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Luminescence thermometry is the most versatile remote temperature sensing technique and can be employed from living cells to large surfaces and from cryogenic temperatures to the melting points of metals. Ongoing research aims to optimize the sensitivity of the ratio between the emission intensity from two coupled excited states. However, this approach is inherently limited to temperature-dependent processes involving only the excited states. Here, we develop a novel measurement technique, called luminescence intensity ratio squared (LIR2) for the Yb3+/Er3+ pair, that combines the temperature sensitivity of ground- and excited-state populations. We use Y3Al5O12:Er3+,Yb3+ nanoparticles as a promising model system with both visible and infrared emissions. To apply our method, we record two luminescence spectra at different excitation wavelengths and determine the LIR2 using one emission in each of the two spectra. The LIR2 testing with Y3Al5O12 nanoparticles showed a sensitivity increase of 70% in the visible region and an impressive 230% increase in the NIR region compared to the conventional LIR method. This enhances the measurement precision by a factor of 1.5–2.5. The LIR2 based on the visible upconversion emission is particularly useful for measurements of high temperatures, while the LIR2 based on the downshifted ∼1.5 μm emission may revolutionize temperature measurements of biological samples in the range of physiological temperatures.

Broad band near infrared (NIR) emission has recently been reported for a wide variety of concentrated Mn2+ phosphors. Typically, Mn2+ emits in the green to red spectral region, depending on...

Many phosphor materials rely on energy transfer (ET) between optically active dopant ions. Typically, a donor species absorbs light of one color and transfers the energy to an acceptor species that emits light of a different color. For many applications, it is beneficial, or even crucial, that the phosphor is of nanocrystalline nature. Much unlike the widely recognized finite-size effects on the optical properties of quantum dots, the behavior of optically active ions is generally assumed to be independent of the size or shape of the optically inactive host material. Here, we demonstrate that ET between optically active dopants is also impacted by finite-size effects: Donor ions close to the surface of a nanocrystal (NC) are likely to have fewer acceptors in proximity compared to donors in a bulk-like coordination. As such, the rate and efficiency of ET in nanocrystalline phosphors are low in comparison to that of their bulk counterparts. Surprisingly, these undesired finite-size effects should be considered already for NCs with diameters as large as 12 nm. If we suppress radiative decay of the donor by embedding the NCs in media with low refractive indices, we can compensate for finite-size effects on the ET rate. Experimentally, we demonstrate these finite-size effects and how to compensate for them in YPO

The unprecedented borate hydride Sr5 (BO3 )3 H and deuteride Sr5 (11 BO3 )3 D crystallizing in an apatite-related structure are reported. Despite the presence of hydride anions, the compound decomposes only slowly in air. Doped with Eu2+ , it shows broad-band orange-red emission under violet excitation owing to the 4f6 5d-4f7 transition of Eu2+ . The observed 1 H NMR chemical shift is in good agreement with previously reported 1 H chemical shifts of ionic metal hydrides as well as with quantum chemical calculations and very different from 1 H chemical shifts usually found for hydroxide ions in similar materials. FTIR and Raman spectroscopy of different samples containing 1 H, 2 H, nat B, and 11 B combined with calculations unambiguously prove the absence of hydroxide ions and the sole incorporation of hydride ions into the borate. The orange-red emission obtained by doping with Eu2+ shows that the new compound class might be a promising host material for optical applications.

This article outlines the main results of a research and development cooperation between Philips Research Eindhoven; Peter the Great St. Petersburg Polytechnic University; Ioffe Institute, St. Petersburg; Utrecht University; and Philips Healthcare. It reviews the properties of garnet ceramics in the (Lu,Gd)3(Ga,Al)5O12:Ce system for medical imaging, especially time-of-flight positron emission tomography (PET). Thermally stimulated luminescence (TSL) peaks are attributed to impurities, verified by intentional codoping of samples. A lately developed method allows extraction of carrier lifetimes, thermal trap depths, and frequency factors from TSL and afterglow measurements. A detailed analysis reveals the presence of a distribution of trap depths, allowing a more accurate afterglow modeling. Activation energies of thermal ionization and trap depths obtained from TSL show the influence of Ga/Al substitution on thermal quenching and on trap position. The resulting nonmonotonic dependence of the conduction band edge with Ga content in (Lu,Gd) garnets is consistent with earlier predictions. Shallow traps determine both signal decay and short-term afterglow. The impact of signal height, rise, and decay times on coincidence resolving time and further on PET image quality is described by analytical models.

The impressive career of George Blasse in the field of luminescence started in 1964. Triggered by the discovery of YVO4:Eu3+ as red phosphor in cathode ray color TVs the management of Philips Research decided to re-establish the luminescence group and George Blasse was asked to lead the group. Very productive years followed in which George discovered many new phosphors, including YAG:Ce3+. In 1970 he made an unusual career switch and accepted the position of professor of Solid State Chemistry at Utrecht University. George Blasse led a highly successful research group for over 25 years and became internationally recognized for his research on luminescent materials. During his career he frequently attended ECS Meetings which has been and continues to be a place where reseaerchers from universities and industry in the field of luminescence materials meet and new phosphors and concepts are presented. I vividly remember attending the ECS meeting in Chicago in 1988 together with George. George Blasse combined groundbreaking fundamental discoveries with societal relevance. He had a deep and also intuitive understanding of luminescence phenomena and often relied on clever chemical variations in host composition and structure to obtain insight in the underlying physics. Simple guidelines and clear design rules based on systematic and careful research of a wide variety of materials were discovered, explaining spectral variations and trends, for example in thermal quenching of luminescence for Eu3+. The constant energy difference between fd states of different lanthanides was pioneered by George Blasse and later became the basis for the now widely used Dorenbos model. Another area where the work of George Blasse was instrumental involved energy transfer and energy migration phenomena. He was able to translate the somewhat complex theoretical papers as those by Förster and Dexter into simple models and equations to understand and quantify energy transfer processes. George Blasse also made important contributions in understanding of the optical properties of s2 ions like Pb2+, Bi3+, Sb3+ and Sn2+. In recent years this work of George receives increasing attention in the light of extensive work on luminescence of these ions in double perovskites. In this presentation I will share memories and highlight the great contributions of George Blasse to our field. Figure 1

The shift of the conduction band (CB) edge for thirty different (Lu,Gd) 3 (Ga,Al) 5 O 12 :Ce compositions, with simultaneous variation in Lu/Gd and Ga/Al content was studied using thermally stimulated luminescence (TSL). Specific TSL peaks were related to impurities of Ta, Cr, Yb, Ti and Eu in Lu 1 Gd 2 Ga 3 Al 2 O 12 :Ce ceramics. The shift of Yb-related peak positions (in temperature and trap depth) with composition modification was investigated as well. In Gd-containing (Lu,Gd) 3 (Ga,Al) 5 O 12 compositions a non-monotonous shift of the CB edge with increasing Ga content has been affirmed. The difference between thermal trap depths evaluated from our TSL experiments and optical trap depths obtained from the literature was explained by the role of lattice relaxation.

Fiducial markers are used in correlated light and electron microscopy (CLEM) to enable accurate overlaying of fluorescence and electron microscopy images. Currently used fiducial markers, e.g. dye‐labelled nanoparticles and quantum dots, suffer from irreversible quenching of the luminescence after electron beam exposure. This limits their use in CLEM, since samples have to be studied with light microscopy before the sample can be studied with electron microscopy. Robust fiducial markers, i.e. luminescent labels that can (partially) withstand electron bombardment, are interesting because of the recent development of integrated CLEM microscopes. In addition, nonintegrated CLEM setups may benefit from such fiducial markers. Such markers would allow switching back from EM to LM and are not available yet. Here, we investigate the robustness of various luminescent nanoparticles (NPs) that have good contrast in electron microscopy; 130 nm gold‐core rhodamine B‐labelled silica particles, 15 nm CdSe/CdS/ZnS core–shell–shell quantum dots (QDs) and 230 nm Y2O3:Eu3+ particles. Robustness is studied by measuring the luminescence of (single) NPs after various cycles of electron beam exposure. The gold‐core rhodamine B‐labelled silica NPs and QDs are quenched after a single exposure to 60 ke− nm–2 with an energy of 120 keV, while Y2O3:Eu3+ NPs are robust and still show luminescence after five doses of 60 ke− nm–2. In addition, the luminescence intensity of Y2O3:Eu3+ NPs is investigated as function of electron dose for various electron fluxes. The luminescence intensity initially drops to a constant value well above the single particle detection limit. The intensity loss does not depend on the electron flux, but on the total electron dose. The results indicate that Y2O3:Eu3+ NPs are promising as robust fiducial marker in CLEM.

The efficiency of single-junction solar cells is limited to about 30% (the Shockley-Queisser limit). Spectral mismatch losses (transparency to low-energy photons, thermalization of high-energy photons) strongly contribute to lowering the maximum efficiency. To reduce thermalization losses, photon splitting is proposed and observed for a variety of lanthanide-doped materials. For ${\mathrm{Er}}^{3+}$, even a one-to-three photon-splitting process has been reported, yielding three IR photons at around 1530 nm following absorption of one blue-green photon. This is especially beneficial for narrow band gap solar cells, such as crystalline $\mathrm{Ge}$. Here, we report on photon splitting for ${\mathrm{Er}}^{3+}$ in ${\mathrm{YVO}}_{4}$. Following absorption in the ${}^{2}{\mathrm{H}}_{11/2}$ and ${}^{4}{\mathrm{S}}_{3/2}$ levels (520--550 nm), efficient cross-relaxation (CR) yields two excited ${\mathrm{Er}}^{3+}$ ions: one in the ${}^{4}{\mathrm{I}}_{9/2}$ state and one in the ${}^{4}{\mathrm{I}}_{13/2}$ state (CR1). A second CR step from the ${}^{4}{\mathrm{I}}_{9/2}$ state, leaving both ${\mathrm{Er}}^{3+}$ ions in the ${}^{4}{\mathrm{I}}_{13/2}$ excited state (CR2), is crucial in realizing efficient three IR photon splitting. It is demonstrated here that the second step has a low efficiency, as a result of competing fast multiphonon relaxation, ${}^{4}{\mathrm{I}}_{9/2}{\ensuremath{\rightarrow}}^{4}{\mathrm{I}}_{11/2}$, and a large energy mismatch, which makes the CR2 step thermally activated. Based on experiments and theory, a maximum quantum efficiency of 170% is calculated for IR emission, following blue-green excitation in ${\mathrm{YVO}}_{4}:{\mathrm{Er}}^{3+}$. An outlook is presented for three-photon splitting in low-phonon-energy hosts, where nonradiative multiphonon relaxation is suppressed. The anti-Stokes nature of the second CR step makes three-photon splitting unlikely and prevents the realization of IR quantum yields above 200%.

Phosphors have been used successfully for both research and commercial applications for decades. Eu

The relaxation pathways of states in the energy region of the fundamental absorption of NaMgF3:Yb2+ and MgF2:Yb2+ are investigated using time-resolved vacuum ultraviolet spectroscopy. For NaMgF3:Yb2+, excitation into intrinsic free exciton states or above the band gap results in emission features associated with self-trapped excitons and impurity-trapped excitons. Excitation into host-related states of MgF2:Yb2+ similarly exhibits self-trapped exciton emission as well as emission from Yb2+ 4f135d states. The excitation features related to Yb2+ 4f14→4f135d transitions are interpreted using semi-empirical crystal field models. For both materials, the interplay between intrinsic distortions and Yb impurity centres, as the excitation wavelength and sample temperature are varied, is presented and analysed. © 2019 Elsevier B.V.

We present a detailed investigation of the dynamics of impurity-trapped excitons in CaF 2 :Yb 2+ using transient photoluminescence enhancement induced via a two-frequency, sequential excitation process employing a UV optical parametric amplifier (OPA) synchronized to an infrared free electron laser (FEL). The temporal behaviour is well approximated by a multi-level rate equation model as relaxation between excited states of the exciton as well as a small contribution from local lattice heating by the FEL which becomes evident due to the 40 cm −1 splitting of the exciton excited states giving rise to the transient photoluminescence enhancement itself.

A new, facile and quick synthesis method for Eu2+ doped the alkaline earth fluorides was developed using ionic liquids as solvent, precursor and capping agent. Reductive atmosphere and very high temperatures were avoided, while still attaining the desired structure, small particle sizes and divalent oxidation state of the lanthanide. This opens the door for the development of new Ln2+ doped nanomaterials. The successful Eu2+ incorporation was proven by optical spectroscopic measurements which showed the spin and parity allowed f-d transitions of Eu2+ in CaF2:Eu2+/BaF2:Eu2+. 4f7-4f7 transitions could be observed at low temperatures (7 K).

The unusual temperature dependence of exciton emission decay in CsPbX3 perovskite nanocrystals (NCs) attracts considerable attention. Upon cooling, extremely short (sub-ns) lifetimes were observed and were explained by an inverted bright–dark state splitting. Here, we report temperature-dependent exciton lifetimes for CsPbCl3 NCs doped with 0–41% Mn2+. The exciton emission lifetime increases upon cooling from 300 to 75 K. Upon further cooling, a strong and fast sub-ns decay component develops. However, the decay is strongly biexponential and also a weak, slow decay component is observed with a ∼40–50 ns lifetime below 20 K. The slow component has a ∼5–10 times stronger relative intensity in Mn-doped NCs compared to that in undoped CsPbCl3 NCs. The temperature dependence of the slow component resembles that of CdSe and PbSe quantum dots with an activation energy of ∼19 meV for the dark–bright state splitting. Based on our observations, we propose an alternative explanation for the short, sub-ns exciton decay time in CsPbX3 NCs. Slow bright–dark state relaxation at cryogenic temperatures gives rise to almost exclusively bright state emission. Incorporation of Mn2+ or high magnetic fields enhances the bright–dark state relaxation and allows for the observation of the long-lived dark state emission at cryogenic temperatures.

Luminescence (nano)thermometry is an important technique for remote temperature sensing. The recent development of lanthanide-doped nanoparticles with temperature-dependent emission has expanded the field of applications, especially for ratiometric methods relying on the temperature variation of relative emission intensities from thermally coupled energy levels. Analysis and calibration of the temperature dependence is based on a Boltzmann equilibrium for the coupled levels. To investigate the validity of this assumption, we analyze and model thermal equilibration for Eu3+ D15 and D05 emission in NaYF4. The results show that for low Eu3+ concentrations, temperature-dependent multiphonon relaxation can accurately explain both the intensity ratio and emission decay dynamics. The analysis also reveals that a Boltzmann equilibrium is not realized in the temperature regime investigated (300-900 K). By increasing the Eu3+ concentration, cross relaxation between neighboring Eu3+ ions enhances D15-D05 relaxation rates and extends the temperature range in which emission intensity ratios can be used for temperature sensing (500-900+ K). The results obtained are important for recognizing, understanding, and controlling deviations from Boltzmann behavior in luminescence (nano)thermometry. By varying the dopant concentration, the range for accurate temperature sensing can be adjusted. These insights are crucial in the development and understanding of reliable temperature sensors.

In some materials, the absorption of a single photon can trigger a chain reaction that produces a large burst of light. The discovery of these photon avalanches in nanostructures opens the way to imaging and sensing applications. Highly nonlinear photon emission from lanthanide-doped nanoparticles.

The luminescence of Bi 2+ has its origin in p–p transitions between different spin–orbit states in the 6s 2 6p 1 electronic configuration. The divalent state is highly unusual for Bi and because of the low stability of Bi 2+ , research on the intraconfigurational p–p transitions of Bi 2+ is limited. Narrow-band emission has been observed in the orange/red spectral region, which makes Bi 2+ luminescence interesting for application in white light LEDs. In this paper we investigate the luminescence of Bi 2+ in a variety of borate host lattices with the aim to tune the emission wavelength and to investigate the influence of the host on the luminescence properties. Bi 2+ was doped into Sr 1− x Ba x B 4 O 7 , α-BaB 2 O 4 , α-BaB 4 O 7 , Ba 2 B 10 O 17 and BaB 8 O 13 . Luminescence was observed for Bi 2+ in all hosts, except α-BaB 2 O 4 , indicating that Bi 2+ cannot be stabilized in this host. In the other borates the emission wavelength varies between 586 nm (in BaB 8 O 13 ) and 671 nm (in α-BaB 4 O 7 ). The shift in emission wavelength is explained by a variation in crystal field splitting and spin–orbit coupling. The luminescence lifetime of the p–p emission is in the μ s range, varying between 6 and 13 μ s , reflecting the parity forbidden character of the p–p transition. Narrow-band red emission at 612 nm (FWHM=35 nm) is observed for Ba 2 B 10 O 17 :Bi 2+ . These luminescence characteristics of Bi 2+ are favorable for application in w-LEDs, but an important drawback is that only low concentrations (in the ppm range) can be stabilized in the divalent state.

${\mathrm{Y}}_{2}{\mathrm{O}}_{3}$ codoped with $\mathrm{B}{\mathrm{i}}^{3+}$ and $\mathrm{Y}{\mathrm{b}}^{3+}$ is considered as an efficient downconversion material combining strong broadband absorption of $\mathrm{B}{\mathrm{i}}^{3+}$ with photon splitting by cooperative energy transfer from $\mathrm{B}{\mathrm{i}}^{3+}$ to two $\mathrm{Y}{\mathrm{b}}^{3+}$ neighbors. However, evidence for photon splitting is lacking. Here we investigate the $\mathrm{B}{\mathrm{i}}^{3+}\text{\ensuremath{-}}\mathrm{to}\text{\ensuremath{-}}\mathrm{Y}{\mathrm{b}}^{3+}$ energy-transfer mechanism. For cooperative energy transfer the $\mathrm{Y}{\mathrm{b}}^{3+}$-concentration-dependent luminescence decay will show clear characteristics of cooperative dipole-dipole transfer. Analysis of $\mathrm{Y}{\mathrm{b}}^{3+}$-concentration and temperature-dependent decay curves however demonstrates that the energy-transfer mechanism is not cooperative but single step, probably through a $\mathrm{B}{\mathrm{i}}^{4+}\text{\ensuremath{-}}\mathrm{Y}{\mathrm{b}}^{2+}$ charge-transfer state. The temperature dependence of the $\mathrm{B}{\mathrm{i}}^{3+}\text{\ensuremath{-}}\mathrm{to}\text{\ensuremath{-}}\mathrm{Y}{\mathrm{b}}^{3+}$ energy-transfer efficiency is unusual as it decreases with temperature, unlike commonly observed thermally activated energy transfer. This is a signature of energy transfer via exchange interaction. The present results provide evidence for the absence of photon splitting in ${\mathrm{Y}}_{2}{\mathrm{O}}_{3}:\mathrm{B}{\mathrm{i}}^{3+},\mathrm{Y}{\mathrm{b}}^{3+}$ and form a convincing demonstration of exchange interaction mediated energy transfer.

The main losses in solar cells result from the incomplete utilization of the solar spectrum. Via the addition of an upconverting layer to the rear side of a solar cell, the otherwise-unused sub-bandgap photons can be utilized. In this paper, we demonstrate an efficiency enhancement of a silicon solar cell under real sunlight due to upconversion of sub-bandgap photons. Sunlight was concentrated geometrically with a lens with a factor of up to 50 suns onto upconverter silicon solar cell devices. The upconverter solar cell devices (UCSCDs) were also measured indoors using a solar simulator. To correct for differences in the spectral distribution between real sunlight and the solar simulator a spectral mismatch correction is required and is especially important to properly predict the performance when a non-linear response (e.g. upconversion) is involved. By applying a spectral mismatch correction, good agreement between the solar simulator measurements and the outdoor measurements using real sunlight was achieved. The method was tested on two different upconverter powders, β-NaYF4: 25% Er3+ and Gd2O2S: 10% Er3+, which were both embedded in a polymer. We determined additional photocurrents due to upconversion of 9.4 mA/cm2 with β-NaYF4 and 8.2 mA/cm2 with Gd2O2S under 94-suns concentration. Our results show i) the applicability of measurements using standard solar cell characterization equipment for predicting the performance of non-linear solar devices, and ii) underline the importance of applying proper mismatch corrections for accurate prediction of the performance of such non-linear devices.

Doping nanocrystals (NCs) with luminescent activators provides additional color tunability for these highly efficient luminescent materials. In CsPbCl

The role of transition metals in chemical reactions is often derived from probing the metal 3d states. However, the relation between metal site geometry and 3d electronic states, arising from multielectronic effects, makes the spectral data interpretation and modeling of these optical excited states a challenge. Here we show, using the well-known case of red ruby, that unique insights into the density of transition metal 3d excited states can be gained with 2p3d resonant inelastic X-ray scattering (RIXS). We compare the experimental determination of the 3d excited states of Cr3+ impurities in Al2O3 with 190 meV resolution 2p3d RIXS to optical absorption spectroscopy and to simulations. Using the crystal field multiplet theory, we calculate jointly for the first time the Cr3+ multielectronic states, RIXS, and optical spectra based on a unique set of parameters. We demonstrate that (i) anisotropic 3d multielectronic interactions causes different scaling of Slater integrals, and (ii) a previously not observed doublet excited state exists around 3.35 eV. These results allow to discuss the influence of interferences in the RIXS intermediate state, of core–hole lifetime broadenings, and of selection rules on the RIXS intensities. Finally, our results demonstrate that using an intermediate excitation energy between L3 and L2 edges allows measurement of the density of 3d excited states as a fingerprint of the metal local structure. This opens up a new direction to pump-before-destroy investigations of transition metal complex structures and reaction mechanisms.

A magnetic-field-dependent spectroscopy study on single perovskite nanocrystals reveals the spectral signatures of an exciton dark state below the bright triplet states.

Luminescence properties of divalent europium in the mixed-anion hydride chloride EuHCl were studied for the first time. Olive-green single crystals of EuHCl (PbFCl-type structure: tetragonal, P4/nmm, a = 406.58(3) pm, c = 693.12(5) pm, c/a = 1.705, Z = 2) resulted from the reaction of elemental europium (Eu), sodium hydride (NaH) and sodium chloride (NaCl), while powder samples were prepared from the binary components europium dihydride (EuH2) and dichloride (EuCl2). Low temperature X-ray powder diffraction proved the absence of phase transitions for 12(2) K ≤ T ≤ 295(2) K. Bright green emission was observed under UV-excitation and assigned to the 4f65d1–4f7 transition of divalent europium. Temperature-dependent luminescence absorption and emission, as well as lifetime measurements were carried out on single crystal and powder samples. Surprisingly, only limited concentration quenching was found. Additionally, two emission bands (485 and 510 nm) are observed, whose intensity ratio depends strongly on temperature. In order to explain this behavior for a single Eu2+ site, we suggest either a dynamical Jahn–Teller effect in the excited 5d1 state or emission from both a 4f65d1 state and a trapped exciton state.

We report the fabrication and characterization of a microfluidic droplet microreactor with potential use for single catalyst particle diagnostics. The aim is to capture Fluid Catalytic Cracking (FCC) particles in droplets and perform a probe reaction that results in a fluorescent output signal. The intensity of such a signal can be used as a measure of the catalytic activity of the particle. The microreactor features a droplet generator, platinum (Pt) microheaters, and Pt micro temperature sensors, and is able to operate at pressures up to at least 5 bar. Fluidic channels are etched in a silicon substrate, and platinum heater and sensor structures embedded in the glass cover. We have mapped the temperature inside the microchannels using nanoparticles that show temperature-dependent luminescence. At various spots on the chip, the temperature deviates by 0.86 degrees Celsius close to the Pt sensor and 5.5 degrees Celsius farther away from it. Experiments with making oil-in-water droplets at various temperatures and pressures result in stable droplets up to 100 degrees Celsius at atmospheric pressure. At this temperature, small gas bubbles are formed in the water phase, and then collected by the oil droplets. At a pressure of 5 bar, the droplets are stable up to at least 120 degrees Celsius. E-cat FCC particles were captured in water droplets at a rate of 150 droplets per second.

In this paper, a systematic study of the decay times of the spin-enabled and spin-forbidden transitions of Yb

We investigated thermally activated ionization and thermally activated crossover as the two possibilities of quenching of $5d$ luminescence in $\mathrm{P}{\mathrm{r}}^{3+}$-doped ${\mathrm{Y}}_{3}\mathrm{A}{\mathrm{l}}_{5\ensuremath{-}x}\mathrm{G}{\mathrm{a}}_{x}{\mathrm{O}}_{12}$. Varying the Ga content $x$ gives the control over the relative energy level location of the $5d$ and $4{f}^{2}:^{3}P_{J}$ states of $\mathrm{P}{\mathrm{r}}^{3+}$ and the host conduction band (CB). Temperature-dependent luminescence lifetime measurements show that the $5d$ luminescence quenching temperature ${T}_{50%}$ increases up to $x=2$ and decreases with further increasing Ga content. This peculiar behavior is explained by a unique transition between the two quenching mechanisms which have an opposite dependence of thermal quenching on Ga content. For low Ga content, thermally activated crossover from the $4f5d$ state to the $4{f}^{2}(^{3}P_{J})$ states is the operative quenching mechanism. With increasing Ga content, the activation energy for thermally activated crossover becomes larger, as derived from the configuration coordinate diagram, while from the vacuum referred binding energy diagram the activation energy of thermal ionization becomes smaller. Based on these results, we demonstrated that the thermal quenching of $\mathrm{P}{\mathrm{r}}^{3+}:5{d}_{1}\ensuremath{-}4f$ luminescence in ${\mathrm{Y}}_{3}\mathrm{A}{\mathrm{l}}_{5\ensuremath{-}x}\mathrm{G}{\mathrm{a}}_{x}{\mathrm{O}}_{12}$ with $x=0,\phantom{\rule{0.28em}{0ex}}1,\phantom{\rule{0.28em}{0ex}}2$ is a thermally activated crossover while for $x=3,\phantom{\rule{0.28em}{0ex}}4,\phantom{\rule{0.28em}{0ex}}5$ it results from the thermal ionization.

Light yield, time response, afterglow, and thermoluminescence of Ce-doped garnet scintillators and persistent luminescent materials are controlled by a complex interplay between recombination and trapping/detrapping processes. Extensive research has contributed to a good qualitative understanding of how traps, impurities, and the presence of Ce4+ affect the materials properties. In this work we present a quantitative model that can explain the thermoluminescence and afterglow behavior of complex garnets. In particular, the model allows the determination of capture rates and effective capture radii for electrons by traps and recombination centers in Lu1Gd2Ga3Al2O12:Ce garnet ceramics. The model relies on solving a set of coupled rate equations describing charge carrier trapping and recombination in garnet ceramics doped with Ce and also codoped with a known concentration of an intentionally added electron trap, Yb3+. The model is supported by analysis of a complete set of experimental data on afterglow, rise-time kinetics, and X-ray excited luminescence which show that thermoluminescence/afterglow are governed by trapping/detrapping processes following interactive kinetics with dominant recombination channel. The underlying reason for dominant recombination is the presence of a small fraction of Ce4+ (≈2 ppm in the 0.2% Ce-doped sample) which have a very high capture cross section (≈2.7 Å effective radius) because of the Coulomb attractive nature of this recombination center. The quantitative insights on capture cross sections and concentrations of Ce4+ help to better understand the optical properties of Ce-doped garnet scintillators and persistent luminescent materials and serve in optimizing synthesis procedures by tuning the Ce3+/Ce4+ ratio by codoping with divalent cations and annealing in an oxygen-containing atmosphere.