressing of precursor nanopowders. The microstructure, phase compositions, and in-line transmittance, as well as the photoluminescence properties were investigated systematically. Highly optical quality Pr,Gd:SrF₂ transparent ceramic with nearly pore-free microstructure was obtained at 800°C for 1.5 hours. The average in-line transmittance of the x at.% Pr, 6 at.% Gd:SrF₂ (x = 0.2, 0.5, 1.0, 2.0) transparent ceramics reached to 87.3 % in the infrared region. The photoluminescence spectra presented intense visible light emissions under the excitation of 444 nm, the main intrinsic emission bands located at 483 and 605 nm, which were attributed to the transitions of Pr³⁺: ³P₀ → ³H₄ and ¹D₂ → ³H₄, respectively. With the co-doping of Gd³⁺ ions, the emission intensity of the Pr:SrF₂ transparent ceramic was greatly enhanced. All the emission bands of x at.% Pr, 6 at.% Gd:SrF₂ transparent ceramics exhibited the highest luminescence intensity with the 1.0 at.% Pr³⁺ doping concentrations, whereas the lifetimes decreased dramatically with the Pr³⁺ doping contents increasing from 0.2 to 2.0 at.% due to its intense concentration quenching effect. The 1 at.% Pr, 6 at.% Gd:SrF₂ transparent ceramic is a promising material for visible luminescent device applications.     

Luminescent thermometry based on Ba4Y3F17:Pr3+ and Ba4Y3F17:Pr3+,Yb3+ nanoparticles

New effective luminescence thermometers based on novel host Ba₄Y₃F₁₇ doped with Pr³⁺ and Pr³⁺/Yb³⁺ in 80–320 K temperature range were studied. The absolute temperature sensitivity (S_a) of both Ba₄Y₃F₁₇: Pr³⁺(0.1 mol %) and Ba₄Y₃F₁₇: Pr³⁺(0.1 mol.%): Yb³⁺(10.0 mol.%)nanothermometers based on luminescence intensity ratio (LIR) between two Pr³⁺ emission bands (³P₁-³H₅ and ³P₀-³H₅) demonstrate a notable value (0.011 K⁻¹ at 300 K) in the 200–320 K range. The Ba₄Y₃F₁₇: Pr³⁺(0.1 mol.%): Yb³⁺(10.0 mol.%)nanothermometers based on LIR between ³P₀→³H₄ of Pr³⁺ and ²F_5/2→ ²F_7/2 of Yb³⁺ emission bands demonstrate high S_a into the 80–200 K range with maximal S_a = 0,0778 K⁻¹ at 100 K. The stability of the phosphors was revealed by thermo-cycling experiments.     

Dual-mode optical ratiometric thermometer based on rare earth ions doped perovskite oxides with tunable luminescence

Rare earth ions doped luminescent materials have drawn considerable attention as they can generate both upconversion and downshifting emissions. Here, the rare earth ions Pr³⁺/Er³⁺ codoped perovskite oxide Bi₄Ti₃O₁₂ is proposed as a dual-mode temperature sensor and anti-counterfeiting material based on its up/down-conversion luminescence. Under 481 nm excitation, the intensity ratio of green emission (～523 nm in Er³⁺) and red emission (～611 nm in Pr³⁺) brings about a very high absolute sensitivity (S_a) of 2% K^?1 at 568 K and a maximum relative sensitivity (S_r) of 1.03% K^?1 at 478 K in the temperature range of 298–568 K. In addition, the upconversion green emissions of Er³⁺ yield a relatively-high S_r of 1.1% K^?1 at 298 K with 980 nm excitation, which can provide self-calibration coupled with down-conversion luminescence temperature sensing mode. Besides, this phosphor also shows tunable luminous colors for the potential application in the anti-counterfeiting field under various excitation wavelengths.     

High thermal stability pyroxene type CaScAlSiO6:Tb3+/Sm3+ ceramics with excellent cryogenic optical thermometry performance

The lanthanide-doped fluorescence intensity ratio (FIR) temperature measurement technology for detecting inaccessible objects has attracted the attention of many scholars. In fact, FIR technology can achieve high-precision and non-contact temperature measurement, which is widely used in the biomedical and industrial fields, among others. In this study, novel pyroxene-type CaScAlSiO₆:Tb³⁺/Sm³⁺ ceramics (CSAC) were synthesized using the traditional high-temperature solid-phase method, with the crystal structure, energy transfer mechanism, and optical behavior of the ceramics subsequently studied in detail. The use of ⁵D₄→⁷F₅ and ⁴G_5/2 → ⁶H_7/2 (I_544nm/I_602nm) energy level transitions as an optical thermometer has great potential, with the maximum absolute and relative sensitivity of the sensor found to be 0.1403 K^?1 and 1.65%K^?1, respectively. In addition, the material was cycled three times in the temperature range of 77–287 K and demonstrated a particularly high degree of credibility, with the luminous intensity having a particularly small effect on the temperature changes. Overall, the results indicate that the green-light-emitting CSAC:0.08Tb³⁺/0.01Sm³⁺ ceramic has great potential for use in optical cryothermometers.     

Synthesis and photoluminescence characterizations of Zn2+ and Tb3+ codoped CeO2 phosphors for temperature sensing applications

Cyan light-emitting Ce_0.985-xZn_xO₂:0.015 Tb³⁺ (x = 0 to 0.2) phosphors were synthesized using the ethylenediaminetetraacetic acid-assisted hydrothermal method. The X-ray diffraction and refinement analyses of the prepared phosphors indicated that the formed face-centered cubic structure remained intact even after the doping of large quantities of Zn²⁺ ions. However, the incorporation of Zn²⁺ ions increased the Ce³⁺/Ce⁴⁺ ratio, resulting in the enhancement of oxygen vacancies in the prepared phosphors. The generation of oxygen vacancies caused the evolution of a broad photoluminescence emission band ranging from 400 to 525 nm with a characteristic Tb³⁺ emission of approximately 543 nm. Two-emission regions in Ce_0.885Zn_0.1O₂:0.015 Tb³⁺ phosphors were utilized for measuring the fluorescence intensity ratio (FIR) as a function of temperature ranging from 303 to 523 K. At 523 K, the FIR values dropped to approximately 40% of the starting temperature value. The variation of FIR values followed the Boltzmann behavior. The Boltzmann fitting demonstrated the feasibility of the present phosphors for temperature sensor applications. The optimum absolute sensor sensitivity of Ce_0.885Zn_0.1O₂:0.015 Tb³⁺ phosphors was measured to be 0.0043 K⁻¹ at 398 K with a resolution of approximately 1 K⁻¹. Moderate temperature sensitivity, negligible hysteresis loop, and excellent reversibility revealed the suitability of Ce_0.885Zn_0.1O₂:0.015 Tb³⁺ phosphors for sensing the temperature in various electronic devices.     

Thermometric analysis of the near-infrared emission of Nd3+ in Y2SiO5 ceramic powder prepared by combustion synthesis

Nd³⁺:Y₂SiO₅ crystalline ceramic powder prepared by combustion synthesis exhibited down-conversion fluorescence when the sample was exposed to CW laser radiation at 532 nm. Three emission bands centered at 879.36 nm, 804.86 nm and 753.21 nm, corresponding to transitions (⁴F_7/2,⁴S_3/2) → ⁴I_9/2, (⁴F_5/2,²H_9/2) → ⁴I_9/2 and ⁴F_3/2 → ⁴I_9/2, were observed. A noticeable change on the relative intensities of the transitions in the temperature range 298–673 K occured due to thermal coupling among the excited electronic states. By means of the fluorescence intensity ratio (FIR) technique, we obtained a maximum absolute sensitivity of 2.5 × 10⁻³ K⁻¹ at 650 K and a relative sensitivity of 2.11% K⁻¹ at 300 K for this material. The Judd-Ofelt standard intensity parameters ratio Ω₄/Ω₆, a spectroscopy quality factor for laser applications, was estimated from the FIR thermometry data and a value of 1.75 was obtained. The potential use of this material in thermal sensing at the first biological window (700–950 nm) and in lasing applications is discussed.     

Temperature-dependent upconversion luminescence in ferroelectric 1.5%Pr/3%Yb:0.75Pb(Mg1/3Nb2/3)O3–0.25PbTiO3 transparent ceramics for non-contact thermometry and optical heating

Non-contact optical thermometry based upon fluorescence intensity ratio (FIR) has attracted much attention because of its excellent accuracy and sensitivity. Recently we reported the upconversion luminescence in Pr³⁺/Yb³⁺ co-doped 0.75Pb(Mg_1/3Nb_2/3)O₃–0.25PbTiO₃ transparent ceramics [Z. Lv et al., Ceramics International 45 (2019) 10924–10929]. In this article we study the temperature dependence of upconversion emissions from the 1.5%Pr/3%Yb:0.75Pb(Mg_1/3Nb_2/3)O₃–0.25PbTiO₃ transparent ceramics. The FIR between the ¹D₂-³H₄ and ³P₀–³H₄ emissions fits a thermally coupled-levels-like equation, and exhibits the maximum relative sensitivity of 1.03% K⁻¹ at the temperature 320 K. Meanwhile, the FIRs of I_Red/I_Green, I_Red/I_Blue and I_Red/I_BG show linear responses versus temperature and illustrate the high constant sensitivities of 1.09%–2.9% K⁻¹, but in a narrow temperature range of 140–240 K. The multiple temperature-sensing performances offer us chances to select distinct response functions on the basis of the practical demands. In addition, laser induced heating was observed in the transparent ceramics. The generated temperature in the ceramics can be accurately monitored and regulated from 329 to 377 K by changing the excitation power from 1.58 to 8.61 W/cm². The high heating efficiency of ~6.83 K cm²/W makes the transparent ceramics suitable for optical heater. The transparent ceramics exhibit excellent piezoelectric and ferroelectric performances, and thus they are promising candidates for multifunctional optical-electro devices besides non-contact thermometer and optical heater.     

Effect of B-site substitution on the luminescence,thermal stability and temperature sensitivity of Ba2NaNb5O15:Eu3+/Er3+ glass ceramics

Transparent glass ceramic with Ba₂NaNb₅O₁₅ as the main crystal phase was prepared, and the appropriate heat treatment condition was selected as 710 °C/150 min through various characterizations. The luminous intensity and thermal stability were enhanced significantly when the glass ceramic was used as the luminous matrix. After introducing Ti⁴⁺ ions as charge compensators, the luminescence performance and thermal stability were further improved, and the reasons for this were analyzed. At 458 K, the luminous intensity of 0.5%Eu³⁺ doped glass ceramic containing 0.5%Ti⁴⁺ can maintain about 65% of room temperature with a chromaticity shift of 5.16 × 10⁻². The relative and absolute sensitivities of 0.7%Er³⁺ doped glass ceramic were 4.04 × 10⁻³ K⁻¹ and 1.31% K⁻¹. Introducing Ti⁴⁺ ions would weaken the population redistribution ability of ²H_11/2 and ⁴S_3/2 levels and reduce the temperature sensitivity. However, the sample containing Ti⁴⁺ shows good thermal stability, its green emission at 458 K has a small chromaticity shift of 6.45 × 10⁻³. The research shows that the glass ceramic can be used as a good luminescent host material, and Eu³⁺/Er³⁺ doped glass ceramic can be used in the fields of LEDs or temperature sensing.     

High-sensitive temperature sensing use NIR upconversion luminescence of Er3+-core@Tm3+-shell with good robustness

In recent years, lanthanide doped materials have been extensively studied in the field of fluorescence temperature sensing due to their abundant emission levels and sensitive thermal response. Temperature sensing based on fluorescence intensity ratio (FIR) of upconversion nanoparticles has the advantages of fast temperature response, non-aggressiveness, and high spatial resolution. However, the most reported FIR sensing has limited sensitivity, probably due to the use of thermal coupling levels. Herein, we report a novel FIR temperature measurement based on non-thermal coupling levels of NaGdF₄:Yb³⁺/Er³⁺@NaGdF₄@NaGdF₄:Yb³⁺/Tm³⁺ core-shell-shell nanostructure, which has high sensitivity and robustness simultaneously. The relative sensitivity based on I₈₀₁/I₆₅₄ and I₈₀₁/I₈₄₁ of Tm³⁺ to Er³⁺ can reach up to 4.56 (303 K) and 3.82% K⁻¹ (313 K), respectively. Between them, FIR of I₈₀₁/I₈₄₁ is independent of excitation power and time. These results show the great potential of FIR based on non-thermal coupling levels in high-sensitive and robust temperature sensors.     

The effects of the solution reactant concentration and temperature on the preparation of Pr3+:BaF2 transparent ceramics

Barium fluoride (BaF₂) crystals are attracting much attention as efficient inorganic scintillator promising for high-energy physics, industrial inspection, and other fields because of the fast component of the decay time (0.6 ns) and high radiation resistance. However, two major drawbacks limit its practical application: (i) a slow decay time of ~600 ns is derived from self-trapping excitons; (ii) the absolute light yield from the fast luminescence component is not competitive. The introducing of rare earth ions and preparation of BaF₂ polycrystalline ceramics is considered to be effective measures to solve these bottlenecks. Pr³⁺ is extremely suitable as the activated ion of scintillation materials, which possess emission peaks located in visible band and the faster 5d–4f transition. In this work, highly sinterable Pr³⁺:BaF₂ precursor powder was synthesized via the coprecipitation method by adjusting the reactant concentration and temperature. The morphology and microstructure of as-synthesized powders were characterized using scanning electron microscopy (SEM) and transmittance electron microscopy analysis. The 5 at.% Pr³⁺:BaF₂ transparent ceramic with a transmittance of 50.7% at the wavelength of 500 nm was fabricated by hot pressing the as-prepared powders at 900°C for 4 h under the axial pressure of 50 MPa. The SEM images of ceramic cross-section show that the residual pore is the main light scattering source. The absorption and emission spectrum of ceramic samples were discussed.     

Realizing particle population inversion of 2.7 μm emission in heavy Er3+/Pr3+ co-doped low hydroxyl fluorotellurite glass for mid-infrared laser

In this work, the population bottleneck of Er³⁺: ⁴I_11/2 → ⁴I_13/2 was overcome for the first time in heavy Er³⁺/Pr³⁺ co-doped TeO₂–BaF₂–La₂O₃–LaF₃ (TBLL) low hydroxyl fluorotellurite glasses. Infrared emission spectra and fluorescence lifetime decay curves reveal that Pr³⁺ ions could deplete the electrons from the Er³⁺: ⁴I_13/2 level faster than those from the Er³⁺: ⁴I_11/2 under 980 nm excitation. Specifically, the energy transfer (ET) efficiency of the Er³⁺: ⁴I_13/2 → Pr³⁺: ³F_3,4 process (ET1) reached 96.27%, while that of the Er³⁺: ⁴I_11/2 → Pr³⁺: ¹G₄ process (ET2) is only 2.17% in the Er³⁺/Pr³⁺ co-doped glass. Additionally, the energy transfer mechanism of Er³⁺ and Pr³⁺ ions was investigated using the Dexter theory, where the energy transfer microscopic parameters C_D-A are 13.21 × 10⁻⁴⁰ cm⁶/s and 0.89 × 10⁻⁴⁰ cm⁶/s for the ET1 and ET2 processes, respectively. Finally, a numerical simulations laser model was developed to discuss the laser properties of the Er³⁺/Pr³⁺ co-doped TBLL fibers. The simulation results indicate that a 2.7 μm laser with a maximum output power of 2.26 W and slope efficiency of 13.89% could be achieved when the fiber background loss is reduced to 0.5 dB/m. The above results suggest that the Er³⁺/Pr³⁺ co-doped TBLL glass has great potential applications in mid-infrared fiber lasers.     

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.     

Large electrostrictive effect and high energy storage performance of Pr3+-doped PIN-PMN-PT multifunctional ceramics in the ergodic relaxor phase

The photoluminescence, dielectric relaxation, ferroelectric hysteresis, and field-induced strain properties of Pr³⁺-doped 0.24Pb(In_1/2Nb_1/2)O₃-0.42Pb(Mg_1/3Nb_2/3)O₃-0.34PbTiO₃ (PIN-PMN-PT:Pr³⁺) multifunctional ceramics have been investigated. It was found that Pr³⁺ doping enhanced the dielectric diffuseness and relaxation behavior of PIN-PMN-PT ceramics. Slim P-E loops and S-E curves appear in PIN-PMN-PT:Pr³⁺ ceramics when the Pr³⁺ doping concentration reaches 1.4 mol%. Local domain configurations associated with phase transitions were investigated by piezoresponse force microscopy (PFM). Large electrostrictive coefficient Q₃₃ (?0.03 m⁴/C²) and high energy-storage efficiency η (92%) were obtained in 2 mol% Pr³⁺-doped PIN-PMN-PT ceramic in the ergodic relaxor (ER) phase at room temperature. The giant electrostrictive effect and excellent energy-storage performance are related to the field-induced dynamic behavior of polar nanoregions (PNRs). The results show that the PIN-PMN-PT:Pr³⁺ system is an excellent multifunctional material for making electromechanical and energy storage devices.     

The negative magnetization and dielectric relaxation in (La0.3Pr0.7)1-xCaxCrO3 ceramics

The (La_0.3Pr_0.7)_1-xCa_xCrO₃ (x = 0, 0.1, 0.3 and 0.5) ceramics have been synthesized by a sol-gel method. The compounds crystalize in orthorhombic structure with space group Pnma at room temperature. The magnetic measurements confirm that the materials form canted antiferromagnetic (AFM) ordering from 254.1, 231.2, 118.5, 49.3 K for the samples of x = 0, 0.1, 0.3 and 0.5, accompanied by a spin reorientation transition at 191.4, 145.1, 55.8, 38.9 K because of the Pr³⁺-Cr³⁺ interaction. Meanwhile, a positive exchange bias effect is observed due to the antiparallel coupling between the Pr³⁺ and the canted AFM structure of Cr³⁺. The Néel temperature, compensation temperature as well as the coupling strength of Pr³⁺-Cr³⁺ monotonously decrease with the increase of Ca²⁺ concentration. For the samples of x = 0.1 and 0.3, there exists a field induced Pr³⁺-Pr³⁺ AFM ordering. By means of dielectric measurements, large permittivity at room temperature and dielectric relaxation are observed in all the samples. The permittivity is decreased and dielectric relaxation moves to low temperature range with the increase of Ca²⁺ concentration. A relaxor-like ferroelectric transition is observed in the measuring temperature range in Ca²⁺ doped samples.     

Exploring the luminescent thermometer and optical storage applications in negative thermal quenching phosphor of BaSc2Ge3O10:Pr3+

The optical contactless thermometer demonstrates remarkable advantages over traditional contact thermometers. However, maintaining a strong optical signal output across a wide temperature range poses a challenge due to the thermal quenching effect. In this study, we aimed to overcome the limitation of low signal intensity in high-temperature applications by developing a Pr³⁺-doped BaSc₂Ge₃O₁₀ (BSGO) phosphor with anti-thermal quenching properties. This phosphor was designed to ensure a stable and sufficiently strong light signal for accurate temperature measurements. The phosphor exhibited a negative thermal quenching effect, where the integrated emission intensity spanning from 450 nm to 800 nm increased from 173 K to 253 K, and the intensity at 353 K still remained stronger than at 173 K. Multiple 4f-4f transition emissions were observed in the phosphor, and the fluorescence intensity ratio (FIR) was utilized as a standard parameter for temperature evaluation ranging from 82 K to 353 K. At 82 K, the phosphor achieved maximum relative sensitivity (S_r) of 2.3% K⁻¹. The presence of multiple traps with depths ranging from 1.09 eV to 1.50 eV not only supported luminescence with a negative thermal quenching effect but also resulted in long persistent luminescence (LPL), which was systematically investigated in this study.     

Color-tunable emissions in Bi3+/Eu3+ activated phosphors for multi-mode optical thermometers

Accurate, self-referential temperature measurements of inaccessible workpieces, bodies in hazardous work environments, or fluids are of tremendous significance. Here, a series of Ca₃Y₂Ge₃O₁₂(CaYGO):1.0%Bi³⁺/yEu³⁺ phosphors were manufactured, which simultaneously presents the characteristic radiation of dopant ions, and the luminescence mechanism was investigated via emission spectroscopy and decay lifetime characterization. Upon excitation at 285 nm, energy transfer (Bi³⁺ → Eu³⁺) of the as-prepared materials exhibited adjustable polychromatic emissions (blue → red). The temperature dependence was evaluated via thermal quenching. The CaYGO phosphors demonstrated a high relative sensitivity of 0.019 K^?1 at 297.8 K according to FIR (the fluorescence intensity ratio) operation rules. Furthermore, temperature-controlled emission spectroscopy displayed a significant chromaticity shift (Δs = 0.05679 for 297.8–480 K). The temperature-dependent lifetime of Eu³⁺ which regarded as a basis of the self-reference measurement provided the maximum relative sensitivity was 0.038%. The excellent signal resolution, high sensitivity values, and results produced by three temperature measurement systems revealed that Ca₃Y₂Ge₃O₁₂: Bi³⁺/Eu³⁺ luminescent materials have excellent development prospects in multi-mode and self-referenced luminous thermometry fields.     

Gd3+ doping induced microstructural evolution and enhanced visible luminescence of Pr3+ activated calcium fluoride transparent ceramics

Transparent Pr³⁺ doped Ca_1-xGd_xF_2+x (x = 0, 0.01, 0.03, 0.06, 0.10, 0.15) polycrystalline ceramics with fine-grained microstructures were prepared by the hot-pressing method. The dependence of microstructure, optical transmittance, luminescence performances and mechanical properties on the Gd³⁺ concentrations for Pr³⁺:Ca_1-xGd_xF_2+x transparent ceramics were investigated. The Gd³⁺ ions show positive effects on the microhardness of Pr³⁺:Ca_1-xGd_xF_2+x transparent ceramics as a result of the decrease in the grain sizes. Excited by the Xenon lamp of 444 nm, typical visible emissions located at 484 nm, 598 nm and 642 nm were observed. Furthermore, the incorporation of Gd³⁺ ions can greatly enhance the photoluminescence performance owing to the improvement in the concentration quenching effect. The quenching concentration of Pr³⁺ ions in CaF₂ transparent ceramics increased to 1 at.% as a result of the positive effect of Gd³⁺ codoping. The energy transfer mechanism of Pr³⁺ in the Pr³⁺:Ca_1-xGd_xF_2+x transparent ceramics has been investigated and discussed.