arXiv subjects

Vitali Prakapenka

Publications and source records attributed to Vitali Prakapenka.

At least 19 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↗

arXiv2026

High-pressure stabilization of Mg2IrH7: Structural proximity to high-Tc superconductivity

Mg$_2$IrH$_6$ is a metastable complex metal hydride with a predicted superconducting transition temperature as high as 170 K at ambient pressure. Following the synthesis of isomorphic, insulating Mg$_2$IrH$_5$ at low pressure, higher-pressure studies were conducted to investigate the phase behavior and compound formation in this system. X-ray diffraction and Raman spectroscopic measurements indicate that cubic Mg$_2$IrH$_7$ is stabilized above ca. 40 GPa and coexists with a related hexagonal hydride with likely composition near Mg$_2$IrH$_5$. Electrical transport measurements show that the cubic Mg$_2$IrH$_7$ is insulating, in agreement with ab initio predictions, and persists during room-temperature decompression until $\sim$20 GPa before reverting back to the cubic Mg$_2$IrH$_5$. The experimental results confirm ground-state structure predictions in the Mg-Ir-H system, and the formation of two nearly identical phases with surrounding compositions opens new opportunities to access superconducting Mg$_2$IrH$_6$ through non-equilibrium processing pathways.

cond-mat.supr-con↗

arXiv2025

Building Wet Planets through High-Pressure Magma-Hydrogen Reactions

Close-in transiting sub-Neptunes are abundant in our galaxy \cite{fulton2017california}. Planetary interior models based on their observed radius-mass relationship suggest that sub-Neptunes contain a discernible amount of either hydrogen (dry planets) or water (wet planets) blanketing a core composed of rocks and metal \cite{bean2021nature}. Water-rich sub-Neptunes have been believed to form farther from the star and then migrate inward to their present orbits \cite{bitsch2021Dry}. Here, we report experimental evidence of reactions between warm dense hydrogen fluid and silicate melt that releases silicon from the magma to form alloys and hydrides at high pressures. We found that oxygen liberated from the silicate melt reacts with hydrogen, producing a significant amount of water up to a few tens of weight percent, which is much greater than previously predicted based on low-pressure ideal gas extrapolation \cite{misener2023Atmospheresa,schlichting2022Chemical}. Consequently, these reactions can generate a spectrum of water contents in hydrogen-rich planets, with the potential to reach water-rich compositions for some sub-Neptunes, implying an evolutionary relationship between hydrogen-rich and water-rich planets. Therefore, detection of a large amount of water in exoplanet atmospheres may not be the optimal evidence for planet migration in the protoplanetary disk, calling into question the assumed link between composition and planet formation location.

astro-ph.EP↗

arXiv2025

Prediction and Synthesis of Mg$_4$Pt$_3$H$_6$: A Metallic Complex Transition Metal Hydride Stabilized at Ambient Pressure

The low-pressure stabilization of superconducting hydrides with high critical temperatures ($T_c$s) remains a significant challenge, and experimentally verified superconducting hydrides are generally constrained to a limited number of structural prototypes. Ternary transition-metal complex hydrides (hydrido complexes)-typically regarded as hydrogen storage materials-exhibit a large range of compounds stabilized at low pressure with recent predictions for high-$T_c$ superconductivity. Motivated by this class of materials, we investigated complex hydride formation in the Mg-Pt-H system, which has no known ternary hydride compounds. Guided by ab initio structural predictions, we successfully synthesized a novel complex transition-metal hydride, Mg$_4$Pt$_3$H$_6$, using laser-heated diamond anvil cells. The compound forms in a body-centered cubic structural prototype at moderate pressures between 8-25 GPa. Unlike the majority of known hydrido complexes, Mg$_4$Pt$_3$H$_6$ is metallic, with formal charge described as 4[Mg]$^{2+}$.3[PtH$_2$]$^{2-}$. X-ray diffraction (XRD) measurements obtained during decompression reveal that Mg$_4$Pt$_3$H$_6$ remains stable upon quenching to ambient conditions. Magnetic-field and temperature-dependent electrical transport measurements indicate ambient-pressure superconductivity with $T_c$ (50%) = 2.9 K, in reasonable agreement with theoretical calculations. These findings clarify the phase behavior in the Mg-Pt-H system and provide valuable insights for transition-metal complex hydrides as a new class of hydrogen-rich superconductors.

cond-mat.supr-con↗

arXiv2025

Observation of Body-Centered Cubic Iron above 200 Gigapascals

The crystallographic structure of iron under extreme conditions is a key benchmark for cutting-edge experimental and numerical methods. Moreover, it plays a crucial role in understanding planetary cores, as it significantly influences the interpretation of observational data and, consequently, insights into their internal structure and dynamics. However, even the structure of pure solid iron under the Earth's core conditions remains uncertain, with the commonly expected hexagonal close-packed structure energetically competitive with various cubic lattices. In this study, iron was compressed in a diamond anvil cell to above 200 GPa, and dynamically probed near the melting point using MHz frequency X-ray pulses from the European X-ray Free Electron Laser. The emergence of an additional diffraction line at high temperatures suggests the formation of an entropically stabilized bcc structure. Rapid heating and cooling cycles captured intermediate phases, offering new insights into iron's phase transformation paths. The appearance of the bcc phase near melting at extreme pressures challenges current understanding of the iron phase diagram under Earth's core conditions.

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

Origin of Pressure-Induced Structural Instability in CsPbX$_3$ Photovoltaic Perovskites

Under external stimuli, lead halide perovskites exhibit large atomic fluctuations, impacting optical and electron transport properties that affect device performance in operational settings. However, a thorough understanding of the atomic basis for the underlying structural instability is still absent. Focusing on the model material CsPbBr$_3$, the inherent lattice softness of halide perovskites is elucidated at the atomic level through in-situ single-crystal X-ray diffraction measurements under pressure complemented by atomic level simulations. We identify and explore the nature of the first-order phase transition to a distorted P21/c phase at 1.3 GPa, induced by the sudden Cs-Br bonds breaking. Unlike classical transition metal oxide perovskites, where the internal energy term dominates, we show explicitly that pressure primarily influences the Gibbs free energy for halide perovskites through the pressure-volume term. As such, strategically mitigating bond strains from volume shrinkage is the key to suppressing the first-order behavior for maintaining the coordinates of PbX$_6$ polyhedral upon external perturbation. Our thermodynamic calculation reveals the demand for high entropy in the -T*del-S term, which can be achieved by exploring a broader spectrum of doped A site and B sites in ABX$_3$ systems, enabling continuous structural changes that facilitate recovery from mechanical damage in practical applications.

cond-mat.mtrl-sci↗

arXiv2024

Crystalline forsterite to 160 GPa: the striking metastability of one of Universe's most abundant minerals

Among Universe's most consequential events are large impacts generating rapidly-evolving extreme pressures and temperatures. Crystalline and amorphous forms of (Mg, Fe)2SiO4 are abundant and widespread, within planets and in space. The behavior of these minerals is expected to deviate form thermodynamic equilibrium in many of the processes that are critical to the formation and evolution of planets, particularly shock events. To further the understanding of the behavior of the silicate under extreme conditions, we statically compressed a crystal of forsterite up to 160.5 GPa, far beyond the compound's stability field, and probed its long-range ordering with synchrotron microdiffraction. We found that forsterite retains long-range ordering up to the highest pressure reached. Forsterite III, emerging at about 58 GPa, persists in compression to 160.5 GPa and in decompression down to about 13 GPa, for a rare combined occurrence of a metastable phase of nearly 150 GPa. These observations dispute earlier reports of pressure-induced amorphization and are a unique testimony of the resilience of the crystalline state in quasi hydrostatic compression. We confirm that highly disordered forsterite can be obtained from the decompression of forsterite III as suggested from the substantial loss of long-range ordering observed at 7 GPa after further decompression. Such kinetic pathway may explain how synthetic olivine glass have been obtained in shock experiments and could be a mechanism of generation of amorphous forsterite in cosmic dust. The 120 GPa Hugoniot discontinuity finds no correspondence in our data, marking a departure from the parallelism between static "cold compression" and dynamic compression.

astro-ph.EP↗

arXiv2024

Low-symmetry polymorph of GaP upends bonding paradigms of metallic high-pressure III-V compounds

The pressure-induced polymorphism of binary octect compounds has long been considered a settled problem although the possible atomic disordering of some phases remains a puzzling observation. Taking GaP as a case study, we conclude, through x-ray microdiffraction and first-principles calculations, that its high-pressure phase II (previously reported as being disordered) adopts in fact an ordered base-centered monoclinic structure previously unknown in this class of compounds. The formation of layered patterns with variable degrees of interlayer dimerization, as observed in GaP, marks a paradigm shift of our understanding of ordering in octect high-pressure phases which calls for a more extensive re-examination. A rich polymorphism with fine tuning of chemical and physical properties can be envisioned.

cond-mat.mtrl-sci↗

arXiv2023

Superconductivity above 70 K observed in lutetium polyhydrides

The binary polyhydrides of heavy rare earth lutetium that shares a similar valence electron configuration to lanthanum have been experimentally discovered to be superconductive. The lutetium polyhydrides were successfully synthesized at high pressure and high temperature conditions using a diamond anvil cell in combinations with the in-situ high pressure laser heating technique. The resistance measurements as a function of temperature were performed at the same pressure of synthesis in order to study the transitions of superconductivity (SC). The superconducting transition with a maximum onset temperature (Tc) 71 K was observed at pressure of 218 GPa in the experiments. The Tc decreased to 65 K when pressure was at 181 GPa. From the evolution of SC at applied magnetic fields, the upper critical field at zero temperature μ0Hc2(0) was obtained to be ~36 Tesla. The in-situ high pressure X-ray diffraction experiments imply that the high Tc SC should arise from the Lu4H23 phase with Pm-3n symmetry that forms a new type of hydrogen cage framework different from those reported for previous light rare earth polyhydride superconductors.

cond-mat.supr-con↗

arXiv2023

Observation of Conventional Near Room Temperature Superconductivity in Carbonaceous Sulfur Hydride

The phenomenon of high temperature superconductivity, approaching room temperature, has been realized in a number of hydrogen-dominant alloy systems under high pressure conditions1-12. A significant discovery in reaching room temperature superconductivity is the photo-induced reaction of sulfur, hydrogen, and carbon that initially forms of van der Waals solids at sub-megabar pressures. Carbonaceous sulfur hydride has been demonstrated to be tunable with respect to carbon content, leading to different superconducting final states with different structural symmetries. A modulated AC susceptibility technique adapted for a diamond anvil cell confirms a Tc of 260 kelvin at 133 GPa in carbonaceous sulfur hydride. Furthermore, direct synchrotron infrared reflectivity measurements on the same sample under the same conditions reveal a superconducting gap of ~85 meV at 100 K in close agreement to the expected value from Bardeen-Cooper-Schrieffer (BCS) theory13-18. Additionally, x-ray diffraction in tandem with AC magnetic susceptibility measurements above and below the superconducting transition temperature, and as a function of pressure at 107-133 GPa, reveal the Pnma structure of the material is responsible for the close to room-temperature superconductivity at these pressures.

cond-mat.supr-con↗

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↗

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↗

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

Helium-hydrogen immiscibility at high pressures

Hydrogen and helium are the most abundant elements in the universe and they constitute the interiors of gas giant planets. Thus, their equations of states, phase, chemical state, and chemical reactivity at extreme conditions are of great interest. Applying Raman spectroscopy, visual observation, and synchrotron X-ray diffraction in diamond anvil cells (DAC), we performed experiments on H2-He 1:1 and D2-He 1:10 compressed gas mixture up to 100 GPa at 300 K. By comparing with the available data on pure bulk materials, we find no sign of miscibility, chemical reactivity, and new compound formation.

cond-mat.mtrl-sci↗