arXiv subjects

Florian Trybel

Publications and source records attributed to Florian Trybel.

16 recordsLinked to original sources

arXiv2026

Electronic structure and oxidation states in high-pressure synthesized isostructural CeCN$_5$ and TbCN$_5$

Understanding the behavior of 4$f$ electrons in materials containing rare earth elements is one of the fundamental questions within condensed matter physics. In this work the electronic properties of isostructural CeCN$_5$ and TbCN$_5$, both recently synthesized at extreme pressure, are investigated using Density Functional Theory (DFT) calculations. We include the on-site Coulomb repulsion between localized 4$f$ states within the static DFT+U framework; the DFT+U results are cross-checked with DFT+dynamical mean-field theory (DMFT) calculations within the quasi-atomic (Hubbard-I) approximation. Despite CeCN$_5$ and TbCN$_5$ being isostructural compounds Ce and Tb show different oxidation states, 4+ and 3+ respectively. This leads to distinctly different electronic properties: the former compound is an insulator, while the latter is a metal. An extra electron which is donated by Ce to the polymeric C-N network is distributed across the network. This leads to a modification of the bond length in CeCN$_5$ compared to TbCN$_5$. Still, the polymeric C-N networks can accommodate the different oxidation states in isostructural lanthanide-carbon-nitrogen (LnCN) compounds. Our results underline that LnCN compounds under high pressure offer a unique platform for probing the interplay between 4$f$-electron behavior and structural complexity.

cond-mat.mtrl-sci↗

arXiv2026

Symmetry-restricted energy landscapes as a benchmark for machine learned interatomic potentials

Machine learned interatomic potentials (MLIPs) are becoming a standard method for DFT-level accurate molecular dynamics simulation and large-scale studies of crystal energetics. Increasingly popular are universal pre-trained potentials, also called foundation models, based one, e.g. the MACE, CHGNet, M3GNet, ORB, and SevenNet architectures. While there are many benchmarks of these models using validation errors and materials discovery tasks, their fidelity in reproducing the detailed features of potential energy surfaces (PES) is not understood to the same degree. We evaluate the accuracy of these potentials by systematically probing their predicted energy landscapes. Two-dimensional slices of the potential energy surface are constructed where the atomic positions are varied along selected Wyckoff degrees of freedom within a fixed crystal symmetry. This approach enables a direct, visual comparison of the interatomic potentials and DFT-calculated surfaces which reveals potential artifacts e.g., arising from unique local environments. Our analysis highlights the strengths and limitations of different potentials in capturing local minima, saddle points, and overall PES topology, offering insights into the physical accuracy of current pre-trained IAPs and providing benchmarks for future model development.

cond-mat.mtrl-sci↗

arXiv2026

Screening 39 billion protostructures for materials discovery

Large-scale computational surveys are increasingly used to map the landscape of stable crystalline materials. We report a high-throughput energy screening of inorganic crystals that enumerates binary and ternary compositions up to a specified unit-cell complexity, yielding 39 billion protostructures. Candidates predicted to lie on or near the convex hull are retained, and their degrees of freedom are explored via Latin hypercube sampling followed by relaxation with machine-learned interatomic potentials. The resulting dataset contains 81 million locally relaxed crystal structures spanning 4495 ternary phase diagrams constructed from elements ranging from lithium to bromine and contains 88,498 crystal prototypes not present in existing crystal-structure databases. The methods are validated both for three well-explored materials systems, Zr-Zn-N, Ti-Zn-N, and Hf-Zn-N, and by comparing with known data for structures resulting from the larger screening. The work provides a systematic map of low-energy compositional-structural space and a large, structured pool of candidates for downstream property evaluation and materials design.

cond-mat.mtrl-sci↗

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↗

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

Direct Hydrogen Quantification in High-pressure Metal Hydrides

High-pressure metal-hydride (MH) research evolved into a thriving field within condensed matter physics following the realisation of metallic compounds showing phonon mediated near room-temperature superconductivity. However, severe limitations in determining the chemical formula of the reaction products, especially with regards to their hydrogen content, impedes a deep understanding of the synthesized phases and can lead to significantly erroneous conclusions. Here, we present a way to directly access the hydrogen content of MH solids synthesised at high pressures in (laser-heated) diamond anvil cells using nuclear magnetic resonance (NMR) spectroscopy. We show that this method can be used to investigate MH compounds with a wide range of hydrogen content, from MHx with x=0.15 (CuH0.15) to x < 6.4 (H 6+-0.4 S 5).

cond-mat.mtrl-sci↗

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

Absence of proton tunneling during the hydrogen bond symmetrization in $δ$-AlOOH

$δ$-AlOOH is of significant crystallochemical interest due to a subtle structural transition near 10 GPa from a $P2_1nm$ to a $Pnnm$ structure, the nature and origin of hydrogen disorder, the symmetrization of the O-H$\cdots$O hydrogen bond and their interplay. We perform a series of density functional theory based simulations in combination with high-pressure nuclear magnetic resonance experiments on $δ$-AlOOH up to 40 GPa with the goal to better characterize the hydrogen potential and therefore the nature of hydrogen disorder. Simulations predict a phase transition in agreement with our nuclear magnetic resonance experiments at $10-11$ GPa and hydrogen bond symmetrization at $14.7$ GPa. Calculated hydrogen potentials do not show any double-well character and there is no evidence for proton tunneling in our nuclear magnetic resonance data.

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

Proton dynamics in high-pressure ice-VII from density functional theory

Using a density-functional-theory-based approach, we explore the symmetrization and proton dynamics in ice-VII, for which recent high-pressure NMR experiments indicate significant proton dynamics in the pressure-range of $20-95$ GPa. We directly sample the potential seen by the proton and find a continuous transition from double- to single-well character over the pressure range of 2 to 130 GPa accompanied by proton dynamics in agreement with the NMR experiments.

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↗

arXiv2019

Pressure induced Hydrogen-Hydrogen interaction in metallic FeH revealed by NMR

Knowledge of the behavior of hydrogen in metal hydrides is the key for understanding their electronic properties. So far, no experimental methods exist to access these properties beyond 100 GPa, where high-Tc superconductivity emerges. Here, we present an 1H-NMR study of cubic FeH up to 200GPa. We observe a distinct deviation from the ideal metallic behavior between 64 and 110 GPa that suggests pressure-induced H-H interactions. Accompanying ab-initio calculations support this interpretation, as they reveal the formation of an intercalating sublattice of electron density, which enhances the hydrogen contribution to the electronic density of states at the Fermi level. This study shows that pressure induced H-H interactions can occur in metal hydrides at much lower compression and larger H-H distances than previously thought and stimulates an alternative pathway in the search for novel high-temperature superconductors.

cond-mat.mtrl-sci↗