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Daniel F. Urban

Publications and source records attributed to Daniel F. Urban.

At least 19 recordsLinked to original sources

The Future of Computing for Materials Science Challenges

Materials discovery increasingly relies on the coordinated use of theory, computation, experiment, data-driven methods, and emerging quantum technologies, yet the full potential of these tools is realised only when they operate within workflows that reflect the complexity of real systems. This perspective summarises current capabilities, limitations, and opportunities across these domains, drawing on contributions from academia, industry, and national laboratories to identify the scientific and structural requirements for more reliable and efficient discovery. Classical simulations provide broad coverage across design spaces, while experimental measurements reveal degradation, heterogeneity, and kinetic processes that determine performance under realistic conditions. Machine learning accelerates exploration when supported by well-curated datasets with clear provenance and uncertainty quantification, and quantum computing offers promising routes into correlated electronic behaviour when aligned with properties that influence engineering decisions. Collectively, these insights highlight the need for reproducible workflows, shared data standards, realistic benchmarks, and a research culture that prepares scientists to work across paradigms. By integrating these methodological and organisational elements, the community can move toward discovery processes that deliver robust predictions, support confident decision making, and shorten the path from conceptual design to deployable materials.

cond-mat.mtrl-sci

Influence of ligand field and correlation on the electronic structure of NiO and CoO from DFT+DMFT calculations

The intriguing physics and rich application potential of strongly correlated first-row transition metal oxide compounds result from the complex interplay of several factors that influence the electronic structure. To shed light on the effect of composition, structure, and correlation strength, we apply a well-established charge self-consistent combination of density functional theory and dynamical mean field theory, which has proven to give electron binding energies in good agreement to experimentally derived excitation spectra. For paramagnetic NiO and CoO, we analyze the effect of rock-salt and zincblende structures and their different ligand fields on the spectral functions. By varying the value of the interaction parameter U, different correlation strengths among the transition-metal 3d electrons are considered, as well as the effect of additionally accounting for correlations in the oxygen 2p orbitals by a self-interaction-correction pseudopotential scheme.

cond-mat.str-el

High-Throughput-Screening Workflow for Predicting Volume Changes by Ion Intercalation in Battery Materials

Mechanical stresses and strains developing locally within the microstructure of active ion-battery-electrode materials during charge-discharge cycles can compromise their long-term stability. In this context, crystalline compounds exhibiting low volume changes are of particular interest. Atomistic simulations can be employed to quantify the volume change of the crystal structure upon intercalation and deintercalation of ions and to elucidate the local mechanisms underlying the global structural response. While density functional theory (DFT) offers a robust and accurate framework for such calculations, its computational cost limits its applicability for large-scale screening of diverse intercalation structures and sites. In this work, we present a workflow designed to prioritize candidate materials for subsequent detailed characterization. The workflow calculates the volume change upon intercalation using atomic-level features and a machine-learning model for bond-length prediction. The bond-length predictions are based on the assumption that bonds between the same ionic species in similar local coordination environments exhibit comparable lengths across different crystallographic structures. The model was trained on a DFT-generated dataset, which inherently defines the chemical space in which reliable predictions can be expected. We demonstrate the workflow's utility by screening approximately 1,175,000 transition-metal oxides and fluorides, followed by DFT validation of the most promising candidates. The proposed workflow enables filtering of large candidate sets and accelerates the potential discovery of low volume change intercalation materials for batteries.

cond-mat.mtrl-sci

Spin coherence in strongly coupled spin baths in quasi-two-dimensional layers

We investigate the spin-coherence decay of NV$^-$-spins interacting with the strongly-coupled bath of nitrogen defects in diamond layers. For thin diamond layers, we demonstrate that the spin-coherence times exceed those of bulk diamond, thus allowing to surpass the limit imposed by high defect concentrations in bulk. We show that the stretched-exponential parameter for the short-time spin-coherence decay is governed by the hyperfine interaction in the bath, thereby constraining random-noise models. We introduce a novel method based on the cluster-correlation expansion applied to strongly-interacting bath partitions. Our results facilitate material development for quantum-technology devices.

quant-ph

Diffusion behavior of Li ions in crystalline and amorphous Li-Zr-O and Li-Nb-O phases

Li containing transition metal oxides are known as good ionic conductors. Performing classical molecular dynamics simulations, the diffusion behavior of Li ions is investigated in crystalline and amorphous phases with the stoichiometries Li$_2$ZrO$_3$ and LiNbO$_3$. We first demonstrate the stability of the crystal structures for the used interatomic potential model and then analyze the amorphous phases, which result from melt-and-quench simulations, in terms of radial distribution functions. Diffusivities of Li ions in those systems are obtained from a statistical Arrhenius analysis of mean square displacement curves at different temperatures. The crystalline phase of Li$_2$ZrO$_3$ exhibits two well-defined migration mechanisms: vacancy-mediated migration is dominant below and a site exchange of Li ions above a crossover region between about 1700 and 1800 K. The latter mechanism also prevails in the amorphous phases of Li$_2$ZrO$_3$ with a strongly reduced activation energy, which is due to a smaller equilibrium separation of Li ions as in the crystal structure. This migration mechanism is found in amorphous LiNbO$_3$, too.

cond-mat.mtrl-sci

Coherence properties of NV-center ensembles in diamond coupled to an electron-spin bath

We investigate nitrogen-vacancy center (NV) ensembles in diamond under the influence of strongly-correlated electron-spin baths. We thoroughly calculate the decoherence properties of the NV central spin for bath concentrations of 0.1-100 ppm using the cluster-correlation expansion (CCE) method. We systematically analyze possible origins of the significant deviations in the values of the $T_2$ coherence time reported in literature. We demonstrate that significant variations can originate from the choice of averaging and fitting procedures used for the ensemble average and we point out the respective aspects that need to be considered, when comparing the various theoretical studies. Our study may ease readers to perform reliable and fast simulations on the central spin problem. It provides an understanding and interpretation of the outcome parameters describing the dynamics of the local bath spins.

quant-ph

Theoretical limits of magnetic detection of structural surface defects at the nanometer scale

We present a theoretical study on the magnetic signals of structural surface defects like cracks or indents combined with rough surfaces or subsurface inclusions of soft ferromagnetic metals like body-centered cubic Fe or amorphous CoFeB. We discuss limits of early detection of small surface defects on the basis of calculated magnetic stray fields few tens of nm above the surface. The considered surface imperfections have extensions of a few nm which correspond to low multiples of the magnetic exchange lengths of Fe or CoFeB. The detection of such small inhomogeneities requires that the sensor is about as close to the surface as the size of the inhomogeneity is. Furthermore, the step width of a scanning sensor must be of the same size as well. Both these requirements may be fulfilled for instance by scanning microscopy with diamond nitrogen-vacancy-center quantum sensors.

cond-mat.mtrl-sci

Influence of interstitial Li on the electronic properties of Li$_{x}$CsPbI$_{3}$ for photovoltaic and battery applications

The integrated device of a perovskite solar cell with a Li-ion battery is an innovative solution for decentralized energy storage in smart electronic devices. In this study, we examine the stability of Li ions intercalated in a CsPbI$_3$ perovskite and their effect on the electronic structure of Li$_x$CsPbI$_3$ compounds using first-principles density functional theory. Our simulations demonstrate that the insertion of Li at concentrations up to $x$ = 1 into CsPbI$_3$ is energetically possible. Moreover, we identify that the distortion of the Pb-I octahedra has the strongest impact on the change in the electronic band gap. Specifically, an increase in the amount of intercalated Li causes larger structural distortions, which in turn lead to an increasing band gap as function of the Li content.

cond-mat.mtrl-sci

Location and migration of interstitial Li ions in CsPbI$_3$ crystals

Halide perovskites are highly promising light-harvesting materials with strong ionic character, enabling in principle the combination of a solar cell and a Li-ion battery in one integrated photo-battery device. Here, we investigate Li ions inside crystals of CsPbI$_3$, as a prototype compound, by means of density-functional-theory calculations. Our findings demonstrate that the interstitial location and migration of Li ions depend strongly on the dynamic nature of the crystal structure of the perovskite compound. We consider two limiting cases for Li in CsPbI$_{3}$,(i) the cubic-symmetry structure as a model for the limit of fast ion motion and (ii) a distorted cubic structure as a model for the limit of slow ion motion. For both limiting cases we obtain moderate energy barriers for migrating Li ions, which highlight the potential of halide perovskites like CsPbI$_3$ for applications in photo-battery devices.

cond-mat.mtrl-sci

Formation energy profiles of oxygen vacancies at grain boundaries in perovskite-type electroceramics

Oxygen vacancy formation energies play a major role in the electric field assisted abnormal grain growth of technologically relevant polycrystalline perovskite phases. The underlying effect on the atomic scale is assumed to be a redistribution of cationic and anionic point defects between grain boundaries and the bulk interior regions of the grains due to different defect formation energies in the structurally different regions, accompanied by the formation of space charge zones. Using atomistic calculations based on classical interatomic potentials, we derive and discuss optimized structures of the symmetric tilt grain boundaries $Σ$5(210)[001] and $Σ$5(310)[001], and of the asymmetric tilt grain boundary (430)[001]||(100)[001] in the electroceramic perovskite materials SrTiO$_3$, BaTiO$_3$, and BaZrO$_3$. We present profiles of oxygen vacancy formation energies across those GBs and discuss their dependence on composition and grain boundary type.

cond-mat.mtrl-sci

Stability and electronic structure of NV centers at dislocation cores in diamond

We present a density functional theory analysis of the negatively charged nitrogen-vacancy (NV) defect complex located at or close to the core of 30$^\circ$ and 90$^\circ$ partial glide dislocations in diamond. Formation energies, electronic densities of states, structural deformations, hyperfine structure and zero-field splitting parameters of NV centers in such structurally distorted environments are analyzed. The formation energies of the NV centers are up to 3 eV lower at the dislocation cores compared to the bulk values of crystalline diamond. We found that the lowest energy configuration of the NV center at the core of a 30$^\circ$ partial glide dislocation is realized when the axis of the NV center is oriented parallel to the dislocation line. This special configuration has a stable triplet ground state. Its hyperfine constants and zero field splitting parameters deviate by only 3% from values of the bulk NV center. Hence, this is an interesting candidate for a self-assembly of a linear array of NV centers along the dislocation line.

cond-mat.mtrl-sci

Electrostatic treatment of charged interfaces in classical atomistic simulations

Artificial electrostatic potentials can be present in supercells constructed for atomistic simulations of surfaces and interfaces in ionic crystals. Treating the ions as point charges, we systematically derive an electrostatic formalism for model systems of increasing complexity, both neutral and charged, and with either open or periodic boundary conditions. This allows to correctly interpret results of classical atomistic simulations which are directly affected by the appearance of these potentials. We demonstrate our approach at the example of a strontium titanite (SrTiO$_3$) supercell containing an asymmetric tilt grain boundary. The formation energies of charged oxygen vacancies and the relaxed interface structure are calculated based on an interatomic rigid-ion potential, and the results are analyzed in consideration of the electrostatic effects.

cond-mat.mtrl-sci

Influence of (N,H)-terminated surfaces on stability, hyperfine structure, and zero-field splitting of NV centers in diamond

We present a density functional theory analysis of the negatively charged nitrogen-vacancy (NV$^-$) defect complex in diamond located in the vicinity of (111)- or (100)-oriented surfaces with mixed (N,H)-terminations. We assess the stability and electronic properties of the NV$^-$ center and study their dependence on the H:N ratio of the surface termination. The formation energy, the electronic density of states, the hyperfine structure and zero-field splitting parameters of an NV$^-$ center are analyzed as function of its distance and orientation to the surface. We find stable NV$^-$ centers with bulk-like properties at distances of at least $\sim8$ Angstroem from the surface provided that the surface termination consists of at least 25\% substitutional nitrogen atoms. Our results indicate that axial NV centers near a flat 100\% N-terminated (111) surface are the optimal choice for NV-based quantum sensing applications as they are the least influenced by the proximity of the surface.

cond-mat.mtrl-sci

Defects and Phase Formation in Non-Stoichiometric LaFeO$_3$: a Combined Theoretical and Experimental Study

The defect chemistry of perovskite compounds is directly related to the stoichiometry and to the valence states of the transition-metal ions. Defect engineering has become increasingly popular as it offers the possibility to influence the catalytic properties of perovskites for applications in energy storage and conversion devices such as solid-oxide fuel- and electrolyzer cells. LaFeO$_3$ (LFO) can be regarded as a base compound of the family of catalytically active perovskites La$_{1-x}$A$_x$Fe$_{1-y}$B$_y$O$_{3-δ}$, for which the defect chemistry as well as the electronic and ionic conductivity can be tuned by substitution on cationic sites. Combining theoretical and experimental approaches, we explore the suitability for A-site vacancy engineering, namely the feasibility of actively manipulating the valence state of Fe and the concentration of point defects by synthesizing La-deficient LFO. Formation energies and concentrations of point defects were determined as a function of processing conditions by first-principles DFT+U calculations. Based on the results, significant compositional deviations from stoichiometric LFO cannot be expected by providing rich or poor conditions of the oxidic precursor compounds (Fe$_2$O$_3$ and La$_2$O$_3$) in a solid-state processing route. In the experimental part, LFO was synthesized with a targeted La-site deficiency. We analyze the resulting phases by X-ray diffraction and scanning electron microscopy, (scanning) transmission electron microscopy in combination with energy-dispersive X-ray spectroscopy, and electron energy-loss spectrometry. Instead of a variation of the La/Fe ratio, a mixture of the two phases Fe$_2$O$_3$ and LFO was observed, resulting in an invariant charge state of Fe, in line with the theoretical results. We discuss our findings with respect to partly differing assumptions made in previous studies on this material system.

cond-mat.mtrl-sci

Stability and magnetic properties of grain boundaries in the inverse Heusler phase Fe$_2$CoGa and in bcc Fe

We investigate grain boundaries (GBs) in the cubic inverse Heusler phase Fe$_2$CoGa by means of first principles calculations based on density functional theory. Besides the energetic stability, the analysis focuses on the magnetic properties of a set of 16 GB structures in this intermetallic phase. We determine the integrated excess magnetization across the GB and analyze it in terms of the projected local magnetic moments of the atoms and their local Voronoi volumes. The results are systematically compared to those of corresponding GBs in body-centered cubic (bcc) Fe. The studied GBs in Fe$_2$CoGa may have a considerably increased magnetization at the GB, up to more than twice as much as in bcc Fe, depending on the GB type, while geometrical quantities like GB widening or local GB excess volume distributions are similar for both phases. We explain this difference by the higher flexibility of the ternary Fe$_2$CoGa phase in compensating the disturbance of a crystal defect by structural relaxation. The GB structures therefore have a lower energy accompanied with increased local magnetic moments of the Co and half of the Fe atoms within a distance of a few angstrom around the GB plane.

cond-mat.mtrl-sci

Atomistic calculations of charged point defects at grain boundaries in SrTiO$_3$

Oxygen vacancies have been identified to play an important role in accelerating grain growth in polycrystalline perovskite-oxide ceramics. In order to advance the fundamental understanding of growth mechanisms at the atomic scale, classical atomistic simulations were carried out to investigate the atomistic structures and oxygen vacancy formation energies at grain boundaries in the prototypical perovskite-oxide material SrTiO$_3$. In this work, we focus on two symmetric tilt grain boundaries, namely $Σ$5(310)[001] and $Σ$5(210)[001]. A one-dimensional continuum model is adapted to determine the electrostatic potential induced by charged lattice planes in atomistic structure models containing grain boundaries and point defects. By means of this model, electrostatic artifacts, which are inherent to supercell models with periodic or open boundary conditions, can be taken into account and corrected properly. We report calculated formation energies of oxygen vacancies on all the oxygen sites across boundaries between two misoriented grains, and we analyze and discuss the formation-energy values with respect to local charge densities at the vacant sites.

cond-mat.mtrl-sci

First-principles calculation of electroacoustic properties of wurtzite (Al,Sc)N

We study the electroacoustic properties of aluminum scandium nitride crystals Al$_{1-x}$Sc$_x$N with the metastable wurtzite structure by means of first-principles calculations based on density functional theory. We extract the material property data relevant for electroacoustic device design, namely the full tensors of elastic and piezoelectric constants. Atomistic models were constructed and analyzed for a variety of Sc concentrations $0\le x\le50$ %. The functional dependence of the material properties on the scandium concentration was extracted by fitting the data obtained from an averaging procedure for different disordered atomic configurations. We give an explanation of the observed elastic softening and the extraordinary increase in piezoelectric response as function of Sc content in terms of an element specific analysis of bond lengths and bond angles.

cond-mat.mtrl-sci

Atomistic analysis of Li migration in $\text{Li}_{1+x} \text{Al}_x \text{Ti}_{2-x} {(\text{P} \text{O}_4)}_3$ (LATP) solid electrolytes

We examine the ionic migration of Li in LATP [$\text{Li}_{1+x} \text{Al}_x \text{Ti}_{2-x} {(\text{P} \text{O}_4)}_3$] solid electrolytes from an atomistic viewpoint by means of density functional theory calculations. We vary the Al content and investigate its effects on the crystal structure of LATP and on the migration energy landscape of interstitial Li ions. The energy profiles governing the Li diffusion are found to be systematically influenced by the position of Al ions in direct vicinity of the migration path, and we derive a simplified classification scheme of three universal energy profile shapes. The overall influence of the Al/Ti-ratio on the Li migration is analyzed by a separation into chemical and geometrical aspects. This work provides a solid basis for a resource-efficient computational examination of the ionic conductivity of Li in LATP with varying Al/Ti concentrations.

cond-mat.mtrl-sci