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The deviation from stoichiometry and the understanding of its consequences are key factors for the application of kesterites as solar cell absorbers. Therefore, this study investigates the local atomic structure of off-stoichiometric Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe) and Cu2ZnGeSe4 (CZGSe) by means of Extended X-ray Absorption Fine Structure Spectroscopy. Temperature dependent measurements yield the bond stretching force constants of all cation-anion bonds in stoichiometric CZTS and CZTSe and nearly stoichiometric CZGSe. Low temperature measurements allow high precision analysis of the influence of off-stoichiometry on the element specific average bond lengths and their variances. The overall comparison between the materials is in excellent agreement with measures like ionic/atomic radii and bond ionicities. Furthermore, the small uncertainties allow the identification of systematic trends in the Cu–Se and Zn–Se bond lengths of CZTSe and CZGSe. These trends are discussed in context of the types and concentrations of certain point defects, which gives insight into the possible local configurations and their influence on the average structural parameters. The findings complement the understanding of the effect of off-stoichiometry on the local structure of kesterites, which affects their electronic properties and thus their application for solar cells. [ABSTRACT FROM AUTHOR]

Under DAPHNE4NFDI, the X‐ray absorption spectroscopy (XAS) reference database, RefXAS, has been set up. For this purpose, we developed a method to enable users to submit a raw dataset, with its associated metadata, via a dedicated website for inclusion in the database. Implementation of the database includes an upload of metadata to the scientific catalogue and an upload of files via object storage, with automated query capabilities through a web server and visualization of the data and files. Based on the mode of measurements, quality criteria have been formulated for the automated check of any uploaded data. In the present work, the significant metadata fields for reusability, as well as reproducibility of results (FAIR data principles), are discussed. Quality criteria for the data uploaded to the database have been formulated and assessed. Moreover, the usability and interoperability of available XAS data/file formats have been explored. The first version of the RefXAS database prototype is presented, which features a human verification procedure, currently being tested with a new user interface designed specifically for curators; a user‐friendly landing page; a full list of datasets; advanced search capabilities; a streamlined upload process; and, finally, a server‐side automatic authentication and (meta‐) data storage via MongoDB, PostgreSQL and (data‐) files via relevant APIs. [ABSTRACT FROM AUTHOR]

Contrary to other semiconductor alloys, incorporation of Ag into CuGaSe2 increases the bandgap, even though the lattice expands, and the Ga--Se bond length is theoretically predicted to decrease rather than increase. Herein, we experimentally confirm this peculiar bond length dependence of (Ag,Cu)GaSe2 using x-ray absorption spectroscopy. We further model the different anion displacements and estimate that their combined contribution to the bandgap bowing is close to zero. These findings differ from those for Cu(In,Ga)Se2 and demonstrate the diversity of chalcopyrite alloys and their properties. [ABSTRACT FROM AUTHOR]

Polyanion‐type phosphate materials, such as M3V2(PO4)3 (M = Li/Na/K), are promising as insertion‐type negative electrodes for monovalent‐ion batteries including Li/Na/K‐ion batteries (lithium‐ion batteries (LIBs), sodium‐ion batteries (SIBs), and potassium‐ion batteries (PIBs)) with fast charging/discharging and distinct redox peaks. However, it remains a great challenge to understand the reaction mechanism of materials upon monovalent‐ion insertion. Here, triclinic Mg3V4(PO4)6/carbon composite (MgVP/C) with high thermal stability is synthesized via ball‐milling and carbon‐thermal reduction method and applied as a pseudocapacitive negative electrode in LIBs, SIBs, and PIBs. In operando and ex situ studies demonstrate the guest ion‐dependent reaction mechanisms of MgVP/C upon monovalent‐ion storage due to different sizes. MgVP/C undergoes an indirect conversion reaction to form Mg0, V0, and Li3PO4 in LIBs, while in SIBs/PIBs the material only experiences a solid solution with the reduction of V3+ to V2+. Moreover, in LIBs, MgVP/C delivers initial lithiation/delithiation capacities of 961/607 mAh g−1 (30/19 Li+ ions) for the first cycle, despite its low initial Coulombic efficiency, fast capacity decay for the first 200 cycles, and limited reversible insertion/deinsertion of 2 Na+/K+ ions in SIBs/PIBs. This work reveals a new pseudocapacitive material and provides an advanced understanding of polyanion phosphate negative material for monovalent‐ion batteries with guest ion‐dependent energy storage mechanisms. [ABSTRACT FROM AUTHOR]

The local ordering in amorphous Ga2O3 and (InxGa1−x)2O3 thin films on Si(111) and its change on crystallization upon annealing are investigated. Synchrotron‐based extended X‐ray absorption fine structure data, as probed across the Ga K‐edge in conjunction with a numerical implementation of the scattering process including multiple scattering events, provide detailed insights into the bonding character of the Ga cations. Bond lengths and coordination numbers indicate the preferential formation of GaO4 tetrahedrons in the amorphous binary and ternary solid solution at the absence of long‐range ordering. Upon annealing there is no tendency toward the formation of long‐range ordering for the ternary alloy, while the amorphous Ga2O3 film turns into monoclinic β‐Ga2O3 crystallites with both tetra‐ and octahedral coordination. [ABSTRACT FROM AUTHOR]

Amorphous Cu–Sn–I is a promising p‐type transparent semiconductor. Therefore, herein, composition and structure of sputtered Cu–Sn–I thin films with varying thickness and Sn content are investigated by different electron microscopy techniques, energy‐dispersive X‐ray spectroscopy, and X‐ray absorption spectroscopy at the Cu, Sn, and I K‐edge. After exposure to air, the sputtered films are found to contain significant amounts of oxygen, leading to a complete phase separation of the resulting Cu–Sn–I–O films. Spatially‐resolved compositional analysis and high‐resolution transmission electron microscopy reveal that one phase consists of crystalline γ‐CuI while the other phase is composed of amorphous Cu–Sn–O, most likely a mixture of SnO2 and Cu2O/CuO. X‐ray absorption spectroscopy confirms that the local structural environment of the Sn and I atoms is similar to that in amorphous SnO2 and crystalline CuI, respectively. In contrast, the X‐ray absorption near edge structure and the extended X‐ray absorption fine structure of the Cu K‐edge both demonstrate that Cu atoms are not only bonded to I but also to O atoms. The incorporation of oxygen into the sputtered films thus completely alters the material and therefore clearly needs to be inhibited. [ABSTRACT FROM AUTHOR]

Fast counting applications with energy discrimination like X-ray absorption spectroscopy require an energy resolution of some hundred eV even at highest count rates and ask for small form factors. The development, fabrication and test of a small number of 7-cell Si-drift detectors have been successfully finished at DESY fitting these claims with a good cost-versus-performance tradeoff. The monolithic 7-cell sensor chip and a corresponding readout chip are the key components of the sensor head. The module's length and wrench size amounts to 21.2 and 1.6 cm, respectively. Its active area is ~ 7 x 7 [mm.sup.2] and main energy range is 2.5 to 18 keV. Between ~5 and 20 keV the nonlinearity in a six-modules setup is well within [+ or -] 2% and 4-0.5% when uncompensated and compensated, respectively. The Cu - [K.sub.[alpha]] line width is (271 [+ or -] 46) eV when operated at 10[degrees]C. With the best SDD chip we achieved (223 [+ or -] 7) eV at 10[degrees]C and (294 [+ or -] 10) eV at 24[degrees]C. By selection of the three best devices the spectral resolution reaches 400 and 600 eV at 10[degrees]C and a sum-count rate of 3.3 * [10.sup.6] [s.sup.-1] and 6.7 * [10.sup.6] [s.sup.-1], respectively. XAFS measurements have shown that the peak shift and line-width variations remain below 5 eV and 17 eV, respectively. Index Terms--Silicon-drift detector, spectral resolution, X-ray absorption spectroscopy, X-ray standing waves.

The electrochemical performance and reaction mechanism of orthorhombic V2O5 in 1 M ZnSO4 aqueous electrolyte are investigated. V2O5 nanowires exhibit an initial discharge and charge capacity of 277 and 432 mA h g−1, respectively, at a current density of 50 mA g−1. The material undergoes quick capacity fading during cycling under both low (50 mA g−1) and high (200 mA g−1) currents. V2O5 can deliver a higher discharge capacity at 200 mA g−1 than that at 50 mA g−1 after 10 cycles, which could be attributed to a different type of activation process under both current densities and distinct degrees of side reactions (parasitic reactions). Cyclic voltammetry shows several successive redox peaks during Zn ion insertion and deinsertion. In operando synchrotron diffraction reveals that V2O5 undergoes a solid solution and two-phase reaction during the 1st cycle, accompanied by the formation/decomposition of byproducts Zn3(OH)2V2O7·2(H2O) and ZnSO4Zn3(OH)6·5H2O. In the 2nd insertion process, V2O5 goes through the same two-phase reaction as that in the 1st cycle, with the formation of the byproduct ZnSO4Zn3(OH)6·5H2O. The reduction/oxidation of vanadium is confirmed by in operando X-ray absorption spectroscopy. Furthermore, Raman, TEM, and X-ray photoelectron spectroscopy (XPS) confirm the byproduct formation and the reversible Zn ion insertion/deinsertion in the V2O5. [ABSTRACT FROM AUTHOR]

Electrochemical CO2 reduction to valuable chemicals represents a green and sustainable approach to close the anthropogenic carbon cycle, but has been impeded by low efficiency and high cost of electrocatalysts. Here, a cost‐effective hybrid catalyst consisting of hemin (chloroprotoporphyrin IX iron(III)), a product recovered from bovine blood, adsorbed onto commercial Vulcan carbon is reported. Upon heat treatment, this material shows significantly improved activity and selectivity for CO2 reduction in water while exhibiting good stability for more than 10 h. The heat treatment leads to consecutive removal of the axial chlorine atom and decomposition of the iron porphyrin ring, restructuring to form atomic Fe sites. The optimized hybrid catalyst obtained at 900 °C shows near‐unity selectivity for reduction of CO2 to CO at a small overpotential of 310 mV. The insight into transformation of adsorbed Fe complexes into single Fe atoms upon heat treatment provides guidance for development of single atom catalysts. [ABSTRACT FROM AUTHOR]

In decimolar aqueous solutions, zirconium oxychloride octahydrate forms several micrometer long and approximately 15 nm wide thin ribbons through the reaction with excess amounts of the sodium salt of 1-hydroxyethane-1,1-diphosphonic acid (HEDP, known as etidronic acid). Primarily deduced from SAXS, TEM, EXAFS and solid-state NMR analyses, a consistent structural model enables congruous explanations for the colloidal behaviour of the purified ribbons as well as of their reaction products with ammonia and amines, respectively. Properties of the lyotropic, liquid crystalline phases are discussed in the light of potential applications in aqueous coatings. [ABSTRACT FROM AUTHOR]

Sulfide compounds are interesting conversion electrode materials for Li-ion batteries, due to their high theoretical capacity. However, they suffer from large volumetric changes and fast capacity fading. To overcome these issues, nanosized zinc sulfide (ZnS) modified with polyelectrolytes and graphene (ZnS-C/G) has been synthesized and investigated as an enhanced conversion-alloying anode material. In situ synchrotron X-ray diffraction and X-ray absorption spectroscopy are used to elucidate the Li storage process during the 1st cycle. In addition, the evolution of internal resistance and the corresponding solid electrolyte interphase (SEI) formation during the 1st cycle are discussed based on electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy. The results reveal that the formation of lithiated products and the SEI layer at different voltages can influence Li+ diffusion into the electrode. Moreover, an artificial carbon layer can not only facilitate Li+ transport but also avoid the direct formation of the SEI layer on the surface of active particles. Compared to bare ZnS, the ZnS-C/G electrode shows outstanding rate capability and cycling capacity (571 mA h g−1 after 120 cycles at a specific current of 1.0 A g−1 with a retention rate of 94.4%). The high capacity at elevated current density is ascribed to the contribution of capacitive charge storage. [ABSTRACT FROM AUTHOR]

Round robin studies have been used across fields of science for quality control testing and to investigate laboratory dependencies and cross‐platform inconsistencies as well as to drive forward the improvement of understanding of experimental systems, systematic effects and theoretical limitations. Here, following the Q2XAFS Workshop and Satellite to IUCr Congress 2017 on `Data Acquisition, Treatment, Storage – quality assurance in XAFS spectroscopy', a mechanism is suggested for a suitable study across XAFS (X‐ray absorption fine‐structure) beamlines and facilities, to enable each beamline to cross‐calibrate, provide representative test data, and to enable collaborative cross‐facility activities to be more productive. [ABSTRACT FROM AUTHOR]

The 6Mg(NH2)2–9LiH–LiBH4 composite system has a maximum reversible hydrogen content of 4.2 wt% and a predicted dehydrogenation temperature of about 64 °C at 1 bar of H2. However, the existence of severe kinetic barriers precludes the occurrence of de/re-hydrogenation processes at such a low temperature (H. Cao, G. Wu, Y. Zhang, Z. Xiong, J. Qiu and P. Chen, J. Mater. Chem. A, 2014, 2, 15816–15822). In this work, Li3N and YCl3 have been chosen as co-additives for this system. These additives increase the hydrogen storage capacity and hasten the de/re-hydrogenation kinetics: a hydrogen uptake of 4.2 wt% of H2 was achieved in only 8 min under isothermal conditions at 180 °C and 85 bar of H2 pressure. The re-hydrogenation temperature, necessary for a complete absorption process, can be lowered below 90 °C by increasing the H2 pressure above 185 bar. Moreover, the results indicate that the hydrogenation capacity and absorption kinetics can be maintained roughly constant over several cycles. Low operating temperatures, together with fast absorption kinetics and good reversibility, make this system a promising on-board hydrogen storage material. The reasons for the improved de/re-hydrogenation properties are thoroughly investigated and discussed. [ABSTRACT FROM AUTHOR]

Structural changes in Li2MnO3 cathode material for rechargeable Li-ion batteries are investigated during the first and 33rd cycles. It is found that both the participation of oxygen anions in redox processes and Li+-H+ exchange play an important role in the electrochemistry of Li2MnO3. During activation, oxygen removal from the material along with Li gives rise to the formation of a layered MnO2-type structure, while the presence of protons in the interslab region, as a result of electrolyte oxidation and Li+-H+ exchange, alters the stacking sequence of oxygen layers. Li re-insertion by exchanging already present protons reverts the stacking sequence of oxygen layers. The re-lithiated structure closely resembles the parent Li2MnO3, except that it contains less Li and O. Mn4+ ions remain electrochemically inactive at all times. Irreversible oxygen release occurs only during activation of the material in the first cycle. During subsequent cycles, electrochemical processes seem to involve unusual redox processes of oxygen anions of active material along with the repetitive, irreversible oxidation of electrolyte species. The deteriorating electrochemical performance of Li2MnO3 upon cycling is attributed to the structural degradation caused by repetitive shearing of oxygen layers. [ABSTRACT FROM AUTHOR]

Owing to a theoretical hydrogen storage capacity of 10.5 wt% H2, Ca(BH4)2+ MgH2, the so-called calcium reactive hydride composite (Ca-RHC), has a great potential as a hydrogen storage material. However, its dehydrogenation temperature (∼623 K) is too high for any mobile applications. By addition of 10 mol% of NbF5 into Ca(BH4)2+MgH2, a decrease of the dehydrogenation onset temperature by ∼120 K is observed. In order to understand the reasons behind this desorption temperature decrement two sets of samples [Ca(BH4)2+MgH2 and Ca(BH4)2+MgH2+0.1NbF5] in different hydrogenation states, were prepared. The structural investigation of the above mentioned sets of samples by means of volumetric measurements, anomalous small-angle X-ray scattering (ASAXS) and X-ray absorption spectroscopy (XAS) is reported here. The XAS results show that after the milling procedure NbB2 is formed and remains stable upon further de/rehydrogenation cycling. The results of Nb ASAXS point to nanometric spherical NbB2 particles distributed in the hydride matrix, with a mean diameter of ∼10 nm. Results from Ca ASAXS indicate Cacontaining nanostructures in the Ca-RHC+0.1NbF5 samples to be ∼50% finer compared to those without additive. Thus, a higher reaction surface area and shorter diffusion paths for the constituents are concluded to be important contributions to the catalytic effect of an NbF5 additive on the hydrogen sorption kinetics of the Ca(BH4)2+MgH2 composite system. [ABSTRACT FROM AUTHOR]

P65 is a new EXAFS beamline to be built at the PETRA III storage ring at DESY in Hamburg. It will mainly be used for standard EXAFS applications with a relatively large beam and moderate flux density. While the beamline optics will be similar to many other standard EXAFS beamlines, the insertion device at such a large high-energy storage ring with low emittance like PETRA III will need special attention. This paper discusses the main design considerations for the construction of such a beamline at a 6 GeV storage ring with an emittance of 1 nm rad. [ABSTRACT FROM AUTHOR]

In this paper results are presented from fluorescence-yield X-ray absorption fine-structure spectroscopy measurements with a new seven-cell silicon drift detector (SDD) module. The complete module, including an integrated circuit for the detector readout, was developed and realised at DESY utilizing a monolithic seven-cell SDD. The new detector module is optimized for applications like XAFS which require an energy resolution of ~250-300 eV (FWHM Mn Kα) at high count rates. Measurements during the commissioning phase proved the excellent performance for this type of application. [ABSTRACT FROM AUTHOR]

A new spectroscopic cell suitable for the analysis of heterogeneous catalysts by fluorescence EXAFS (extended X-ray absorption fine structure), transmission EXAFS and X-ray diffraction during in situ treatments and during catalysis is described. Both gas-phase and liquid-phase reactions can be investigated combined with on-line product analysis performed either by mass spectrometry or infrared spectroscopy. The set-up allows measurements from liquid-nitrogen temperature to 973 K. The catalysts are loaded preferentially as powders, but also as self-supporting wafers. The reaction cell was tested in various case studies demonstrating its flexibility and its wide applicability from model studies at liquid-nitrogen temperature to operando studies under realistic reaction conditions. Examples include structural studies during (i) the reduction of alumina-supported noble metal particles prepared by flame-spray pyrolysis and analysis of alloying in bimetallic noble metal particles (0.1%Pt–0.1%Pd/Al2O3, 0.1%Pt–0.1%Ru/Al2O3, 0.1%Pt–0.1%Rh/Al2O3, 0.1%Au–0.1%Pd/Al2O3), (ii) reactivation of aged 0.8%Pt–16%BaO–CeO2 NO x storage–reduction catalysts including the NO x storage/reduction cycle, and (iii) alcohol oxidation over gold catalysts (0.6%Au–20%CuO–CeO2). [ABSTRACT FROM AUTHOR]

A new focusing spectrometer employing Johann geometry has been built and permanently installed at the wiggler beamline W1 at the Hamburger Synchrotron Strahlungslabor (HASYLAB) am Deutschen Elektronen Synchrotron (DESY). It is now available for user operation. The design of the spectrometer is optimized for the use of a large source spot size at the DORIS storage ring as well as for simple operation and robustness under user-mode conditions. Nevertheless, the ideal source for enlarging the number of possible applications in the future would be an undulator at one of the PETRA III beamlines. In the following the spectrometer design is described and boundary conditions such as the available energy range for future experiments are discussed. Two benchmark experiments, resonant inelastic X-ray scattering (RIXS) and X-ray absorption fluorescence spectroscopy (XAFS), on samples with complicated matrices demonstrate the performance of the new instrument. [ABSTRACT FROM AUTHOR]

In this work, the effect of Li+ substitution in Li3V2(PO4)3 with a large divalent ion (Ca2+) toward lithium insertion is studied. A series of materials, with formula Li3−2xCaxV2(PO4)3/C (x = 0, 0.5, 1, and 1.5) is synthesized and studied in the potential region 3–0.01 V versus Li+/Li. Synchrotron diffraction demonstrates that Li3V2(PO4)3/C has a monoclinic structure (space group P21/n), while Ca1.5V2(PO4)3/C possesses a rhombohedral structure (space group R‐3c). The intermediate compounds, Li2Ca0.5V2(PO4)3/C and LiCaV2(PO4)3/C, are composed of two main phases, including monoclinic Li3V2(PO4)3/C and rhombohedral Ca1.5V2(PO4)3/C. Cyclic voltammetry reveals five reduction and oxidation peaks on Li3V2(PO4)3/C and Li2Ca0.5V2(PO4)3/C electrodes. In contrast, LiCaV2(PO4)3/C and Ca1.5V2(PO4)3/C have no obvious oxidation and reduction peaks but a box‐type voltammogram. This feature is the signature for capacitive‐like mechanism, which involves fast electron transfer on the surface of the electrode. Li3V2(PO4)3/C undergoes two solid‐solution and a short two‐phase reaction during lithiation and delithiation processes, whereas Ca1.5V2(PO4)3/C only goes through capacitive‐like mechanism. In operando X‐ray absorption spectroscopy confirms that, in both Li3V2(PO4)3/C and Ca1.5V2(PO4)3/C, V ions are reduced during the insertion of the first three Li ions. This study demonstrates that the electrochemical characteristic of polyanionic phosphates can be easily tuned by replacing Li+ with larger divalent cations. [ABSTRACT FROM AUTHOR]

In this work novel garnet persistent luminescence phosphor Mg 3 Y 2 Ge 3 O 12 : Tb3+ (MYG: Tb3+) with bright blue to green tuneable emission is reported. MYG: Tb3+ samples with Tb3+ content ranging from 0 % to 100 % were analysed. X-ray absorption spectroscopy analysis showed that Tb3+ ions incorporate in the garnet structure by replacing Y3+ ions. Optimal dopant content was determined to achieve bright afterglow lasting more than ten hours. Tuneability of persistent luminescence was attributed to the deviations in populations of 5D 4 and 5D 3 emitting states of Tb3+ due to multiphonon relaxation and cross-relaxation processes. Photoluminescence, thermostimulated luminescence (TSL) and electron paramagnetic resonance (EPR) methods were used to determine the dominant persistent luminescence processes in the investigated material. The analysis of intrinsic defects in the material using EPR spectroscopy showed the presence of three distinct paramagnetic V-type centres and at least one F-type centre. The role of Tb3+ as a charge trap was discussed. MYG: Tb3+ exhibited luminescence tuneability by doping and temperature, which is promising for advanced anti-counterfeiting applications. • Blue to green tuneable persistent luminescence was achieved in Mg 3 Y 2 Ge 3 O 12 : Tb3+. • Tuneability results from deviation of population of 5D 3 and 5D 4 states of Tb3+. • Changes in population of the states is achieved by Tb3+ content and by temperature. • Introduction of Tb3+ in host suppresses the formation of paramagnetic hole centres. • The use of Mg 3 Y 2 Ge 3 O 12 : Tb3+ for anti-counterfeiting is demonstrated. [ABSTRACT FROM AUTHOR]

NaAlH 4 was melt infiltrated within a CO 2 activated carbon aerogel, which had been preloaded with TiCl 3 . Nanoconfinement was verified by Small Angle X-Ray Scattering (SAXS) and the nature of the Ti was investigated with Anomalous SAXS (ASAXS) and X-Ray Absorption Near Edge Structure (XANES) to determine its size and chemical state. The Ti is found to be in a similar state to that found in the bulk Ti-doped NaAlH 4 system where it exists as Al 1− x Ti x nanoalloys. Crystalline phases exist within the carbon aerogel pores, which are analysed by in-situ Powder X-Ray Diffraction (PXD) during hydrogen cycling. The in-situ data reveals that the hydrogen release from NaAlH 4 and its hydrogen uptake occurs through the Na 3 AlH 6 intermediate when confined at this size scale. The hydrogen capacity from the nanoconfined NaAlH 4 is found to initially be much higher in this CO 2 activated aerogel compared with previous studies into unactivated aerogels. [ABSTRACT FROM AUTHOR]

Abstract: The existence of the different R2+/R3+/RIV (R: rare earth) ions as well as the modifications in the structural environment around the dopant and co-dopants in CaAl2O4:Eu2+,R3+ persistent luminescence materials was studied by LIII edge X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) measurements at Hamburger Synchrotronstrahlungslabor (HASYLAB) at Deutsches Elektronen-Synchrotron (DESY) (Hamburg, Germany). The measurements were carried out at 10 and 296K for selected rare earth (co-)dopants (Eu2+; Ce3+, Nd3+, Sm3+, and Yb3+). The XANES results indicated the co-existence of both divalent and trivalent europium in all co-doped CaAl2O4:Eu2+,R3+ materials, but only divalent europium in CaAl2O4:Eu2+. The measurement temperature did not affect the XANES results. The interatomic distances extracted from the EXAFS data of the CaAl2O4:Eu2+,R3+ materials indicated that co-doping creates distortions around the Eu2+ ions suggesting dopant aggregation. [Copyright &y& Elsevier]

The studies of polaronic centers in a homologous series of scheelite-type compounds A WO 4 ( A = Ca, Sr, Ba) were performed using the W L 3 -edge and Sr K-edge X-ray absorption spectroscopy combined with the reverse Monte Carlo simulations, X-ray photoelectron spectroscopy (XPS), and first-principles calculations. Protonated scheelites H x A WO 4 were produced using acid electrolytes in a one-step route at ambient conditions. The underlying mechanism behind this phenomenon can be ascribed to the intercalation of H + into the crystal structure of tungstate, effectively resulting in the reduction of W 6+ to W 5+ , i.e., the formation of polaronic centers, and giving rise to a characteristic dark blue-purple color. The emergence of the W 5+ was confirmed by XPS experiments. The relaxation of the local atomic structure around the W 5+ polaronic center was determined from the analysis of the extended X-ray absorption fine structures using the reverse Monte Carlo method. The results obtained suggest the displacement of the W 5+ ions from the center of [W 5+ O 4 ] tetrahedra in the structure of A WO 4 scheelite-type tungstates. This finding was also supported by the results of the first-principles calculations.