Advanced approach to the local structure reconstruction and theory validation on the example of the W L3-edge extended X-ray absorption fine structure of tungsten

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 CAMART

High-temperature X-ray absorption spectroscopy study of thermochromic copper molybdate

Financial support provided by Scientific Research Project for Students and Young Researchers Nr. SJZ/2017/5 and SJZ/2018/1 realized at the Institute of Solid State Physics, University of Latvia is greatly acknowledged. The work was also supported by philanthropist MikroTik and administrated by the University of Latvia Foundation . The experiment at the Elettra synchrotron was performed within the project No. 20150303 .X-ray absorption spectroscopy at the Cu and Mo K-edges was used to study the effect of heating on the local atomic structure and dynamics in copper molybdate (α-CuMoO4) in the temperature range from 296 to 973 K. The reverse Monte-Carlo (RMC) method was successfully employed to perform accurate simulations of EXAFS spectra at both absorption edges simultaneously. The method allowed us to determine structural models of α-CuMoO4 being consistent with the experimental EXAFS data. These models were further used to follow temperature dependencies of the local environment of copper and molybdenum atoms and to obtain the mean-square relative displacements for Cu–O and Mo–O atom pairs. Moreover, the same models were able to interpret strong temperature-dependence of the Cu K-edge XANES spectra. We found that the local environment of copper atoms is more affected by thermal disorder than that of molybdenum atoms. While the MoO4 tetrahedra behave mostly as the rigid units, a reduction of correlation in atomic motion between copper and axial oxygen atoms occurs upon heating. This dynamic effect seems to be the main responsible for the temperature-induced changes in the O2−→Cu2+ charge transfer processes and, thus, is the origin of the thermochromic properties of α-CuMoO4 upon heating above room temperature.Scientific Research Project for Students and Young Researchers Nr. SJZ/2017/5 and SJZ/2018/1 at the Institute of Solid State Physics, University of Latvia; MikroTik, University of Latvia Foundation; 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 CAMART

Studies of Local Structure Relaxation in Nanomaterials

Elektroniskā versija nesatur pielikumusAnotācija Rentgenabsorbcijas spektroskopija ir unikāla tieša metode materiālu lokālās struktūras noteikšanai, kas ir pielietojama jebkādiem materiāliem, sākot no lieliem kristāliem līdz nanomateriāliem, šķidrumiem un gāzēm. Šajā darbā mēs pielietojām rentgenabsorbcijas spektra sīkstruktūru (RASS), lai pētītu niķeļa oksīda (NiO), kobalta volframāta (CoWO4), vara volframāta (CuWO4) un svina sulfīda (PbS) nanodaļiņu atomāro struktūru. Mēs šo nanomateriālu atomāro struktūru salīdzinājām ar atbilstošo savienojumu makro materiālu struktūru ar mērķi identificēt atomārās struktūras relaksāciju (izmaiņas atomārā struktūrā), kuru izraisījusi daļiņu izmēru samazināšanās līdz nanomērogam. Mēs esam pielāgojuši nanomateriāliem nesen izstrādāto kompleksās modelēšanas pieeju, turpmāk apzīmētu kā MD-RASS, kas apvieno ab initio (no pirmajiem principiem) daudzkārtējās izkliedes RASS aprēķinus ar klasisko molekulāro dinamiku (MD). MD-RASS metodes priekšrocība ir būtiski samazināts brīvo parametru skaits. Nepieciešami tikai tādi parametri, kas apraksta nanoobjekta ģeometriju un MD izmantoto spēka lauku. Jaunā metode tika pielietota NiO nanodaļiņu un plāno kārtiņu atomārās struktūras analīzē un deva lielisku sakritību ar eksperimentālajiem datiem. Iegūtie rezultāti mums ļāva identificēt Ni vakanču daudzumu un to lomu NiO struktūras relaksācijā. Atslēgas vārdi: Rentgenabsorbcijas spektroskopija; nanomateriāli; atomārā struktūra; molekulārā dinamikaAbstract The X-ray absorption spectroscopy is a unique tool for direct local structure determination which is suitable for any material starting from bulk crystals ending with nanomaterials, liquids and gasses. In this study we have applied the extended x-ray absorption fine structure (EXAFS) spectroscopy to probe the atomic structure of NiO, CoWO4, CuWO4 and PbS nanoparticles. We have compared the atomic structure of these nanomaterials with that of the corresponding bulk compounds, in order to identify the atomic structure relaxation (changes in atomic structure) caused by a reduction of the particle size down to nanoscale. We have adopted a recently developed complex modeling approach, combining ab initio multiple-scattering EXAFS calculations with classical molecular dynamics (MD), further referenced as MD-EXAFS, to the nanomaterials. The advantage of the MD-EXAFS method is a significant reduction of a number of free model parameters, which are required to describe the structure and dynamics of nanoobjects. Thus, a set of the parameters is restricted to that related to the geometry of the nanoobject and to the forcefield model utilized in the MD simulations. The novel approach has been tested on NiO nanoparticles and thin films. The obtained results allowed us to identify the amount and the role of the Ni vacancies in the structure relaxation of NiO. Keywords: X-ray absorption spectroscopy; nanomaterials; atomic structure; molecular dynamic

Treatment of disorder effects in X-ray absorption spectra beyond the conventional approach

The contribution of static and thermal disorder is one of the largest challenges for the accurate determination of the atomic structure from the extended X-ray absorption fine structure (EXAFS). Although there are a number of generally accepted approaches to solve this problem, which are widely used in the EXAFS data analysis, they often provide less accurate results when applied to outer coordination shells around the absorbing atom. In this case, the advanced techniques based on the molecular dynamics and reverse Monte Carlo simulations are known to be more appropriate: their strengths and weaknesses are reviewed here.Latvian Council of Science project no. lzp-2018/2-0353; 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 CAMART

Thermal disorder and correlation effects in anti-perovskite-type copper nitride

This work has been supported by the Latvian National Research Program IMIS2. The EXAFS experiment has been financed from the European Community's Seventh Framework Programme under grant agreement No. 226716 (Project I-20100098 EC). J.T. also gratefully acknowledges support from the National Science Foundation under the DMREF program Grant No. CHE-1534184.Reverse Monte Carlo simulations coupled with evolutionary algorithm were employed for the analysis of the temperature dependent (10–300 K) Cu K-edge extended X-ray absorption fine structure (EXAFS) spectra of polycrystalline copper nitride (Cu3N) with the goal to extract information on the thermal disorder and interatomic correlations in anti-perovskite-type crystal lattice. The obtained results are discussed in comparison with metallic copper and perovskite-type rhenium trioxide. The analysis of EXAFS spectra suggests that the anisotropy of copper atom vibrations is significantly enhanced upon increasing temperature, leading to pronounced tilting motion of NCu6 octahedra. Strong correlation in the motion of atoms was found along –N–Cu–N– atomic chains but it reduces rapidly with an increase of interatomic distance. Finally, anticorrelated motion of neighboring Cu atoms occurs along Cu–Cu bonds and is consistent with breathing-type motion of NCu6 octahedra.National Science Foundation CHE-1534184; Seventh Framework Programme 226716; 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 CAMART

Influence of Pressure and Temperature on X-Ray Induced Photoreduction of Nanocrystalline CuO

The authors are grateful to Prof. Alain Polian for providing NDAC cell. Parts of the present research have been carried out at the ODE beamline at SOLEIL.X-ray absorption spectroscopy at the Cu K-edge is used to study X-ray induced photoreduction of copper oxide to metallic copper. Although no photoreduction has been observed in microcrystalline copper oxide, we have found that the photoreduction kinetics of nanocrystalline CuO depends on the crystallite size, temperature and pressure. The rate of photoreduction increases for smaller nanoparticles but decreases at low temperature and higher pressure.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 CAMART

Kinetic Monte Carlo modeling of Y2O3 nano-cluster formation in radiation resistant matrices

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.As known, Y2O3 nano-clusters considerably increase radiation resistance of reactor construction materials. To model the nano-cluster formation kinetics, we propose the simplest possible mathematical model and perform kinetic Monte Carlo (KMC) simulations. We extended the KMC simulated results to the experimentally relevant times using autoregressive integrated moving average forecasting. Within the model, we have studied prototypical attractive interaction energies and particle concentrations, and compared the simulations with experiments. We have observed the standard Lifshitz-Slyozov-Wagner (LSW) theory, predicting the average cluster radius growth with time, , with in the long-time limit, for weak (0.1 eV) mutual particle attraction. However, the respective cluster growth rates in these KMC simulations are overestimated compared to the experiments. The best agreement with experiment is obtained for a medium (0.3 eV) and strong (0.5 eV) attractions, when nano-cluster formation occurs during intermediate asymptotic time scale, where power order p ranges from 5 to 7.6 depending on interaction, without reaching actually the LSW long-time limit. Such a stronger interaction leads also to a more compact {110}–faceted nano-clusters.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 CAMART

Interpretation of the Cu K-edge EXAFS spectra of Cu3N using ab initio molecular dynamics

Financial support provided by ERDF project No. 1.1.1.2/VIAA/l/16/147 (1.1.1.2/16/I/001) under the activity “Post-doctoral research aid” realized at the Institute of Solid State Physics, University of Latvia is greatly acknowledged. This work was supported by a grant from the Swiss National Supercomputing Centre (CSCS) under the project ID s681 .Cubic copper nitride (Cu3N) has anti-perovskite structure, and its properties are strongly affected by anisotropic thermal vibrations of copper atoms. Ab initio molecular dynamics (AIMD) simulations were performed in the temperature range from 300 K to 700 K in order to probe the details of Cu3N lattice dynamics. The Cu K-edge extended X-ray absorption fine structure (EXAFS) spectrum of bulk Cu3N was used to validate AIMD simulations at 300 K. The AIMD results suggest strong anharmonicity of the Cu–N and Cu–Cu bonds, the rigidity of NCu6 octahedra and strong correlation in atomic motion within –N–Cu–N– atom chains as well as support anisotropy of copper thermal vibrations.National Centre for Supercomputing Applications; Institute of Solid State Physics, Chinese Academy of Sciences; European Regional Development Fund 1.1.1.2/16/I/001,1.1.1.2/VIAA/l/16/147; Swiss National Supercomputing Centre grant under the project ID s681; 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 CAMART

Pressure-induced structural changes in α-MoO3 probed by X-ray absorption spectroscopy

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 CAMART

Recent progress in high-pressure X-ray absorption spectroscopy studies at the ODE beamline

I.J. and A.K. are grateful to the Latvian Council of Science project no. lzp-2018/2-0353 for financial support. The research leading to these results has been partially supported by the project CALIPSOplus under the Grant Agreement No. 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020.High pressure energy-dispersive X-ray absorption spectroscopy is a valuable structural technique, especially, when combined with a nano-polycrystalline diamond anvil cell. Here we present recent results obtained using the dispersive setup of the ODE beamline at SOLEIL synchrotron. The effect of pressure and temperature on the X-ray induced photoreduction is discussed on the example of nanocrystalline CuO. The possibility to follow local environment changes during pressure-induced phase transitions is demonstrated for α-MoO (Formula presented.) based on the reverse Monte Carlo simulations.Horizon 2020 project CALIPSOplus under the Grant Agreement No. 73087; Latvian Council of Science project no. lzp-2018/2-0353; 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 CAMART