Showing 1–11 of 11 results for author: Pudza, I

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

Authors: Vitalijs Dimitrijevs, Pjotrs Žguns, Inga Pudza, Aleksandr Kalinko, Alexei Kuzmin

Abstract: 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 de… ▽ More 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. △ Less

Submitted 7 August, 2026; originally announced August 2026.

Journal ref: Physica B 739 (2026) 418945

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

Authors: Pjotrs Žguns, Inga Pudza, Alexei Kuzmin

Abstract: 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 isostruc… ▽ More 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. △ Less

Submitted 10 September, 2025; originally announced September 2025.

Journal ref: J. Chem. Theory Comput. 2025, 21, 16, 8142-8150

Scalable solution chemical synthesis and comprehensive analysis of Bi2Te3 and Sb2Te3

Authors: Bejan Hamawandi, Parva Parsa, Inga Pudza, Kaspars Pudzs, Alexei Kuzmin, Sedat Ballikaya, Edmund Welter, Rafal Szukiewicz, Maciej Kuchowicz, Muhammet S. Toprak

Abstract: 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 tec… ▽ More 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. △ Less

Submitted 12 March, 2025; originally announced March 2025.

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

Authors: Alexei Kuzmin, Vitalijs Dimitrijevs, Inga Pudza, Aleksandr Kalinko

Abstract: 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 invers… ▽ More 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. △ Less

Submitted 25 October, 2024; originally announced October 2024.

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

Authors: Inga Pudza, Boris Polyakov, Kaspars Pudzs, Edmund Welter, Alexei Kuzmin

Abstract: 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 temperatu… ▽ More 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$_2$. △ Less

Submitted 29 January, 2024; originally announced January 2024.

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

Authors: Inga Pudza, Dmitry Bocharov, Andris Anspoks, Matthias Krack, Aleksandr Kalinko, Edmund Welter, Alexei Kuzmin

Abstract: 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… ▽ More 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. △ Less

Submitted 2 June, 2023; originally announced June 2023.

Journal ref: Mater. Today Commun. 35 (2023) 106359

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

Authors: Inga Pudza, Kaspars Pudzs, Andrejs Tokmakovs, Normunds Ralfs Strautnieks, Aleksandr Kalinko, Alexei Kuzmin

Abstract: 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 CaW… ▽ More 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. △ Less

Submitted 10 February, 2023; originally announced February 2023.

Journal ref: Materials 16 (2023) 667

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

Authors: Inga Pudza, Andris Anspoks, Giuliana Aquilanti, Alexei Kuzmin

Abstract: 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-scat… ▽ More 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) $f(\varphi)$. 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. △ Less

Submitted 27 May, 2022; originally announced May 2022.

Journal ref: Mater. Res. Bull. 153 (2022) 111910

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

Authors: Alexei Kuzmin, Inga Pudza, Konstantin Klementiev

Abstract: 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 nanocryst… ▽ More 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. △ Less

Submitted 28 March, 2022; v1 submitted 14 March, 2022; originally announced March 2022.

Journal ref: Physica Status Solidi (b) 259 (2022) 2200001

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

Authors: Inga Pudza, Andris Anspoks, Arturs Cintins, Aleksandr Kalinko, Edmund Welter, Alexei Kuzmin

Abstract: The influence of zinc ions on the thermochromic properties of polycrystalline Cu$_{1-x}$Zn$_x$MoO$_4$ ($x$=0.10, 0.50, 0.90) solid solutions was studied by X-ray absorption spectroscopy at the Cu, Zn and Mo K-edges. Detailed structural information on the local environment of metal ions was obtained from the simultaneous analysis of EXAFS spectra measured at three metal absorption edges using the r… ▽ More The influence of zinc ions on the thermochromic properties of polycrystalline Cu$_{1-x}$Zn$_x$MoO$_4$ ($x$=0.10, 0.50, 0.90) solid solutions was studied by X-ray absorption spectroscopy at the Cu, Zn and Mo K-edges. Detailed structural information on the local environment of metal ions was obtained from the simultaneous analysis of EXAFS spectra measured at three metal absorption edges using the reverse Monte Carlo method. Thermochromic phase transition with the hysteretic behaviour between $α$ and $γ$ phases was observed in Cu$_{0.90}$Zn$_{0.10}$MoO$_4$ solid solution. It was found that the local environment of molybdenum ions is most susceptible to the substitution of copper for zinc and, upon cooling, transforms from tetrahedral MoO$_4$ to distorted octahedral MoO$_6$. △ Less

Submitted 5 July, 2021; originally announced July 2021.

Journal ref: Materials Today Communications 28 (2021) 102607

Study of the thermochromic phase transition in CuMo$_{1-x}$W$_x$O$_4$ solid solutions at the W L$_3$-edge by resonant X-ray emission spectroscopy

Authors: I. Pudza, A. Kalinko, A. Cintins, A. Kuzmin

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 splitti… ▽ More 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 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 CuMo$_{1-x}$W$_x$O$_4$ solid solutions on cooling and heating, even at low ($x$<0.10) tungsten content. It was found that tungsten ions have octahedral coordination for 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. △ Less

Submitted 30 December, 2020; originally announced December 2020.

Journal ref: Acta Materialia 205 (2021) 116581