The luminescence properties of ZnO nanopowders

Pure and Al3+ doped ZnO nanopowders were studied by means of time-resolved luminescence spectroscopy. The powders were synthesized by hydrothermal and plasma methods. These powders were used as a raw material for vaporization-condensation process inside the Solar reactor. The commercially available ZnO nanopowder was studied for a comparison. Exciton to defect band luminescence intensity ratio was estimated in different types of ZnO nanopowders. It was found that nanopowders with whiskers morphology show superlinear luminescence intensity depending on excitation density. The observed effect depends on the average nanoparticle size and on the powder morphology.

Low temperature X-ray absorption spectroscopy study of $CuMoO_{4}$ and $CuMo_{0.90}W_{0.10}O_4$ using reverse Monte-Carlo method

Radiation physics and chemistry 175, 108411 (2020). doi:10.1016/j.radphyschem.2019.108411

Time-resolved cathodoluminescence and photoluminescence of nanoscale oxides

The nanostructured oxide materials such as ZnO, ZrO2, and Y3Al5O12 (YAG) are perspective materials for transparent scintillating and/or laser ceramics. The luminescence properties of single crystals, nanopowders and ceramic were compared. Nominally pure and rare-earth doped nanopowders and ceramics have been studied by means of time-resolved luminescence spectroscopy. The fast blue luminescence band was studied in ZnO ceramics sintering from different raw materials. The luminescence centres of ZrO2:Y were compared in a single crystal, ceramic and nanopowder. It is shown that ceramic sintering parameters have a strong influence on time-resolved luminescence characteristics in cerium-doped YA…

Extended x-ray absorption fine structure spectroscopy and first-principles study of SnWO4

The local atomic structure in α- and β-SnWO 4 was studied bysynchrotron radiation W L 3 -edge X-ray absorption spectroscopy at 10 and 300 K.Strongly distorted WO 6 octahedra were found in α-SnWO 4 , whereas nearly regularWO 4 tetrahedra were observed in β-SnWO 4 , confirming previous results. Thestructural results obtained were supported by the first-principles calculations,suggesting that the second-order Jahn-Teller effect is responsible for octahedraldistortion.

Study of the thermochromic phase transition in CuMo1−xWxO4 solid solutions at the W L3-edge by resonant X-ray emission spectroscopy

Abstract Polycrystalline CuMo 1 − x W x O 4 solid solutions were studied by resonant X-ray emission spectroscopy (RXES) at the W L 3 -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 t…

Probing NiO nanocrystals by EXAFS spectroscopy

Abstract The structure relaxation in nanocrystalline NiO (nano-NiO, 13 nm crystallite size) has been studied by X-ray absorption spectroscopy at the Ni K-edge at 300 K. Conventional single-scattering analysis of the EXAFS signals from the first two coordination shells showed a lattice volume expansion by about 1% and a contraction of the Ni–O bonds by about 0.5% in nano-NiO compared to microcrystalline NiO. A more sophisticated approach, based on a combination of classical molecular dynamics and ab initio multiple-scattering EXAFS theory, allowed us to interpret both static relaxation and lattice dynamics in nano-NiO.

Local Structure of Multiferroic $\mathrm{MnWO_4}$ and $\mathrm{Mn_{ 0.7} Co_{0.3}WO_4}$ Revealed by the Evolutionary Algorithm

A novel reverse Monte Carlo/evolutionary algorithm scheme was applied to the analysis of the W L3-edge and Mn(Co) K-edges EXAFS spectra from multiferroic MnWO4 and Mn0.7Co0.3WO4. A 3D structural model, consistent with the experimental data, was obtained, and the influence of composition and temperature on the local structure of tungstates is discussed.

Study of the thermochromic phase transition in CuMo1−xWxO4 solid solutions at the W L3-edge by resonant X-ray emission spectroscopy

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.

Origin of Pressure-Induced Metallization in Cu 3 N: An X-ray Absorption Spectroscopy Study

The authors are grateful to Professor Alain Polian for providing the NDAC cell.

Synthesis and characterization of GaN/ReS2, ZnS/ReS2 and ZnO/ReS2 core/shell nanowire heterostructures

This research was funded by the ERDF project “Smart Metal Oxide Nanocoatings and HIPIMS Technology”, project number: 1.1.1.1/18/A/073. 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 Use of X-ray Absorption Spectra for Validation of Classical Force-Field Models

Abstract Extended X-ray absorption fine structure (EXAFS) spectroscopy and molecular dynamics (MD) simulations are two complementary techniques widely used to study the atomic structure of materials. Their combined use, known as the MD-EXAFS approach, allows one to access the structural information, encoded in EXAFS, far beyond the nearest coordination shells and to validate the accuracy of the interaction potential models. In this study we demonstrate the use of the MD-EXAFS method for a validation of several force-field models on an example of the cubic-perovskite SrTiO3 and hexagonal wurtzite-type ZnO crystals.

Evidence of nickel ions dimerization in NiWO4 and NiWO4-ZnWO4 solid solutions probed by EXAFS spectroscopy and reverse Monte Carlo simulations

Abstract The existence of exchange-coupled Ni 2 + ions – the so-called magnetic dimers – in wolframite-type NiWO 4 and Zn c Ni 1 − c WO 4 solid solutions with high nickel content was discovered by X-ray absorption spectroscopy combined with reverse Monte Carlo (RMC) simulations. Temperature- (10–300 K) and composition-dependent X-ray absorption spectra were measured at the Ni K-edge, Zn K-edge, and W L 3 -edge of microcrystalline NiWO 4 , Zn c Ni 1 − c WO 4 and ZnWO 4 . Structural models were obtained from simultaneous analysis of the extended X-ray absorption fine structure (EXAFS) spectra at three metal absorption edges using RMC simulations. The obtained radial distribution functions for…

Ab initio LCAO study of the atomic, electronic and magnetic structures and the lattice dynamics of triclinic CuWO4

Abstract The electronic, structural and phonon properties of antiferromagnetic triclinic CuWO 4 have been studied using the first-principles spin-polarized linear combination of atomic orbital (LCAO) calculations based on the hybrid exchange–correlation density functional (DFT)/Hartree–Fock (HF) scheme. In addition, the local atomic structure around both Cu and W atoms has been probed using extended X-ray absorption fine structure (EXAFS) spectroscopy. We show that, by using the hybrid DFT–HF functional, one can accurately and simultaneously describe the atomic structure (the unit cell parameters and the atomic fractional coordinates), the band gap and the phonon frequencies. In agreement w…

Blue luminescence in ZnO single crystals, nanopowders, ceramic

The luminescence spectra and luminescence decay processes were studied in a ZnO single crystal, nanopowders and ceramic at liquid helium and room temperature under VUV synchrotron radiation as well as under pulsed laser excitation. The exciton-exciton and exciton-multiphonon processes were compared in different ZnO nanopowders (commercial powder, powders obtained by vaporization-condensation technique) and ceramic. The possibility of luminescence decay time modification by Al3+ doping was shown.

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 .

Local structure of copper nitride revealed by EXAFS spectroscopy and a reverse Monte Carlo/evolutionary algorithm approach

Physica scripta 91(5), 054003 (2016). doi:10.1088/0031-8949/91/5/054003

Orthorhombic boron oxide under pressure: in situ study by X-ray diffraction and Raman scattering

High-pressure phase of boron oxide, orthorhombic \b{eta}-B2O3, has been studied in situ by synchrotron X-ray diffraction to 22 GPa and Raman scattering to 46 GPa at room temperature. The bulk modulus of \b{eta}-B2O3 has been found to be 169(3) GPa that is in good agreement with our ab initio calculations. Raman and IR spectra of \b{eta}-B2O3 have been measured at ambient pressure, all experimentally observed bands have been attributed to the theoretically calculated ones, and the mode assignment has been performed. Based on the data on Raman shift as a function of pressure, combined with equation-of-state data, the Gr\"uneisen parameters of all experimentally observed Raman bands have been …

Local Structure of Multiferroic MnWO4 and Mn0.7Co0.3WO4 Revealed by the Evolutionary Algorithm

A novel reverse Monte Carlo/evolutionary algorithm scheme was applied to the analysis of the W L3-edge and Mn(Co) K-edges EXAFS spectra from multiferroic MnWO4 and Mn0.7Co0.3WO4. A 3D structural model, consistent with the experimental data, was obtained, and the influence of composition and temperature on the local structure of tungstates is discussed.

Pressure-induced insulator-to-metal transition in α-SnWO4

In-situ high-pressure W L1 and L3 edges x-ray absorption and mid-infrared spectroscopies complemented by first-principles calculations suggest the existence of pressure- induced insulator-to-metal transition in α-SnWO4 in the range of 5-7 GPa. Its origin is explained by a symmetrization of metal-oxygen octahedra due to a strong interaction of Sn 5s, W 5d and O 2p states along the b-axis direction, leading to a collapse of the band gap.

Chemical-state analyses of Ni, Zn, and W ions in NiWO$_4$-ZnWO$_4$ solid solutions by X-ray photoelectron spectroscopy

The chemical states of Ni, Zn, and W in microcrystalline NiWO$_4$-ZnWO$_4$ solid solutions were studied by X-ray photoelectron spectroscopy. The recorded spectra of the Ni 2p, Zn 2p, and W 4f photoelectron lines and Ni L$_2$M$_{23}$M$_{45}$, Zn L$_3$M$_{45}$M$_{45}$, and W N$_4$N$_{67}$N$_{7}$ Auger-transition lines show pronounced changes with increasing Zn concentration. The positions of the resolved photoelectron and Auger-transition lines were combined to construct so-called chemical-state plots (Wagner or Auger-parameter plots) for metal ions in solid solutions. With increasing Zn concentration, the Auger parameter increases for Ni and decreases for W, thus evidencing a lowering and an…

Ab initio study of the electronic and atomic structure of the wolframite-type ZnWO4

Abstract Ab initio quantum chemistry calculations of the structural and electronic properties of monoclinic wolframite-type ZnWO 4 crystal have been performed within the periodic linear combination of atomic orbitals (LCAO) method using six different Hamiltonians, based on density functional theory (DFT) and hybrid Hartree-Fock-DFT theory. The obtained results for optimized structural parameters, band gap and partial density of states are compared with available experimental data, and the best agreement is observed for hybrid Hamiltonians. The calculations show that zinc tungstate is a wide band gap material, with the direct gap about 4.6 eV, whose valence band has largely O 2p character, w…

Molecular dynamics and reverse Monte Carlo modeling of scheelite-type AWO4(A = Ca, Sr, Ba) WL3-edge EXAFS spectra

Classical molecular dynamics (MD) and reverse Monte Carlo methods coupled with ab initio multiple-scattering extended x-ray absorption fine structure (EXAFS) calculations were used for modeling of scheelite-type AWO4 (A = Ca, Sr, Ba) W L 3-edge EXAFS spectra. The two theoretical approaches are complementary and allowed us to perform analysis of full EXAFS spectra. Both methods reproduce well the structure and dynamics of tungstates in the outer coordination shells, however the classical MD simulations underestimate the W–O bond MSRD due to a neglect of quantum zero-point-motion. The thermal vibration amplitudes, correlation effects and anisotropy of the tungstate structure were also estimat…

Luminescence Properties of ZnO Nanocrystals and Ceramics

The luminescence excitation spectra, luminescence spectra and the nanosecond-scale decay kinetics were studied. The ZnO and ZnO:Al nanopowders were prepared by vaporization-condensation in a solar furnace using different raw powders: commercial, hydrothermal and those obtained by plasma synthesis. Exciton-phonon as well as exciton-exciton interaction processes in nanopowders, a bulk crystal and ZnO ceramics were studied and compared. The fast decay and low afterglow intensity of ZnO nanopowders and ceramics support these materials for scintillators.

High-pressure x-ray absorption spectroscopy study of tin tungstates

Room-temperature pressure-dependent (0-25 GPa) x-ray absorption spectroscopy at the W -edges of α-SnWO4 and β-SnWO4 was performed using a dispersive setup and a high-pressure nanodiamond anvil cell. The detailed analysis of experimental x-ray absorption near-edge structure and extended x-ray absorption fine structure data suggests that upon increasing pressure, a displacement of tungsten atoms by about 0.2 A toward the center of the WO6 octahedra occurs in α-SnWO4, whereas the coordination of tungsten atoms changes from tetrahedral to distorted octahedral in β-SnWO4.

Electronic excitations in ZnWO4 and ZnxNi1−x WO4 (x = 0.1 − 0.9) using VUV synchrotron radiation

The photoluminescence spectra and luminescence excitation spectra of pure microcrystalline and nano-sized ZnWO4 as well as the Zn x Ni1−x WO4 solid solutions were studied using vacuum ultraviolet (VUV) synchrotron radiation. The samples were also characterized by x-ray powder diffraction. We found that: (i) the shape of the photoluminescence band at 2.5 eV, being due to radiative electron transitions within the [WO6]6− anions, becomes modulated by the optical absorption of Ni2+ ions in the Zn x Ni1−x WO4 solid solutions; and (ii) no significant change in the excitation spectra of Zn0.9Ni0.1WO4 is observed compared to pure ZnWO4. At the same time, a shift of the excitonic bands to smaller en…

Temperature dependence of the local structure and lattice dynamics of wurtzite-type ZnO

Temperature-dependent (10–300 K) Zn K-edge extended X-ray absorption fine structure (EXAFS) spectra of polycrystalline wurtzitetype ZnO were analyzed using ab initio multiple-scattering theory and taking into account anisotropy of the crystallographic structure and thermal disorder. We employed two different simulation approaches: classical molecular dynamics (MD) and reverse Monte Carlo coupled with an evolutionary algorithm (RMC/EA method). The accuracy of several force-field models, which are commonly used in the MD simulations of bulk and nanostructured ZnO, was tested based on a comparison between the experimental and simulated Zn K-edge EXAFS spectra. It was found that available force…

Study of Copper Nitride Thin Film Structure

Abstract X-ray diffraction and x-ray absorption spectroscopy at the Cu K-edge were used to study the atomic structure in copper nitride (Cu3N) thin films. Textured nanocrystalline films are obtained upon dc magnetron sputtering on substrates heated at about 190 °C, whereas amorphous films having strongly disordered structure already in the second coordination shell of copper are deposited in the absence of heating.

Understanding the Conversion Process of Magnetron-Deposited Thin Films of Amorphous ReO$_x$ to Crystalline ReO$_3$ upon Thermal Annealing

Crystal growth & design 20(9), 6147 - 6156 (2020). doi:10.1021/acs.cgd.0c00848

Static and dynamic structure of $ZnWO_4$ nanoparticles

Abstract Static and dynamic structure of ZnWO 4 nanoparticles, synthesized by co-precipitation technique, has been studied by temperature dependent x-ray absorption spectroscopy at the Zn K-edge and W L 3 -edge. Complementary experimental techniques, such as x-ray powder diffraction, Raman and photoluminescence spectroscopies, have been used to understand the variation of vibrational, optical, and structural properties of nanoparticles, compared to microcrystalline ZnWO 4 . Our results indicate that the structure of nanoparticles experiences strong relaxation leading to the significant distortions of the WO 6 and ZnO 6 octahedra, being responsible for the changes in optical and vibrational …

FIRST-PRINCIPLES LCAO CALCULATIONS ON 5D TRANSITION METAL OXIDES: ELECTRONIC AND PHONON PROPERTIES

ABSTRACT First-principles quantum chemistry calculations within the periodic linear combination of atomic orbitals (LCAO) formalism have been used to probe electronic and phonon properties of crystalline 5d transition metal oxides ReO3, ZnWO4 and CaWO4. The obtained equilibrium crystal structure is in good agreement with known crystallographic data. Rhenium trioxide is correctly predicted to be a metal, whereas both tungstates—a wide gap insulating compounds. The phonon frequencies, calculated by the frozen phonon method, agree rather well with those obtained by infrared and Raman spectroscopies.

Ab initio molecular dynamics simulations of the Sc K-edge EXAFS of scandium trifluoride

Scandium fluoride ScF3 has a simple cubic structure and attracts attention due to its large negative thermal expansion (NTE) over a wide range of temperatures (0-1100 K). In this study we present ab initio molecular dynamics (AIMD) simulations of ScF3 and their validation using the Sc K-edge EXAFS spectra in the temperature range from 300 K to 1000 K measured at the XAFS beamline of ELETTRA. The obtained results allow an assessement of the employed AIMD model and provide insight into the local structure and the lattice dynamics of ScF3 beyond the harmonic approximation. A strong anisotropy of the fluorine atom vibrations in the direction orthogonal to the -Sc-F-Sc- chain is observed. An exp…

X-ray absorption and Raman spectroscopy studies of tungstates solid solutions ZncNi1-cWO4 (c=0.0-1.0)

G.B. acknowledges the financial support provided by the State Education Development Agency for project No. 1.1.1.2/VIAA/3/19/444 (agreement No. 1.1.1.2/16/I/001) realized at the Institute of Solid State Physics, University of Latvia. A.K. and A.K. would like to thank the support of the Latvian Council of Science project No. lzp-2019/1-0071. 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.

Interpretation of EXAFS in scheelite-type AWO4(A = Ca, Sr, Ba) compounds using molecular dynamics simulations

In this work we successfully interpret the W L3-edge EXAFS spectra in scheelite-type AWO4 (A = Ca, Sr, Ba) compounds using a combination of classical NVT molecular dynamics (MD) and ab initio multiple-scattering (MS) theory. The configuration-averaged EXAFS spectra show good agreement with our room temperature experimental data supporting the reliability of the developed force-field models. The contributions from all coordination shells up to 6 A are elucidated. The contribution of the MS effects into the total EXAFS signal in AWO4 compounds is small, being around 10%.

Raman and photoluminescence spectroscopy of zinc tungstate powders

ZnWO4 powders, synthesized using co-precipitation technique and annealed in air at different temperatures in the range of 80-800 � C, were studied by Raman and photoluminescence spectroscopy. ZnWO4 single crystal was used for comparison. The interpretation of the observed variations of the Raman spectra and intrinsic photoluminescence band upon annealing is suggested.

Nanocrystalline CaWO$_4$ and ZnWO$_4$ Tungstates for Hybrid Organic-Inorganic X-ray Detectors

The experiment at the DESY PETRA-III synchrotron was performed within project No. I-20211105 EC at the Institute of Solid State Physics, University of Latvia, as the Cen ter of Excellence has received funding from the European Union’s Horizon 2020 Framework Pro gramme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2.

Effect of Pressure and Temperature on the Local Structure and Lattice Dynamics of Copper(II) Oxide

Abstract Microcrystalline and nanocrystalline (6 nm) CuO were studied in situ by the Cu K-edge X-ray absorption spectroscopy as a function of pressure (0-20 GPa) and temperature (10-300 K). Pressure dependence of X-ray absorption near edge structure (XANES) was interpreted within the full-multiple-scattering formalism based on the relaxed atomic structure determined by ab initio linear combination of atomic orbital (LCAO) calculations. Temperature dependence of the mean-square relative displacement (MSRD) for the four shortest Cu–O distances was obtained from the analysis of extended X-ray absorption fine structure (EXAFS) and described by the correlated Einstein model with the characterist…

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.

Nanosession: Valence Change Memories - A Look Inside

Probing the Thermochromic Phase Transition in CuMoO 4 by EXAFS Spectroscopy

The work was supported by philanthropist MikroTik and administrated by the University of Latvia Foundation. The experiment at HASYLAB/DESY was performed within the project I-20160149 EC.

Discovery of new boron-rich chalcogenides: Orthorhombic B6X (X=S, Se)

The authors thank T. Chauveau (LSPM) for help with Rietveld analysis, A. Jamali (LRCS) for assistance with SEM measurements, and Drs. Y. Tange (SPring-8) and N. Guignot (SOLEIL) for help in synchrotron experiments that were carried out during beamtimes allocated to proposals 2017A1047 & 2018A1121 at SPring-8 and proposal 20170092 at SOLEIL. Ab initio calculations have been performed using Rurik and Arkuda supercomputers. This work was financially supported by the European Union’s Horizon 2020 Research and Innovation Programme under Flintstone2020 project (grant agreement No. 689279). Z.W. thanks the National Science Foundation of China (grant No. 11604159). A.R.O. thanks the Russian Ministr…

Interpretation of unexpected behavior of infrared absorption spectra ofScF3beyond the quasiharmonic approximation

Scandium fluoride (${\mathrm{ScF}}_{3}$), having cubic ${\mathrm{ReO}}_{3}$-type structure, has attracted much scientific attention due to its rather strong negative thermal expansion (NTE) in the broad temperature range from 10 to 1100 K. Here we use the results of diffraction and extended x-ray absorption fine-structure (EXAFS) spectroscopy to interpret the influence of NTE on the temperature dependence of infrared absorption spectra of ${\mathrm{ScF}}_{3}$. Original infrared absorption and EXAFS experiments in a large temperature range are presented and interpreted using ab initio lattice dynamics simulations within and beyond quasiharmonic approximations. We demonstrate that ab initio e…

Local structure of perovskites ReO3and ScF3with negative thermal expansion: interpretation beyond the quasiharmonic approximation

We propose an approach beyond the quasiharmonic approximation for interpretation of EXAFS and XRD data and for ab initio calculations of electronic and vibration properties of materials with negative thermal expansion. Ab initio electronic structure and lattice dynamics calculations for cubic and distorted ScF3 were performed using the linear combination of atomic orbitals (LCAO) method. The band gap obtained in calculations for ScF3 is equal to 10.54 eV and agree well with the expected value. The calculated infrared spectra of F displaced (FD) cubic ScF3 allow us to predict that its mean Sc-F-Sc angle within NTE deviates from 180 degree.

X-ray absorption near edge spectroscopy of thermochromic phase transition in CuMoO4

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 s…

Spectroscopic study of the electric field induced valence change of Fe-defect centers in SrTiO(3)

The electrochemical changes induced by an electric field in Fe-doped SrTiO(3) have been investigated by X-ray absorption spectroscopy (XANES and EXAFS), electron paramagnetic resonance (EPR) and Raman spectroscopy. A detailed study of the Fe dopant in the regions around the anode and cathode reveals new insights into the local structure and valence state of Fe in SrTiO(3) single crystals. The ab initio full multiple-scattering XANES calculations give an evidence of the oxygen vacancy presence in the first coordination shell of iron. Differences in the length and disorder of the Fe-O bonds as extracted from EXAFS are correlated to the unequivocal identification of the defect type by compleme…

Analysis of extended x-ray absorption fine structure data from copper tungstate by the reverse Monte Carlo method

The static disorder and lattice dynamics of crystalline materials can be efficiently studied using reverse Monte Carlo simulations of extended x-ray absorption fine structure spectra (EXAFS). In this work we demonstrate the potentiality of this method on an example of copper tungstate CuWO4. The simultaneous analysis of the Cu K and W L3 edges EXAFS spectra allowed us to follow local structure distortion as a function of temperature.

First-principles LCAO study of phonons in NiWO4

Abstract The electronic, structural and phonon properties of antiferromagnetic wolframite-type NiWO4 have been studied using first-principles spin-polarized LCAO calculations based on the hybrid Hartree-Fock (HF)/density functional (DFT) scheme. The influence of different percentages of HF contribution, i.e. different correlation strength, on the structure and phonon frequencies has been investigated and compared with the available experimental data.

External pressure and composition effects on the atomic and electronic structure of SnWO4

The atomic and electronic structure of tin tungstates, α-SnWO4, α-Sn1.03W0.99O4 and β-SnWO4 ,w as studied by the W L3-edge X-ray absorption spectroscopy and first-principles linear combination of atomic orbital (LCAO) calculations based on the hybrid exchange-correlation density functional (DFT)/ Hartree–Fock (HF) scheme. It was found that the crystal structure of both α-phases is built up of strongly distorted WO6 octahedra, whereas that of β-SnWO4 is composed of nearly regular WO4 tetrahedra. In addition, there are distorted SnO6 octahedra in both α- and β-phases. The metal–oxygen octahedra distortion is explained by the second-order Jahn–Teller effect. The influence of pressure on the st…

Interpretation of EXAFS in ReO3using molecular dynamics simulations

Temperature dependent Re L3-edge EXAFS signals from perovskite-type cubic ReO3 have been successfully interpreted using a combination of classical NVT molecular dynamics (MD) and ab initio multiple-scattering approach. The force field model, required for MD simulations, has been determined by fitting the Re-O and O-O pairwise interatomic potentials to a set of experimental data (lattice parameter, elastic constants and bulk modulus) and phonon frequencies, theoretically calculated from the first principles at high symmetry points of the Brillouin zone. The MD simulations reproduce well the anisotropy of thermal vibration for oxygen atoms and confirm a deviation of the mean Re-O-Re angle fro…

X-ray absorption spectroscopy of Cu-doped $WO_{3}$ films for use inelectrochemical metallization cell memory

Abstract We have performed the first synchrotron radiation X-ray absorption spectroscopy (EXAFS/XANES) study of the local atomic and electronic structure around Cu and W ions in WO 3 /Cu/WO 3 /Si and WO 3 /Cu/Si multilayered structures, aimed for the application in the electrochemical metallization cell memory. The influence of low-temperature annealing at 135 °C has been investigated in details, and a structural model of Cu-doped WO 3 films is proposed.

The influence of Sb doping on the local structure and disorder in thermoelectric ZnO:Sb thin films

The experiment at HASYLAB/DESY was performed within the project I-20200161 EC. 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. 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 carried out in part through the use of the INL Advanced Electron Microscopy, Imaging and Spectroscopy Facility. This work (proposal ID 2018–020-022469) was car…

The role of Ga and Bi doping on the local structure of transparent zinc oxide thin films

The experiment at HASYLAB/DESY was performed within the project I-20180036 EC. 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. Filipe Correia is grateful to the Fundação para a Ciência e Tecnologia (FCT, Portugal) for the Ph.D. Grant SFRH/BD/111720/2015. Joana Ribeiro is grateful to the Project WinPSC - POCI-01-0247-FEDER-017796, for the research grant from the Agência Nacional de Inovação, co-funded by the European Regional Development Fund (ERDF), through the Operational Programme for Competitiveness and Internationalisation (COMPETE 2020), under the…

Local structure relaxation in nanosized tungstates

Abstract The atomic structure of nanosized and microcrystalline tungstates MeWO 4 (Me=Co, Ni, Cu, Zn), synthesized by co-precipitation technique, has been studied by x-ray absorption spectroscopy at the W L 3 -edge and Co/Ni/Cu/Zn K-edges, x-ray diffraction, Raman and Fourier transform infrared spectroscopies. The distortion of metal–oxygen octahedra is caused by the electron–lattice coupling and is further enhanced in nanosized tungstates due to formation of the double tungsten–oxygen bonds at the nanoparticle surface.

Local structure and dynamics of wurtzite‐type ZnO from simulation‐based EXAFS analysis

Conventional methods of EXAFS data analysis are often limited to the nearest coordination shells of the absorbing atom due to the difficulties in accurate accounting for the so-called multiple-scattering effects. Besides, it is often difficult to resolve the non-equivalent groups of atoms in a single coordination shell due to strong correlation between structural parameters. In this study we overcome these problems by applying two different simulation-based methods, i.e., classical molecular dynamics (MD) and reverse Monte with evolutionary algorithm (EA), to the analysis of the Zn K-edge EXAFS data for wurtzite-type bulk ZnO. The RMC/EA-EXAFS method allowed us to separate the contributions…

High-pressure synthesis of boron-rich chalcogenides B12S and B12Se

The authors thank Drs. I. Dovgaliuk and T. Chauveau for assistance with Rietveld analysis; and Drs. V. Bushlya and A. Jamali for help with EDX/SEM measurements. This work was financially supported by the European Union's Horizon 2020 Research and Innovation Program under Flintstone2020 project (grant agreement No 689279).

The influence of Zn$^{2+}$ ions on the local structure and thermochromic properties of Cu$_{1-x}$Zn$_x$MoO$_4$ solid solutions

I. P., A. K. and A. K. would like to thank the support of the Latvian Council of Science project No. lzp-2019/1-0071. I.P. acknowledges the L‘OREAL Baltic “For Women In Science” Program with the support of the Latvian National Commission for UNESCO and the Latvian Academy of Sciences. The experiment at the PETRA III synchrotron was performed within project No. I-20190277 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. Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2…

Atomic structure relaxation in nanocrystalline NiO studied by EXAFS spectroscopy: Role of nickel vacancies

Nanocrystalline NiO samples have been studied using the Ni K-edge extended x-ray absorption fine structure (EXAFS) spectroscopy and recently developed modeling technique, combining classical molecular dynamics with ab initio multiple-scattering EXAFS calculations (MD-EXAFS). Conventional analysis of the EXAFS signals from the first two coordination shells of nickel revealed that (i) the second shell average distance R(Ni–Ni2) expands in nanocrystalline NiO compared to microcrystalline NiO, in agreement with overall unit cell volume expansion observed by x-ray diffraction; (ii) on the contrary, the first shell average distance R(Ni–O1) in nanocrystalline NiO shrinks compared to microcrystall…