arXiv subjects

Artem R. Oganov

Publications and source records attributed to Artem R. Oganov.

At least 73 records · Page 4Linked to original sources

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↗

arXiv2020

Hydrocarbons under pressure: phase diagrams and surprising new compounds in the C-H system

Understanding the high-pressure behavior of C-H system is of great importance due to its key role in organic, bio-, petroleum and planetary chemistry. We have performed a systematic investigation of the pressure-composition phase diagram of the C-H system at pressures up to 400 GPa using evolutionary structure prediction coupled with ab initio calculations and discovered that only saturated hydrocarbons are thermodynamically stable. Several stable methane-hydrogen co crystals are predicted: 2CH4 * H2, earlier obtained experimentally, is predicted to have I4/m space group and 2-90 GPa stability range at 0 K, and two new thermodynamically stable compounds 2CH4 * 7H2 (P-3m1 space group) and CH4 * 9H2 (Cm space group), as potential energy storage materials. P21/c phase of methane is predicted to be stable at pressures < 8 GPa; bulk graphane (CH) was shown to be thermodynamically stable at 7-18 and 18-50 GPa and 0 K in the P-3m and Cmca phases, respectively; polyethylene is shown to have a narrow field of stability. We report the p-T-x phase diagram of the C-H system and p-T phase diagram of CH4.

cond-mat.mtrl-sci↗

arXiv2019

Superconducting Praseodymium Superhydrides

Superhydrides have complex hydrogenic sublattices and are important prototypes for studying metallic hydrogen and high-temperature superconductors. Encouraged by the results on LaH10, in consideration of the differences between La and Pr, Pr-H system is especially worth studying because of the magnetism and valence-band f-electrons in element Pr. Here we successfully synthesized praseodymium superhydrides (PrH9) in laser-heated diamond anvil cells. Synchrotron X-ray diffraction (XRD) analysis demonstrated the presence of previously predicted F43m-PrH9 and unexpected P63/mmc-PrH9 phases. Moreover, Fm3m-PrH3, P4/nmm-PrH(3-δ) and Fm3m-PrH(1+x) were found below 52 GPa. F43m-PrH9 and P63/mmc-PrH9 were stable above 100 GPa in experiment. Experimental studies of electrical resistance in the PrH9 sample showed the emergence of superconducting transition (Tc) below 9K and a dependent Tc on applied magnetic field. Theoretical calculations indicate that magnetic order and electron-phonon interaction coexist in a very close range of pressures in the PrH9 sample which may contribute to its low superconducting temperature Tc. Our results highlight the intimate connections among hydrogenic sublattices, density of states, magnetism and superconductivity in Pr-based superhydrides.

cond-mat.mtrl-sci↗

arXiv2019

mDCThermalC: A program for calculating thermal conductivity quickly and accurately

mDCThermalC is a program written in Python for computing lattice thermal conductivity of crystalline bulk materials using the modified Debye-Callaway model. Building upon the traditional Debye-Callaway theory, the modified model obtains the lattice thermal conductivity by averaging the contributions from acoustic and optical branches based on their specific heat. The only inputs of this program are the phonon spectrum, phonon velocity and Gruneisen parameter, all of which can be calculated using third-party ab initio packages, making the method fully parameter-free. This leads to a fast and accurate evaluation and enables high-throughput calculations of lattice thermal conductivity even in large and complex systems. In addition, this program calculates the specific heat and phonon relaxation times for different scattering processes, which will be beneficial for understanding the phonon transfer behavior.

cond-mat.mtrl-sci↗

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↗

arXiv2019

High-Temperature Superconductivity in the Ti--H System at High Pressures

Search for stable high-pressure compounds in the Ti--H system reveals the existence of titanium hydrides with new stoichiometries, including Ibam-Ti$_2$H$_5$, I4/m-Ti$_5$H$_{13}$, I$\bar{4}$-Ti$_5$H$_{14}$, Fddd-TiH$_4$, Immm-Ti$_2$H$_{13}$, P$\bar{1}$-TiH$_{12}$, and C2/m-TiH$_{22}$. Our calculations predict I4/mmm $\rightarrow$ R$\bar{3}$m and I4/mmm $\rightarrow$ Cmma transitions in TiH and TiH$_2$, respectively. Phonons and the electron--phonon coupling of all searched titanium hydrides are analyzed at high pressure. It is found that Immm-Ti$_2$H$_{13}$ rather than the highest hydrogen content C2/m-TiH$_{22}$, exhibits the highest superconducting critical temperature T$_{c}$. The estimated T$_{c}$ of Immm-Ti$_2$H$_{13}$ and C2/m-TiH$_{22}$ are respectively 127.4--149.4 K ($μ^{*}$=0.1-0.15) at 350 GPa and 91.3--110.2 K at 250 GPa by numerically solving the Eliashberg equations. One of the effects of pressure on T$_{c}$ can be attributed to the softening and hardening of phonons with increasing pressure.

cond-mat.mtrl-sci↗

arXiv2019

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

New boron-rich sulfide B6S and selenide B6Se have been discovered from high pressure - high temperature synthesis combined with ab initio evolutionary crystal structure prediction, and studied by synchrotron X-ray diffraction and Raman spectroscopy at ambient conditions. As it follows from Rietveld refinement of powder X-ray diffraction data, both chalcogenides have orthorhombic symmetry and belongs to Pmna space group. All experimentally observed Raman bands have been attributed to the theoretically calculated phonon modes, and the mode assignment has been performed. Prediction of mechanical properties (hardness and elastic moduli) of new boron-rich chalcogenides have been made using ab initio routines, and both compounds were found to be members of a family of hard phases (Hv ~ 31 GPa).

cond-mat.mtrl-sci↗

arXiv2019

On Distribution of Superconductivity in Metal Hydrides

Using the data on the superconducting critical temperature ($T_{C}$) for a number of metal hydrides, we found a rule that makes it possible to predict the maximum $T_{C}$ based only on the information about the electronic structure of metal atoms. Using this guiding principle, we explored the hydride systems for which no reliable information existed, predicted new higher hydrides in the K-H, Zr-H, Hf-H, Ti-H, Mg-H, Sr-H, Ba-H, Cs-H, and Rb-H systems at high pressures, and calculated their $T_{C}$. Results of the study of actinides and lanthanides show that they form highly symmetric superhydrides $XH_{7-9}$. However, actinide hydrides do not exhibit high-temperature superconductivity (except Th-H system) and might not be considered as promising materials for experimental studies, as well as all $d^m$-elements with m > 4, including metal hydrides of the noble elements. Designed neural network allowing the prediction of $T_{C}$ of various hydrides shows good accuracy and was used to estimate upper limit for $T_{C}$ of the materials with absence of the data. The developed rule, based on regular behavior of the maximum achievable critical temperature as a function of number of $d+f$ electrons, enables targeted predictions about the existence of new high-$T_{C}$ superconductors.

cond-mat.supr-con↗

arXiv2019

Synthesis of clathrate cerium superhydride CeH9 below 100 GPa with atomic hydrogen sublattice

Hydrogen-rich superhydrides are believed to be very promising high-Tc superconductors as they are expected to mimic characteristics of metallic hydrogen. Recent experiments discovered superhydrides at very high pressures, e.g. FeH5 at 130 GPa and LaH10 at 170 GPa. With the motivation of discovering new hydrogen-rich high-Tc superconductors at lowest possible pressure, here we report the prediction and experimental synthesis of cerium superhydride CeH9 below 100 GPa in the laser-heated diamond anvil cell. Ab-initio calculations were carried to evaluate the detailed chemistry of the Ce-H system and to understand the structure, stability and superconductivity of CeH9. CeH9 crystallizes in a P63/mmc clathrate structure with a substantially dense 3-dimensional hydrogen sublattice at 100 GPa. These findings shed a new light on the search for superhydrides in close proximity with atomic hydrogen within a feasible pressure range. Discovery of superhydride CeH9 provides a practical platform to further investigate and understand conventional superconductivity in hydrogen rich superhydrides.

cond-mat.mtrl-sci↗

arXiv2019

Accelerating crystal structure prediction by machine-learning interatomic potentials with active learning

In this letter we propose a new methodology for crystal structure prediction, which is based on the evolutionary algorithm USPEX and the machine-learning interatomic potentials actively learning on-the-fly. Our methodology allows for an automated construction of an interatomic interaction model from scratch replacing the expensive DFT with a speedup of several orders of magnitude. Predicted low-energy structures are then tested on DFT, ensuring that our machine-learning model does not introduce any prediction error. We tested our methodology on a problem of prediction of carbon allotropes, dense sodium structures and boron allotropes including those which have more than 100 atoms in the primitive cell. All the the main allotropes have been reproduced and a new 54-atom structure of boron have been found at very modest computational efforts.

cond-mat.mtrl-sci↗

arXiv2019

Magnetic borophenes from evolutionary search

A computation methodology based on ab initio evolutionary algorithms and the spin-polarized density functional theory was developed to predict two-dimensional (2D) magnetic materials. Its application to a model system borophene reveals an unexpected rich magnetism and polymorphism. A stable borophene with nonzero thickness was an antiferromagnetic (AFM) semiconductor from first-principles calculations, which can be further turned into a half metal by finite electron doping. In this borophene, the buckling and coupling among three atomic layers are not only responsible for the magnetism, but also result in an out-of-plane negative Poissons ratios under uniaxial tension, making it the first elemental material possessing auxetic and magnetic properties simultaneously.

cond-mat.mtrl-sci↗

arXiv2019

New two-dimensional phase of tin chalcogenides: candidates for high-performance thermoelectric materials

Tin-chalcogenides SnX (X = Te, Se and S) have been arousing research interest due to their thermoelectric physical properties. The two-dimensional (2D) counterparts, which are expected to enhance the property, nevertheless, have not been fully explored because of many possible structures. Generating variable composition of 2D Sn$_{1-x}$X$_{x}$ systems (X = Te, Se and S) has been performed using global searching method based on evolutionary algorithm combining with density functional calculations. A new hexagonal phase named by $β'$-SnX is found by Universal Structure Predictor Evolutionary Xtallography (USPEX), and the structural stability has been further checked by phonon dispersion calculation and the elasticity criteria. The $β'$-SnTe is the most stable among all possible 2D phases of SnTe including those experimentally available phases. Further, $β'$ phases of SnSe and SnS are also found energetically close to the most stable phases. High thermoelectronic (TE) performance has been achieved in the $β'$-SnX phases, which have dimensionless figure of merit (ZT) as high as $\sim$0.96 to 3.81 for SnTe, $\sim$0.93 to 2.51 for SnSe and $\sim$1.19 to 3.18 for SnS at temperature ranging from 300 K to 900 K with practically attainable carrier concentration of 5$\times$10$^{12}$ cm$^{-2}$. The high TE performance is resulted from a high power factor which is attributed to the quantum confinement of 2D materials and the band convergence near Fermi level, as well as low thermal conductivity mainly from both low elastic constants due to weak inter-Sn bonding strength and strong lattice anharmonicity.

cond-mat.mtrl-sci↗

arXiv2018

Simultaneous prediction of atomic structure and stability of nanoclusters in a wide area of compositions

We present a universal method for the large-scale prediction of the atomic structure of clusters. Our algorithm performs the joint evolutionary search for all clusters in a given area of the compositional space and takes advantage of structural similarities frequently observed in clusters of close compositions. The resulting speedup is up to 50 times compared to current methods. This enables the first-principles studies of multi-component clusters with full coverage of a wide range of compositions. As an example, we report an unprecedented first-principles global optimization of 315 SinOm clusters with n<=15 and m<=20. The obtained map of Si-O cluster stability shows the existence of both expected (SiO2)n and unexpected (e.g. Si4O18) stable ("magic") clusters, which can be important for miscellaneous applications.

cond-mat.mtrl-sci↗

arXiv2018

Old puzzle of incommensurate crystal structure of calaverite AuTe$_2$ and predicted stability of novel AuTe compound

It is shown that the long-standing puzzle of incommensurate crystal structure of AuTe$_2$ can be solved, if this material is considered as a negative charge-transfer system. Using modern computational methods, we demonstrate that charge redistribution associated with incommensurate modulations of crystal structure occurs not so much on Au, but predominantly on Te sites. This substantially reduces Coulomb energy costs for creating such a unique crystal structure. The same mechanism also explains superconductivity of doped AuTe$_2$. Exploring different Au-Te compositions, we also discovered a previously unknown compound AuTe, which theoretically is very stable, and we predict its crystal structure.

cond-mat.str-el↗

arXiv2018

High-Temperature Superconductivity in Th-H System at Pressure Conditions

New stable thorium decahydride $Fm\bar{3}m$-$ThH_{10}$, a record high-temperature superconductor with $T_C$ up to 241 K ($-32^{\circ}C$), critical field $H_C$ up to 71T and superconducting gap ${Δ_0}$ = 52 meV at 80-100 GPa was predicted by evolutionary algorithm USPEX. Another phase ${P2_1c}$-$ThH_7$ was found to be a high-temperature superconductor with $T_C$ of 65 K. Analysis of superconducting state was performed within Eliashberg formalism and the dependencies of $H_C(T)$, ${Δ_0}(T)$, $T_C(P)$ together with jump in the specific heat at critical temperature were calculated. Several other new thorium hydrides were predicted to be stable under pressure including $ThH_3$, $Th_3H_{10}$, $ThH_4$, $ThH_6$. Thorium (which has $s^2d^2$ electronic configuration) forms high-$T_C$ polyhydrides similar to those formed by $s^2d^1$ metals (Y-La-Ac). Thorium - is the next member in Mg-Ca-Sc-Y-La-Ac family of elements forming high-$T_C$ superconducting hydrides.

cond-mat.supr-con↗

arXiv2018

Novel high-pressure calcium carbonates

Calcium and magnesium carbonates are believed to be the host compounds for most of the oxidized carbon in the Earth's mantle. Here, using evolutionary crystal structure prediction method USPEX, we systematically explore the MgO-CO2 and CaO-CO2 systems at pressures ranging from 0 to 160 GPa to search for thermodynamically stable magnesium and calcium carbonates. While MgCO3 is the only stable magnesium carbonate, three calcium carbonates are stable under pressure: well-known CaCO3, and newly predicted Ca3CO5 and CaC2O5. Ca3CO5 polymorphs are found to contain isolated orthocarbonate (CO4)4- tetrahedra, and are stable at relatively low pressures (>11 GPa), whereas CaC2O5 is stable above 33 GPa and its polymorphs feature polymeric motifs made of CO4 tetrahedra. Detailed analysis of chemical stability of CaCO3, Ca3CO5 and CaC2O5 in the environment typical of the Earth's lower mantle reveals that none of these compounds can exist in the Earth's lower mantle. We conclude that MgCO3 is the main host of oxidized carbon throughout the lower mantle.

cond-mat.mtrl-sci↗

arXiv2018

Simple and accurate model of fracture toughness of solids

Fracture toughness $K_{IC}$ plays an important role in materials design. Along with numerous experimental methods to measure fracture toughness of materials, its understanding and theoretical prediction is very important. However, theoretical prediction of fracture toughness is challenging. By investigating the correlation between fracture toughness and elastic properties of materials, we have constructed a fracture toughness model for covalent and ionic crystals. Furthermore, by introducing an enhancement factor, which is determined by the density of states at the Fermi level and atomic electronegativities, we have constructed a universal model of fracture toughness for covalent and ionic crystals, metals and intermetallics. The predicted fracture toughnesses are in good agreement with experimental values for a series of materials. All the parameters in the proposed model of fracture toughness can be obtained from first-principles calculations, which makes it suitable for practical applications.

cond-mat.mtrl-sci↗

arXiv2018

Titanium-hydrogen interaction at megabar pressure

The process of transport of metal particles ($\mathit{ejecta}$) in gases is the subject of recent works in the field of nuclear energetics. We studied the process of dissolution of titanium ejecta in warm dense hydrogen at megabar pressure. Thermodynamic and kinetic properties of the process were investigated using classical and quantum molecular dynamics methods. We estimated the dissolution time of ejecta, the saturation limit of titanium atoms with hydrogen and the heat of dissolution. It was found that particles with a radius of 1 $μm$ dissolve in hydrogen in time of $1.5 \cdot 10^{-2} \ μs$, while the process of mixing can be described by diffusion law. The presented approach demonstrates the final state of the titanium-hydrogen system as a homogenized fluid with completely dissolved titanium particles. This result can be generalized to all external conditions under which titanium and hydrogen are atomic fluids.

cond-mat.mtrl-sci↗