Research of Efficient and Fast Scintillator Garnet Crystals: The Role of Ce4+ in Ce3+, Mg2+-Co-Doped Gd3Al2Ga3O12 from Spectroscopic and XANES Characterizations

Herein, the most advanced oxide scintillator Ce3+, Mg2+‐co‐doped Gd3Al2Ga3O12 (GAGG) garnet host, grown using the Czochralski method is reported. The charge transfer absorption band in UV, such as in other Ce3+, Mg2+‐co‐doped oxides, reveals the creation of stable Ce4+ oxidation state, in addition to the usual Ce3+ one, admitted as responsible of the scintillation mechanism improvement displayed by this garnet. Strangely, the confirmation of quantitative values of Ce4+ concentration has never been done, and the goal is mainly focused on the presence of Ce4+ and the evaluation of the Ce3+/Ce4+ ratio measured by X‐ray absorption near edge spectroscopy (XANES) spectroscopy at the Ce LIII threshold of the European Synchrotron Radiation Facility (ESRF). This result is compared with those obtained for Ce3+, Mg2+‐co‐doped Lu3Al5O12 (LuAG) and, also, in garnets without any Mg2+ ions such as Ce3+‐doped GAGG, Ce3+‐doped LuAG, and Ce3+‐doped YAG. The role of Li+ ion in place of Mg2+ one is also considered in Ce3+, Li+‐co‐doped LuAG single crystal.

The development of scintillation materials often requires significant efforts in defect engineering. A quick screening approach of the possible dopant elements is urgently needed. Lu2SiO5 (LSO) is a scintillation material with wide applications in nuclear medicine, public security, and high‐energy physics. Ca doping and Al doping have been demonstrated to enhance the scintillation properties of Ce‐doped LSO. To further carry out such an approach, it is expected to use first‐principles calculations to assist the defect engineering. Unfortunately, the direct calculation of defect formation energy is a complicated process. Herein, the use of the relaxation energy of a dopant in the host lattice as a descriptor for the defect formation energy is proposed. It is suited for high‐throughput calculations. 32 elements are selected for doping LSO and the difficulty of substituting Lu and Si sites with these elements was analyzed. For 15 doping cases, the defect formation energies are directly calculated and compared with the corresponding relaxation energies. The results suggest that the relaxation energy could serve the purpose stated above. This descriptor is expected to play a more important role in larger‐scale screening for defect engineering in LSO and other inorganic nonmetallic materials, for example, when codoping is planned for.

X-ray computed tomography devices and X-ray diffraction techniques are powerful tools: the former provide volumetric data of samples during a non-destructive examination for biology and material science, and the latter measure grain orientation and strain, as well as crystalline phase identification and structure refinement. Today, the European Synchrotron Radiation Facility (ESRF) provides increasingly higher energy beams, up to 150 keV combined with higher brilliance (1013 X-ray photons/sec). This means that detectors suffer from low X-ray absorption at high spatial resolution (1-10 μm) and from radiation damage in tomography and diffraction applications. In addition, more and more experiments in medicine require the absorbed dose by the sample to be reduced. In this context, more efficient scintillators are developed and evaluated at the ESRF. In order to perform sub-micrometer and micrometer resolution imaging scintillators 1 μm to 500 μm thin are required. Single Crystal Film scintillators (SCF), 1 μm to 100 μm can be obtained via Liquid Phase Epitaxy for sub-micrometer resolution. Transparent ceramics, 100 μm to 500 μm thick are promising candidates for X-ray imaging requiring high X-ray absorption and good contrast with micrometer resolution. Commonly available scintillators, such as CdWO4 and YAG:Ce suffer from low efficiency, therefore new scintillators with higher light yield and stopping-power are required. A first test was carried out to evaluate an Europium doped Lutetium Oxide ceramic for micrometer resolution and new SCFs of Lu3Ga5-xInxO12:Eu for sub-micrometer resolution are investigated. Performance of Lu2O3 and LuInGG, i.e absorption, light yield, afterglow, spatial resolution will be presented and compared to standard screens (YAG, GGG). First results will be illustrated with X-ray images and will demonstrate the absorption efficiency improvement at high spatial resolution.

High optical quality Lu2SiO5 (LSO) and (Lu0.5Gd0.5)2SiO5 (LGSO) laser crystals codoped with Er3+ and Yb3+ have been fabricated by the Czochralski method. Intense upconversion (UC) and infrared emission (1543nm) are observed under excitation of 975nm. The luminescence processes are explained and the emission efficiencies are quantitatively obtained by measuring the UC efficiency and calculating the emission cross section. The temperature-dependent optical properties of the crystals are also investigated. Our study indicates that Er3+–Yb3+:LSO and Er3+–Yb3+:LGSO crystals are promising gain media for developing the solid-state 1.5μm optical amplifiers and tunable UC lasers.

The crystal structure of monoclinic Lu2SiO5(LSO) crystals, grown by the Czochralski method, was determined at room temperature by X-ray diffraction. The unit-cell parameters area= 10.2550 (2),b= 6.6465 (2),c= 12.3626 (4) Å, β = 102.422 (1)° in space groupI2/a. The linear thermal expansion tensor was determined along thea,b,candc* directions over the temperature range from 303.15 to 768.15 K, and the principal coefficients of the thermal expansion tensor are found to be αI= −1.0235 × 10−6 K, αII= 4.9119 × 10−6 K and αIII= 10.1105 × 10−6 K. The temperature dependence of the cell volume and monoclinic angle were also evaluated. In addition, the specific heat and the thermal diffusivity were measured over the temperature ranges from 293.15 to 673.15 K and from 303.15 to 572.45 K, respectively. As a result, the anisotropic thermal conductivity could be calculated and is reported for the first time, to the best of the authors' knowledge. The specific heat capacity of LSO is 139.54 J mol−1 K−1, and the principal components of the thermal conductivity arekI= 2.26 W m−1 K−1,kII= 3.14 W m−1 K−1andkII= 3.67 W m−1 K−1at 303.15 K. A new structure model was proposed to better understand the relationships between the crystal structure and anisotropic thermal properties. In comparison with other laser matrix crystals, it is found that LSO possesses relatively large anisotropic thermal properties, and owing to its small heat capacity it has a moderate thermal conductivity, which is similar to those of the tungstates but lower than those of the vanadates.

LISA (Linea Italiana per la Spettroscopia di Assorbimento di raggi X) is the new Italian Collaborating Research Group (CRG) beamline at the European Synchrotron Radiation Facility (ESRF) dedicated to X-ray absorption spectroscopy (XAS). The beamline covers a wide energy range, 4 < E < 90 keV, which offers the possibility for probe the K and L edges of elements that are heavier than Ca. A liquid He/N2 cryostat and a compact furnace are available for measurements in a wide temperature range (10–1000 K), allowing for in situ chemical treatments and measurements under a controlled atmosphere. The sub-millimetric beam size, the high photon flux provided, and the X-ray fluorescence detectors available (HP-Ge, SDD) allow for the study of liquid and highly diluted samples. Trace elements in geological or environmental samples can be analyzed, even for very small sample areas, gaining information on oxidation states and host phases.

The oxidation state of vanadium in natural and synthetic Fe–Ti oxides is determined using high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS). Eleven natural magnetite-bearing samples from a borehole of the Main Magnetite Layer of the Bushveld Complex (South Africa), five synthetic Fe oxide samples, and three natural hematite-bearing samples from Dharwar supergroup (India) are investigated. V K edge spectra were recorded on the ID26 beamline at the European Synchrotron Radiation Facility (Grenoble, France), and the pre-edge features were used to determine the local environment and oxidation state of vanadium. In the case of the magnetite samples (natural and synthetic), we show that vanadium is incorporated in the octahedral site of the spinel structure under two oxidation states: +III and +IV. The variations of the pre-edge area are interpreted as various proportions in V3+ and V4+ (between 9.5 and 16.3% of V4+), V3+ being the main oxidation state. In particular, the variations of the V4+/V3+ ratio along the profile of the Main Magnetite Layer seem to follow the crystallization sequence of the layer. In the case of the hematite samples from India, the pre-edge features indicate that vanadium is substituted to Fe and mainly incorporated as V4+ (between 40 and 72% of V4+). We also demonstrate the potentiality of HERFD-XAS for mineralogical studies, since it can filter out the unwanted fluorescence and give better resolved spectra than conventional XAS.

High energy Li-Ion batteries rely on the pairing of a transition metal oxide cathode with a graphite anode. Pushing the upper cut-off voltage and state of charge (SOC) to higher values, the cycle-life of the battery is drastically reduced. Besides electrochemical electrolyte oxidation leading to poor coulombic efficiencies and a large growth of cell impedance, leaching of transition metal ions from the cathode active materials is one of the major obstacles to increase the power density of the cell without change of active materials. While the capacity loss which can be ascribed to the overall loss of active material is minor, cell aging is largely aggravated by liberated transition metal ions present in the electrolyte, as they diffuse through the separator and deposit on the graphite anode due to its low potential. A severe impedance growth of the anode and loss of active lithium by chemical delithiation of the graphite has been observed by many studies. Especially for manganese, the amount of active lithium loss has been found to be a multifold higher than the total amount of manganese ions present in the electrolyte and deposited onto the graphite anode, indicating a catalytic role of transition metal ions on active lithium loss (1). Comparisons of the transition metal leaching behavior for different cathode active materials is missing so far, as studies vary widely in test procedures (potential range, SOC) and environment (electrolyte and temperature). We herein present operando analysis of the intrinsic stability of several cathode active materials upon charge to high SOC and potential with our unique spectroscopic cell enabling spatially resolved operando X-ray absorption spectroscopy, monitoring the concentration and oxidation state of transition metals in the electrolyte and in the graphite anode (2, 3). Aside layered mixed transition metal oxide materials (Li1+w[NixCoyMnz]1-xO2) also the spinel structure is tested to better understand the effect of crystal destabilization at high SOC. The formerly standard layered oxide NCM111 (x=y=z=0.33) was already studied earlier, focusing on manganese dissolution (3). In this talk we present time and potential resolved analysis of all transition metals released from NCM111 (see Fig. 1) in comparison to a nickel-rich NCM811 (x=0.8, y=z=0.1), a lithium- and manganese-rich layered oxide material (HE-NCM; w=0.17, x=0.22, y=0.12, z=0.66 (4)) and the high-voltage spinel LiNi0.5Mn1.5O4, (LNMO). This data allows conclusions on the extent of the two major causes for transition metal leaching from oxide based cathode materials: chemical decomposition induced by protic electrochemical electrolyte oxidation products (5) and crystal lattice destabilization of layered transition metal oxides caused by high degrees of delithiation (high SOC). References J. A. Gilbert, I. A. Shkrob and D. P. Abraham, Journal of The Electrochemical Society, 164, A389 (2017). Y. Gorlin, A. Siebel, M. Piana, T. Huthwelker, H. Jha, G. Monsch, F. Kraus, H. A. Gasteiger and M. Tromp, Journal of the Electrochemical Society, 162, A1146 (2015).J. Wandt, A. Freiberg, R. Thomas, Y. Gorlin, A. Siebel, R. Jung, H. A. Gasteiger and M. Tromp, J. Mater. Chem. A, 4, 18300 (2016). B. Strehle, K. Kleiner, R. Jung, F. Chesneau, M. Mendez, H. A. Gasteiger and M. Piana, Journal of The Electrochemical Society, 164, A400 (2017). M. Metzger, B. Strehle, S. Solchenbach and H. A. Gasteiger, Journal of The Electrochemical Society, 163, A798 (2016). Acknowledgements Financial support for S. S. and B. S. by the BASF SE through its Research Network on Electrochemistry and Batteries is gratefully acknowledged. XAS data were gathered at the European Synchrotron Radiation Facility (ESRF) and SOLEIL Synchrotron (France).We are grateful to Dr. Debora Motta Meira at the ESRF, Grenoble (France) for providing assistance in using beamline BM 23 as well as Dr. Emiliano Fonda for assistance on the SAMBA beamline at SOLEIL Synchrotron. Fig. 1: Transition metal dissolution of an NCM111 (5.5 mAh/cm2)//C6 cell upon cycling in the conventional potential range (0-6.5 h: 60% SOC until ≈4.2 VLi) and upon high degrees of delithiation (7-11 h: ≈100% SOC until 5.1 VLi) measured in our operando XAS cell using glass fiber separators and standard LP57 electrolyte (1 M LiPF6 in EC:EMC [3:7]) at 25 °C. The concentration of transition metal ions within the graphite electrode (filled symbols) and in the electrolyte (hollow stars, shown as electrolyte share in the graphite pores [concentration measured in the separator multiplied by the porosity of the graphite anode]) is monitored operando. Figure 1

This research aims to understand colouring technologies in 5th–7th centuries glass imported to Atlantic Britain by correlating the iron (Fe) and manganese (Mn) ratios and oxidation states with colour. Despite having a similar matrix chemical composition and concentrations of Fe and Mn oxides, these vessels display different colours (from green to yellow/amber, sometimes with purple streaks). Colour changes can be induced by controlling the reduction-oxidation reactions that occur during glass production, which are influenced by the raw materials, furnace and melt atmosphere, and recycling. To evaluate these parameters, reference glasses were prepared, following the composition of Late Antique archaeological glass recovered from Tintagel (UK) and Whithorn (UK). A corpus of archaeological and experimental glass samples was analysed using bulk Fe and Mn K-edge x-ray absorption near edge structure (XANES) spectroscopy, micro-XANES and micro x-ray fluorescence (μ-XRF) at beamline ID21, at the European Synchrotron Radiation Facility. Fe and Mn XANES spectra of the archaeological glass indicate that Fe and Mn are in a similar oxidation state in all the yellow samples, predominantly Fe3+ and Mn2+. No detectable difference in Mn and Fe oxidation state occurs in the purple streaks compared to the yellow glass bulk but μ-XRF maps of the distribution of Fe and Mn show that Mn is more concentrated in the purple streaks. This indicates that the purple colour of the streaks is mainly due to a higher Mn/Fe ratio and persistence of more oxidised manganese in the purple areas, even though it is difficult to detect. Many archaeological fragments appear pale green in transmitted light but amber in reflected light. XANES studies detected the presence of surface layers where manganese is more oxidised. This layer is believed to scatter transmitted and reflected light differently and might be responsible for the optical features of the archaeological glass.

Bulk crystal growth technologies originate from the Czochralski (CZ) and Vertical Bridgman (VB) methods developed almost one century ago. Both methods were applied to prepare single crystals of many kind of inorganic materials, for example, semiconductors, halides and many oxides. In the VB process, molten raw materials are wetting the crucible wall easily. This phenomenon causes the sticking of grown crystals with crucibles and often leads to the cracking of the crystal and crucible. These issues prohibit us from obtaining high quality single crystals. Therefore, practical application of VB method is limited only on several materials such as CaF₂ and GaAs single crystals. The issue of crucible’s wetting is present in the CZ method as well. For example, the purity of silicon single crystals is degraded from 11N raw material to 5~6N level by the oxygen and carbon contamination caused by the wetting between silicon melt and quartz crucible. These issues are yet to be solved in VB and CZ methods. Many molten materials reach the spherical shape driven by the surface tension when a residual moisture (H₂O) is completely removed from the raw material, the crucible, and atmosphere. We denote this condition as the “Liquinert” state meaning “liquid being in an inert state”, non-wetting and non-reactive with the crucible at high temperature. The author has prepared many high-quality bulk crystals of mainly metallic halides and semiconductors, except oxides, by VB method when applying the “Liquinert” process. This technology is applicable to high quality bulk crystal growth of silicon as well as other inorganic materials of huge industrial interest. In this review, the “Liquinert” process, its background, methodology, examples of applications in fundamental research, and practical development are exposed. In addition, we also discuss the future of this industrial process on bulk silicon crystals for semiconductors and solar cells.

Oxygen evolution reaction (OER) takes place in various types of electrochemical devices that are pivotal for the conversion and storage of renewable energy. This paper describes a strategy in the design of solid-state structures of OER electrocatalysts through controlling the cation substitution on the active metal site and consequently valence band center position of site-mixed Y2(YxRu1-x)2O7-δ pyrochlore to achieve high catalytic activity. We found that partially replacing the B-site Ru4+ cation with A-site Y3+ in pyrochlore-structured Y2Ru2O7-δ modifies the oxidation state of B-site Ru from 4+ to 5+, as observed by electron paramagnetic resonance (EPR) spectroscopy but does not continuously increase the oxygen vacancy concentration in these oxygen substoichiometric compositions, as quantified by thermogravimetric analysis (TGA) decomposition studies. We found the increased Ru oxidation state leads to a downshift in valence band center. X-ray photoelectron spectroscopy (XPS) analysis was performed to quantitatively determine the optimal band center to be ∼1.27 eV below the Fermi energy level based on the analysis of the valence band edge of these Ru-based Y2(YxRu1-x)2O7-δ OER electrocatalysts. This work highlights that defect engineering can be a practical, effective approach to the optimization of oxidation state and electronic band center for high OER catalytic performance in a quantitative manner.

Rare Earth (RE) such as Y, La, Sc and Gd ions doped Ce:Lu2SiO5 (LSO) crystals, were grown by Czochralski method. The concentrations of these RE ions in LSO crystals were measured, and their segregation coefficients were calculated according to the concentrations. La3+ was found to be helpful to increase the segregation coefficients of Ce3+ in LSO crystals. Transmission, UV and X-ray excited emission spectra, and UV light emission decay curves of RE-doped LSO crystals were tested at room temperature. No significant difference in emission spectra and decay time can be identified among these RE-doped LSO crystals. Doping with La3+ may improve the energy resolution and photoelectron yield of LSO crystals.

Effect of temperature and excitation wavelength on luminescent characteristics of Lu2SiO5–Gd2SiO5 solid solution crystals co-doped with Ce3+ and Sm3+

In this work the comparative analysis of thermoluminescent properties of undoped and Ce-doped single crystals (SC) and single crystalline films (SCF) of Lu2SiO5 (LSO) and Y2SiO5 (YSO) orthosilicates were performed. The SC samples were prepared with the Czochralski method, and SCF were grown by the liquid phase epitaxy technique. We show that such different methods of material preparation resulted in different thermoluminescent properties of undoped and Ce3+ doped LSO and YSO SC and SCF caused by the presence host defects (first of all, oxygen vacancies) as trapping centers in SC and formation of Pb2+-Ce4+ pair centers in SCF.

Spectral characteristics of visible luminescence in Gd2SiO5–Lu2SiO5 (LGSO) solid solution crystals co-doped with Ce3+ and Dy3+