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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]

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.