arXiv subjects

Artem R. Oganov

Publications and source records attributed to Artem R. Oganov.

At least 127 records · Page 7Linked to original sources

arXiv2014

Synthesis of ultra-incompressible sp3-hybridized carbon nitride

Search of materials with C-N composition hold a great promise in creating materials which would rival diamond in hardness due to the very strong and relatively low-ionic C-N bond. Early experimental and theoretical works on C-N compounds were based on structural similarity with binary A3B4 structural- types; however, the synthesis of C3N4 remains elusive. Here we explored an unbiased synthesis from the elemental materials at high pressures and temperatures. Using in situ synchrotron X-ray diffraction and Raman spectroscopy we demonstrate synthesis of highly incompressible Pnnm CN compound with sp3 hybridized carbon is synthesized above 55 GPa and 7000 K. This result is supported by first principles evolutionary search, which finds that Pnnm CN is the most stable compound above 10.9 GPa. On pressure release below 6 GPa the synthesized CN compound amorphizes reattaining its 1:1 stoichiometry as confirmed by Energy-Dispersive X-ray Spectroscopy. This work underscores the importance of understanding of novel high-pressure chemistry rules and it opens a new route for synthesis of superhard materials.

cond-mat.mtrl-sci↗

arXiv2014

Phase stability, chemical bonding and mechanical properties of titanium nitrides: A first-principles study

We have performed first-principles evolutionary searches for all stable titanium nitrides and have found, in addition to the well-known rocksalt-type TiN, new ground states Ti$_3$N$_2$, Ti$_4$N$_3$, Ti$_6$N$_5$ at atmospheric pressure, and Ti$_2$N and TiN$_2$ at higher pressures. The latest nitrogen-rich structure presents encapsulated N$_2$ dumbbells with a N-N distance of 1.348 Å at 60 GPa and TiN$_2$ is predicted to be mechanically stable (quenchable). Our calculations of the mechanical properties (bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and hardness) are in excellent agreement with the available experimental data and show that the hardness of titanium nitrides increases with increasing nitrogen content. The hardness of titanium nitrides is enhanced by strengthening directional covalent bonds and disappearance of Ti-Ti metallic bonds. Among the predicted compounds, TiN$_2$ has the highest hardness of 27.2 GPa.

cond-mat.mtrl-sci↗

arXiv2014

Unexpected Reconstruction of the alpha-Boron (111) Surface

We report on a novel reconstruction of the alpha-boron (111) surface, discovered using an ab initio evolution structure search, and reveal that it has an unexpected neat structure and much lower surface energy than the recently proposed (111)-I_R,(a) surface. For this reconstruction, every single interstitial boron atom forms bridges with the unique polar-covalent bonds between neighboring B_12 icosahedra, which perfectly meet the electron counting rule and are responsible for the reconstruction-induced metal-semiconductor transition. The peculiar charge transfer between the interstitial atoms and the icosahedra plays an important role in stabilizing the surface.

cond-mat.mtrl-sci↗

arXiv2014

Backbone NxH Compounds at High Pressures

Optical and synchrotron x-ray diffraction diamond anvil cell experiments have been combined with first principles theoretical structure predictions to investigate mixed N2 and H2 up to 55 GPa. We found the formation of oligomeric NxH (x>1) compounds using mechano- and photochemistry at pressures above 47 and 10 GPa, respectively, and room temperature. These compounds can be recovered to ambient pressure at T<130 K, whereas at room temperature, they can be metastable down to 3.5 GPa. Our results suggest new pathways for synthesis of environmentally benign high energy-density materials and alternative planetary ice.

cond-mat.mtrl-sci↗

arXiv2014

Formation of Stoichiometric CsF$_n$ Compounds

Alkali halides $MX$, have been viewed as typical ionic compounds, characterized by 1:1 ratio necessary for charge balance between M$^+$ and X$^-$. It was proposed that group I elements like Cs can be oxidized further under high pressure. Here we perform a comprehensive study for the CsF-F system at pressures up to 100 GPa, and find extremely versatile chemistry. A series of CsF$_n$ ($n$ $\geq$ 1) compounds are predicted to be stable already at ambient pressure. Under pressure, 5$p$ electrons in Cs atoms become active, with growing tendency to form Cs$^{3+}$ and Cs$^{5+}$ valence states at fluorine-rich conditions. Although Cs$^{2+}$ and Cs$^{4+}$ are not energetically favoured, the interplay between two mechanisms (polyfluoride anions and polyvalent Cs cations) allows CsF$_2$ and CsF$_4$ compounds to be stable under pressure. The estimated defluorination temperatures of CsF$_n$ (n=2,3,5) compounds at atmospheric pressure (218 $^\circ$C, 150 $^\circ$C, -15 $^\circ$C, respectively), are attractive for fluorine storage applications.

cond-mat.mtrl-sci↗

arXiv2014

Exploration of stable compounds, crystal structures, and superconductivity in the Be-H system

Using first-principles variable-composition evolutionary methodology, we explored the high-pressure structures of beryllium hydrides between 0 and 400 GPa. We found that BeH$_2$ remains the only stable compound in this pressure range. The pressure-induced transformations are predicted as $Ibam$ $\rightarrow $ $P\bar{3}m1$ $\rightarrow $ $R\bar{3}m$ $ \rightarrow $ $Cmcm$ $ \rightarrow $ $P4/nmm$, which occur at 24, 139, 204 and 349 GPa, respectively. $P\bar{3}m1$ and $R\bar{3}m$ structures are layered polytypes based on close packings of H atoms with Be atoms filling octahedral voids in alternating layers. $Cmcm$ and $P4/nmm$ structures have 3D-networks of strong bonds, but also feature rectanular and squre, respectively, layers of H atoms with short H-H distances. $P\bar{3}m1$ and $R\bar{3}m$ are semiconductors while $Cmcm$ and $P4/nmm$ are metallic. We have explored superconductivity of both metallic phases, and found large electron-phonon coupling parameters of $ λ$=0.63 for $Cmcm$ (resulting in a $T_c$ of 32.1-44.1 K) at 250 GPa and $ λ$=0.65 for $P4/nmm$ ($T_c$ = 46.1-62.4 K) at 400 GPa. The dependence of $T_c$ on pressure indicates that $T_c$ initially increases to a maximum of 45.1 K for $Cmcm$ at 275 GPa and 97.0 K for $P4/nmm$ at 365 GPa, and then decreases with increasing pressure for both phases.

cond-mat.mtrl-sci↗

arXiv2014

Stability of numerous novel potassium chlorides at high pressure

K-Cl is a simple system displaying all four main types of bonding, as it contains (i) metallic potassium, (ii) elemental chlorine made of covalently bonded Cl2 molecules held together by van der Waals forces, and (iii) an archetypal ionic compound KCl. Well-known octet rule (and its special case, the charge balance rule, assigning classical charges of +1 to K and -1 to Cl) predicts that no compounds other than KCl are possible. However, our quantum-mechanical variable-composition evolutionary simulations predict new stable compounds. The phase diagram of the K-Cl system turned out to be extremely complicated, featuring new thermodynamically compounds K3Cl, K2Cl, K3Cl2, K4Cl3, K5Cl4, K3Cl5, KCl3, KCl7. Of particular interest are 2D-metallic homologs Kn+1Cln, the presence of positively charged Cl atoms in KCl7, and the predicted stability of KCl3 already at the atmospheric pressure.

cond-mat.mtrl-sci↗

arXiv2014

Predicted Novel Hydrogen Hydrate Structures under Pressure from First Principles

Gas hydrates are systems of prime importance. In particular, hydrogen hydrates are potential materials of icy satellites and comets, and may be used for hydrogen storage. We explore the H2O-H2 system at pressures in the range 0-100 GPa with ab initio variable-composition evolutionary simulations. According to our calculation and previous experiments, the H2O-H2 system undergoes a series of transformations with pressure, and adopts the known open-network clathrate structures (sII, C0), dense "filled ice" structures (C1, C2) and two novel hydrate phases. One of these is based on the hexagonal ice framework and has the same H2O:H2 ratio (2:1) as the C0 phase at low pressures and similar enthalpy (we name this phase Ih-C0). The other newly predicted hydrate phase has a 1:2 H2O:H2 ratio and structure based on cubic ice. This phase (which we name C3) is predicted to be thermodynamically stable above 38 GPa when including van der Waals interactions and zero-point vibrational energy, and explains previously mysterious experimental X-ray diffraction and Raman measurements. This is the hydrogen-richest hydrate and this phase has a remarkable gravimetric density (18 wt.%) of easily extractable hydrogen.

cond-mat.mtrl-sci↗

arXiv2014

Semimetallic Two-Dimensional Boron Allotrope with Massless Dirac Fermions

It has been widely accepted that planar boron structures, composed of triangular and hexagonal motifs are the most stable two dimensional (2D) phases and likely precursors for boron nanostructures. Here we predict, based on ab initio evolutionary structure search, novel 2D boron structure with non-zero thickness, which is considerably, by 50 meV/atom lower in energy than the recently proposed alpha-sheet structure and its analogues. In particular, this phase is identified for the first time to have a distorted Dirac cone, after graphene and silicene the third elemental material with massless Dirac fermions. The buckling and coupling between the two sublattices not only enhance the energetic stability, but also are the key factors for the emergence of the distorted Dirac cone.

cond-mat.mtrl-sci↗

arXiv2013

Unexpected stable stoichiometries of sodium chlorides

Sodium chloride (NaCl), or rocksalt, is well characterized at ambient pressure. Due to the large electronegativity difference between Na and Cl atoms, it has highly ionic chemical bonding, with stoichiometry 1:1 dictated by charge balance, and B1-type crystal structure. Here, by combining theoretical predictions and diamond anvil cell experiments we show that new materials with different stoichiometries emerge at pressure as low as 20 GPa. Compounds such us Na3Cl, Na2Cl, Na3Cl2, NaCl3 and NaCl7 are theoretically stable and have unusual bonding and electronic properties. To test this prediction, at 55-80 GPa we synthesized cubic and orthorhombic NaCl3 at 55-70 GPa and 2D-metallic tetragonal Na3Cl. This proves that novel compounds, violating chemical intuition, can be thermodynamically stable even in simplest systems at non-ambient conditions.

cond-mat.mtrl-sci↗

arXiv2013

Prediction of stable hafnium carbides: their stoichiometries, mechanical properties, and electronic structure

Hafnium carbides are studied by a systematic search for possible stable stoichiometric compounds in the Hf-C system at ambient pressure using variable-composition ab initio evolutionary algorithm implemented in the USPEX code. In addition to well-known HfC, we predicted two additional compounds Hf3C2 and Hf6C5. The structure of Hf6C5 with space group C2/m contains 11 atoms in the primitive cell and this prediction revives the earlier proposal by A. I. Gusev. The stable structure of Hf3C2 also has space group C2/m, and is more energetically favorable than the Immm, P-3m1, P2 and C2221 structures put forward by A. I. Gusev. Dynamical and mechanical stability of the newly predicted structures have been verified by calculations of their phonons and elastic constants. The bulk and shear moduli of Hf3C2 are 195.8 GPa and 143.1 GPa, respectively, while for Hf6C5 they are 227.9 GPa and 187.2 GPa, respectively. Their mechanical properties are inferior to those of HfC due to the presence of structural vacancies. Chemical bonding, band structure, and Bader charge are presented and discussed.

cond-mat.mtrl-sci↗

arXiv2013

Photovoltaic effect in BiFeO3/TiO2 heterostructures tuned with epitaxial strain and an electric field

The photovoltaic effect in the BiFeO3/TiO2 heterostructures can be tuned by epitaxial strain and an electric field in the visible-light region which is manifested by the enhancement of absorption activity in the heterojunction under tensile strain and an electric field based on the first-principles calculations. It is suggested that there are coupling between photon, spin carrier, charge, orbital, and lattice in the interface of the bilayer film which makes the heterojunction an intriguing candidate towards fabricating the multifunctional photoelectric devices based on spintronics. The microscopic mechanism involved in the heterostruces is related deeply with the spin transfer and charge rearrangement between the Fe 3d and O 2p orbitals in the vicinity of the interface.

cond-mat.mtrl-sci↗

arXiv2013

Evolutionary search for new high-k dielectric materials: methodology and applications to hafnia-based oxides

High-k dielectric materials are important as gate oxides in microelectronics and as potential dielectrics for capacitors. In order to enable computational discovery of novel high-k dielectric materials, we propose a fitness model (energy storage density) that includes the dielectric constant, bandgap, and intrinsic breakdown field. This model, used as fitness function in conjunction with first-principles calculations and global optimization evolutionary algorithm USPEX, efficiently leads to practically important results. We found a number of high-fitness structures of SiO2 and HfO2, some of which correspond to known phases and some are new. The results allow us to propose characteristics (genes) common to high-fitness structures - these are the coordination polyhedra and their degree of distortion. Our variable-composition searches in the HfO2-SiO2 system uncovered several high-fitness states. This hybrid algorithm opens up a new avenue of discovering novel high-k dielectrics with both fixed and variable compositions, and will speed up the process of materials discovery.

cond-mat.mtrl-sci↗

arXiv2013

Evolutionary Method for Predicting Surface Reconstructions with Variable Stoichiometry

We present a specially designed evolutionary algorithm for the prediction of surface reconstructions. This new technique allows one to automatically explore all the low-energy configurations with variable surface atoms and variable surface unit cells through the whole chemical potential range. The power of evolutionary search is demonstrated by the efficient identification of diamond 2*1 (100) and 2*1 (111) surfaces with a fixed number of surface atom and a fixed cell size. With further variation of surface unit cells, we study the reconstructions of the polar surface MgO (111). Experiment has detected an oxygen trimer (ozone) motif (Plass et al, 1998). We predict a new version of this motif which can be thermodynamically stable at extreme oxygen rich condition. Finally, we perform a variable stoichiometry search for a complex ternary system: semi-polar GaN (101bar1) with and without adsorbed oxygen. The search yields a non-intuitive reconstruction based on N3-trimers. These examples demonstrate that an automated scheme to explore the energy landscape of surfaces will improve our understanding of surface reconstructions. The method presented in this report can be generally applied to binary and multi-component systems.

cond-mat.mtrl-sci↗

arXiv2012

First-Principles Determination of the Structure of Magnesium Borohydride

The energy landscape of Mg(BH4)2 under pressure is explored by ab initio evolutionary calculations. Two new tetragonal structures, with space groups P-4 and I41/acd are predicted to be lower in enthalpy, by 15.4 kJ/mol and 21.2 kJ/mol, respectively, than the earlier proposed P42nm phase. We have simulated X-ray diffraction (XRD) spectra, lattice dynamics, and equations of state (EOS) of these phases. The density, volume contraction, bulk modulus, and the simulated XRD patterns of I41/acd and P-4 structures are in excellent agreement with the experimental results.

cond-mat.mtrl-sci↗

arXiv2012

Unexpected Stoichiometries in Mg-O System under High Pressure

Using ab initio evolutionary simulations, we explore all the possible stoichiometries for Mg-O system at pressures up to 850 GPa. In addition to MgO, our calculations find that two extraordinary compounds MgO2 and Mg3O2 become thermodynamically stable at 116 GPa and 500 GPa, respectively. Detailed chemical bonding analysis shows large charge transfer in all magnesium oxides. MgO2 contains peroxide ions [O-O]2-, while non-nuclear electron density maxima play the role of anions in the electride compound Mg3O2. The latter compound is calculated to have a much narrower band gap compared to MgO and MgO2. We discuss conditions at which MgO2 and Mg3O2 might exist in planetary conditions.

cond-mat.mtrl-sci↗

arXiv2012

Stability of xenon oxides at high pressures

Xenon, which is quite inert under ambient conditions, may become reactive under pressure. The possibility of formation of stable xenon oxides and silicates in the interior of the Earth could explain the atmospheric missing xenon paradox. Using the ab initio evolutionary algorithm, we predict the thermodynamical stabilization of Xe-O compounds at high pressures (XeO, XeO2 and XeO3 at pressures above 83, 102 and 114 GPa, respectively). Our calculations indicate large charge transfer in these oxides, suggesting that large electronegativity difference and pressure are the key factors favoring the formation of xenon compounds. Xenon compounds in the Earth's mantle, however, cannot directly explain the missing xenon paradox: xenon oxides are unstable in equilibrium with metallic iron in the Earth's lower mantle, while xenon silicates are predicted to spontaneously decompose at all mantle pressures (<136 GPa). This does not preclude Xe atoms from being retained in defects of mantle silicates and oxides.

cond-mat.mtrl-sci↗

arXiv2012

Unexpected stable stoichiometries of sodium chlorides

At ambient pressure, sodium, chlorine, and their only known compound NaCl, have well-understood crystal structures and chemical bonding. Sodium is a nearly-free-electron metal with the bcc structure. Chlorine is a molecular crystal, consisting of Cl2 molecules. Sodium chloride, due to the large electronegativity difference between Na and Cl atoms, has highly ionic chemical bonding, with stoichiometry 1:1 dictated by charge balance, and rocksalt (B1-type) crystal structure in accordance with Pauling's rules. Up to now, Na-Cl was thought to be an ultimately simple textbook system. Here, we show that under pressure the stability of compounds in the Na-Cl system changes and new materials with different stoichiometries emerge at pressure as low as 25 GPa. In addition to NaCl, our theoretical calculations predict the stability of Na3Cl, Na2Cl, Na3Cl2, NaCl3 and NaCl7 compounds with unusual bonding and electronic properties. The bandgap is closed for the majority of these materials. Guided by these predictions, we have synthesized cubic NaCl3 at 55-60 GPa in the laser-heated diamond anvil cell at temperatures above 2000 K.

cond-mat.mtrl-sci↗