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Thermochromic phase transition was studied in CuMoO4 using the Cu and Mo K-edge x-ray absorption spec-troscopy in the temperature range of 10-300 K. The hysteretic behavior has been evidenced from the tempera-ture dependence of the pre-edge shoulder intensity at the Mo K-edge, indicating that the transition from brown-ish-red γ-CuMoO4 to green α-CuMoO4 occurs in the temperature range of 230-280 K upon heating, whereas the α-to-γ transition occurs between 200 and 120 K upon cooling. Such behavior of the pre-edge shoulder at the Mo K-edge correlates with the change of molybdenum coordination between distorted tetrahedral in α-CuMoO4 and distorted octahedral in γ-CuMoO4. This result has been supported by ab initio full-multiple-scattering x-ray ab-sorption near edge structure (XANES) calculations. ; Financial support provided by Scientific Research Project for Students and Young Researchers Nr. SJZ/2017/5 realized at the Institute of Solid State Physics, University of Latvia is greatly acknowledged. The experiment at HASYLAB/DESY was performed within the project I-20160149 EC; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

Björn Matthey (Fraunhofer IKTS, Dresden) is acknowledged for providing HfO2 and ZrO2 powders on short notice after DESY's renowned customs office punished us. Parts of this research were carried out at Petra III at DESY, a member of the Helmholtz Association (HGF). The experiments on single Si:HfO2 thin film samples were performed at the CLAESS beamline at ALBA Synchrotron with the collaboration of ALBA staff. We would like to thank Edmund Welter for assistance (in using beamline P65) and DESY for enabling this research for proposal no. 20160591 and for travel support. T.S. acknowledges the German Research Foundation (DFG) for funding this work in the frame of the project "Inferox" (project no. MI 1247/11-2). B.J., J.L.J., and U.S. acknowledge funding from the Army Research Office through contract number W911NF-15-1-0593. This work was performed in part at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the U.S. National Science Foundation (award number ECCS-1542015). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI). ; Despite increasing attention for the recently found ferro- and antiferroelectric properties, the polymorphism in hafnia- and zirconia-based thin films is still not sufficiently understood. In the present work, we show that it is important to have a good quality X-ray absorption spectrum to go beyond an analysis of the only the first coordination shell. Equally important is to analyze both EXAFS and XANES spectra in combination with theoretical modelling to distinguish the relevant phases even in bulk materials and to separate structural from chemical effects. As a first step toward the analysis of thin films, we start with the analysis of bulk references. After that, we successfully demonstrate an approach that allows us to extract high-quality spectra also for 20 nm thin films. Our analysis extends to the second coordination shell and includes effects created by chemical substitution of Hf with Zr to unambiguously discriminate the different polymorphs. The trends derived from X-ray absorption spectroscopy agree well with X-ray diffraction measurements. In this work we clearly identify a gradual transformation from monoclinic to tetragonal phase as the Zr content of the films increases. We separated structural effects from effects created by chemical disorder when ration of Hf:Zr is varied and found differences for the incorporation of the substitute atoms between powders and thin films, which we attribute to the different fabrication routes. This work opens the door for further in-depth structural studies to shine light into the chemistry and physics of these novel ferroelectric thin films that show high application relevance. ; DESY proposal no. 20160591; German Research Foundation MI 1247/11-2; Army Research Office W911NF-15-1-0593; State of North Carolina and the U.S. National Science Foundation (award number ECCS-1542015); Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

With the examples of the C K-edge in graphite and the B K-edge in hexagonal boron nitride, we demonstrate the impact of vibrational coupling and lattice distortions on the X-ray absorption near-edge structure (XANES) in two-dimensional layered materials. Theoretical XANES spectra are obtained by solving the Bethe–Salpeter equation of many-body perturbation theory, including excitonic effects through the correlated motion of the core hole and excited electron. We show that accounting for zero-point motion is important for the interpretation and understanding of the measured X-ray absorption fine structure in both materials, in particular for describing the σ*-peak structure. ; Funding agencies: Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Knut and Alice Wallenbergs Foundation; Swedish Energy Research [43606-1]; Carl Trygger Foundation [CTS16:303, CTS14:310]; JSPS KA

The authors are grateful to Professor Alain Polian for providing the NDAC cell. ; High-pressure (0–26.7 GPa) Cu K-edge X-ray absorption spectroscopy is used to study possible structural modifications of anti-perovskite-type copper nitride (Cu3N) crystal lattice. The analysis of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), based on theoretical full-multiple-scattering and single-scattering approaches, respectively, suggests that at all pressures the local atomic structure of Cu3N remains close to that in cubic (Formula presented.) phase. Therefore, the transition to metal state above 5 GPa, observed previously using pressure-dependent electrical resistance and optical absorption measurements, is explained by the band gap collapse due to a decrease of the unit cell volume. We found that the lattice parameter of Cu3N is reduced by ≈2% upon increasing pressure up to 26.7 GPa, and the structure is restored upon pressure release. ; This study was supported by Latvian National Research program IMIS2. The experiment at the SOLEIL synchrotron radiation facility received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) CALIPSO under Grant agreement no. 312284; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

The measurements of Sr K-edge XAFS were performed under the approval of Proposal No. 97G042 of Photon Factory (KEK) and partially supported by the Research Grants of Hirosaki University. This work was supported by Bruce Ravel providing data for BTO. Boby Joseph acknowledges IISc Bangalore and ICTP Trieste for financial support through the award of the IISc-ICTP fellowship. ; The ferroelectric distortions in perovskites were a subject of numerous investigations for a long time. However, some controversial results still exist, coming from the analysis of diffraction (X-ray, neutron or electron) data and X-ray absorption spectra. In this study, our goal was to revisit these classical materials using recently developed methods without imposing any predefined structural model. Local environment around A-type atom in ABO3 perovskites (SrTiO3, BaTiO3, EuTiO3) was studied by X-ray absorption spectroscopy (XAS) in a wide range of temperatures (20–400 K). Using reverse Monte Carlo method enhanced by evolutionary algorithm, the 3D structure was extracted from the extended X-ray absorption fine structure (EXAFS) and interpreted in terms of the radial distribution functions (RDFs). Our findings show that both diffraction and XAS data are consistent, but reflect the structure of the material from different points of view. In particular, when strong correlations in the motion of certain atoms are present, the information obtained by XAS might lead to a different from expected shape of the RDF. At the same time, the average positions of all atoms are in good agreement with those given by diffraction. This makes XAS an important technique for studying interatomic correlations and lattice dynamics. ; Abdus Salam International Centre for Theoretical Physics; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

The authors are grateful to Prof. Alain Polian for providing NDAC cell. The research leading to this result has been supported by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. The work was supported by philanthropist MikroTik and administrated by the University of Latvia Foundation. ; Energy-dispersive X-ray absorption spectroscopy at the Mo K-edge was used to study pressure-induced (up to 36 GPa) changes in the local atomic structure of 2D layered oxide α-MoO3. A linear combination analysis based on the low and high-pressure X-ray absorption near edge structure (XANES) spectra shows clear evidence of two high-pressure phases, existing at 18-25 GPa and above 32 GPa. The first transition is due to gradual decrease of the interlayer gap, whereas the second one - to its collapse and oxide structure reconstruction. The local atomic structure around molybdenum atoms at 0.2, 18.5 and 35.6 GPa was determined from the extended X-ray absorption fine structure (EXAFS) using reverse Monte Carlo calculations. ; Project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

The local structure and chemical bonding in two-phase amorphous Cr1−xCx nanocomposite thin films are investigated by Cr K-edge (1s) X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies in comparison to theory. By utilizing the computationally efficient stochastic quenching (SQ) technique, we reveal the complexity of different Cr-sites in the transition metal carbides, highlighting the need for large scale averaging to obtain theoretical XANES and EXAFS spectra for comparison with measurements. As shown in this work, it is advantageous to use ab initio theory as an assessment to correctly model and fit experimental spectra and investigate the trends of bond lengths and coordination numbers in complex amorphous materials. With sufficient total carbon content (≥30 at. %), we find that the short-range coordination in the amorphous carbide phase exhibit similarities to that of a Cr7C3 ± y structure, while excessive carbons assemble in the amorphous carbon phase. ; Funding agencies:We would like to thank the staff at MAX-lab for experimental support and U. Jansson and M. Andersson for providing the samples. This work was supported by the Swedish Research Council (VR) Linnaeus Grant LiLi-NFM, the FUNCASE project supported Swedish Strategic Research Foundation (SSF). W.O. acknowledges financial support from VR Grant No. 621-2011-4426, the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU No 2009 00971), Knut and Alice Wallenbergs Foundation project Strong Field Physics and New States of Matter 2014-2019 (COTXS). B.A. would like to thank E. Holmstrom and R. Lizarraga for support with the SQ method and acknowledges financial support by the Swedish Research Council (VR) through the young researcher Grant No. 621-2011-4417 and the international career Grant No. 330-2014-6336 and Marie Sklodowska Curie Actions, Cofund, Project INCA 600398. The calculations were performed using supercomputer resources provided by the Swedish National Infrastructure for Computing (SNIC) at the National Supercomputer Centre (NSC) and Center for Parallel Computing (PDC).

This is the peer reviewed version of the following article: I. Pudza, A. Kalinko, A. Cintins, A. Kuzmin, Acta Mater. 205 (2021) 116581, which has been published in final form at https://www.sciencedirect.com/science/article/abs/pii/S1359645420310181 This article may be used for non-commercial purposes in accordance with Elsevier Terrms and Conditions for Self-Archiving. ; Polycrystalline CuMo1−xWxO4 solid solutions were studied by resonant X-ray emission spectroscopy (RXES) at the W L3-edge to follow a variation of the tungsten local atomic and electronic structures across thermochromic phase transition as a function of sample composition and temperature. The experimental results were interpreted using ab initio calculations. The crystal-field splitting parameter Δ for the 5d(W)-states was obtained from the analysis of the RXES plane and was used to evaluate the coordination of tungsten atoms. Temperature-dependent RXES measurements were successfully employed to determine the hysteretic behaviour of the structural phase transition between the α and γ phases in CuMo1−xWxO4 solid solutions on cooling and heating, even at low (x 0.15 in the whole studied temperature range (90-420 K), whereas their coordination changes from tetrahedral to octahedral upon cooling for smaller (x ≤ 0.15) tungsten content. Nevertheless, some amount of tungsten ions was found to co-exists in the octahedral environment at room temperature for x < 0.15. The obtained results correlate well with the color change in these solid solutions. ; Financial support provided by Scientific Research Project for Students and Young Researchers Nr. SJZ/2019/1 realized at the Institute of Solid State Physics, University of Latvia is greatly acknowledged. The used infrastructure of the von Hamos spectrometer was realized in the frame of projects FKZ 05K13UK1 and FKZ 05K14PP1. The experiment at the PETRA III synchrotron was performed within the project No. I-20180615 EC.The synchrotron experiments have been supported by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. The experiment at the Elettra synchrotron was performed within the project No. 20150303. Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2.

This work was supported by a grant from the Swiss National Supercomputing Centre (CSCS) under the project ID s444. The resource allocation within the PSI share at CSCS and on the PSI compute cluster Merlin4 is also acknowledged. D. B. is grateful for a fellowship within the Sciex-NMS programme. A. K. was supported by Latvian Science Council Grant no. 187/2012. ; Uranium L3-edge X-ray absorption spectroscopy was used to study the atomic structure of uranium dioxide (UO2). The extended X-ray absorption fine structure (EXAFS) was interpreted within the ab initio multiple-scattering approach combined with classical molecular dynamics to account for thermal disorder effects. Nine force-field models were validated, and the role of multiple-scattering contributions was evaluated. ; Swiss National Supercomputing Centre project ID s444; Latvian Science Council grant no. 187/2012; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

School of Studies in Physics, Vikram University, Ujjain-456 010 Department of Physics, Government College, Neemuch-458 441 Department of Physics, Government Arts & Science College, Ratlam-457 001 Department of Chemistry, Faculty of Science, M. S. University, Vadodara-390 002 Manuscript received 24 October 1991, revised 4 February 1993, accepted 31 March 1993 X-Ray K-absorption spectra of one mononuclear and two binuclear tetradentate Schiff base copper(ll) complexes have been recorded. The observed edge-shifts, edge-widths and shift of the principal absorption maximum have been correlated with the nature of the bond, ionicity and other structural characteristics of the complexes. The X-ray absorption near edge structure (XANES) features have been observed in the absorption curves. The small absorption maximum appearing on the low energy side of the main peak A has been assigned the transition Is → 4s*. The band gap energies have also been estimated.

The chemical bonding within the transition-metal carbide materials MAX phase Ti3AlC2 and MXene Ti3C2Txis investigated by x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine-structure(EXAFS) spectroscopies. MAX phases are inherently nanolaminated materials that consist of alternating layersof Mn+1Xn and monolayers of an A-element from the IIIA or IVA group in the Periodic Table, where M is atransition metal and X is either carbon or nitrogen. Replacing the A-element with surface termination speciesTx will separate the Mn+1Xn-layers forming two-dimensional (2D) flakes of Mn+1XnTx. For Ti3C2Tx the Tx corresponds to fluorine (F) and oxygen (O) covering both sides of every single 2D Mn+1Xn-flake. The Ti K-edge(1s) XANES of both Ti3AlC2 and Ti3C2Tx exhibit characteristic preedge absorption regions of C 2p-Ti 3dhybridization with clear crystal-field splitting while the main-edge absorption features originate from the Ti1s → 4p excitation, where only the latter shows sensitivity toward the fcc-site occupation of the terminationspecies. The coordination numbers obtained from EXAFS show that Ti3AlC2 and Ti3C2Tx are highly anisotropicwith a strong in-plane contribution for Ti and with a dynamic out-of-plane contribution from the Al monolayersand termination species, respectively. As shown in the temperature-dependent measurements, the O contributionshifts to shorter bond length while the F diminishes as the temperature is raised from room temperature up to 750 °C. ; Funding agencies: Swedish Research CouncilSwedish Research Council [2018-07152]; Swedish Governmental Agency for Innovation SystemsVinnova [2018-04969]; FormasSwedish Research Council Formas [2019-02496]; Swedish Research Council (VR) LiLi-NFM Linnaeus EnvironmentSwedish R

AIF acknowledge support by the US Department of Energy, Office of Basic Energy Sciences under Grant No. DE-FG02 03ER15476. AIF acknowledges support by the Laboratory Directed Research and Development Program through LDRD 18-047 of Brookhaven National Laboratory under U.S. Department of Energy Contract No. DE-SC0012704 for initiating his research in machine learning methods. The help of the beamline staff at ELETTRA (project 20160412) synchrotron radiation facility is acknowledged. RMC-EXAFS and MD-EXAFS simulations were performed on the LASC cluster-type computer at Institute of Solid State Physics of the University of Latvia. ; The knowledge of the coordination environment around various atomic species in many functional materials provides a key for explaining their properties and working mechanisms. Many structural motifs and their transformations are difficult to detect and quantify in the process of work (operando conditions), due to their local nature, small changes, low dimensionality of the material, and/or extreme conditions. Here we use an artificial neural network approach to extract the information on the local structure and its in situ changes directly from the x-ray absorption fine structure spectra. We illustrate this capability by extracting the radial distribution function (RDF) of atoms in ferritic and austenitic phases of bulk iron across the temperature-induced transition. Integration of RDFs allows us to quantify the changes in the iron coordination and material density, and to observe the transition from a body-centered to a face-centered cubic arrangement of iron atoms. This method is attractive for a broad range of materials and experimental conditions. ; Laboratory Directed Research and Development LDRD 18-047; U.S. Department of Energy DE-FG02 03ER15476; Brookhaven National Laboratory DE-SC0012704; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

The authors gratefully acknowledge the assistance of the ELETTRA XAFS beamline staff members during the EXAFS experiment No 20150303. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. ; Atomistic simulations of the experimental W L3-edge extended x-ray absorption fine structure (EXAFS) of bcc tungsten at T = 300 K were performed using classical molecular dynamics (MD) and reverse Monte Carlo (RMC) methods. The MD-EXAFS method based on the results of MD simulations allowed us to access the structural information, encoded in EXAFS, beyond the first coordination shell and to validate the accuracy of two interaction potential models—the embedded atom model potential and the second nearest-neighbor modified embedded atom method potential. The RMC-EXAFS method was used for more elaborate analysis of the EXAFS data giving access to thermal disorder effects. The results of both methods suggest that the correlation in atomic motion in bcc tungsten becomes negligible above 8 Å. This fact allowed us to use the EXAFS data to determine not only mean-square relative displacements of atomic W–W pair motion but also mean-square displacements of individual tungsten atoms, which are usually accessible from diffraction data only. ; EUROfusion Consortium, Euratom research and training programme 2014-2018 under grant agreement No 633053;Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²