arXiv subjects

Christoph Freysoldt

Publications and source records attributed to Christoph Freysoldt.

At least 19 recordsLinked to original sources

arXiv2026

Electrostatic Phenomenology Benchmarks for Machine-Learned Interatomic Potentials in Electrochemistry: Beyond the Energy-Force Metric

Accurate treatment of long-range interactions in machine learning interatomic potentials (MLIPs) is essential for electrochemical simulations. However, aggregate energy and force errors alone are insufficient to establish an MLIP's physical accuracy since they do not detect qualitative inconsistencies in the model such as the prediction of image-charge attraction, dielectric screening, or charge transfer. We introduce a benchmark suite EPhEct (Electrostatic Phenomena for Electrochemistry) of focused test cases designed to evaluate MLIPs on electrochemically relevant physical phenomena. The tests probe for image-charge attraction at a metal electrode, the splitting between longitudinal and transverse optical phonons as a probe of ionic and electronic screening, the dipole moment of interfacial water, and Fermi-level pinning during ion discharge. These tests establish a qualitative diagnostic routine complementary to aggregate energy-force metrics.

cond-mat.mtrl-sci↗

arXiv2026

Conditional spinodal decomposition in Li-Mg anodes for lithium metal batteries

The development of batteries with high energy density, short charging times and use of sustainable materials is critical for decarbonization. Magnesium (Mg)-based anodes for lithium (Li) metal batteries promote homogeneous Li plating, thereby avoiding the formation of Li dendrites that cause short circuits and battery failure. However, microstructural modifications induced by Li-alloying and their influence on battery operation remain elusive. Here, we unveil the previously unknown formation of an ordered B2 phase, which creates a conditional spinodal decomposition with the \b{eta}-body-centered cubic phase. Chemical fluctuations characteristic of spinodal decomposition give rise to uniformly dispersed Li-rich \b{eta}-BCC and Li-poor B2 continuous interconnected phases, with the former providing a fast diffusion pathway for Li diffusion towards the anode, hence decreasing the propensity for dendrite formation at elevated current density. This is achieved using Earth-abundant and inexpensive Mg.

cond-mat.mtrl-sci↗

arXiv2026

Spatial resolution(s) in atom probe tomography

Atom probe tomography (APT) is often quoted to provide "atomic-scale" analysis of materials in three dimensions. Despite efforts to quantify APT's spatial resolution, misunderstanding remain regarding its true spatial performance. If the depth resolution was once reported to be 20 pm, quoting this value outside of its specific context is misleading and should be avoided. The resolution achievable in pure metals, at one specific location within one reconstructed dataset, does not generally apply across materials or analysis conditions, or even throughout a single tomographic reconstruction. Here, we review various efforts at defining and measuring the spatial resolution in the study of single phase and single element materials - i.e. pure metals - in field-ion microscopy (FIM) and APT. We also report on the degradation of the resolution arising from ion optical devices used to improve the mass-resolution. We aim to offer some perspective as to how reported resolutions may be or may not be of any relevance to most of the materials characterisation efforts by APT, including cases of precipitates in a matrix that emphasise the need to consider an effective resolution. Finally, we discuss concepts to improve the spatial accuracy of the technique in a relatively distant future.

cond-mat.mtrl-sci↗

arXiv2025

Identifying grain boundary and intragranular pinning centres in Sm2(Co,Fe,Cu,Zr)17 permanent magnets to guide performance optimisation

Permanent magnets draw their properties from a complex interplay of chemical composition and phase, each with their associated intrinsic magnetic properties. Gaining an understanding of these interactions is the key to deciphering the origins of a permanent magnets' magnetic performance and facilitate the engineering of much improved-performing magnets. Here, we use advanced multiscale microscopy and microanalysis on a bulk Sm2(Co,Fe,Cu,Zr)17 pinning-type high-performance magnet with outstanding thermal and chemical stability. Comparison of the microstructure in regions of different composition, we demonstrate that the pinning of magnetic domains, imaged by nanoscale magnetic induction mapping, is controlled by the composition and atomic arrangement of copper. This is confirmed by micromagnetic simulations. Contrary to the belief that grain boundaries are "weak links" in magnetic materials, we demonstrate grain boundaries undergo magnetization reversal at relatively low fields (0.1-0.3 T), but this remains confined to these regions and does not significantly impact the magnet's coercivity. Our results showcase that it is the optimal microstructure within the grain itself that is crucial for achieving the desired magnetic properties.

cond-mat.mtrl-sci↗

arXiv2025

Domain Boundaries in a Metallic Distortive Polar Metal

Polar metals are an underexplored material class combining two properties that are typically incompatible, namely a polar crystal structure and reasonable electrical conductivity. These intriguing materials offer a unique combination of properties, potentially relevant to optoelectronics, catalysis, memory devices, among other applications. The distortive polar metal (DPM) subclass forms through a symmetry-lifting phase transformation into a non-centrosymmetric polar crystal structure. In the process, domains with uniform geometric polar directions form, oftentimes separated by domain boundaries with polarity discontinuities arranged in "charged" head-to-head (H-H) or tail-to-tail (T-T) morphologies. To date, only metallic oxide DPM microstructures have been studied. Here we reveal, in the intermetallic DPM Mn$_{5}$Al$_{8}$, different surface interactions and electron transfer reactivity at domain boundaries depending on their H-H or T-T character. Variable surface reactivity suggests localized changes in electronic work functions due to an increase (H-H) or decrease (T-T) in the electronic density of states. These findings suggest that metallic DPMs may offer functionalizable domain boundaries and deserve increased attention, given that they allow tunable chemistries and various thermomechanical processing or transformation protocols. Ultimately, this study proposes unconventional metal physics, propelling the discovery and design of advanced electronic materials and devices.

cond-mat.mtrl-sci↗

arXiv2025

User-defined Electrostatic Potentials in DFT Supercell Calculations: Implementation and Application to Electrified Interfaces

Introducing electric fields into density functional theory (DFT) calculations is essential for understanding electrochemical processes, interfacial phenomena, and the behavior of materials under applied bias. However, applying user-defined electrostatic potentials in DFT is nontrivial and often requires direct modification to the specific DFT code. In this work, we present an implementation for supercell DFT calculations under arbitrary electric fields and discuss the required corrections to the energies and forces. The implementation is realized through the recently released VASP-Python interface, enabling the application of user-defined fields directly within the standard VASP software and providing great flexibility and control. We demonstrate the application of this approach with diverse case studies, including molecular adsorption on electrified surfaces, field ion microscopy, electrochemical solid-water interfaces, and implicit solvent models.

cond-mat.mtrl-sci↗

arXiv2025

Computing Solvation Shell Dynamics and Energetics in Electron Transfer Reactions via Molecular Dynamics Simulations

Marcus theory is fundamental to describing electron transfer reactions and quantifying their rates, effectively representing the energy surface associated with an electron transfer from the reactant to the product ionic state via parabolas within a reaction coordinate diagram. Here, we present an intuitive and computationally efficient generalised reaction coordinate amenable to molecular dynamics simulations. By utilising the nuclear charge of the ion, we are able to quantify in a targeted approach changes in the ion's solvation shell, thereby efficiently obtaining the free energy profile associated with the electron transfer, the transition state geometry and the evolution of the water network.

physics.chem-ph↗

arXiv2025

Machine learning enhanced atom probe tomography analysis: a snapshot review

Atom probe tomography (APT) is a burgeoning characterization technique that provides compositional mapping of materials in three-dimensions at near-atomic scale. Since its significant expansion in the past 30 years, we estimate that one million APT datasets have been collected, each containing millions to billions of individual ions. Their analysis and the extraction of microstructural information has largely relied upon individual users whose varied level of expertise causes clear and documented bias. Current practices hinder efficient data processing, and make challenging standardization and the deployment of data analysis workflows that would be compliant with FAIR data principles. Over the past decade, building upon the long-standing expertise of the APT community in the development of advanced data processing or data mining techniques, there has been a surge of novel machine learning (ML) approaches aiming for user-independence, and that are efficient, reproducible, and robust from a statistics perspective. Here, we provide a snapshot review of this rapidly evolving field. We begin with a brief introduction to APT and the nature of the APT data. This is followed by an overview of relevant ML algorithms and a comprehensive review of their applications to APT. We also discuss how ML can enable discoveries beyond human capability, offering new insights into the mechanisms within materials. Finally, we provide guidance for future directions in this domain.

cond-mat.mtrl-sci↗

arXiv2025

Electric Field-Induced Formation of a 2D Adatom Gas on Cryogenic Li Surfaces

Intense electrostatic fields, such as those able to break bonds and cause field-ion emission, can fundamentally alter the behaviour of atoms at and on the surface. Using density functional theory (DFT) calculations on the Li (110) surface under high electrostatic fields, we identify a critical field strength at which surface atoms occupying a kink site become thermodynamically unstable against adatom formation. This mechanism leads to the formation of a highly concentrated two-dimensional (2D) adatom gas on the surface. Moreover, the applied field reverses the stability of preferred adsorption sites, enabling barrierless diffusion of lithium atoms even well below the threshold required for field evaporation. The here identified mechanisms offer a unified explanation for experimental observations in atom probe tomography and for understanding high electric field phenomena in systems such as battery interfaces and electrochemical environments.

cond-mat.mtrl-sci↗

arXiv2024

Roadmap on Data-Centric Materials Science

Science is and always has been based on data, but the terms "data-centric" and the "4th paradigm of" materials research indicate a radical change in how information is retrieved, handled and research is performed. It signifies a transformative shift towards managing vast data collections, digital repositories, and innovative data analytics methods. The integration of Artificial Intelligence (AI) and its subset Machine Learning (ML), has become pivotal in addressing all these challenges. This Roadmap on Data-Centric Materials Science explores fundamental concepts and methodologies, illustrating diverse applications in electronic-structure theory, soft matter theory, microstructure research, and experimental techniques like photoemission, atom probe tomography, and electron microscopy. While the roadmap delves into specific areas within the broad interdisciplinary field of materials science, the provided examples elucidate key concepts applicable to a wider range of topics. The discussed instances offer insights into addressing the multifaceted challenges encountered in contemporary materials research.

cond-mat.mtrl-sci↗

arXiv2024

Understanding atom probe's analytical performance for iron oxides using correlation histograms and ab initio calculations

Field evaporation from ionic or covalently bonded materials often leads to the emission of molecular ions. The metastability of these molecular ions, particularly under the influence of the intense electrostatic field (1010 Vm-1), makes them prone to dissociation with or without an exchange of energy amongst them. These processes can affect the analytical performance of atom probe tomography (APT). For instance, neutral species formed through dissociation may not be detected at all or with a time of flight no longer related to their mass, causing their loss from the analysis. Here, we evaluated the changes in the measured composition of FeO, Fe2O3 and Fe3O4 across a wide range of analysis conditions. Possible dissociation reactions are predicted by density-functional theory (DFT) calculations considering the spin states of the molecules. The energetically favoured reactions are traced on to the multi-hit ion correlation histograms, to confirm their existence within experiments, using an automated Python-based routine. The detected reactions are carefully analysed to reflect upon the influence of these neutrals from dissociation reactions on the performance of APT for analysing iron oxides.

cond-mat.mtrl-sci↗

arXiv2023

Accurate computation of chemical contrast in field ion microscopy

We present a computational approach to simulate local contrast observed in Field Ion Microscopy (FIM). It is based on density-functional theory utilizing the Tersoff-Hamann approach as done in Scanning Tunneling Microscopy (STM). A key requirement is the highly accurate computation of the surface states' wave-function tails. To refine the Kohn-Sham states from standard iterative global solvers we introduce and discuss the EXtrapolation of Tails via Reverse integration Algorithm (EXTRA). The decaying tails are obtained by reverse integration (from outside in) using a Numerov-like algorithm. The starting conditions are then iteratively adapted to match the values of plane-wave Kohn-Sham wave functions close to the surface. We demonstrate the performance of the proposed algorithm by analysing and showing the chemical contrast for Ta at Ni surface.

cond-mat.mtrl-sci↗

arXiv2023

A machine learning framework for quantifying chemical segregation and microstructural features in atom probe tomography data

Atom probe tomography (APT) is ideally suited to characterize and understand the interplay of chemical segregation and microstructure in modern multicomponent materials. Yet, the quantitative analysis typically relies on human expertise to define regions of interest. We introduce a computationally efficient, multistage machine learning strategy to identify chemically distinct domains in a semi automated way, and subsequently quantify their geometric and compositional characteristics. In our algorithmic pipeline, we first coarse grain the APT data into voxels, collect the composition statistics, and decompose it via clustering in composition space. The composition classification then enables the real space segmentation via a density based clustering algorithm, thus revealing the microstructure at voxel resolution. Our approach is demonstrated for a Sm(Co,Fe)ZrCu alloy. The alloy exhibits two precipitate phases with a plate-like, but intertwined morphology. The primary segmentation is further refined to disentangle these geometrically complex precipitates into individual plate like parts by an unsupervised approach based on principle component analysis, or a U-Net based semantic segmentation trained on the former. Following the chemical and geometric analysis, detailed chemical distribution and segregation effects relative to the predominant plate-like geometry can be readily mapped without resorting to the initial voxelization.

physics.chem-ph↗

arXiv2022

Ab initio vacancy formation energies and kinetics at metal surfaces under high electric field

Recording field ion microscope images under field evaporating conditions and subsequently reconstructing the underlying atomic configuration, called three-dimensional field ion microscopy (3D-FIM) is one of the few techniques capable of resolving crystalline defects at an atomic scale. However, the quantification of the observed vacancies and their origins are still a matter of debate. It was suggested that high electric fields (1-5 V/Å) used in 3D-FIM could introduce artefact vacancies. To investigate such effects, we used density functional theory (DFT) simulations. Stepped Ni and Pt surfaces with kinks were modelled in the repeated slab approach with a (971) surface orientation. An electrostatic field of up to 4 V/Å was introduced on one side of the slab using the generalized dipole correction. Contrary to what was proposed, we show that the formation of vacancies on the electrified metal surface is more difficult compared to a field-free case. We also find that the electric field can introduce kinetic barriers to a potential vacancy-annihilation mechanism. We rationalize these findings by comparing to insights from field evaporation models.

cond-mat.mtrl-sci↗

arXiv2022

Impact of metastable defect structures on carrier recombination in solar cells

The efficiency of a solar cell is often limited by electron-hole recombination mediated by defect states within the band gap of the photovoltaic (PV) semiconductor. The Shockley-Read-Hall (SRH) model considers a static trap that can successively capture electrons and holes. In reality however, true trap levels vary with both the defect charge state and local structure. Here we consider the role of metastable structural configurations in capturing electrons and holes, taking the tellurium interstitial in CdTe as an illustrative example. Consideration of the defect dynamics, and symmetry-breaking, changes the qualitative behaviour and activates new pathways for carrier capture. Our results reveal the potential importance of metastable defect structures in non-radiative recombination, in particular for semiconductors with anharmonic/ionic-covalent bonding, multinary compositions, low crystal symmetries or highly-mobile defects.

cond-mat.mtrl-sci↗

arXiv2021

Reflections on the spatial performance of atom probe tomography in the analysis of atomic neighbourhoods

Atom probe tomography is often introduced as providing "atomic-scale" mapping of the composition of materials and as such is often exploited to analyse atomic neighbourhoods within a material. Yet quantifying the actual spatial performance of the technique in a general case remains challenging, as they depend on the material system being investigated as well as on the specimen's geometry. Here, by using comparisons with field-ion microscopy experiments and field-ion imaging and field evaporation simulations, we provide the basis for a critical reflection on the spatial performance of atom probe tomography in the analysis of pure metals, low alloyed systems and concentrated solid solutions (i.e. akin to high-entropy alloys). The spatial resolution imposes strong limitations on the possible interpretation of measured atomic neighbourhoods, and directional neighbourhood analyses restricted to the depth are expected to be more robust. We hope this work gets the community to reflect on its practices, in the same way, it got us to reflect on our work.

cond-mat.mtrl-sci↗

arXiv2021

Charged Vacancy Defects in Black Phosphorus Monolayer Phosphorene

The two-dimensional semiconductor phosphorene has attracted extensive research interests for potential applications in optoelectronics, spintronics, catalysis, sensors, and energy conversion. To harness phosphorene's potential requires a better understanding of how intrinsic defects control carrier concentration, character, and mobility. Using density-functional theory and a charge correction scheme to account for the appropriate boundary conditions, we conduct a comprehensive study of the effect of structure on the formation energy, electronic structure, and charge transition level of the charged vacancy point defects in phosphorene. We predict that the neutral vacancy exhibits a 9-5 ring structure with a formation energy of 1.7 eV and transitions to a negatively charged state at a Fermi level 1.04 eV above the valence band maximum. The corresponding optical charge transitions display sizeable Frank-Condon shifts with a large Stokes shift of 0.3 eV. Phosphorene vacancies should become negatively charged in n-doped phosphorene, which would passivate the dopants and reduce the charge carrier concentration and mobility.

cond-mat.mtrl-sci↗

arXiv2021

Revealing atomic-scale vacancy-solute interaction in nickel

Imaging individual vacancies in solids and revealing their interactions with solute atoms remains one of the frontiers in microscopy and microanalysis. Here we study a creep-deformed binary Ni-2 at.% Ta alloy. Atom probe tomography reveals a random distribution of Ta. Field ion microscopy, with contrast interpretation supported by density-functional theory and time-of-flight mass spectrometry, evidences a positive correlation of tantalum with vacancies. Our results support solute-vacancy binding, which explains improvement in creep resistance of Ta-containing Ni-based superalloys and helps guide future material design strategies.

cond-mat.mtrl-sci↗