Search

Purpose Fuel demand forecast is a fundamental tool to guide private planning actions and public policies aim to guarantee energy supply. This paper aims to evaluate different forecasting methods to project the consumption of light fuels in Brazil (fuel used by vehicles with internal combustion engine). Design/methodology/approach Eight different methods were implemented, besides of ensemble learning technics that combine the different models. The evaluation was carried out based on the forecast error for a forecast horizon of 3, 6 and 12 months. Findings The statistical tests performed indicated the superiority of the evaluated models compared to a naive forecasting method. As the forecast horizon increase, the heterogeneity between the accuracy of the models becomes evident and the classification by performance becomes easier. Furthermore, for 12 months forecast, it was found methods that outperform, with statistical significance, the SARIMA method, that is widely used. Even with an unprecedented event, such as the COVID-19 crisis, the results proved to be robust. Practical implications Some regulation instruments in Brazilian fuel market requires the forecast of light fuel consumption to better deal with supply and environment issues. In that context, the level of accuracy reached allows the use of these models as tools to assist public and private agents that operate in this market. Originality/value The study seeks to fill a gap in the literature on the Brazilian light fuel market. In addition, the methodological strategy adopted assesses projection models from different areas of knowledge using a robust evaluation procedure.

Luminescent nanoparticles of Y2 O3 doped with europium (Eu) and/or titanium (Ti) were synthesized using modified sol-gel routes. The crystalline cubic phase was confirmed using X-ray powder diffraction (XRD). Particle morphology and size were evaluated using scanning and transmission electron microscopy. High-resolution transmission electron microscopy showed that the synthesis method affected the average particle size and the Fourier transform of the images showed the lattice plane distances, indicating that the samples presented high crystallinity degree in accordance with the XRD pattern. The Ti valence was investigated using X-ray absorption near edge spectroscopy and the tetravalent form was the dominant oxidizing state in the samples, mainly in Eu and Ti co-doped Y2 O3 . Optical behaviour was investigated through X-ray excited optical luminescence and photoluminescence under ultraviolet-visible (UV-vis) and vacuum ultraviolet (VUV) excitation. Results indicated that Eu3+ is the emitting centre in samples doped with only Eu and with both Eu and Ti with the 5 D0 →7 F2 transition as the most intense, indicating Eu3+ in a noncentrosymmetric site. Finally, in the Eu,Ti-doped Y2 O3 system, Ti3+ (or TiIV ) excitation was observed but no Ti emission was present, indicating a very efficient energy transfer process from Ti to Eu3+ . These results can aid the development of efficient nanomaterials, activated using UV, VUV, or X-rays.

In the last few years, few studies using upconversion to charge persistent luminescence were reported in the literature [1] , [2] . One of the suggested ways to do so is to synthesize an upconverting material and a persistent luminescent material and transfer energy radiatively, taking advantage of an overlap between the upconversion emission and persistent luminescence excitation [2] . Within this scheme, we synthesized Zn 1.33 Ga 1.34 Sn 0.33 O 4 :0.5%Cr 3+ (ZGSO), a recently known persistent luminescent material [3] at the nanoscale for the first time through hydrothermal method adapted from Maldiney et al. [4] and NaGdF 4 :Yb 3+ ,Er 3+ nanoparticles by precipitation, adapted from Palo et al. [5] . Both nanoparticles were mixed together by dry impregnation. TEM images of the NaGdF 4 -decorated particles at two scales are shown in Fig. 1a and 1b . The smaller hexagonal-shaped particles are NaGdF 4 :Yb,Er nanoparticles, while the bigger squares are ZGSO nanoparticles.

An alternative green route using a domestic microwave oven to synthesize NaGdF4:Yb,Tm and NaYF4:Yb,Er was successfully developed. Decreasing dopants concentration increased the β-NaREF4/α-NaREF4 ratio in NaGdF4. The scanning electron microscopy (SEM) images of NaGdF4 showed particles ranging from 500 nm to 1 μm with formation of aggregates up to 100 μm, with two different surface morphologies. The NaGdF4:Yb,Tm emission spectrum showed white-like emission due to Er3+ and Eu3+ contamination. The bandgaps of undoped NaGdF4 and NaYF4 were measured using synchrotron radiation vacuum-UV (VUV) diffuse reflectance, with values of 10.20 eV for the cubic phase and 10.80 eV for the hexagonal phase for both materials. The determination of the RE2+/3+ energy level positions in these fluorides sheds new light on the design of efficient upconversion materials.

Hackmanites, a variety of sodalite with the general formula Na$_{8}$Al$_{6}$Si$_{6}$O$_{24}$(Cl,S)$_{2}$, are a family of nature-based smart materials having the ability for reversible photochromism upon UV or X-ray exposure. Being nontoxic, cheap, and durable, hackmanite would be an optimal material for the visual detection of the presence of X-rays in simple portable systems. However, its X-ray-induced coloring abilities are so far known only qualitatively. In this work, a combination of experimental and computational methods is used to reveal the mechanism of X-ray-induced color changing in these materials. Finally, their use is demonstrated both in color intensity-based X-ray dosimetry and photochromic X-ray imaging.

The synthesis of persistent luminescent materials usually requires a multi-step long time annealing at high temperatures (>1200°C) in a resistive oven, causing a huge energy consumption. Also, to achieve reduced oxidation states of emitter ions (e.g., Eu3+ → Eu2+ ), the H2(g) atmosphere is often used, which can be dangerous and increase the costs of the process. Therefore, the development of a quick and new single-step green strategy, using in-situ low-risk atmosphere (e.g., CO(g)) and a microwave-assisted solid-state (MASS) method has been encouraged. In this work, we present a single-step method to synthesize the compound Sr2MgSi2O7:Eu2+,Dy3+ using the MASS method and the results were compared with those prepared by a conventional ceramic method. The luminescent material was prepared in 25 min of synthesis using carbon as a microwave susceptor and CO(g) atmosphere source at the same time. A higher concentration of Eu2+ emitter was identified by XANES in the MASS method product, which has a significant effect on the luminescence efficiency, as well as an improvement in the optical properties, leading to an emission 100 times more intense. Furthermore, to understand the Eu3+ reduction process under CO(g) atmosphere, we present here the innovative results of in-situ XANES analysis for the Sr2MgSi2O7:Eu2+,Dy3+ material. Finally, the MASS method makes it possible to prepare the materials with less than 5% of the ceramic method's duration in time. The energy-saving and better-quality persistent luminescent properties obtained in the MASS method provide viable applications on anti-counterfeiting markers, solar cell sensitizers, and other luminescent technologies.

The localized surface plasmon resonance (LSPR) excitation in plasmonic nanoparticles (NPs) in the visible and near-infrared ranges is currently at the forefront of improving photocatalytic performances via plasmonic photocatalysis. One bottleneck of this field is that the NPs that often display the best optical properties in the visible and near-infrared ranges are based on expensive noble metals such as silver (Ag) and gold (Au). While earth-abundant plasmonic materials have been proposed together with catalytic metals in antenna-reactor systems, their performances remain limited by their optical properties. Importantly, the synthesis of plasmonic photocatalysts remains challenging in terms of scalability while often requiring several steps, high temperatures, and special conditions. Herein, we address these challenges by developing a one-pot, gram-scale, room-temperature synthesis of earth-abundant plasmonic photocatalysts while improving their activities beyond what has been dictated by the LSPR excitation of the plasmonic component. We describe the mechanochemical synthesis of earth-abundant plasmonic photocatalysts by using MoO3 (antenna) and Au (reactor) NPs as a proof-of-concept example and demonstrate that the dual plasmonic excitation of antenna and reactor sites enables the tuning of plasmonic photocatalytic performances toward the reductive coupling of nitrobenzene to azobenzene as a model reaction. In addition to providing a pathway to the facile and gramscale synthesis of plasmonic photocatalysts, the results reported herein may open pathways to improved activities in plasmonic catalysis.

The use of luminescence in biological systems allows us to diagnose diseases and understand cellular processes. Persistent luminescent materials have emerged as an attractive system for application in luminescence imaging of biological systems; the afterglow emission grants background-free luminescence imaging, there is no need for continuous excitation to avoid tissue and cell damage due to the continuous light exposure, and they also circumvent the depth penetration issue caused by excitation in the UV-Vis. This review aims to provide a background in luminescence imaging of biological systems, persistent luminescence, and synthetic methods for obtaining persistent luminescent materials, and discuss selected examples of recent literature on the applications of persistent luminescent materials in luminescence imaging of biological systems and photodynamic therapy. Finally, the challenges and future directions, pointing to the development of compounds capable of executing multiple functions and light in regions where tissues and cells have low absorption, will be discussed.

Here, the upconversion behaviour of Yb3+/Er3+-codoped layered Dion–Jacobson calcium-niobium perovskites was investigated. This lanthanide pair is majorly responsible for the efficient green and red anti-Stokes-like emissions in rare-earth fluorides. Layered perovskites of the composition KCa2Nb3O10 were synthesized by the ceramic method by replacing 1% per mol of Ca2+ by lanthanide ions in such a way that the Yb3+:Er3+ molar proportion varied from 2 : 1 to 20 : 1. Non-doped and four doped materials were characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and emission spectroscopy. All materials showed similar XRD profiles, which indicated the isolation of well crystalline single phases. On the other hand, the morphologies accessed by SEM were different: microparticles with well-defined edges were replaced by particles with poorly defined edges, rounded features, and intergrowth aspect when the niobate was doped. The Raman spectra showed modifications in the bands below 200 cm−1 attributed to low-phonon-energy transitions. The doped layered niobates presented almost pure-green emission (excitation at 980 nm), which could be related to the low energy values of the vibrational levels, precluding energy loss. Additionally, the results indicated that the defects generated by the lanthanide ion isomorphic-aliovalent substitution in Ca2+ sites were mainly in the interlayer region. This coupled behaviour reduced the non-radiative decays of the upconversion centres, leading to the formation of an efficient emitting material. The spectroscopic emission data showed a two-photon major mechanism to excite the emitting levels via an energy transfer upconversion (ETU) population process. The lifetime results indicated that increasing the Yb3+ concentration did not lead to non-radiative decay paths. The high efficiency and the versatility of the layered niobate hosts can provide new insights into the design of efficient morphology-controlled materials.

Persistent luminescence materials are a reality in several applications. However, there is still a lack of efficient red-emitting materials. SrS:Eu2+ phosphor is a potential candidate since its strong nephelauxetic effect shifts Eu2+4f65d1 → 4f7 to red, and its weak bond between strontium and sulphide, due to the soft base-hard acid character, generates a high number of intrinsic defects. SrS:Eu2+,RE3+ materials were efficiently prepared by two rounds of 22 min microwave-assisted solid-state synthesis. The highly crystalline purity and the material organization at the micro-scale were observed with X-ray powder diffraction and scanning electron microscopy, respectively. X-ray absorption spectroscopy revealed a low amount of Eu2+ compared to Eu3+ due to the efficient Eu2+ photo-oxidation by X-ray irradiation in the high storage capability SrS host matrix. Electron paramagnetic resonance spectra confirmed that at least 50% of Eu2+ ions in the material are photo-oxidized during excitation, reinforcing the previously established mechanisms. The RE2+ energy level positioned very close to or into the conduction band led to an abnormal co-doping effect, with similar effects independent of the co-dopant. The high concentration of intrinsic defects in SrS indicates that the soft–hard pair host is an excellent approach to develop efficient persistent luminescence materials.

This study is intended to characterize the economic and financial status of the sugarcane energy industry in Brazil, providing relevant information for new investors, directors of companies and government to make more appropriate decisions. The data used in the study was obtained from financial statements issued by about 150 plants in the 2014/2015 and 2015/2016 harvest seasons. Following the logic proposed by the approach known as financial ratio analysis, financial indicators were calculated to measure the liquidity, profitability and debt of the plants. A cluster analysis was applied to define representative groups of companies in the 2014/2015 harvest season and a discriminant analysis was used to classify companies in the 2015/2016 crop season. The results showed high heterogeneity in the industry, marked by the presence of four different groups of companies. The first group includes high-liquidity and low-debt units. The fourth group is characterized by companies with high debt, low liquidity and negative operating margins. The two intermediate groups are made up of plants with moderate leverage and liquidity, but with operational performance levels varying from high to low. It should be noted that higher revenues in the 2015/2016 harvest season made it possible to improve the financial performance of the companies, except of those with in the fourth group. The results indicate difficulties to expand production capacity in the short term, but also recovery possibilities in the medium and long term if market conditions are favorable and the institutional framework in the country's fuel industry is lasting and predictable.

The synthesis of efficient nanosized persistent luminescence materials remains a challenge for the community. Paradoxically, due to the dependence of the point lattice defects and the persistent luminescence efficiency, the control of the defect formation, favorable when the materials are prepared at high temperatures, normally leads to particle growth and sintering. In this work, efficient nanosized rare earth doped disilicates Sr2MgSi2O7:Eu2+,Dy3+ were synthesized via three different wet-chemical methods taking advantages of the microwave-assisted reduction process as a support step to produce high-quality polycrystalline materials. The crystallite size of the sample showed to be smaller when the decomposition temperature of the precursors is higher and close to the phase formation energy. The excitation VUV spectroscopy indicated that despite being nanocrystalline, the materials optical band gap has just a small difference compared one to another. The reduction of Eu3+ to Eu2+ was successfully obtained, since the f-d interconfigurational transitions of Eu2+ 4f65d1→4f7 emission were observed in the blue region of the spectra. The persistent luminescence efficiency measured through its lasting decay time was close to the commercial materials references and with the advantage of having size control during the synthesis method that can lead to the size dependent applications of photonic materials.

This work aims the investigation of lithium excess influence on the X-ray Excited Optical Luminescence (XEOL) and X-ray Absorption Spectroscopy (XAS) of Eu-doped LiAl5O8 samples. XEOL spectra excited at Eu LIII-edge is composed of emission peaks attributed to the emission of Eu2+, Eu3+ and Cr3+ ions. The results showed that lithium excess contributes to important differences concerning the relative intensity of Eu2+ and Eu3+ emissions, suggesting that can Li+ or Li vacancies act to create defects able to stabilize the Eu2+ on the host lattice. Scanning electronic microscopy images showed that the Li+ excess did not contribute to significant changes on the particles' morphology and size. Based on the XAS measurements, we suggest that defects such as V’Li may be created in the LiAl5O8 structure contributing to the Eu2+ stabilization. Consequently, the Li+ excess favors the Eu3+ stabilization and the preparation of red emitting phosphors.

RE2O2S:Yb3+ and RE2O2S:Yb3+,Ti,Mg2+ near-infrared persistent luminescence materials (RE3+: La, Gd, Y) were designed based on the efficient red-emitting RE2O2S:Eu3+,Ti,Mg2+, due to the similar ground state level positions between Eu3+ and Yb3+. The Y2O2S:Yb3+,Ti,Mg2+ showed the longest near-infrared (~983 nm) persistent luminescence time (~1 h) when excited at the charge transfer transition, demonstrating that Ti,Mg2+ co-doping enhances the persistent luminescence only for the Y2O2S host. The unprecedented relationship between Eu3+ and Yb3+ persistent luminescence opens a new path in the systematic design of efficient Yb3+-activated persistent luminescence materials e.g. Y2O2S:Yb3+,Ti,Mg2+, which present potential for photonic applications such as bioimaging probes.

In this work, the microwave-assisted solid-state (MASS) synthesis is reported as a rapid, cost-effective and environmental-friendly way to prepare Gd2O2S:Tb3+ X-ray scintillation standards. The preparation procedure employs active charcoal as the microwave susceptor, using a domestic microwave oven without a setup for special gases. The MASS method drastically reduced the synthesis time and energy consumption (up to 97% in nominal power) over previous reports using conventional solid-state methods. A single-step synthesis of 10 min is enough to yield highly crystalline powders starting from Gd2O3 and Tb4O7 precursors and elemental sulfur. In order to obtain 100% pure materials with superior scintillation efficiency, two microwave treatments of 25 min each are required. Due to the reducing CO atmosphere generated in situ by the active charcoal during the microwave synthesis, no Tb4+ impurity was found according to Synchrotron Radiation X-ray Absorption (SR-XAS) experiments. Spectroscopic studies were carried out using SR Vacuum-Ultraviolet (VUV) photoluminescence and SR X-ray Excited Optical Luminescence (XEOL). The material prepared by two-step synthesis exhibited a better scintillation performance owing to higher crystal purity and emission intensity, presenting versatility for technological applications such as X-ray imaging and scintillation bio-probing.

Rare earth silicate based MnMgSi2O5+n (M = Ca, Sr or Ba and n=1-2) phosphors, have attracted interest of researchers due to their high efficiency as a host, excellent thermal and chemical stability and high brightness adding to their low cost. These phosphors showed great potential in various applications such as fluorescent lamps, white light emitting diodes, and display components. High temperature solid-state reactions are usually employed to synthesize those compounds. This paper proposes an alternative method of obtaining nanophosphor host based on Eu-doped CaMgSi2O6 (CMS:Eu), persistent luminescence phosphor. Sol gel technique combined to a modified molten salt method was used. The resulted powder was calcined for 3h under an atmosphere of 5% H2 and 95% Ar2. Phase identification by XRD and the measurements of photoluminescence (PL) and photoluminescence excitation (PLE) were performed. Single phased CMS:Eu with persistent luminescence characteristics was prepared.

The impurities and dopants' inappropriate valences may deteriorate the performance of luminescent materials, cause waste of the precious rare earth (R) material and thus incur financial losses. The usual methods to detect the valence of rare earths; XPS, Mossbauer and XANES spectroscopies, are not sensitive enough for low concentrations and the EPR methods are not very suitable for powders. In this work, the comparison between the theoretical and experimental temperature-dependent paramagnetic susceptibilities was used to obtain quantitatively the concentrations of the impurity valence in Eu 2 O 2 S and Tb 2 O 2 SO 4 , both containing nominally only R 3+ . Minute (ppm level) Eu 2+ impurities could be analyzed because of the huge difference in the paramagnetic susceptibility between Eu 2+ and Eu 3+ at low temperatures. However, temperatures below 50 K are then needed whilst the Tb IV impurity in a Tb 3+ matrix can be observed already at higher temperatures. The latter method based on comparing the slopes of the Tb 3+ /Tb IV paramagnetic susceptibility vs temperature curves for the Tb 3+ /Tb IV couple is less sensitive than for the Eu 2+ /Eu 3+ one. Finally, the host independent temperature evolution of the paramagnetic susceptibility was calculated for Gd 3+ (or Eu 2+ or Tb IV ) to yield a simple analytical expression to be used universally.

Persistent luminescence materials Lu2O3:R(3+),M (Pr,Hf(IV); Eu; or Tb,Ca(2+)) were successfully and rapidly (22 min) prepared by microwave-assisted solid-state synthesis (MASS) using a carbon microwave susceptor and H3BO3 as flux. Reaction times are reduced by up to 93% over previous synthetic methods, without special gases application and using a domestic microwave oven. All materials prepared with H3BO3 flux exhibit LuBO3 impurities that were quantified by Rietveld refinement from synchrotron radiation X-ray powder diffraction patterns. The flux does not considerably affect the crystalline structure of the C-Lu2O3, however. Scanning electron micrographs suggest low surface area when H3BO3 flux is used in the materials' synthesis, decreasing the amount of surface hydroxyl groups in Lu2O3 and improving the luminescence intensity of the phosphors. The carbon used as the susceptor generates CO gas, leading to complete reduction of Tb(IV) to Tb(3+) and partial conversion of Pr(IV) to Pr(3+) present in the Tb4O7 and Pr6O11 precursors, as indicated by X-ray absorption near-edge structure data. Persistent luminescence spectra of the materials show the red/near-IR, reddish orange, and green emission colors assigned to the 4f(n) → 4f(n) transitions characteristics of Pr(3+), Eu(3+), and Tb(3+) ions, respectively. Differences between the UV-excited and persistent luminescence spectra can be explained by the preferential persistent luminescence emission of R(3+) ion in the S6 site rather than R(3+) in the C2 site. In addition, inclusion of Hf(IV) and Ca(2+) codopants in the Lu2O3 host increases the emission intensity and duration of persistent luminescence due to generation of traps caused by charge compensation in the lattice. Photonic materials prepared by MASS with H3BO3 flux show higher persistent luminescence performance than those prepared by the ceramic method or MASS without flux. Color tuning of persistent luminescence in Lu2O3:R(3+),M provides potential applications in bioimaging as well as in solar cell sensitizers.

Trivalent europium (Eu3+ ) and terbium (Tb3+ ) ions are important activator centers used in different host lattices to produce red and green emitting materials. The current work shows the design of new clay minerals to act as host lattices for rare earth (RE) ions. Based on the hectorite structure, nano-chlorohectorites and nano-fluorohectorites were developed by replacing the OH- present in the hectorite structure with Cl- or F- , thus avoiding the luminescence quenching expected due to the OH- groups. The produced matrices were characterized through X-ray powder diffraction (XPD), transmission electron microscopy (TEM), FT-IR, 29 Si MAS (magic angle spinning) NMR, nitrogen sorption, thermogravimetry-differential scanning calorimetry (TGA-DSC) and luminescence measurements, indicating all good features expected from a host lattice for RE ions. The nano-clay materials were successfully doped with Eu3+ and/or Tb3+ to yield materials preserving the hectorite crystal structure and showing the related luminescence emissions. Thus, the present work shows that efficient RE3+ luminescence can be obtained from clays without the use of organic 'antenna' molecules.

The NaYF4:Yb3+,R3+ (R: none, Pr, Nd, Sm, Eu, Tb or Dy) materials were prepared with selected dopant concentrations, using co-precipitation synthesis, to study the effect of the dopant and its concentration on the structure of these materials. The thermal behaviour of the as-prepared materials was studied with differential scanning calorimetry (DSC) and thermogravimetry. The structures prior to and after annealing were identified with X-ray powder diffraction. The materials were mainly hexagonal with occasional slight cubic impurity. The DSC curves revealed the cubic-to-hexagonal phase transition at 400–450 °C which temperature changes irregularly with the R3+ dopant and its concentration. The specific enthalpy of this transition varies also in a complicated way but may be correlated with the completeness of the transition. The hexagonal-to-cubic transition temperature (ca. 670 °C) is rather constant regardless of the R3+ dopant or its concentration. The temperatures and specific enthalpies of the phase transitions are useful when choosing the optimum dopant concentrations. It is also possible to estimate the structure prior to annealing with significant savings in use of resources. Only with Sm3+ and Dy3+ doping, no visible up-conversion luminescence was observed—in addition to the Er3+ and Tm3+ impurity emission. Eventually, it was found that the hexagonal form gives much stronger up-conversion luminescence and changes in the rare earth concentration allows the formation of this form.

The R 2 O 3 :Eu 3+ (R 3+ : Y, Gd and Lu) nanophosphors were synthesized with a low temperature thermolysis method using rare earth 1,3,5-benzenetricarboxylate (TMA) complexes as precursors. The R(TMA):Eu 3+ (1 mole %) complexes were synthesized using the co-precipitation method which complexes were then annealed at temperatures from 500 to 1000 °C for 1 h, yielding the cubic C-type R 2 O 3 :Eu 3+ materials. The XPD patterns revealed no impurities in C-R 2 O 3 obtained at any temperature. The Scherrer’s equation based on powder diffraction data confirmed an increase in crystallite size from 6 to 52 nm with increasing annealing temperature from 500 to 1000 °C for Y 2 O 3 , respectively, agreeing with the SEM and TEM data. Similar results were obtained for the other R 2 O 3 :Eu 3+ . The excitation spectra show a broad oxide to europium charge transfer absorption band at 260 nm, i.e. O 2− (2p) → Eu 3+ (4f 6 ) LMCT, as well as the narrow lines assigned to the 4f–4f transitions of the Eu 3+ ion. The characterization of the photoluminescence properties of the phosphors was also based on the analysis of the emission spectra and luminescence decay curves. The experimental intensity parameters ( Ω λ ), lifetimes ( τ ), as well as radiative ( A rad ) and non-radiative ( A nrad ) decay rates were calculated and discussed.

Trivalent rare-earth (RE3+) phenolate complexes have been receiving growing attention owing to their remarkable structural, luminescent, and magnetic properties. The RE3+ ions play an important role in the synthetic mechanisms acting as template, leading to a large number of homodinuclear and heterodinuclear coordination compounds presenting macrocyclic and macroacyclic phenolate ligands. The photoluminescent behavior of these kinds of complexes has been attributed to an efficient intramolecular energy transfer from the ligands to the RE3+ ions. The RE phenolate complexes have opened new possibilities for practical applications such as in RE3+ separation, in tunable photonic devices, in fluoroimmunoassays, and as paramagnetic contrast agents in magnetic resonance imaging. Keywords: the chemistry of metal phenolate complexes; structural properties of rare-earth phenolate complexes; RE3+ compounds containing acyclic and cyclic phenolate ligands; rare-earth–hydroxyquinolinate, -hydroxypyridine, -hydroxylpicolinate, -hydroxynicotinate cryptate, and -oxazolylphenol complexes; RE3+ luminescence properties

The up-converting ZrO2:Yb3+,E3+ nanomaterials were prepared with the combustion and sal-gel methods. The structure of the materials was cubic except for the minor monoclinic and tetragonal impurities associated with the use of the sal-gel method. The XANES results revealed only the trivalent form for both the Yb and Er dopants. The distance distributions calculated from EXAFS confirm that Er3+ and Yb3+ occupy the Zr-IV site in the structure. The nanomaterials show red (650-700 nm) and very weak green (520-560 nm) up-conversion luminescence due to the F-4(9/2) -> I-4(15/2) and (H-2(11/2),S-4(3/2)) -> I-4(15/2) transitions of Er3+, respectively. The nanomaterials obtained with the combustion synthesis yield the most intense luminescence though showing significant afterglow, as well. The thermoluminescence measurements revealed the existence of traps with depths between 0.68 and 1.03 eV well matching to room temperature persistent emission. Finally, the mechanism for the persistent up-conversion luminescence was introduced based on the experimental results and discussed. (C) 2014 Elsevier B.V. All rights reserved. (Less)

Photon up-conversion which yields higher energy emission by stacking lower energy photons is possible only with specific rare earth ions. Despite this, it has several potential applications. NaYF4 with Yb3+,Er3+ co-doping has been recognized as one of the most feasible materials for efficient up-conversion luminescence. In this work, the up-conversion luminescence of the Pr3+–Er3+ combination was studied using sensitization by Yb3+. Emission was observed in the visible including blue, green, yellow, orange and red. These are due to the 3P0,1 → 3H4–6,3F2–4 and 1D2 → 3H4 (Pr3+) as well as 2H11/2,4S3/2,4F9/2 → 4I15/2 (Er3+) transitions. Concentration quenching of the Pr3+ luminescence was observed already with 1 mol-% due to many cross-relaxation processes. With naked eye, the up-conversion luminescence was seen as white light. The CIE chromaticity coordinates are close to those of the standard illuminant F4 which represents warm white.

The R3+ (rare earth) co-dopants may have a surprisingly important role in persistent luminescence – enhancement of up to 1–3 orders of magnitude may be obtained in the performance of these phosphor materials – depending strongly on the R3+ ion, of course. In this work, the effects of the R3+ co-dopants in the BaAl2O4:Eu2+,R3+ materials were studied using mainly thermoluminescence (TL) and synchrotron radiation XANES methods. In BaAl2O4, the conventional and persistent luminescence both arise from the 4f7→4f65d1 transition of Eu2+, yielding blue–green emission color. The former, in the presence of humidity, turns to more bluish because of creation of an additional Eu2+ luminescence centre which is not, however, visible in persistent luminescence. The trap structure in the non-co-doped BaAl2O4:Eu2+ is rather complex with 4–5 TL bands above room temperature. With R3+ co-doping, this basic structure is modified though no drastic change can be observed. This underlines the fact that even very small changes in the trap depths can produce significant modifications in the persistent luminescence efficiency. It should be remembered that basically the persistent luminescence performance is controlled by the Boltzmann population law depending exponentially on both the temperature and trap depth. Some mechanisms for persistent luminescence have suggested the presence of either divalent R2+ or tetravalent RIV during the charging of the Eu2+ doped materials. The present XANES measurements on BaAl2O4:Eu2+,R3+ confirmed the presence of only the trivalent form of the R3+ co-dopants excluding both of these pathways. It must thus be concluded, that the energy is stored in intrinsic and extrinsic defects created by the synthesis conditions and charge compensation due to R3+ co-doping. Even though the effect of the R3+ co-dopants was carefully exploited and characterized, the differences in the effect of different R3+ ions with very similar chemical and spectroscopic properties could not be explained in a satisfactory manner. More work is – and perhaps a completely new approach may be – needed.

The synthesis conditions of the Yb 3+ and Tb 3+ co-doped NaYF 4 were optimized by reducing the number of washings to include only ethanol. The avoidance of the loss of amorphous NaF prior to post-annealing of the as-prepared materials resulted in the enhancement of the otherwise rather weak up-conversion from Tb 3+ by 1–2 orders of magnitude. At the same time, the temperature of formation of the hexagonal NaRF 4 phase with high up-conversion could be lowered by 100 °C down to 350 °C. This improvement in up-conversion was concluded to result from the better stoichiometry of the material without washing with water. The deficit of Na + would result in the excess of fluoride which, although not as fatal to the luminescence as the fluoride vacancies, has serious implications to the up-conversion intensity. A further enhancement in the up-conversion luminescence was observed to be due to the Er 3+ ion impurity frequently associated with high-concentration Yb 3+ materials. The mechanism involving the unintentional Er 3+ sensitizer and the resonance energy transfer in the Yb 3+ –Er 3+ –Tb 3+ co-doped NaYF 4 were discussed based on the energy level schemes of the Yb 3+ , Er 3+ , and Tb 3+ ions in NaYF 4 .

The NaYF4:Yb3+, Tb3+ (xYb: 0.20, xTb: 0.04) materials were prepared using the co-precipitation method. The as-prepared material was washed either with or without water in addition to ethanol and thereafter annealed for 5 h at 500 °C. This resulted in materials with moderate or very high up-conversion luminescence intensity, respectively. The structural study carried out with X-ray powder diffraction revealed microstrains in the rare earth (R) sublattice that were relaxed for the material with very high up-conversion intensity thus decreasing energy losses. The local structural details were investigated with R LIII and Y K edge extended X-ray absorption fine structure (EXAFS) using synchrotron radiation. Around 10 mol.% of the Yb3+ ions were found to occupy the Na site in the material with very high up-conversion intensity. These Yb species formed clusters with the Tb3+ ions occupying the regular Na/R sites. Such clustering enhanced the energy transfer between Yb3+ and Tb3+ thus intensifying the up-conversion emission.

Similar to many other Eu2+,RE3+-co-doped persistent luminescence materials, for Sr2MgSi2O7:Eu2+,RE3+ the initial intensity and duration of persistent luminescence was also found to depend critically on the rare-earth (RE) co-doping. An enhancement of 1 - 2 orders of magnitude in these properties could be obtained by Dy3+ co-doping whereas total quenching of persistent luminescence resulted from the use of Sm3+ and Yb3+. To solve this drastic disparity, the effects of the individual RE3+ ions were studied with thermoluminescence (TL) spectroscopy to derive information about the formation of traps storing the excitation energy. The charge compensation defects were concluded to be the origin of the complex TL glow curve structure. The tuning of the band gap of the Sr2MgSi2O7 host and especially the position of the bottom of the conduction band due to the Eu2+,RE3+ co-doping was measured with the synchrotron radiation vacuum UV (VUV) excitation spectra of the Eu2+ dopant. The model based on the evolution of the band gap energy with RE3+ co-doping was found to explain the intensity and duration of the persistent luminescence.

Layered Silicates (LS) such as hectorite clays have been used as host matrices for optically functional species due to their tunable structure. Luminescence has been reported for different phosphors using LS as a host host intercalated with rare earth containing compounds. However, the LS composition has OH- ions which may result in severe luminescence quenching. In the present work, a new LS material based on the hectorite structure was synthesized by replacing OH- with F-. This LS produced without doping strong blue/green emission. Also persistent luminescence was observed for this material with the main emission lifetime about 321 ms. It is the first time that such kind of luminescence has been reported for trioctahedral smectites. The origin of the luminescence was proved to be a Ti3+ impurity - originally as TiIV in the SiO2 precursor. Both conventional and persistent luminescence mechanisms were suggested for this “non-doped” system.

Highly luminescent rare earth (R3+) doped alkaline-earth tungstates MWO4:R3+ (M2+: Ca, Sr and Ba, R3+: Eu, Tb, Gd) were prepared with a room temperature coprecipitation method. The phosphors were characterized by X-ray powder diffraction (XPD), thermal analysis (TG), infrared absorption spectroscopy (FTIR) and UV excited photoluminescence. The as-prepared MWO4:R3+ particles belong to the tetragonal scheelite phase, and are well crystallized and are of the average size of 16–48 nm. The excitation and emission spectra of the materials were recorded at 300 and 77 K temperatures. The luminescent materials exhibit intense red (Eu3+) and green (Tb3+) colors under UV excitation. The excitation spectra of the Eu3+ doped materials show broad bands arising from the ligand-to-metal charge transfer transitions (O2−→WVI and O2−→Eu3+) as well as narrow bands from 4f–4f intraconfigurational transitions of Eu3+. 4f–4f emission data of the Eu3+ and Tb3+ in the MWO4 host matrices as well as the values of emission quantum efficiencies of the 5D0 level and the 4f–4f experimental intensity parameters of Eu3+ ion are presented and discussed.

The persistent luminescence of CdSiO3:Tb3+ was investigated with photoluminescence, thermoluminescence (TL), synchrotron radiation X-ray absorption (XANES and EXAFS) and UV-VUV spectroscopies. Only the typical intraconfigurational 4f8–4f8 transitions of the Tb3+ ion were observed with no traces of band emission in either the conventional UV excited or persistent luminescence spectra. The trap structure from TL with three traps from 0.65 to 0.85 eV is ideal for room-temperature persistent luminescence similar to, e.g., Sr2MgSi2O7:Eu2+,R3+. Despite the rather low band gap energy, 5.28 eV, the persistent luminescence from Tb3+ is produced only under UV irradiation due to the inauspicious position of the 7F6 ground level deep in the band gap of CdSiO3. This confirms the role of electrons as the charge carriers in the mechanism of Tb3+ persistent luminescence. The XANES spectra indicated the presence of only the trivalent Tb3+ species, thus excluding the direct Tb3+ → TbIV oxidation during the charging process...

The methaneseleninate and 1,10-phenanthroline were used as ligands in the synthesis of new lanthanide complexes. The photostability, emission quantum yield (q) and quantum efficiency (η) of the 5D0 emitting level of the Eu3+ ion were determined. An energy level diagram was used to establish the most relevant channels involved in the ligand-to-metal intramolecular energy transfer process. The nephelauxetic effect was investigated to assess the covalency of the ligand–metal chemical bond. The values of the experimental 4f–4f intensity parameters, suggest that this ion is in a chemical environment less polarisable than in the case of complexes with β-diketonates as ligands.

The details of the mechanism of persistent luminescence were probed by investigating the trap level structure of Sr2MgSi2O7:Eu2+,R3+ materials (R: Y, La-Lu, excluding Pm and Eu) with thermoluminescence (TL) measurements and Density Functional Theory (DFT) calculations. The TL results indicated that the shallowest traps for each Sr2MgSi2O7:Eu2+,R3+ material above room temperature were always ca. 0.7 eV corresponding to a strong TL maximum at ca. 90 °C. This main trap energy was only slightly modified by the different co-dopants, which, in contrast, had a significant effect on the depths of the deeper traps. The combined results of the trap level energies obtained from the experimental data and DFT calculations suggest that the main trap responsible for the persistent luminescence of the Sr2MgSi2O7:Eu2+,R3+ materials is created by charge compensation lattice defects, identified tentatively as oxygen vacancies, induced by the R3+ co-dopants.

The persistent luminescence materials, barium aluminates doped with Eu2+ and Dy3+ (BaAl2O4:Eu2+,Dy3+), were prepared with the combustion synthesis at temperatures between 400 and 600 °C as well as with the solid state reaction at 1500 °C. The concentrations of Eu2+/Dy3+ (in mol% of the Ba amount) ranged from 0.1/0.1 to 1.0/3.0. The electronic and defect energy level structures were studied with thermoluminescence (TL) and synchrotron radiation (SR) spectroscopies: UV–VUV excitation and emission, as well as with X-ray absorption near-edge structure (XANES) methods. Theoretical calculations using the density functional theory (DFT) were carried out in order to compare with the experimental data.

New lanthanide complexes with benzeneseleninic (ABSe) and 4-chloro-benzeneseleninic (ABSeCl) acids have been synthesized and characterized by elemental analysis, infrared and UV–visible spectroscopies. The emission spectra of the trivalent europium complexes presented the typical electronic 5 D 0 → 7 F J transitions of the ion ( J =0–4). The ground-state geometries of the europium complexes have been calculated by using the Sparkle/AM1 model. From these results, the 4f–4f intensity parameters and energies of the ligand singlet and triplet excited states have been obtained. The lower emission quantum yield for the [Eu(ABSe) 3 (H 2 O) 2 ](H 2 O) 2 compound, as compared to the [Eu(ABSeCl) 3 (H 2 O) 2 ] one, can be associated to the higher numbers of water molecules, in the first and second coordination spheres, that contribute to the luminescence quenching. The [Eu(ABSe) 3 (H 2 O) 2 ](H 2 O) 2 complex presents an intermediate state whose energy difference with respect to the first excited singlet state is resonant with three phonons from the water molecules, favouring a multiphonon relaxation process from the singlet state followed by a fast internal conversion process; this effect is less pronounced in the complex with the ABSeCl ligand. The luminescence decay curves of the gadolinium complexes indicate that the level responsible for the intramolecular energy transfer process has a triplet character for both compounds. The nephelauxetic effect in these compounds was investigated under the light of a recently proposed covalency scale based on the concept of overlap polarizability of the chemical bond.

Polycrystalline Eu2+ and Dy3+ doped barium aluminate materials, BaAl2O4:Eu2+,Dy3+, were prepared with solid state reactions at temperatures between 700 and 1500 °C. The influence of the thermal treatments on the stability, homogeneity and structure as well as to the UV-excited and persistent luminescence of the materials was investigated by X-ray powder diffraction, SEM imaging and infrared spectroscopies as well as by steady state luminescence spectroscopy and persistent luminescence decay curves, respectively. The IR spectra of the materials prepared at 250, 700, and 1500 °C follow the formation of BaAl2O4 composition whereas the X-ray powder diffraction of compounds revealed how the hexagonal structure was obtained. The morphology of the materials at high temperatures indicated important aggregation due to sintering. The luminescence decay of the quite narrow Eu2+ band at ca. 500 nm shows the presence of persistent luminescence after UV irradiation. The dopant (Eu2+) and co-dopant (Dy3+) concentrations affect the crystallinity and luminescence properties of the materials.

Materiais com luminescência persistente são normalmente baseados em aluminatos e silicatos dopados com íons Eu2+, portanto buscam-se novos materiais com diferentes dopantes. Consequentemente, os materiais BaAl2O4:Eu2+,TR3+ (R: La-Lu) foram preparados pelo método de combustão com temperaturas de ignição entre 400 e 600 °C, assim como pelo método cerâmico a 1500 °C. Os sistemas mais versáteis CdSiO3:TR3+ foram preparados apenas pelo método cerâmico a 950 °C. Para estudar a estrutura e a pureza de fase dos materiais utilizou-se espectroscopia de absorção no infravermelho, difração de raios X (método do pó) e microscopia eletrônica de varredura. Os estados de oxidação e o ambiente ao redor dos íons terras raras foram investigados com a espectroscopia de absorção de raios X com radiação Síncrotron (XANES e EXAFS). As propriedades da luminescência persistente dos fósforos foram investigadas com as técnicas de termoluminescência (TL), fotoluminescência e espectroscopia com radiação Síncrotron na região do UV-UV vácuo. Baseado nas medidas de TL, a estrutura dos defeitos dos materiais BaAl2O4:Eu2+,TR3+ (TR: Nd e Dy) e CdSiO3:TR3+ (TR: Pr, Gd e Tb) mostrou-se ideal para a luminescência persistente a temperatura ambiente. Os espectros de XANES indicaram a presença dos estados divalente e trivalente do Eu dopado na matriz BaAl2O4, enquanto que apenas íons TR3+ co-dopantes foram observados (exceto CeIV). O íon Eu3+ pode estar presente devido ao processo de migração do elétron do íon Eu2+ para a banda de condução durante o processo de excitação. Em contraste, para o sistema CdSiO3:TR3+, foram observadas apenas as espécies trivalentes TR3+, excluindo, portanto, a oxidação direta TR3+→TRIV. Apenas a transição interconfiguracional 4f65d1→4f7 do íon Eu2+ foi observada na luminescência persistente dos materiais BaAl2O4:Eu2+,TR3+. Por outro lado, para a matriz CdSiO3 dopada com íons Pr3+ e Tb3+, apenas as transições 4fn-4fn foram observadas, sem a presença da banda de emissão defeitos observada tanto no espectro de emissão sob excitação no UV como no espectro de luminescência persistente do CdSiO3:Gd3+. O band gap das matrizes BaAl2O4 (6,5 eV) e CdSiO3 (5,28 eV) foi obtido com a espectroscopia de excitação no UV-UV vácuo. A posição dos níveis de energia dos íons TR2+/3+ determinada com o auxílio da banda de transferência de carga ligante metal (LMCT) O2-(2p)Eu3+ confirmou o papel dos elétrons como carregadores de carga no mecanismo da luminescência persistente dos íons Eu2+, Pr3+ e Tb3+. Por fim, os mecanismos da luminescência persistente foram desenvolvidos, e, apesar de serem diferentes, os mecanismos para os íons Eu2+, Tb3+ e Pr3+ apresentaram princípios similares. Esta semelhança confirma a solidez da interpretação dos dados experimentais dos materiais com luminescência persistentes dopados com Eu2+ e encoraja a expansão de modelos similares para outros materiais com luminescência persistente no futuro. Cálculos teóricos com, por exemplo, método DFT poderão no futuro ser utilizados para refinar o modelo, diminuir o trabalho experimental e para descobrir novos modelos para sistemas que o atual modelo não pode ser aplicado, como no caso dos materiais dopados com Eu3+. Persistent luminescent materials are usually based on Eu2+ doped aluminates or silicates, though novel materials with different dopants are seeked for. Accordingly, BaAl2O4:Eu2+,R3+ (R: La-Lu) materials were prepared by the combustion method with ignition temperatures between 400 and 600 °C as well as with the solid state reaction at 1500 °C. The more versatile CdSiO3:R3+ systems were prepared only with the solid state reaction at 950 °C. Infrared absorption spectroscopy, X-ray powder diffraction and Scanning Electron Microscopy techniques were used to study the structure and phase purity of the materials. The oxidation states of the rare earths and their environment were investigated with synchrotron radiation X-ray absorption spectroscopy (XANES and EXAFS). The persistent luminescence properties of the phosphors were investigated with thermoluminescence (TL), photoluminescence and synchrotron radiation UV-VUV spectroscopy. Based on the TL measurements, the trap structure for BaAl2O4:Eu2+,R3+ (R: Nd and Dy) and CdSiO3:R3+ (R: Pr, Gd and Tb) is ideal for room temperature persistent luminescence. The XANES spectra indicated the presence of both divalent and trivalent europium doped in the BaAl2O4 host, while only trivalent rare earth co-dopants were observed (except CeIV). Eu3+ may be present because of in situ charging of persistent luminescence. In contrast for the CdSiO3:R3+ system, only the trivalent R3+ species were observed, thus excluding the direct R3+→RIV (R: Pr and Tb) oxidation. Only the typical interconfigurational 4f65d1→4f7 transition of Eu2+ ion was observed in the persistent luminescence of BaAl2O4:Eu2+,R3+ materials. On the other hand, for the Pr3+ and Tb3+ doped CdSiO3, only 4fn-4fn transitions were observed with no traces of the defect band emission observed in either the conventional UV excited or persistent luminescence spectra of CdSiO3:Gd3+. The band gap energy for the hosts BaAl2O4 (6.5 eV) and CdSiO3 (5.28 eV) were obtained with the UV-VUV excitation spectroscopy. The R2+/3+ energy levels positions determined with the aid of the O2-(2p)→Eu3+ ligand to metal charge transfer transition (LMCT) confirmed the role of electrons as the charge carriers in the mechanism of Eu2+, Pr3+ and Tb3+ persistent luminescence. Finally, the persistent luminescence mechanisms were constructed and, despite the differences, the mechanisms for Eu2+, Tb3+ and Pr3+ proved to be rather similar. This similarity confirms the solidity of the interpretation of experimental data for the Eu2+ doped persistent luminescence materials and encourages the expansion of similar models for other persistent luminescence materials in the future. Theoretical calculations by e.g. DFT method can be used to refine the model, to decrease experimental work and to find different models for systems where the present model evidently fails, e.g. Eu3+ doped materials.

Optical energy storage materials can store energy when exposed to radiation and subsequently release it as light after thermal or optical stimulation. Such materials are thus employed in, e.g., detectors, dosimetry, self-lit signs, and imaging. In, e.g., dosimetry, the response of the material is correlated with the absorbed energy, but no distinction of different radiation energies can be achieved. In this work, $Sr_3MgSi_2O_8:Eu^{2+},Dy^{3+}$ was studied with thermoluminescence (TL) initiated by irradiating the material with photon energies between 2.6 (480) and 5.4 eV (230 nm). The TL glow curves revealed that the material has two main traps. Both the overall TL intensity and the TL intensity ratio between the two traps strongly depend on the photon energy of the irradiation. A mechanism of energy storage and charge carrier release in this material was constructed from the results obtained.

In this paper, we investigate a new alternative route for the preparation of phosphate glasses with persistent luminescence properties. Phosphate glasses within the P 2 O 5 Na 2 O CaO and P 2 O 5 Na 2 O SrO systems were prepared by a standard melting process in air by adding Sr 4 Al 14 O 25 :Eu 2+ ,Dy 3+ microparticles in the glass batch before melting. We found that all the investigated glasses show persistent luminescence. It is clearly shown that conventional melting in air of Sr 4 Al 14 O 25 :Eu 2+ ,Dy 3+ microparticles in phosphate glass batch can be a new technique to prepare phosphate glasses with persistent luminescence properties.

The existence and effect of different rare earth (R2+/3+/IV) ions in SrAl2O4:Eu2+, R3+ and M2MgSi2O7:Eu2+, R3+ (M: Sr, Ba) persistent luminescence materials was studied with XANES (x-ray absorption near edge structure) measurements at HASYLAB/DESY (Hamburg, Germany) and MAX-lab (Lund, Sweden). The experiments were carried out at 298K for selected rare earth (co-) dopants (Eu2+; Ce3+, Nd3+, Sm3+, Dy3+ and Yb3+). The co-existence of Eu2+ and Eu3+ was observed in all materials. The co-dopants were always in the trivalent form.

The effect of the band gap on the persistent luminescence of cadmium meta- and orthosilicates was studied with VUV–UV–vis photoluminescence and x-ray absorption spectroscopies (XANES and EXAFS). The narrower band gap of Cd2SiO4 is favourable for persistent luminescence from Pr3+ but not from Tb3+ due to the trivalent rare-earth energy level positions in the band gap.

The influence of lithium excess and calcination temperature on the luminescence properties of the LiAl5O8:Eu under VUV excitation was investigated. The presence of both broad bands and sharp peaks in the VUV and X-ray emission spectra suggests the presence of Eu2+, even in absence of reducing atmosphere in the synthesis. The VUV excitation spectra indicated a band gap of 8.5 eV while the UV excited one showed the Eu3+ charge transfer transition starting at 3.9 eV. These values indicate that Eu2+ is stable in this host since its ground state is below the Fermi level of the host (ca. 4.1 eV). The relation between the intensity of Eu2+ and Eu3+ emissions showed that the reduction is favored at higher temperatures and lower Li content, leading to the proposition of a reduction mechanism based on the incorporation of charge compensation defects formed in the aliovalent doping of Eu3+ in Li+ sites.

Non-doped as well as titanium and lutetium doped zirconia (ZrO2) materials were synthesized via the sol-gel method and structurally characterized with X-ray powder diffraction. The addition of Ti in the zirconia lattice does not change the crystalline structure whilst the Lu doping introduces a small fraction of the tetragonal phase. The UV excitation results in a bright white-blue luminescence at ca. 500 nm for all the materials which emission could be assigned to the Ti3+eg → t2g transition. The persistent luminescence originates from the same Ti3+ center. The thermoluminescence data shows a well-defined though rather similar defect structures for all the zirconia materials. The kinetics of persistent luminescence was probed with the isothermal decay curve analyses which indicated significant retrapping. The short duration of persistent luminescence was attributed to the quasi-continuum distribution of the traps and to the possibility of shallow traps even below the room temperature.

The structural distortions resulting from the size mismatch between the Eu2+ luminescent centre and the host Ba2+ cation as well as the electronic structure of BaAl2O4:Eu2+(,Dy3+) were studied using density functional theory (DFT) calculations and synchrotron radiation (SR) luminescence spectroscopy. The modified interionic distances as well as differences in the total energies indicate that Eu2+ prefers the smaller of the two possible Ba sites in the BaAl2O4 host. The calculated Eu2+ 4f7 and 4f65d1 ground level energies confirm that the excited electrons can reach easily the conduction band for subsequent trapping. In addition to the green luminescence, a weak blue emission band was observed in BaAl2O4:Eu2+,Dy3+ probably due to the creation of a new Ba2+ site due to the effect of water exposure on the host.

The fading of persistent luminescence in Sr2MgSi2O7:Eu2+,R3+ (R: Y, La-Nd, Sm-Lu) was studied combining thermoluminescence (TL) and room temperature (persistent) luminescence measurements to gain more information on the mechanism of persistent luminescence. The TL glow curves showed the main trap signal at ca. 80 °C, corresponding to 0.6 eV as the trap depth, with every R co-dopant. The TL measurements carried out with different irradiation times revealed the general order nature of the TL bands. The results obtained from the deconvolutions of the glow curves allowed the prediction of the fading of persistent luminescence with good accuracy, though only when using the Becquerel decay law.