arXiv subjects

Alexei Kuzmin

Publications and source records attributed to Alexei Kuzmin.

At least 19 recordsLinked to original sources

arXiv2026

Impact of interstitial carbon on local lattice distortions in CoCrFeMnNi high-entropy alloys

Here, we explore component-dependent local lattice distortions in polycrystalline, equiatomic, face-centered cubic CrMnFeCoNi high-entropy alloys and their modifications induced by dilute interstitial carbon. Multi-edge extended X-ray absorption fine structure spectroscopy combined with reverse Monte Carlo analysis reveals that the Cr component experiences the most substantial local distortions, independent of the temperature of prolonged annealing treatments (993 K or 1373 K) and the nominal carbon content (0 to 0.8 at.%). The static disorder around Cr atoms was found to increase markedly and monotonically upon carbon alloying, whereas Mn, Fe, Co, and Ni demonstrate weaker and non-monotonic tendencies. The carbon-induced lattice distortions extend over several coordination shells, indicating the pronounced effect of the carbon presence on the local environment around Cr absorbers. First-principles density functional theory and finite-temperature molecular dynamics simulations confirm the greater impact of carbon on the local lattice distortions around Cr than around the other $3d$ constituent elements, based on the previous finding that carbon preferentially occupies Cr-rich interstitial sites. These results provide decisive hints towards the atomistic origin of the non-monotonic diffusion behavior previously reported for carbon-doped CrMnFeCoNi alloys, and are noticeable for understanding the carbon-induced phase transitions in compositionally complex systems.

cond-mat.mtrl-sci↗

arXiv2026

Local lattice dynamics of hcp zinc from EXAFS and machine-learning interatomic potentials

The lattice dynamics of hexagonal close-packed (hcp) zinc, a prototypical anisotropic metal, is studied using temperature-dependent Zn K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy combined with atomistic simulations. The reverse Monte Carlo method enable the extraction of mean-square relative displacements (MSRDs) for eight coordination shells, providing a shell-resolved description of thermal motion. The MSRD temperature dependence, analysed using the correlated Einstein model, yields effective interatomic force constants and reveals pronounced anisotropy between in-plane and out-of-plane interactions. This anisotropy is further quantified by the ratio of MSRDs for the first and second coordination shells, which closely matches the anisotropic displacement parameters from diffraction experiments. Molecular dynamics simulations using the CHGNet universal machine-learning interatomic potential show that the original model overestimates thermal disorder, while a fine-tuned version substantially improves agreement with experimental EXAFS spectrum and radial distribution function. Overall, EXAFS-informed analysis is effective for validating and refining machine-learning interatomic potentials.

cond-mat.mtrl-sci↗

arXiv2025

Benchmarking CHGNet Universal Machine Learning Interatomic Potential Against DFT and EXAFS: Case of Layered WS2 and MoS2

Universal machine learning interatomic potentials (uMLIPs) deliver near ab initio accuracy in energy and force calculations at low computational cost, making them invaluable for materials modeling. Although uMLIPs are pre-trained on vast ab initio datasets, rigorous validation remains essential for their ongoing adoption. In this study, we use the CHGNet uMLIP to model thermal disorder in isostructural layered 2Hc-WS2 and 2Hc-MoS2, benchmarking it against ab initio data and extended X-ray absorption fine structure (EXAFS) spectra, which capture thermal variations in bond lengths and angles. Fine-tuning CHGNet with compound-specific ab initio (DFT) data mitigates the systematic softening (i.e., force underestimation) typical of uMLIPs and simultaneously improves alignment between molecular dynamics-derived and experimental EXAFS spectra. While fine-tuning with a single DFT structure is viable, using ~100 structures is recommended to accurately reproduce EXAFS spectra and achieve DFT-level accuracy. Benchmarking the CHGNet uMLIP against both DFT and experimental EXAFS data reinforces confidence in its performance and provides guidance for determining optimal fine-tuning dataset sizes.

cond-mat.mtrl-sci↗

arXiv2025

Identification of the high-pressure phases of alpha-SnWO4 combining x-ray diffraction and crystal structure prediction

We have characterized the high-pressure behavior of alpha-SnWO4. The compound has been studied up to 30 GPa using a diamond-anvil cell and synchrotron powder X-ray diffraction. We report evidence of two structural phase transitions in the pressure range covered in our study, and we propose a crystal structure for the two high-pressure phases. The first one, observed around 12.9 GPa, has been obtained combining indexation using DICVOL and density-functional theory calculations. The second high-pressure phase, observed around 17.5 GPa, has been determined by using the CALYPSO code, the prediction of which was supported by a Le Bail fit to the experimental X-ray diffraction patterns. The proposed structural sequence involves two successive collapses of the unit-cell volume and an increase in the coordination number of Sn and W atoms. The room-temperature equations of state, the principal axes of compression and their compressibility, the elastic constants, and the elastic moduli are reported for {\alpha}-SnWO4 and for the two high-pressure phases.

cond-mat.mtrl-sci↗

arXiv2025

Pressure-Induced Decomposition of beta-SnWO4

This study reports the decomposition of beta-SnWO4 into Sn, SnO2, and WO3 induced by static compression. We performed high-pressure synchrotron powder angle-dispersive X-ray diffraction measurements and found that decomposition occurs at a pressure of 13.97(5) GPa and is irreversible. This result contradicts a previous study that, based on density-functional theory calculations and crystal-chemistry arguments, predicted a pressure-driven transition from beta-SnWO4 to alpha-SnWO4. Our analysis indicates that the observed decomposition is unrelated to mechanical or dynamic instabilities. Instead, it likely stems from frustration of the beta-alpha transition, as this transformation requires a change in Sn coordination from octahedral to tetrahedral. The assessment of how pressure influences the volume of the unit cell provided an accurate determination of the room-temperature pressure-volume equation of state for beta-SnWO4. Furthermore, the elastic constants and moduli, as well as the pressure dependence of Raman and infrared modes of beta-SnWO4, were derived from density-functional theory calculations. Several phonon modes exhibited softening, and three cases of phonon anti-crossing were observed.

cond-mat.mtrl-sci↗

arXiv2025

Scalable solution chemical synthesis and comprehensive analysis of Bi2Te3 and Sb2Te3

Thermoelectric (TE) materials can directly convert heat into electrical energy. However, they sustain costly production procedures and batch-to-batch performance variations. Therefore, developing scalable synthetic techniques for large-scale and reproducible quality TE materials is critical for advancing TE technology. This study developed a facile, high throughput, solution-chemical synthetic technique. Microwave-assisted thermolysis process, providing energy-efficient volumetric heating, was used for the synthesis of bismuth and antimony telluride (Bi2Te3, Sb2Te3). As-made materials were characterized using various techniques, including XRPD, SEM, TEM, XAS, and XPS. Detailed investigation of the local atomic structure of the synthesized Bi2Te3 and Sb2Te3 powder samples was conducted through synchrotron radiation XAS experiments. The sintered TE materials exhibited low thermal conductivity, achieving the highest TE figure-of-merit values of 0.7 (573 K) and 0.9 (523 K) for n-type Bi2Te3 and p-type Sb2Te3, respectively, shifted significantly to the high-temperature region when compared to earlier reports, highlighting their potential for power generation applications. The scalable, energyand time-efficient synthetic method developed, along with the demonstration of its potential for TE materials, opens the door for a wider application of these materials with minimal environmental impact.

physics.app-ph↗

arXiv2024

6 -- X-ray absorption spectroscopy in high-entropy material research

This chapter introduces the use of X-ray absorption spectroscopy (XAS) in studying the local electronic and atomic structure of high-entropy materials. The element selectivity of XAS makes it particularly suitable to address the challenges posed by the study of multicomponent compounds. By analysing different parts of the X-ray absorption spectra for each element, one can obtain information on its chemical state from the X-ray absorption near-edge structure (XANES) and its local environment, distortions, and lattice dynamics from the extended X-ray absorption fine structure (EXAFS). The theoretical background underlying X-ray absorption spectra and existing data analysis procedures are briefly described, with particular emphasis on advanced atomistic modelling techniques that enable more reliable extraction of structural information. Finally, an overview of the applications of the XAS technique in studying high-entropy materials is presented.

cond-mat.mtrl-sci↗

arXiv2024

The use of the correlated Debye model for EXAFS-based thermometry in bcc and fcc metals

Extended X-ray absorption fine structure (EXAFS) spectra are sensitive to thermal disorder and are often used to probe local lattice dynamics. Variations in interatomic distances induced by atomic vibrations are described by the temperature-dependent mean-square relative displacement (MSRD), also known as the Debye-Waller factor. In this study, we evaluated the feasibility of addressing the inverse problem, i.e., determining the sample temperature from the analysis of its EXAFS spectrum using the multiple-scattering formalism, considering contributions up to the 4th-7th coordination shell. The method was tested on several monatomic metals (bcc Cr, Mo, and W; fcc Cu and Ag), where the correlated Debye model of lattice dynamics provides a fairly accurate description of thermal disorder effects up to distant coordination shells. We found that the accuracy of the method strongly depends on the temperature range. The method fails at low temperatures, where quantum effects dominate and MSRD values change only slightly. However, it becomes more accurate at higher temperatures, where the MSRD shows a near-linear dependence on temperature.

cond-mat.mtrl-sci↗

arXiv2024

Temperature-dependent local structure and lattice dynamics of 1T-TiSe$_2$ and 1T-VSe$_2$ probed by X-ray absorption spectroscopy

The local atomic structure and lattice dynamics of two isostructural layered transition metal dichalcogenides (TMDs), 1T-TiSe$_2$ and 1T-VSe$_2$, were studied using temperature-dependent X-ray absorption spectroscopy at the Ti, V, and Se K-edges. Analysis of the extended X-ray absorption fine structure (EXAFS) spectra, employing reverse Monte Carlo (RMC) simulations, enabled tracking the temperature evolution of the local environment in the range of 10-300 K. The atomic coordinates derived from the final atomic configurations were used to calculate the partial radial distribution functions (RDFs) and the mean-square relative displacement (MSRD) factors for the first ten coordination shells around the absorbing atoms. Characteristic Einstein frequencies and effective force constants were determined for Ti-Se, Ti-Ti, V-Se, V-V, and Se-Se atom pairs from the temperature dependencies of MSRDs. The obtained results reveal differences in the temperature evolution of lattice dynamics and the strengths of intralayer and interlayer interactions in TiSe$_2$ and VSe$_2$.

cond-mat.mtrl-sci↗

arXiv2023

Unraveling the interlayer and intralayer coupling in two-dimensional layered MoS$_2$ by X-ray absorption spectroscopy and ab initio molecular dynamics simulations

Understanding interlayer and intralayer coupling in two-dimensional layered materials (2DLMs) has fundamental and technological importance for their large-scale production, engineering heterostructures, and development of flexible and transparent electronics. At the same time, the quantification of weak interlayer interactions in 2DMLs is a challenging task, especially, from the experimental point of view. Herein, we demonstrate that the use of X-ray absorption spectroscopy in combination with reverse Monte Carlo (RMC) and ab initio molecular dynamics (AIMD) simulations can provide useful information on both interlayer and intralayer coupling in 2DLM 2H$_c$-MoS$_2$. The analysis of the low-temperature (10-300 K) Mo K-edge extended X-ray absorption fine structure (EXAFS) using RMC simulations allows for obtaining information on the means-squared relative displacements $σ^2$ for nearest and distant Mo-S and Mo-Mo atom pairs. This information allowed us further to determine the strength of the interlayer and intralayer interactions in terms of the characteristic Einstein frequencies $ω_E$ and the effective force constants $κ$ for the nearest ten coordination shells around molybdenum. The studied temperature range was extended up to 1200 K employing AIMD simulations which were validated at 300 K using the EXAFS data. Both RMC and AIMD results provide evidence of the reduction of correlation in thermal motion between distant atoms and suggest strong anisotropy of atom thermal vibrations within the plane of the layers and in the orthogonal direction.

cond-mat.mtrl-sci↗

arXiv2023

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

Hybrid materials combining an organic matrix and high-Z nanomaterials show potential for applications in radiation detection, allowing unprecedented device architectures and functionality. Herein, novel hybrid organic-inorganic systems were produced using a mixture of tungstate (CaWO$_4$ or ZnWO$_4$) nanoparticles with a P3HT:PCBM blend. The nano-tungstates with a crystallite size of 43 nm for CaWO$_4$ and 30 nm for ZnWO$_4$ were synthesized by the hydrothermal method. Their structure and morphology were characterized by X-ray diffraction and scanning electron microscopy. The hybrid systems were used to fabricate direct conversion X-ray detectors able to operate with zero bias voltage. The detector performance was tested in a wide energy range using monochromatic synchrotron radiation. The addition of nanoparticles with high-Z elements improved the detector response to X-ray radiation compared with that of a pure organic P3HT:PCBM bulk heterojunction cell. The high dynamic range of our detector allows for recording X-ray absorption spectra, including the fine X-ray absorption structure located beyond the absorption edge. The obtained results suggest that nanocrystalline tungstates are promising candidates for application in direct organic-inorganic X-ray detectors.

cond-mat.mtrl-sci↗

arXiv2022

Peculiarities of the local structure in new medium- and high-entropy, low-symmetry tungstates

New monoclinic ($P2$/$c$) tungstates - a medium-entropy tungstate, (Mn,Ni,Cu,Zn)WO$_4$, and a high-entropy tungstate, (Mn,Co,Ni,Cu,Zn)WO$_4$ - were synthesized and characterized. Their phase purity and solid solution nature were confirmed by powder X-ray diffraction and Raman spectroscopy. X-ray absorption spectroscopy was used to probe the local structure around metal cations. The atomic structures based on the ideal solid solution model were optimized by a simultaneous analysis of the extended X-ray absorption fine structure spectra at multiple metal absorption edges - five for (Mn,Ni,Cu,Zn)WO$_4$ and six for (Mn,Co,Ni,Cu,Zn)WO$_4$ - by means of reverse Monte Carlo simulations. In both compounds, Ni$^{2+}$ ions have the strongest tendency to organize their local environment and form slightly distorted [NiO$_6$] octahedra, whereas Mn$^{2+}$, Co$^{2+}$, and Zn$^{2+}$ ions have a strongly distorted octahedral coordination. The most intriguing result is that the shape of [CuO$_6$] octahedra in (Mn,Ni,Cu,Zn)WO$_4$ and (Mn,Co,Ni,Cu,Zn)WO$_4$ differs from that found in pure CuWO$_4$, where a strong Jahn-Teller distortion is present: [CuO$_6$] octahedra become more regular with increasing degree of dilution.

cond-mat.mtrl-sci↗

arXiv2022

Microwave-assisted synthesis and characterization of undoped and manganese doped zinc sulfide nanoparticles

Undoped and Mn-doped ZnS nanocrystals were produced by the microwave-assisted solvothermal method and characterized by X-ray diffraction, photoluminescence spectroscopy and scanning electron microscopy with energy-dispersive X-ray spectroscopy. All samples have the cubic zinc blende structure with the lattice parameter in the range of $a$ = 5.406-5.411 Å, and the average size of crystallites is in the range of 6-9 nm. These nanoparticles agglomerate and form large grains with an average size of up to 180 nm. The photoluminescence of the undoped ZnS sample shows a broad emission band located at 530 nm, attributed to the defects at the surface of nanoparticles. In all Mn-doped samples, the emission peak at 598 nm was observed assigned to the characteristic forbidden transition between excited ($^4$T$_1$) and ground ($^6$A$_1$) levels of Mn$^{2+}$. Synchrotron radiation X-ray absorption spectroscopy at the Zn and Mn K-edges combined with reverse Monte Carlo (RMC) simulations based on the evolutionary algorithm confirms that manganese ions substitute zinc ions. However, the difference in the ion sizes ($R$(Mn$^{2+}$(IV)) = 0.66 Å and $R$(Zn$^{2+}$(IV)) = 0.60 Å) is responsible for the larger interatomic distances Mn-S (2.40(2) Å) compared to Zn-S (2.33(2) Å). The static structural relaxations in ZnS:Mn nanoparticles are responsible for the large values of the mean-square displacements factors for Zn, S and Mn atoms obtained by RMC simulations.

cond-mat.mtrl-sci↗

arXiv2022

Inner relaxations in equiatomic single-phase high-entropy cantor alloy

The superior properties of high-entropy multi-functional materials are strongly connected with their atomic heterogeneity through many different local atomic interactions. The detailed element-specific studies on a local scale can provide insight into the primary arrangements of atoms in multicomponent systems and benefit to unravel the role of individual components in certain macroscopic properties of complex compounds. Herein, multi-edge X-ray absorption spectroscopy combined with reverse Monte Carlo simulations was used to explore a homogeneity of the local crystallographic ordering and specific structure relaxations of each constituent in the equiatomic single-phase face-centered cubic CrMnFeCoNi high-entropy alloy at room temperature. Within the considered fitting approach, all five elements of the alloy were found to be distributed at the nodes of the fcc lattice without any signatures of the additional phases at the atomic scale and exhibit very close statistically averaged interatomic distances (2.54-2.55 Å) with their nearest-neighbors. Enlarged structural displacements were found solely for Cr atoms. The macroscopic magnetic properties probed by conventional magnetometry demonstrate no opening of the hysteresis loops at 5 K and illustrate a complex character of the long-range magnetic order after field-assisted cooling in $\pm$5 T. The observed magnetic behavior is assigned to effects related to structural relaxations of Cr. Besides, the advantages and limitations of the reverse Monte Carlo approach to studies of multicomponent systems like high-entropy alloys are highlighted.

cond-mat.mtrl-sci↗

arXiv2022

The role of Al2O3 interlayer in the synthesis of ZnS/Al2O3/MoS2 core-shell nanowires

During the synthesis of heterostructured nanomaterials, unwanted structural and morphological changes in nanostructures may occur, especially when multiple sequential growth steps are involved. In this study, we describe a synthesis strategy of heterostructured ZnS/Al2O3/MoS2 core-shell nanowires (NWs), and explore the role of the Al2O3 interlayer during synthesis. Core-shell NWs were produced via a four-step route: 1) synthesis of ZnO NWs on a silicon wafer, 2) deposition of thin Al2O3 layer by ALD, 3) magnetron deposition of MoO3 layer, and 4) annealing of the sample in the sulphur atmosphere. During sulphurization, ZnO is converted into ZnS, and MoO3 into MoS2, while the Al2O3 interlayer preserves the smooth surface of an NW required for the growth of a continuous MoS2 shell. The resulting ZnS/Al2O3/MoS2 core-shell NWs were characterized by transmission electron microscopy, X-ray diffraction and photoelectron spectroscopy, Raman spectroscopy, and optical photoluminescence spectroscopy. A reported strategy can be used for the synthesis of other core-shell NWs with a transition metal dichalcogenides (TMDs) shell to protect the NW core material that may otherwise be altered or damaged by the reactive chalcogenides at high temperatures.

cond-mat.mtrl-sci↗

arXiv2022

Revealing the local structure of CuMo$_{1-x}$W$_x$O$_4$ solid solutions by multi-edge X-ray absorption spectroscopy

The effect of tungsten substitution with molybdenum on the structure of CuMo$_{1-x}$W$_x$O$_4$ ($x$ = 0.20, 0.30, 0.50, 0.75) solid solutions was studied by multi-edge X-ray absorption spectroscopy. The simultaneous analysis of EXAFS spectra measured at several (Cu K-edge, Mo K-edge and W L$_3$-edge) absorption edges was performed by the reverse Monte Carlo method taking into account multiple-scattering effects. The degree of distortion of the coordination shells and its dependence on the composition were estimated from partial radial distribution functions (RDFs) $g(r)$ and bond angle distribution functions (BADFs) $f(φ)$. The analysis of partial RDFs suggests that the structure of solid solutions is mainly determined by the tungsten-related sublattice, while molybdenum atoms adapt to a locally distorted environment. As a result, the coordination of both tungsten and molybdenum atoms remains octahedral as in CuWO$_4$ for all the studied compositions.

cond-mat.mtrl-sci↗

arXiv2022

In situ study of zinc peroxide decomposition to zinc oxide by X-ray absorption spectroscopy and reverse Monte-Carlo simulations

The Zn K-edge X-ray absorption spectroscopy has been used to investigate in situ the decomposition of zinc peroxide (ZnO$_2$) to zinc oxide (ZnO). Principal component and linear combination analyses of the EXAFS spectra have been employed to identify the phase composition of the oxide upon heating to 900$^\circ$C. Only the ZnO$_2$ phase has been found up to 180$^\circ$C, whereas only the nanocrystalline ZnO phase has occurred above 250$^\circ$C. Detailed structural information on the temperature dependence of the local environment of zinc atoms has been obtained using the reverse Monte Carlo simulations. A strong increase of disorder has been found upon approaching the decomposition temperature, evidenced by the broadening of Zn-O and Zn-Zn pair distribution functions and related mean-square relative displacements.

cond-mat.mtrl-sci↗

arXiv2021

EDA: EXAFS Data Analysis software package

The EXAFS data analysis software package EDA consists of a suite of programs running under Windows operating system environment and designed to perform all steps of conventional EXAFS data analysis such as the extraction of the XANES/EXAFS parts of the x-ray absorption coefficient, the Fourier filtering, the EXAFS fitting using the Gaussian and cumulant models. Besides, the package includes two advanced approaches, which allow one to reconstruct the radial distribution function (RDF) from EXAFS based on the regularization-like method and to calculate configurationalaveraged EXAFS using a set of atomic configurations obtained from molecular dynamics or Monte Carlo simulations.

cond-mat.mtrl-sci↗