arXiv subjects

Alexander G. Kvashnin

Publications and source records attributed to Alexander G. Kvashnin.

At least 19 recordsLinked to original sources

arXiv2026

Insights of Ammonia Decomposition on W--B Nanoclusters by Computational Simulations

Tungsten-boride nanoclusters represent a promising class of materials for catalytic applications, yet their structural stability and reactivity remain poorly understood. The evolutionary algorithm combined with density functional theory (DFT) are used to systematically explore the ground-state structures and stability landscape of W$_m$B$_n$ nanoclusters with up to 43 atoms. The resulting stability maps reveal a highly non-monotonic landscape characterized by isolated "magic" compositions, including WB$_{16}$, W$_2$B$_8$, W$_7$B$_{24}$, and W$_{11}$B$_{22}$, which exhibit pronounced local stability maxima. We further investigate the adsorption and initial decomposition step of ammonia on these clusters as a probe of their catalytic potential. Molecular NH$_3$ adsorption occurs exclusively on tungsten sites with energies ranging from -0.54 to -1.78 eV (average -1.43 eV), comparable to Pt$_n$ and Fe$_n$ clusters. Atomic hydrogen adsorption spans a broader range from +0.49 to -1.46 eV, reflecting high site sensitivity. Nudged elastic band calculations for the first N--H bond cleavage reveal forward barriers of 1.1-1.4 eV, with the dissociated NH$_2^*$ + H$^*$ state lying below the molecular adsorption state for most compositions. Notably, the activation barrier depends critically on the local environment available for stabilizing the detached hydrogen atom. These findings establish W--B nanoclusters as tunable catalysts for ammonia decomposition and provide a structural foundation for their rational design.

cond-mat.mtrl-sci↗

arXiv2025

Stability and Superconductivity of Ternary Polyhydrides

We review five years of experimental and theoretical attempts (2020-2025) to enhance the superconducting critical temperature ($\textit{T$_c$}$) of hydrogen-rich compounds by alloying binary superhydrides with additional elements. Despite predictions of higher $\textit{T$_c$}$ in ternary systems such as La-Y-H, La-Ce-H, and Ca-Mg-H, experiments consistently show that the maximum $\textit{T$_c$}$ in disordered ternary superhydrides does not exceed that of the best binary parent hydrides within experimental uncertainty. Instead, alloying primarily stabilizes high-symmetry polyhydride phases at lower pressures, enabling $\textit{T$_c$}$ = 200 K near 110-120 GPa, while also improving vortex pinning and upper critical fields. Magnetic dopants suppress $\textit{T$_c$}$, whereas nonmagnetic additives leave it nearly unchanged, reminiscent of Anderson's theorem. These findings indicate that alloying is unlikely to raise $\textit{T$_c$}$, but can reduce the pressures required to stabilize high-$\textit{T$_c$}$ phases. We propose that fully ordered ternary hydrides, synthesized via controlled hydrogenation of intermetallic precursors, offer a promising route toward this goal. One of the most promising compounds of this kind is the recently discovered LaSc$_2$H$_{24}$.

cond-mat.supr-con↗

arXiv2024

Physically Intuitive Anisotropic Model of Hardness

The hardness of materials plays an important role in material design. There are numerous experimental methods to measure the hardness of materials, but theoretical prediction of hardness is challenging. By investigating the correlation between hardness and the elastic properties of materials, namely shear and bulk moduli, the pressure derivative of bulk modulus, we have constructed a simple and physically intuitive hardness model. By introducing the spatial variation of the shear modulus, it is possible to predict the hardness anisotropy of materials to define the minimum and maximum values of hardness possessed by a particular material. Furthermore, by using the equation of states to define the pressure derivative of the bulk modulus, it is possible to determine the temperature dependencies of hardness for given materials. All quantities in the model can be obtained directly from accurate first-principles calculations or from experiments, making it suitable for practical applications.

cond-mat.mtrl-sci↗

arXiv2024

Material Hardness Descriptor Derived by Symbolic Regression

Hardness is a materials' property with implications in several industrial fields, including oil and gas, manufacturing, and others. However, the relationship between this macroscale property and atomic (i.e., microscale) properties is unknown and in the last decade several models have unsuccessfully tried to correlate them in a wide range of chemical space. The understanding of such relationship is of fundamental importance for discovery of harder materials with specific characteristics to be employed in a wide range of fields. In this work, we have found a physical descriptor for Vickers hardness using a symbolic-regression artificial-intelligence approach based on compressed sensing. SISSO (Sure Independence Screening plus Sparsifying Operator) is an artificial-intelligence algorithm used for discovering simple and interpretable predictive models. It performs feature selection from up to billions of candidates obtained from several primary features by applying a set of mathematical operators. The resulting sparse SISSO model accurately describes the target property (i.e., Vickers hardness) with minimal complexity. We have considered the experimental values of hardness for binary, ternary, and quaternary transition-metal borides, carbides, nitrides, carbonitrides, carboborides, and boronitrides of 61 materials, on which the fitting was performed. The found descriptor is a non-linear function of the microscopic properties, with the most significant contribution being from a combination of Voigt-averaged bulk modulus, Poisson's ratio, and Reuss-averaged shear modulus. Results of high-throughput screening of 635 candidate materials using the found descriptor suggest the enhancement of material's hardness through mixing with harder yet metastable structures (e.g., metastable VN, TaN, ReN$_2$, Cr$_3$N$_4$, and ZrB$_6$ all exhibit high hardness).

cond-mat.mtrl-sci↗

arXiv2023

Generating and grading 34 Optimized Norm-Conserving Vanderbilt Pseudopotentials for Actinides and Super Heavy Elements in the PseudoDojo

In the last decades, material discovery has been a very active research field driven by the need to find new materials for many different applications. This has also included materials with heavy elements, beyond the stable isotopes of lead, as most actinides exhibit unique properties that make them useful in various applications. Furthermore, new heavy elements beyond actinides, collectively referred to as super-heavy elements (SHEs), have been synthesized, filling previously empty space of Mendeleev periodic table. Their chemical bonding behavior, of academic interest at present, would also benefit of state-of-the-art modeling approaches. In particular, in order to perform first-principles calculations with planewave basis sets, one needs corresponding pseudopotentials. In this work, we present a series of scalar- and fully-relativistic optimized norm-conserving Vanderbilt pseudopotentials (ONCVPs) for thirty-four actinides and super-heavy elements, for three different exchange-correlation functionals (PBE, PBEsol and LDA). The scalar-relativistic version of these ONCVPs is tested by comparing equations of states for crystals, obtained with \textsc{abinit} 9.6, with those obtained by all-electron zeroth-order regular approximation (ZORA) calculations, without spin-orbit coupling, performed with the Amsterdam Modeling Suite \textsc{band} code. $Δ$-Gauge and $Δ_1$-Gauge indicators are used to validate these pseudopotentials. This work is a contribution to the PseudoDojo project, in which pseudopotentials for the whole periodic table are developed and systematically tested. The pseudopotential files are available on the PseudoDojo web-interface pseudo-dojo.org in psp8 and UPF2 formats, both suitable for \textsc{abinit}, the latter being also suitable for Quantum ESPRESSO.

cond-mat.mtrl-sci↗

arXiv2023

Mechanical properties of single and polycrystalline solids from machine learning

Calculations of elastic and mechanical characteristics of non-crystalline solids are challenging due to high computation cost of $ab$ $initio$ methods and low accuracy of empirical potentials. We propose a computational technique towards efficient calculations of mechanical properties of polycrystals, composites, and multi-phase systems from atomistic simulation with high accuracy and reasonable computational cost. It is based on using actively learned machine learning interatomic potentials (MLIPs) trained on a local fragments of the polycrystalline system for which forces, stresses and energies are computed by using $ab$ $initio$ calculations. Developed approach is used for calculation the dependence of elastic moduli of polycrystalline diamond on the grain size. This technique allows one to perform large-scale calculations of mechanical properties of complex solids of various compositions and structures with high accuracy making the transition from ideal (single crystal) systems to more realistic ones.

cond-mat.mtrl-sci↗

arXiv2023

Non-Fermi-Liquid Behavior of Superconducting SnH$_4$

We studied chemical interaction of Sn with H$_2$ by X-ray diffraction methods at pressures of 180-210 GPa. A previously unknown tetrahydride SnH$_4$ with a cubic structure (${fcc}$) exhibiting superconducting properties below ${T}$$_C$ = 72 K was obtained; the formation of a high molecular ${C2/m}$-SnH$_{14}$ superhydride and several lower hydrides, ${fcc}$ SnH$_2$ and ${C2}$-Sn$_{12}$H$_{18}$, was also detected. The temperature dependence of critical current density ${J}$$_C$(T) in SnH$_4$ yields the superconducting gap 2$Δ$(0) = 23 meV at 180 GPa. SnH$_4$ has unusual behavior in strong magnetic fields: ${B,T}$-linear dependences of magnetoresistance and the upper critical magnetic field ${B}$$_{C2}$(T) $\propto$ (${T}$$_C$ - ${T}$). The latter contradicts the Wertheimer-Helfand-Hohenberg model developed for conventional superconductors. Along with this, the temperature dependence of electrical resistance of ${fcc}$ SnH$_4$ in non-superconducting state exhibits a deviation from what is expected for phonon-mediated scattering described by the Bloch-Grüneisen model, and is beyond the framework of the Fermi liquid theory. Such anomalies occur for many superhydrides, making them much closer to cuprates than previously believed.

cond-mat.supr-con↗

arXiv2022

Machine-learning Driven Synthesis of TiZrNbHfTaC5 High-Entropy Carbide

Synthesis of high-entropy carbides (HEC) requires high temperatures that can be provided by electric arc plasma method. However, the formation temperature of a single-phase sample remains unknown. Moreover, under some temperatures multi-phase structures can emerge. In this work we developed an approach for a controllable synthesis of HEC TiZrNbHfTaC5 based on theoretical and experimental techniques. We used canonical Monte Carlo (CMC) simulations with the machine learning interatomic potentials to determine the temperature conditions for the formation of single-phase and multi-phase samples. In full agreement with the theory, the single-phase sample, produced with electric arc discharge, was observed at 2000 K. Below 1200 K the sample decomposed into (Ti-Nb-Ta)C and a mixture of (Zr-Hf-Ta)C, (Zr-Nb-Hf)C, (Zr-Nb)C, and (Zr-Ta)C. Our results demonstrate the conditions for the formation of HEC and we anticipate that our approach can pave the way towards targeted synthesis of multicomponent materials.

cond-mat.mtrl-sci↗

arXiv2022

Effect of paramagnetic impurities on superconductivity in polyhydrides: $\textit{s}$-wave order parameter in Nd-doped LaH$_{10}$

Polyhydrides are a novel class of superconducting materials with extremely high critical parameters, which is very promising for applications. On the other hand, complete experimental study of the magnetic phase diagram for the best so far known superconductor, lanthanum decahydride LaH$_{10}$, encounters a serious complication because of the large upper critical magnetic field $\textit{H}$$_{C2}$(0), exceeding 120-160 T. Partial replacement of La atoms by magnetic Nd atoms results in a decrease of the upper critical field, which makes it attainable for existing pulse magnets. We found that addition of neodymium leads to significant suppression of superconductivity in LaH$_{10}$: each atomic % of Nd causes decrease in $\textit{T}$$_{C}$ by 10-11 K. Using strong pulsed magnetic fields up to 68 T, we constructed the magnetic phase diagram of the ternary (La,Nd)H$_{10}$ superhydride, which appears to be surprisingly linear with $\textit{H}$$_{C2}$ $\propto$ |$\textit{T}$ - $\textit{T}$$_C$|. The pronounced suppression of superconductivity in LaH$_{10}$ by magnetic Nd atoms and the robustness of $\textit{T}$$_C$ with respect to nonmagnetic impurities (e.g., Y, Al, C) under Anderson's theorem indicate the isotropic ($\textit{s}$-wave) character of conventional electron-phonon pairing in the synthesized superhydrides.

cond-mat.supr-con↗

arXiv2021

Sr-Doped Molecular Hydrogen: Synthesis and Properties of SrH$_{22}$

Recently, several research groups announced reaching the point of metallization of hydrogen above 400 GPa. Following the mainstream of extensive investigations of compressed polyhydrides, in this work we demonstrate that small (4 atom %) doping of molecular hydrogen by strontium leads to a dramatic reduction in the metallization pressure to about 200 GPa. Studying the high-pressure chemistry of the Sr-H system at 56-180 GPa, we observed the formation of several previously unknown compounds: C2/m-Sr$_3$H$_{13}$, pseudocubic SrH$_6$, SrH$_9$ with cubic F-43m Sr sublattice, and pseudotetragonal P1-SrH$_{22}$, the metal hydride with the highest hydrogen content discovered so far. Unlike Ca and Y, strontium forms molecular semiconducting polyhydrides, whereas calcium and yttrium polyhydrides are high-Tc superconductors with an atomic H sublattice. The latter phase, SrH$_{22}$ or Sr$_{0.04}$H$_{0.96}$, may be considered as a convenient model of the consistent bandgap closure and metallization of hydrogen. Using the impedance measurements in diamond anvil cells at 300-440 K, we estimated the direct bandgap of the Pm-3n-like compound P1-SrH$_6$ to be 0.44-0.51 eV at 150 GPa, and its metallization pressure to be 220 GPa. Together with the machine learning interatomic potentials, the impedance spectroscopy allowed us to estimate the diffusion coefficients of hydrogen D$_H$ = 1.0-2.8 E-10 m$^2$/s in SrH$_6$ and 1.2-2.1 E-9 m$^2$/s in P1-SrH$_{22}$ at 500-600 K.

cond-mat.mtrl-sci↗

arXiv2020

Novel Strongly Correlated Europium Superhydrides

We conducted a joint experimental-theoretical investigation of the high-pressure chemistry of europium polyhydrides at pressures of 86-130 GPa. We discovered several novel magnetic Eu superhydrides stabilized by anharmonic effects: cubic $EuH_{9}$, hexagonal $EuH_{9}$, and an unexpected cubic (Pm-3n) clathrate phase, $Eu_{8}H_{46}$. Monte Carlo simulations indicate that cubic $EuH_{9}$ has antiferromagnetic ordering with T(Neel) up to 24 K, whereas hexagonal $EuH_{9}$ and Pm-3n-$Eu_{8}H_{46}$ possess ferromagnetic ordering with T(Curie) = 137 and 336 K, respectively. The electron-phonon interaction is weak in all studied europium hydrides, and their magnetic ordering excludes s-wave superconductivity, except, perhaps, for distorted pseudohexagonal $EuH_{9}$. The equations of state predicted within the DFT+U approach (the Hubbard corrections were found within linear response theory) are in close agreement with the experimental data. This work shows the great influence of the atomic radius on symmetry-breaking distortions of the crystal structures of superhydrides and on their thermodynamic stability.

cond-mat.str-el↗

arXiv2020

Superconductivity at 253 K in lanthanum-yttrium ternary hydrides

Polyhydrides offer intriguing perspectives as high-temperature superconductors. Here we report the high-pressure synthesis of a series of lanthanum-yttrium ternary hydrides: cubic hexahydride $(La,Y)H_{6}$ with a critical temperature $T_{C}$ = 237 +/- 5 K and decahydrides $(La,Y)H_{10}$ with a maximum $T_{C}$ ~${253 K}$ and an extrapolated upper critical magnetic field $B_{C2(0)}$ up to ${135 T}$ at 183 GPa. This is one of the first examples of ternary high-$T_{C}$ superconducting hydrides. Our experiments show that a part of the atoms in the structures of recently discovered ${Im3m}$-$YH_{6}$ and ${Fm3m}$-$LaH_{10}$ can be replaced with lanthanum (~70 %) and yttrium (~25 %), respectively, with a formation of unique ternary superhydrides containing incorporated $La@H_{24}$ and $Y@H_{32}$ which are specific for ${Im3m}$-$LaH_{6}$ and ${Fm3m}$-$YH_{10}$. Ternary La-Y hydrides were obtained at pressures of 170-196 GPa via the laser heating of $P6_{3}$${/mmc}$ lanthanum-yttrium alloys in the ammonia borane medium at temperatures above 2000 K. A novel tetragonal $(La,Y)H_{4}$ was discovered as an impurity phase in synthesized cubic $(La,Y)H_{6}$. The current-voltage measurements show that the critical current density $J_{C}$ in $(La,Y)H_{10}$ may exceed $2500 A/mm^{2}$ at 4.2 K, which is comparable with that for commercial superconducting wires such as ${NbTi}$, $Nb_{3}$${Sn}$. Hydrides that are unstable in a pure form may nevertheless be stabilized at relatively low pressures in solid solutions with superhydrides having the same structure.

cond-mat.supr-con↗

arXiv2020

Anomalous high-temperature superconductivity in YH$_6$

Pressure-stabilized hydrides are a new rapidly growing class of high-temperature superconductors which is believed to be described within the conventional phonon-mediated mechanism of coupling. Here we report the synthesis of yttrium hexahydride Im3m-YH$_6$ that demonstrates the superconducting transition with T$_c$ = 224 K at 166 GPa, much lower than the theoretically predicted (>270 K). The measured upper critical magnetic field B$_c$$_2$(0) of YH$_6$ was found to be 116-158 T, which is 2-2.5 times larger than the calculated value. A pronounced shift of T$_c$ in yttrium deuteride YD$_6$ with the isotope coefficient 0.4 supports the phonon-assisted superconductivity. Current-voltage measurements showed that the critical current I$_c$ and its density J$_c$ may exceed 1.75 A and 3500 A/mm$^2$ at 0 K, respectively, which is comparable with the parameters of commercial superconductors, such as NbTi and YBCO. The superconducting density functional theory (SCDFT) and anharmonic calculations suggest unusually large impact of the Coulomb repulsion in this compound. The results indicate notable departures of the superconducting properties of the discovered YH$_6$ from the conventional Migdal-Eliashberg and Bardeen-Cooper-Schrieffer theories.

cond-mat.supr-con↗

arXiv2020

High-Pressure Synthesis of Barium Superhydrides: Pseudocubic BaH12

Following the discovery of high-temperature superconductivity in the La-H system, where for the recently discovered fcc-LaH10 a record critical temperature Tc = 250 K was achieved [Drozdov et al., Nature, 569, 528 (2019) and Somayazulu et al., Phys. Rev. Lett. 122, 027001 (2019)], we studied the formation of new chemical compounds in the barium-hydrogen system at pressures up to 173 GPa. Using in situ generation of hydrogen from NH3BH3, we synthesized previously unknown superhydride BaH12 with a pseudocubic (fcc) Ba sublattice, which was observed in a wide range of pressures from 75 to 173 GPa in four independent experiments. DFT calculations indicate a close agreement between the theoretical and experimental equations of state. In addition to BaH12, we identified previously known P6/mmm BaH2 and possibly BaH10 and BaH6 as impurities in the samples. Ab initio calculations show that newly discovered semimetallic BaH12 contains H2, H3 molecular units and detached H12 chains. Barium dodecahydride is a unique molecular hydride with metallic conductivity which demonstrates a superconducting transition around 20 K at 140 GPa in agreement with calculations (19-32 K). The interpretation of the multiphase XRD data was possible thanks to the development of new Python scripts for postprocessing the results of evolutionary searches. These scripts help quickly identify the theoretical structures that explain the experimental data in the best way, among thousands of candidates.

cond-mat.supr-con↗

arXiv2020

Structure, Stability and Mechanical Properties of Boron-Rich Mo-B Phases: A Computational Study

Molybdenum borides were studied theoretically using first-principles calculations, empirical total energy model and global optimization techniques to determine stable crystal structures. Our calculations reveal the structures of known Mo-B phases, attaining close agreement with experiment. Following our developed lattice model, we describe in detail the crystal structure of boron-rich $MoB_x$ phases with 3<x<9 as the hexagonal $P6_3/mmc$-$MoB_3$ structure with Mo atoms partially replaced by triangular boron units. The most energetically stable arrangement of these $B_3$ units corresponds to their uniform distribution in the bulk of the crystal structure, which leads to the formation of a disordered nonstoichiometric phase, with ordering arising at compositions close to x=5 due to a strong repulsive interaction between neighboring $B_3$ units. The most energetically favorable structures of $MoB_x$ correspond to the compositions 4<x<5, with $MoB_5$ being the boron-richest stable phase. The estimated hardness of $MoB_5$ is 37-39 GPa, suggesting that the boron-rich phases are potentially superhard.

cond-mat.mtrl-sci↗

arXiv2020

High-Pressure Synthesis of Magnetic Neodymium Polyhydrides

The current search for room-temperature superconductivity is inspired by the unique properties of the electron-phonon interaction in metal superhydrides. Encouraged by the recently found highest-$T_C$ superconductor fcc-$LaH_{10}$, here we discover several superhydrides of another lanthanide - neodymium. We identify three novel metallic Nd-H phases at pressure range from 85 to 135 GPa: $I4/mmm$-$NdH_4$, $C2/c$-$NdH_7$, $P6_3/mmc$-$NdH_9$, synthesized by laser-heating metal samples in NH3BH3 media for in situ generation of hydrogen. A lower trihydride $Fm\bar{3}m$-$NdH_3$ is found at pressures from 2 to 52 GPa. $I4/mmm$-$NdH_4$ and $C2/c$-$NdH_7$ are stable from 135 down to 85 GPa, and $P6_3/mmc$-$NdH_9$ from 110 to 130 GPa. Measurements of the electrical resistance of NdH9 demonstrate a possible superconducting transition at ~4.5 K in $P6_3/mmc$-$NdH_9$. Our theoretical calculations predict that all the neodymium hydrides have antiferromagnetic order at pressures below 150 GPa and represent one of the first discovered examples of strongly correlated superhydrides with large exchange spin-splitting in the electron band structure (> 450 meV). The critical N$é$el temperatures for new neodymium hydrides are estimated using the mean-field approximation as about 4 K ($NdH_4$), 251 K ($NdH_7$) and 136 K ($NdH_9$).

cond-mat.supr-con↗

arXiv2020

Superconductivity of $LaH_{10}$ and $LaH_{16}$ polyhydrides

We explore high-pressure phase stability and superconductivity of lanthanum hydrides $LaH_m$ (m=4-11,16). We predict stability of a hitherto unreported polyhydride $P6/mmm$-$LaH_{16}$ at pressures above 150 GPa; at 200 GPa its predicted superconducting $T_C$ is 156 K, critical field $μ_0$$H_C$(0) ~ 35 T and superconducting gap is up to 35 meV. We revisit superconductivity of the recently discovered $LaH_{10}$ and find its $T_C$ to be up to 259 K (170 GPa) from solving the Eliashberg equation and 271 K from solving the gap equation in SCDFT which also allowed us to compute the Coulomb pseudopotential $μ^*$ for $LaH_{10}$ and $LaH_{16}$. Presence of several polymorph modifications of LaH10 may explain the variety in the experimentally measured $T_C$ values for $LaH_{10}$ [1,2]

cond-mat.supr-con↗

arXiv2019

Superconductivity at 161 K in Thorium Hydride $ThH_{10}$: Synthesis and Properties

Here we report targeted high-pressure synthesis of two novel high-$T_C$ hydride superconductors, $P6_3/mmc$-$ThH_9$ and $Fm\bar{3}m$-$ThH_{10}$, with the experimental critical temperatures ($T_C$) of 146 K and 159-161 K and upper critical magnetic fields ($μ$$H_C$) 38 and 45 Tesla at pressures 170-175 Gigapascals, respectively. Superconductivity was evidenced by the observation of zero resistance and a decrease of $T_C$ under external magnetic field up to 16 Tesla. This is one of the highest critical temperatures that has been achieved experimentally in any compounds, along with such materials as $LaH_{10}$, $H_3S$ and $HgBa_2Ca_xCu_2O_{6+z}$. Our experiments show that $fcc$-$ThH_{10}$ has stabilization pressure of 85 GPa, making this material unique among all known high-$T_C$ metal polyhydrides. Two recently predicted Th-H compounds, $I4/mmm$-$ThH_4$ (> 86 GPa) and $Cmc2_1$-$ThH_6$ (86-104 GPa), were also synthesized. Equations of state of obtained thorium polyhydrides were measured and found to perfectly agree with the theoretical calculations. New phases were examined theoretically and their electronic, phonon, and superconducting properties were calculated.

cond-mat.mtrl-sci↗