arXiv subjects

Leonid Dubrovinsky

Publications and source records attributed to Leonid Dubrovinsky.

At least 19 recordsLinked to original sources

arXiv2025

Halogen Chains with One-Dimensional Semi-Metallic Electronic Structure and Peierls Physics in Polymorphs of Na4X5 (X = I, Br, Cl) Compounds

Since the pioneering works of Peierls, one-dimensional materials have attracted great attention. Still, the synthesis of truly monoatomic chains remains elusive. In this study, we explore a novel path of experimental synthesis of monoatomic one-dimensional chains by their chemical stabilization in ionic compounds. We demonstrate that in synthesized at high pressure sodium halides Na4X5 (X = I, Br, Cl) with hP18 Ga4Ti5-type structures, transfer of valence electrons from cations to anions leads to the formation of halogen chains connected with other atoms only by ionic interaction and having one-dimensional electronic structure. The Peierls physics in the systems is confirmed by theoretical calculations, newly synthesized incommensurately modulated i-hP18-Na4X5 (X = I, Br, Cl) compounds, as well as by the discovered hP36 phases of Na4Cl5 and Na4Br5.

cond-mat.mtrl-sci↗

arXiv2023

Polytypism of Incommensurately Modulated Structures of Crystalline Bromine upon Molecular Dissociation under High Pressure

Polytypism of incommensurately modulated structures was hitherto unobserved. Here, we found the phenomenon in simple halogen systems of bromine and iodine upon molecular dissociation in the solids under pressure. Single-crystal synchrotron X-ray diffraction in laser heated diamond anvil cells pressurised up to 112 GPa revealed a number of allotropes of bromine and iodine including polytypes of Br-IIIγ (Fmmm(00γ)s00) with γ varying within 0.18 to 0.3.

cond-mat.mtrl-sci↗

arXiv2022

Synthesis of Ultra-Incompressible Carbon Nitrides Featuring Three-Dimensional Frameworks of CN4 Tetrahedra Recoverable at Ambient Conditions

More than thirty years ago, carbon nitrides featuring 3D frameworks of tetrahedral CN4 units were identified as one of the great aspirations of materials science, expected to have a hardness greater than or comparable to diamond. Since then, no unambiguous experimental evidence of their existence has been delivered. Here, we report the high-pressure high-temperature synthesis of the long-sought-after covalent carbon nitrides, tI14-C3N4, hP126-C3N4, and tI24-CN2, in laser-heated diamond anvil cells. Their structures were solved and refined using synchrotron single-crystal X-ray diffraction. In these solids, carbon atoms, all sp3-hybridized, and nitrogen atoms are fully saturated, forming four and three covalent bonds, respectively, leading to three-dimensional arrangements of corner-sharing CN4 tetrahedra. These carbon nitrides are ultra-incompressible, with hP126-C3N4 and tI24-CN2 even rivalling diamond's incompressibility, and superhard. These novel compounds are recoverable to ambient conditions in crystalline form and chemically stable in air. Being wide-band gap semiconductors with intriguing features in their electronic structure, they are expected to exhibit multiple exceptional functionalities besides their mechanical properties, opening new perspectives for materials science.

cond-mat.mtrl-sci↗

arXiv2022

High-Pressure Synthesis of Seven Lanthanum Hydrides with a Significant Variability of Hydrogen Content

The lanthanum-hydrogen system has attracted significant attention following the report of superconductivity in LaH10 at near-ambient temperatures and high pressures. Here, we present the results of our single-crystal X-ray diffraction studies on this system, supported by density functional theory calculations, which reveal an unexpected chemical and structural diversity of lanthanum hydrides synthesized in the range of 50 to 180 GPa. Seven lanthanum hydrides were produced, LaH3, LaH~4, LaH4+δ, La4H23, LaH6+δ, LaH9+δ, and LaH10+δ, and the atomic coordinates of lanthanum in their structures determined. The regularities in rare-earth element hydrides unveiled here provide clues to guide the search for other synthesizable hydrides and candidate high-temperature superconductors. The hydrogen content variability in lanthanum hydrides and the samples' phase heterogeneity underline the challenges related to assessing potentially superconducting phase(s) and the nature of electronic transitions in high-pressure hydrides.

cond-mat.mtrl-sci↗

arXiv2022

Revealing Phosphorus Nitrides up to the Megabar Regime: Synthesis of alpha-P3N5, delta-P3N5 and PN2

Non-metal nitrides are an exciting field of chemistry, featuring a significant number of compounds that can possess outstanding material properties. This characteristic relies on maximizing the number of strong covalent bonds, with crosslinked XN6 octahedra frameworks being particularly intriguing. In this study, the phosphorus-nitrogen system was studied up to 137 GPa in laser-heated diamond anvil cells and three previously unobserved phases were synthesized and characterized by single-crystal X-ray diffraction, Raman spectroscopy measurements and density functional theory calculations. Delta-P3N5 and PN2 were found to form at 72 and 134 GPa, respectively, and both feature dense 3D networks of the so far elusive PN6 units. The two are ultra-incompressible, having a bulk modulus of K0 = 322 GPa for delta-P3N5 and K0 = 339 GPa for PN2. Upon decompression below 7 GPa, delta-P3N5 undergoes a transformation into a novel alpha'-P3N5 solid, stable at ambient conditions, that has a unique structure type based on PN4 tetrahedra. The formation of alpha'-P3N5 underlines that a phase space otherwise inaccessible can be explored through high-pressure formed phases.

cond-mat.mtrl-sci↗

arXiv2022

Symmetry protected 1D chains in mixed-valence iron oxides

During the last decade of high-pressure research a whole new series of iron oxides was discovered, like Fe$_4$O$_5$, Fe$_5$O$_6$, Fe$_7$O$_9$ etc., featuring closely related structures with arrays of one-dimensional (1D) chains of trigonal prisms embedded between slabs of octahedra. Here, we develop a unified approach to the series based on a specific crystallographic generation mechanism which predicts the structures of these oxides and naturally classifies them in terms of the slab cycle. When including magnetic interactions, we show that the 1D chains have a symmetry protection against magnetic perturbations from the iron ions in the slabs, and that the slab size determines the type of magnetic order, which is either ferromagnetic or antiferromagnetic. Dynamical mean-field theory calculations reveal the orbitally selective Mott state of the Fe ions and tendency of conductivity to low-dimensional behavior with particular enhancement along the 1D chains. Across the series, the decoupling of the chains increases, and so with the inherent charge ordering of the slabs, these structures have the potential to allow experimental realization of the model system of coupled 1D wires. We point out the possibility to stabilize these compounds in the thin-film form that, together with a wide range of possible ionic substitutions and fact that these compounds are recoverable at ambient pressure, makes them a very promising platform to engineer physical systems with interesting magnetotransport phenomena, as corroborated by the recent discovery of quantum Hall effect in ZrTe$_5$.

cond-mat.str-el↗

arXiv2022

Universal Hydrogen Bond Symmetrisation Dynamics Under Extreme Conditions

The experimental study of hydrogen bonds and their symmetrisation under extreme conditions is predominantly driven by diffraction methods, despite challenges of localising or probing the hydrogen subsystems directly. Until recently, H-bond symmetrisation has been addressed in terms of either nuclear quantum effects, spin crossovers or direct structural transitions; often leading to contradictory interpretations when combined. Here, we present high-resolution \textit{in-situ} $^1$H-NMR experiments in diamond anvil cells investigating a wide range of hydrogen bonded systems at pressure ranges of up to 90 GPa covering their respective H-bond symmetrisation. We found pronounced minima in the pressure dependence of the NMR resonance line-widths associated with a maximum in hydrogen mobility, precursor to a localisation of hydrogen atoms. These minima, independent of of the chemical environment of the linear O -- H-O unit, can be found in a narrow range of oxygen-oxygen distances between 2.44 and 2.45 Å, leading to an average critical oxygen-oxygen distance of $\bar{r}_{\rm OO}^{crit}=2.443(1)$ Å.

physics.chem-ph↗

arXiv2021

Aromatic Hexazine [N6]4- Anion Revealed in the Complex Structure of the High-Pressure Potassium Nitride K9N56

Recent high-pressure synthesis of pentazolates and subsequent stabilization of the aromatic [N5]- anion at atmospheric pressure had an immense impact on nitrogen chemistry. Here, we present the first synthesis of an aromatic hexazine [N6]4- anion realized in high-pressure potassium nitride K9N56 at 46 and 61 GPa. The extremely complex structure of K9N56 was solved based on synchrotron single-crystal X-ray diffraction and corroborated by density functional theory calculations. This result resolves a long-standing question of the aromatic hexazine stability and the possibility of its synthesis.

cond-mat.mtrl-sci↗

arXiv2021

Thermodynamic and electronic properties of ReN$_2$ polymorphs at high-pressure

High pressure synthesis of rhenium nitride pernitride ReN$_2$ with crystal structure unusual for transition metal dinitrides and high values of hardness and bulk modulus attracted significant attention to this system. We investigate the thermodynamic and electronic properties of the P2$_1$/c phase of ReN$_2$ and compare them with two other polytypes, C2/m and P4/mbm phases suggested in the literature. Our calculations of the formation enthalpy at zero temperature show that the former phase is the most stable of the three up to pressure p=170 GPa, followed by the stabilization of the P4/mbm phase at higher pressure. The theoretical prediction is confirmed by diamond anvil cell synthesis of the P4/mbm ReN$_2$ at $\approx$175 GPa. Considering the effects of finite temperature in the quasi-harmonic approximation at p=100 GPa we demonstrate that the P2$_1$/c phase has the lowest free energy of formation at least up to 1000 K. Our analysis of the pressure dependence of the electronic structure of the rhenium nitride pernitride shows a presence of two electronic topological transitions around 18 GPa, when the Fermi surface changes its topology due to an appearance of a new electron pocket at the high-symmetry Y$_2$ point of the Brillouin zone while the disruption of a neck takes place slightly off from the $Γ$-A line.

cond-mat.mtrl-sci↗

arXiv2021

Fe0.79Si0.07B0.14 metallic glass gaskets for high-pressure research beyond 1 Mbar

A gasket is an important constituent of a diamond anvil cell (DAC) assembly, responsible for the sample chamber stability at extreme conditions for x-ray diffraction studies. In this work, we studied the performance of gaskets made of metallic glass Fe0.79Si0.07B0.14 in a number of high-pressure x-ray diffraction (XRD) experiments in DACs equipped with conventional and toroidal-shape diamond anvils. The experiments were conducted in either uniaxial or radial geometry with x-ray beams of micron to sub-micron size. We report that the Fe0.79Si0.07B0.14 metallic glass gaskets offered stable sample environment under compression exceeding one megabar in all XRD experiments described here, even in those involving inter- or small-molecule gases (e.g. Ne, H2) used as pressure transmitting media or in those with laser heating in a DAC. These emphasize the material's importance for a great number of delicate experiments conducted under extreme conditions. Our results indicate that the application of Fe0.79Si0.07B0.14 metallic glass gaskets in XRD experiments of both uniaxial and radial geometries substantially improves the signal-to-noise ratio in comparison to that with conventional gaskets made of Re, W, steel or other crystalline metals.

cond-mat.mtrl-sci↗

arXiv2021

In-situ High Pressure Nuclear Magnetic Resonance Crystallography

Our recent developments in in-situ nuclear magnetic resonance (NMR) spectroscopy under extreme conditions led to the observation of a wide variety of physical phenomena not accessible with standard high pressure experimental probes. However, inherent di- or quadrupolar line broadening in diamond anvil cell (DAC) based NMR experiments often limit detailed investigations of local atomic structures, especially if different phases or local environments are coexisting. Here, we present our progress in the development of high resolution NMR experiments in DACs using one and two-dimensional homonuclear decoupling experiments at pressures up to the Mbar regime. Using this technique, spectral resolutions in the order of 1 ppm and below have been achieved, enabling high pressure structural analysis. Several examples will demonstrate the wide applicability of this method for extreme conditions research.

cond-mat.mtrl-sci↗

arXiv2021

High-Pressure Na3(N2)4, Ca3(N2)4, Sr3(N2)4, and Ba(N2)3 Featuring Nitrogen Dimers with Non-Integer Charges and Anion-Driven Metallicity

Charged nitrogen dimers are ubiquitous in high-pressure binary metal-nitrogen systems. They are known to possess integer formal charges x varying from one through four. Here, we present the investigation of the binary alkali- and alkaline earth metal-nitrogen systems, Na-N, Ca-N, Sr-N, Ba-N to 70 GPa. We report on compounds-Na3(N2)4, Ca3(N2)4, Sr3(N2)4, and Ba(N2)3-featuring charged nitrogen dimers with paradigm-breaking non-integer charges, x = 0.67, 0.75 and 1.5. The metallic nature of all four compounds is deduced from ab initio calculations. The conduction electrons occupy the pi* antibonding orbitals of the charged nitrogen dimers that results in anion-driven metallicity. Delocalization of these electrons over the pi* antibonding states enables the non-integer electron count of the dinitrogen species. Anion-driven metallicity is expected to be found among a variety of compounds with homoatomic anions (e.g., polynitrides, carbides, and oxides), with the conduction electrons playing a decisive role in their properties.

cond-mat.mtrl-sci↗

arXiv2021

High-pressure synthesis of Dirac materials: layered van der Waals bonded BeN$_4$ polymorph

High pressure chemistry is known to inspire the creation of unexpected new classes of compounds with exceptional properties. Here we report the synthesis at ~90 GPa of novel beryllium polynitrides, monoclinic and triclinic BeN4. The triclinic phase, upon decompression to ambient conditions, transforms into a compound with atomic-thick BeN4 layers interconnected via weak van der Waals bonds consisting of polyacetylene-like nitrogen chains with conjugated π-systems and Be atoms in square-planar coordination. Theoretical calculations for a single BeN4 layer show that its electronic lattice is described by a slightly distorted honeycomb structure reminiscent of the graphene lattice and the presence of Dirac points in the electronic band structure at the Fermi level. The BeN4 layer, i.e. beryllonitrene, represents a qualitatively new class of 2D materials that can be built of a metal atom and polymeric nitrogen chains and host anisotropic Dirac fermions.

cond-mat.mtrl-sci↗

arXiv2020

Nuclear Spin Crossover in Dense Molecular Hydrogen

The laws of quantum mechanics are often tested against the behaviour of the lightest element in the periodic table, hydrogen. One of the most striking properties of molecular hydrogen is the coupling between molecular rotational properties and nuclear spin orientations, giving rise to the spin isomers ortho- and para-hydrogen. At high pressure, as intermolecular interactions increase significantly, the free rotation of H2 molecules is increasingly hindered, and consequently a modification of the coupling between molecular rotational properties and the nuclear spin system can be anticipated. To date, high-pressure experimental methods have not been able to observe nuclear spin states at pressures approaching 100 GPa and consequently the effect of high pressure on the nuclear spin statistics could not be directly measured. Here, we present in-situ high-pressure nuclear magnetic resonance data on molecular hydrogen in its hexagonal phase I up to 123 GPa at room temperature. While our measurements confirm the presence of I=1 ortho-hydrogen at low pressures, above 70 GPa, where inter- and intramolecular distances become comparable, we observe a crossover in the nuclear spin statistics from a spin-1 quadrupolar to a spin-1/2 dipolar system, evidencing the loss of spin isomer distinction. These observations represent a unique case of a nuclear spin crossover phenomenon in quantum solids.

cond-mat.mtrl-sci↗

arXiv2020

Metallic Hydrogen Sublattice and Proton Mobility in Copper Hydride at High Pressure

Atomic and electronic structures of Cu2H and CuH have been investigated by high pressure NMR spectroscopy, X-ray diffraction and ab-initio calculations. Metallic Cu2H was synthesized at a pressure of 40 GPa, and semi-metallic CuH at 90 GPa, found stable up to 160 GPa. Experiments and computations suggest the formation of a metallic 1H-sublattice as well as a high 1H mobility of ~10-7 cm2/s in Cu2H. Comparison of Cu2H and FeH data suggests that deviations from Fermi gas behavior, formation of conductive hydrogen networks, and high 1H mobility could be common features of metal hydrides.

cond-mat.mtrl-sci↗

arXiv2020

High-pressure polymeric nitrogen allotrope with the black phosphorus structure

Studies of polynitrogen phases are of great interest for fundamental science and for the design of novel high energy density materials. Laser heating of pure nitrogen at 140 GPa in a diamond anvil cell led to the synthesis of a polymeric nitrogen allotrope with the black phosphorus structure, bp-N. The structure was identified in situ using synchrotron single-crystal X-ray diffraction and further studied by Raman spectroscopy and density functional theory calculations. The discovery of bp-N brings nitrogen in line with heavier pnictogen elements, resolves incongruities regarding polymeric nitrogen phases and provides insights into polynitrogen arrangements at extreme densities.

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↗

arXiv2019

Revisiting spin state crossover in (MgFe)O by means of high resolution X-ray diffraction from a single crystal

(MgFe)O is a solid solution with ferrous iron undergoing the high to low spin state (HS-LS) crossover under high pressure. The exact state of the material in the region of the crossover is still a mystery, as domains with different spin states may coexist over a wide pressure range without changing the crystal structure neither from the symmetry nor from the atomic positions point of view. At the conditions of the crossover, (MgFe)O is a special type of microscopic disorder system. We explore the influences of (a) stress-strain relations in a diamond anvil cell, (b) time relaxation processes, and (c) the crossover itself on the characteristic features of a single crystal (111) Bragg spot before, during and after the transformation. Using high resolution X-ray diffraction as a novel method for studies of unconventional processes at the conditions of suppressed diffusion, we detect and discuss subtle changes of the (111) Bragg spot projections which we measure and analyze as a function of pressure. We report changes of the spot shape which can be correlated with the HS-LS relative abundance. In addition, we report the formation of structural defects as an intrinsic material response. These static defects are accumulated during transformation of the material from HS to LS.

cond-mat.mtrl-sci↗