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Subject: search.filters.subject.530Institute: search.filters.UBSInstitut.Institut für MaterialwissenschaftStart date: 2020End date: 2022

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Now showing 1 - 10 of 24
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    Finite-temperature interplay of structural stability, chemical complexity, and elastic properties of bcc multicomponent alloys from ab initio trained machine-learning potentials
    (2021) Gubaev, Konstantin; Ikeda, Yuji; Tasnádi, Ferenc; Neugebauer, Jörg; Shapeev, Alexander V.; Grabowski, Blazej; Körmann, Fritz
    An active learning approach to train machine-learning interatomic potentials (moment tensor potentials) for multicomponent alloys to ab initio data is presented. Employing this approach, the disordered body-centered cubic (bcc) TiZrHfTax system with varying Ta concentration is investigated via molecular dynamics simulations. Our results show a strong interplay between elastic properties and the structural ω phase stability, strongly affecting the mechanical properties. Based on these insights we systematically screen composition space for regimes where elastic constants show little or no temperature dependence (elinvar effect).
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    Thermally-activated dislocation mobility in bcc metals : an accelerated molecular dynamics study
    (2021) Grabowski, Blazej; Zotov, Nikolay
    Plastic deformation in metals is controlled by dislocation density and mobility. In bcc metals the mobility of screw dislocations, which takes place by temperature- and stress-driven nucleation of critical kink-pairs, is most essential for deformation. However, the critical resolved shear stresses at low temperatures, as determined from molecular dynamics (MD) simulations performed at constant strain rate, are typically 2–3 times larger than the yield stresses measured experimentally. Here, an accelerated MD procedure is developed and employed to investigate the onset of dislocation mobility in the prototypical system bcc Nb. The method combines constant strain and temperature MD with hyperdynamics, using a bond-boost potential. We demonstrate, with a careful statistical analysis, that the method enables nucleation of critical kink-pairs and the determination of the Gibbs energy of kink-pair formation from accelerated MD simulations at experimentally-measured shear stresses.
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    Li5Sn, the most lithium-rich binary stannide : a combined experimental and computational study
    (2022) Stelzer, Robert U.; Ikeda, Yuji; Srinivasan, Prashanth; Lehmann, Tanja S.; Grabowski, Blazej; Niewa, Rainer
    From reaction of excess lithium with tin, we isolate well-crystallized Li5Sn and solve the crystal structure from single-crystal X-ray diffraction data. The orthorhombic structure (space group Cmcm) features the same coordination polyhedra around tin and lithium as previously predicted by electronic structure calculations for this composition, however differently arranged. An extensive ab initio analysis, including thermodynamic integration using Langevin dynamics in combination with a machine-learning potential (moment tensor potential), is conducted to understand the thermodynamic stability of this Cmcm Li5Sn structure observed in our experiments. Among the 108 Li5Sn structures systematically derived using the structure enumeration algorithm, including the experimental Cmcm structure and those obtained in previous ab initio studies, another new structure with the space group Immm is found to be energetically most stable at 0 K. This computationally discovered Immm structure is also found to be thermodynamically more stable than the Cmcm structure at finite temperatures, indicating that the Cmcm Li5Sn structure observed in our experiments is favored likely due to kinetic reasons rather than thermodynamics.
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    Structural and magnetic properties of newly found BaFeO2.667 synthesized by oxidizing BaFeO2.5 obtained via nebulized spray pyrolysis
    (2021) Wollstadt, Stephan; Ikeda, Yuji; Sarkar, Abhishek; Vasala, Sami; Fasel, Claudia; Alff, Lambert; Kruk, Robert; Grabowski, Blazej; Clemens, Oliver
    A new vacancy-ordered perovskite-type compound Ba3Fe3O8 (BaFeO2.667) was prepared by oxidizing BaFeO2.5 (P21/c) with the latter compound obtained by a spray-pyrolysis technique. The structure of Ba3Fe3O8 was found to be isotypic to Ba3Fe3O7F (P21/m) and can be written as Ba3Fe3+2Fe4+1O8. Mössbauer spectroscopy and ab initio calculations were used to confirm mixed iron oxidation states, showing allocation of the tetravalent iron species on the tetrahedral site and octahedral as well as square pyramidal coordination for the trivalent species within a G-type antiferromagnetic ordering. The uptake and release of oxygen was investigated over a broad temperature range from RT to 1100 °C under pure oxygen and ambient atmosphere via a combination of DTA/TG and variable temperature diffraction measurements. The compound exhibits a strong lattice enthalpy driven reduction to monoclinic and cubic BaFeO2.5 at elevated temperatures.
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    Ab initio surface free energies of tungsten with full account of thermal excitations
    (2022) Forslund, Axel; Ruban, Andrei
    The surface free energies of seven different facets of tungsten (W) are obtained up to the melting point with full account of all the relevant thermal excitations; in particular, thermal atomic vibrations, electronic excitations, and their mutual coupling. The latter is done using ab initio molecular dynamics simulations coupled with the thermodynamic integration technique. In this way, the calculations contain almost no error but the one related to the used exchange-correlation functional, which makes the results truly first principles. The obtained results are compared with previous quasiharmonic calculations for the surface free energies of W and experimental data. The anharmonic contribution is, as expected, important for open surfaces at high temperatures, which leads to a temperature dependence of the surface energy anisotropy. The calculated Wulff shapes and surface energies are in excellent agreement with experimental data close to the melting point, where the crystalline structure of the surface layers is destroyed by a dramatic mobility of the atoms there.
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    Ab initio simulations of the surface free energy of TiN(001)
    (2021) Forslund, Axel; Zhang, Xi; Grabowski, Blazej; Shapeev, Alexander V.; Ruban, Andrei V.
    The temperature dependence of the surface free energy of the industrially important TiN(001) system has been investigated by means of an extended two-stage upsampled thermodynamic integration using Langevin dynamics (TU-TILD) methodology, to include the fully anharmonic vibrational contribution, as obtained from ab initio molecular dynamics (AIMD). Inclusion of the fully anharmonic behavior is crucial, since the standard low-temperature quasiharmonic approximation exhibits a severe divergence in the surface free energy due to a high-temperature dynamical instability. The anharmonic vibrations compensate for the quasiharmonic divergence and lead to a modest overall temperature effect on the TiN(001) surface free energy, changing it from around 78 meV Å-2 at 0 K to 73 meV Å-2 at 3000 K. The statistical convergence of the molecular dynamics is facilitated by the use of machine-learning potentials, specifically moment tensor potentials, fitted for TiN(001) at finite temperature. The surface free energy obtained directly from the fitted machine-learning potentials is close to that obtained from the full AIMD simulations.
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    Crystal structure and phase stability of Co2N: a combined first-principles and experimental study
    (2021) Ikeda, Yuji; Lehmann, Tanja S.; Widenmeyer, Marc; Coduri, Mauro; Grabowski, Blazej; Niewa, Rainer
    The crystal structure and phase stability of Co2N are revisited based on experiments and first-principles calculations. Powder X-ray diffraction (PXRD) measurements and Rietveld refinements clearly confirm that the stable crystal structure of Co2N is an isotype of η-Fe2C and Co2C with the space group Pnnm rather than the closely related ζ-Fe2N with the space group Pbcn. The refined lattice parameters of Co2N in the Pnnm structure are a = 4.6108(1) Å, b = 4.3498(1) Å, c = 2.85592(7) Å, obtained from X-ray diffraction using synchrotron radiation. Furthermore, differential scanning calorimetry (DSC) with subsequent diffraction experiments reveal an endothermal transition to an ε-type order at 398 °C followed by an exothermal decomposition at 446 °C. First-principles density-functional-theory (DFT) calculations including the Hubbard U correction (DFT+U) demonstrate that it is essential for transition metal nitrides to consider strong electron correlation to predict the correct experimental structure and magnetic state. In particular, an effective value of Ueff = 2.75 eV can be utilized to obtain an antiferromagnetic Pnnm phase of Co2N in agreement with experiments.
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    Application of machine-learning for construction of bias potential: a case study of add-atom hyperdynamics and straight screw dislocation migration
    (2021) Novikov, Ivan S.; Grabowski, Blazej; Zotov, Nikolay
    In this report we describe a bias potential for add-atom global hyperdynamics on the basis of machine-learning (ML) interatomic potential (namely, Moment Tensor Potential, MTP). We compare the results obtained using the ML-bias potential with the ones obtained with conventional bond-boost bias potential. We also discuss possibilities for construction of a ML-bias potential for acceleration a migration of straight screw dislocation.
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    Three dimensional analytical study of thin film battery electrodes
    (2021) Abdelkarim, Ahmed; Schmitz, Guido (Prof. Dr. Dr. h. c.)
    Li ions play a major role in batteries for energy storage. On the other hand, Li is notoriously challenging to be reliably detected with most microscpic techniques.Owing to its weak scattering form factor, low X-ray emission or peak overlaps in EELS spectroscopy, precise microscopic analysis of Li in battery materials is delicate. The aim of this study was to investigate a very sensitive analysis of the ionic transport on a microscopic, even on an atomic level scale. Well controlled amorphous lithium iron phosphate (LFP) thin films were prepared by ion-beam sputter deposition. In a subsequent annealing step, amorphous films were then crystallized. The electrochemical performance of both LFP phases is checked in cyclo-voltammetry, while structure and microstructure are confirmed by XRD and TEM, respectively. Cycling reversibility over 7000 cycles with a retention more than 92 % is accomplished for the crystalline LFP, whereas the amorphous phase is electrochemically nonfunctional. Intercalation of LFP thin films was studied as a function of film thickness (25 - 250 nm). The intercalation kinetics is systematically quantified over a wide range of scanning rates (0.004 to 400 mV s-1 in cyclic voltammetry experiments. Two different diffusion regimes for the material undergoing two phase reaction were explained with the help of the modified Randles-Sevcik equation. Slow Li diffusion in the thickest films was recorded. Dependence of the peak current on the layer thickness is explained in terms of increasing the grain boundary (GB) area. Opposite to the peak fluxes, the overpotential was interestingly found to be independent of the layer thickness. Less electrical driving force is required to force the same current in thick film. The grain boundaries represent an electroactive interface at which the overpotential appears. And hence, the grain boundaries work as fast conduction paths for faster Li ions diffusion. Thus, the total current is controlled by the total grain boundary area rather than the thin film surface. LiFePO4 (LFP) is then 3-dimensionally studied by laser-assisted Atom Probe Tomography (APT). The effects of laser power on the quantitative analysis of the amorphous phase by atom probe tomography were considered. The systematic investigation of amorphous samples presented herein demonstrates quantification of constituent elements, particularly lithium. Stoichiometric ratios relative to all elements (Li+Fe+P+O) and to the stable element (Fe) were calculated; P and O reveal reverse behavior against laser power. Li, on the other hand, after considering its migration, increases with rising laser power. Even though APT measurements at cryogenic temperatures (60 K) were performed, migration of Li ions in some LFP states was observed. In response to the applied measurement fields, Li ions are undoubtedly redistributed. In the amorphous LFP material, we observe a strong Li gradient towards the tip front, which hinders reliable analysis. Obviously, during measurement, Li is drawn towards the tip front and this effect increases with increasing laser power. The remaining host elements, Fe, P, and O, remain homogeneously distributed. New unique insights into the mechanisms of Li movements are provided. Li is pulled and Li enrichment/depletion regions are observed. A new term "Li shooting" is addressed to describe these Li movements. It is demonstrated that the ions indeed experience a field-dependent drift. By mathematically modelling the resulting composition profiles, the Li diffusivity is quantitatively evaluated. In a direct comparison between the amorphous and the crystalline LFP films of identical chemical composition, it is shown that the diffusivity of the amorphous structure is orders of magnitude faster than that of the crystalline state at a temperature of 60 K. Most notably, this is the first study to investigate the capabilities of APT in LFP at different de-/lithiation states. Li compositions show a wave-like distribution as a result of existence the Li rich/poor phases. 3D iso-surfaces and 2D orthoslices were provided to differentiate between the two phases. Li, in the fully lithiated phase, reaches its ideal stoichiometric ratio, while it is overestimated in the fully delithiated phase. Obviously, the thin films include inactive LFP regions. They were highlighted in this thesis for the first time by atom probe analysis. To quantify the two-phase nature of LFP, statistical analyses of the dis/charged LFP at different lithiation states were performed. Observed frequency distributions of the concentrations of small clusters were compared to the binomial distributions and discussed in detail. Deviations between observed and binomial distributions were represented in the Pearson coefficient to demonstrate the phase separation in the atom probe analysis. Our results provide an evidence to statistically understand the local microstructure evolution in battery materials, which is a pivotal characteristic of battery performance. On the other hand, APT has shown some constraints to microscopy differentiate between the two phases. Although APT measurements were performed at cryo-temperature, Li showed a displacement during measurements. Contrastingly, most of materials are fractured early at ultra low temperatures.
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    Magnetic bands producing a monoclinic magnetic structure in NiO, FeO, MnO, and a tetragonal one in CoO
    (2022) Krüger, Ekkehard
    In a foregoing paper, the author reported evidence that the multi-spin-axis magnetic structure proposed in 1964 by van Laar is realized in antiferromagnetic CoO. Within the nonadiabatic Heisenberg model, this tetragonal body-centered structure is generated by atomic-like electrons in a special magnetic band of CoO, a mechanism that may emerge only because the nonadiabatic Heisenberg model goes beyond the adiabatic approximation. This paper compares the band structures of the transition-metal monoxides NiO, CoO, FeO, and MnO, and shows that only CoO possesses a magnetic band which may produce the tetragonal magnetic structure proposed by van Laar. The magnetic bands of the other monoxides, NiO, FeO, and MnO, are clearly related to the monoclinic base-centered magnetic structure experimentally observed in these materials.