arXiv subjects

Elena Bykova

Publications and source records attributed to Elena Bykova.

17 recordsLinked to original sources

arXiv2026

Two New Molecular Nitrogen Phases near Megabar Pressures

Molecular nitrogen exhibits remarkable structural diversity near the polymeric transition, where multiple phases are metastable. Here, we report two new molecular phases. The first, $tζ$-N$_2$, is a polytype of monoclinic $C2/c$ $ζ$-N$_2$, characterized by a tripled $c$ axis and 96 atoms per unit cell. The second, $ξ$-N$_2$, is a previously unreported hexagonal phase ($P6cc$) containing 112 atoms per unit cell. Both phases were synthesized in a diamond anvil cell by laser heating $ζ$-N$_2$ to 1800--2500~K at pressures of 78--98~GPa. Their crystal structures were determined using single-crystal X-ray diffraction, corroborated by Raman spectroscopy, and supported by first-principles calculations. The $tζ$-N$_2$ phase likely corresponds to the previously reported $κ$-N$_2$ phase.

cond-mat.mtrl-sci↗

arXiv2025

Hydrogen-rich hydrate at high pressures up to 104 GPa

Gas hydrates are considered fundamental building blocks of giant icy planets like Neptune and similar exoplanets. The existence of these materials in the interiors of giant icy planets, which are subject to high pressures and temperatures, depends on their stability relative to their constituent components. In this study, we reexamine the structural stability and hydrogen content of hydrogen hydrates, (H2O)(H2)n, up to 104 GPa, focusing on hydrogen-rich materials. Using synchrotron single-crystal X-ray diffraction, Raman spectroscopy, and first-principles theoretical calculations, we find that the C2-filled ice phase undergoes a transformation to C3-filled ice phase over a broad pressure range of 47 - 104 GPa at room temperature. The C3 phase contains twice as much molecular H2 as the C2 phase. Heating the C2-filled ice above approximately 1500 K induces the transition to the C3 phase at pressures as low as 47 GPa. Upon decompression, this phase remains metastable down to 40 GPa. These findings establish new stability limits for hydrates, with implications for hydrogen storage and the interiors of planetary bodies.

cond-mat.mtrl-sci↗

arXiv2024

Polymorphism of monatomic iodine

We applied synchrotron single-crystal X-ray diffraction in a diamond anvil cell at 48-51 GPa and first-principles theoretical calculations to study the crystal structure of solid atomic iodine at high pressure. We report the synthesis of two phases of atomic iodine at 48-51 GPa via laser heating of I-N2 mixtures. Unlike the familiar monatomic I4/mmm structure, which consists of crystallographically equivalent atoms, a new Pm-3n structure is of inclusion type, featuring two distinct kinds of atoms: a central detached one and peripheral ones forming the linear chains. Moreover, we observe crystallization of the familiar high-pressure face centered cubic (fcc) structure, albeit at much lower pressures compared to cold compressed iodine. The discovery of Pm-3n structure in iodine marks an important step in understanding of the pressure induced phase transition sequence in halogens.

cond-mat.mtrl-sci↗

arXiv2023

Structural diversity of molecular nitrogen on approach to polymeric states

Nitrogen represents an archetypal example of material exhibiting a pressure driven transformation from molecular to polymeric state. Detailed investigations of such transformations are challenging because of a large kinetic barrier between molecular and polymeric structures, making the transformation largely dependent on kinetic stimuli. In the case of nitrogen, additional complications occur due to the rich polymorphism in the vicinity of the transition. Here, we report the observation of both molecular (θ) and polymeric (BP) phases, crystallized upon temperature quenching of fluid nitrogen to room temperature at 97-114 GPa. Synchrotron single-crystal X-ray diffraction, Raman spectroscopy, and first-principles theoretical calculations have been used for diagnostics of the phases and determination of their structure and stability. Molecular θ-nitrogen is the most stable among molecular phases bordering the stability field of polymeric phases, partially settling a previously noted discrepancy between theory and experiment concerning the thermodynamic stability limit of molecular phases.

cond-mat.mtrl-sci↗

arXiv2023

Assessing the stability fields of molecular and polymeric CO2

We investigated the stability of polymeric CO2 over a wide range of pressures, temperatures, and chemical environments. We find that the I-42d polymeric structure, consisting of a three-dimensional network of corner sharing CO4 tetrahedra, forms at 40-140 GPa and from a CO-N2 mixture at 39 GPa. An exceptional stability field of 0 to 286 GPa and 100 to 2500 K is documented for this structure, making it a viable candidate for planetary interiors. The stability of the tetrahedral polymeric motif of CO2-V is a consequence of the rigidity of sp3 hybridized orbitals of carbon in a closed-packed oxygen sublattice.

cond-mat.mtrl-sci↗

arXiv2023

Structural evolution of iodine on approach to the monatomic state

We applied single-crystal X-ray diffraction and Raman spectroscopy in a diamond anvil cell up to 36 GPa and first principles theoretical calculations to study the molecular dissociation of solid iodine at high pressure. Unlike previously reported, we find that the familiar Cmce molecular phase transforms to a Cmc21 molecular structure at 16 GPa, and then to an incommensurate dynamically disordered Fmmm(00γ)s00 structure at 20 GPa, which can be viewed as a stepwise formation of polymeric zigzag chains of three iodine atoms following by the formation of the dynamically dissociated, incommensurately modulated i-Fmmm phase, and the truly monatomic Immm phase at higher pressures.

cond-mat.mtrl-sci↗

arXiv2021

Structure and vibrational properties of methane up to 71 GPa

Single-crystal synchrotron X-ray diffraction, Raman spectroscopy, and first principles calculations have been used to identify the structure of the high-pressure (HP) phase of molecular methane above 20 GPa up to 71 GPa. The structure of HP phase is trigonal R3, which can be represented as a distortion of the cubic phase B, previously documented at 7-15 GPa and confirmed here. The positions of hydrogen atoms in HP phase have been 19 obtained from first principles calculations. The molecules occupy four different crystallographic sites in phases B and eleven sites in the HP phase, which result in splitting of molecular stretching modes detected in Raman spectroscopy and assigned here based on 22 a good agreement with the Raman spectra calculated from the first principles.

cond-mat.mtrl-sci↗

arXiv2021

Synthesis and structure of carbon doped H3S compounds at high pressure

Understanding of recently reported putative close-to-room-temperature superconductivity in C-S-H compounds at 267 GPa demands reproducible synthesis protocol as well as knowledge of its structure and composition. We synthesized C-S-H compounds with various carbon composition at high pressures from elemental C and methane CH4, sulfur S, and molecular hydrogen H2. Here we focus on compounds synthesized using methane as these allow a straightforward determination of their structure and composition by combining single-crystal X-ray diffraction (XRD) and Raman spectroscopy. We applied a two-stage synthesis of ((CH4)x(H2S)(1-x))2H2 compounds by first reacting sulfur and mixed methane-hydrogen fluids and forming CH4 doped H2S crystals at 0.5-3 GPa, and then by growing single crystals of the desired hydrogen rich compound. Raman spectroscopy applied to this material shows the presence of the CH4 molecules incorporated into the lattice and allows to determine the CH4 content, while single-crystal X-ray diffraction results suggest that the methane molecules substitute H2S ones. The structural behavior of these compounds is very similar to the previously investigated methane free compounds demonstrating a transition from Al2Cu type I4/mcm structure to a modulated structure at 20-30 GPa and back to the same basic structure in an extended modification with greatly modified Raman spectra. This latter phase demonstrates a distortion into Pnma structure at 132-159 GPa and then transforms into a common Im-3m H3S phase at higher pressures, however, no structural anomaly is detected near 220 GPa, where a sharp upturn in Tc has been reported.

cond-mat.mtrl-sci↗

arXiv2020

Structure and composition of C-S-H compounds up to 143 GPa

We synthesized two C-S-H compounds from a mixture of carbon and sulfur in hydrogen and from sulfur in mixed methane-hydrogen fluids at 4 GPa. X-ray synchrotron single-crystal diffraction and Raman spectroscopy have been applied to these samples up to 58 and 143 GPa, respectively. Both samples show a similar Al2Cu type I4/mcm basic symmetry, while the hydrogen subsystem evolves with pressure via variously ordered molecular and extended modifications. The methane bearing sample lowers symmetry to an orthorhombic Pnma structure after laser heating to 1400 K at 143 GPa. The results suggest that superconducting C-S-H compounds are structurally different from a common Im-3m H3S.

cond-mat.supr-con↗

arXiv2020

Stabilization of hexazine rings in potassium polynitride at high pressure

Polynitrogen molecules represent the ultimate high energy-density materials as they have a huge potential chemical energy originating from their high enthalpy. However, synthesis and storage of such compounds remain a big challenge because of difficulties to find energy efficient synthetic routes and stabilization mechanisms. Compounds of metals with nitrogen represent promising candidates for realization of energetic polynitrogen compounds, which are also environmentally benign. Here we report the synthesis of polynitrogen planar N6 hexazine rings, stabilized in K2N6 compound, which was formed from K azide upon laser heating in a diamond anvil cell at high pressures in excess of 45 GPa and remains metastable down to 20 GPa. Synchrotron X-ray diffraction and Raman spectroscopy are used to identify this material, also exhibiting metallic luster, being all consistent with theoretically predicted structural, vibrational and electronic properties. The documented here N6 hexazine rings represent new highly energetic polynitrogens, which have a potential for future recovery and utilization.

cond-mat.mtrl-sci↗

arXiv2020

Novel rhenium carbides at 200 GPa

Laser heating of rhenium in a diamond anvil cell to 3000 K at about 200 GPa results in formation of two previously unknown rhenium carbides, hexagonal WC-type structured ReC and orthorhombic TiSi2-type structured ReC2. The Re-C slid solution formed at multimegabar pressure has the carbon content of ca. 20 at%. Unexpectedly long C-C distances (ca. 1.76-1.85 A) in 'graphene-like' carbon nets in the structure of ReC2 cannot be explained by a simple covalent bonding between carbon atoms and suggest that at very high pressures the mechanism of interaction between carbon atoms in inorganic compounds may be different from that considered so far.

cond-mat.mtrl-sci↗

arXiv2018

Evidence for a pressure-induced spin transition in olivine-type LiFePO$_{4}$ triphylite

We present a combination of first-principles and experimental results regarding the structural and magnetic properties of olivine-type LiFePO$_4$ under pressure. Our investigations indicate that the starting $Pbnm$ phase of LiFePO$_4$ persists up to 70 GPa. Further compression leads to an isostructural transition in the pressure range of ~70-75 GPa, inconsistent with a former theoretical study. Considering our first-principles prediction for a high-spin to low-spin transition of Fe$^{2+}$ close to 72 GPa, we attribute the experimentally observed isostructural transition to a change on the spin state of Fe$^{2+}$ in LiFePO$_4$. Compared to relevant Fe-bearing minerals, LiFePO$_4$ exhibits the largest onset pressure for a pressure-induced spin state transition.

cond-mat.mtrl-sci↗

arXiv2017

Structural stability and mechanism of compression of stoichiometric B13C2 up to 68 GPa

Boron carbide is a ceramic material with unique properties widely used in numerous, including armor, applications. Its mechanical properties, mechanism of compression, and limits of stability are of both scientific and practical value. Here, we report the behavior of the stoichiometric boron carbide B13C2 studied on single crystals up to 68 GPa. As revealed by synchrotron X-ray diffraction, B13C2 maintains its crystal structure and does not undergo phase transitions. Accurate measurements of the unit cell and B12 icosahedra volumes as a function of pressure led to conclusion that they reduce similarly upon compression that is typical for covalently bonded solids. A comparison of the compressional behavior of B13C2 with that of alpha-B, gamma-B, and B4C showed that it is determined by the types of bonding involved in the course of compression. Neither molecular-like nor inversed-molecular-like solid behavior upon compression was detected that closes a long-standing scientific dispute.

cond-mat.mtrl-sci↗

arXiv2017

First non-icosahedral boron allotrope synthesized at high pressure and high temperature

Theoretical predictions of pressure-induced phase transformations often become long-standing enigmas because of limitations of contemporary available experimental possibilities. Hitherto the existence of a non-icosahedral boron allotrope has been one of them. Here we report on the first non-icosahedral boron allotrope, which we denoted as ζ-B, with the orthorhombic α-Ga-type structure (space group Cmce) synthesized in a diamond anvil cell at extreme high-pressure high-temperature conditions (115 GPa and 2100 K). The structure of ζ-B was solved using single-crystal synchrotron X-ray diffraction and its compressional behavior was studied in the range of very high pressures (115 GPa to 135 GPa). Experimental validation of theoretical predictions reveals the degree of our up-to-date comprehension of condensed matter and promotes further development of the solid state physics and chemistry.

cond-mat.mtrl-sci↗

arXiv2014

Novel non-magnetic hard boride Co5B16 synthesized under high pressure

A first cobalt boride with the Co:B ratio below 1:1, Co5B16, was synthesized under high-pressure high-temperature conditions. It has a unique orthorhombic structure (space group Pmma, a = 19.1736(12), b = 2.9329(1), and c = 5.4886(2) Å, R1 (all data) = 0.037). The material is hard, paramagnetic, with a weak temperature dependence of magnetic susceptibility.

cond-mat.mtrl-sci↗

arXiv2013

Peierls distortion, magnetism, and high hardness of manganese tetraboride

We report crystal structure, electronic structure, and magnetism of manganese tetraboride, MnB4, synthesized under high-pressure high-temperature conditions. In contrast to superconducting FeB4 and metallic CrB4, which are both orthorhombic, MnB4 features a monoclinic crystal structure. Its lower symmetry originates from a Peierls distortion of the Mn chains. This distortion nearly opens the gap at the Fermi level, but despite the strong dimerization and the proximity of MnB4 to the insulating state, we find indications for a sizable paramagnetic effective moment of about 1.7 muB/f.u., ferromagnetic spin correlations and, even more surprisingly, a prominent electronic contribution to the specific heat. However, no magnetic order has been observed in standard thermodynamic measurements down to 2 K. Altogether, this renders MnB4 a structurally simple but microscopically enigmatic material; we argue that its properties may be influenced by electronic correlations.

cond-mat.mtrl-sci↗

arXiv2013

Discovery of a superhard iron tetraboride superconductor

Single crystals of novel orthorhombic (space group Pnnm) iron tetraboride FeB4 were synthesised at pressures above 8 GPa and high temperatures. Magnetic susceptibility measurements demonstrated bulk superconductivity below 2.9 K. The putative isotope effect on the superconducting critical temperature indicates that FeB4 is likely a phonon-mediated superconductor, which is unexpected in the light of previous knowledge on Fe-based superconductors. The discovered iron tetraboride is highly incompressible and has the nanoindentation hardness of 65(5) GPa, thus, it opens a new class of highly desirable materials combining advanced mechanical properties and superconductivity.

cond-mat.mtrl-sci↗