arXiv subjects

Rotraut Merkle

Publications and source records attributed to Rotraut Merkle.

4 recordsLinked to original sources

arXiv2026

Impact of charge transition levels on grain boundary properties in acceptor doped oxide ceramics: A phase-field study

Advanced doping strategies enable oxide ceramic functionalities by tailoring bulk defect chemistry and space-charge-layer (SCL) behavior at interfaces. Charge transition levels (CTLs), defined as the Fermi level at which a defect changes its stable charge state, play a central role. Their alignment governs bulk defect chemistry, while their bending within SCLs induces additional charge-state transitions. Incorporating CTLs is therefore essential for a consistent description of defect equilibria and SCL formation. In this work, we propose a defect-chemistry-consistent phase-field model explicitly coupled with CTLs to investigate their role in SCL evolution. The model includes multivalent oxygen vacancies, multivalent acceptor dopants, electrons, and holes. It is applied to Fe-doped SrTiO3 over wide ranges of oxygen partial pressure and temperature, capturing both symmetric SCLs at stationary grain boundaries and asymmetric SCLs during migration. Two distinct grain boundary types, slow and fast boundaries, emerge during migration, consistent with experimental observations. Simulations reveal that CTL-governed bulk defect chemistry, together with CTL-induced charge-state transitions within SCLs, critically determine SCL characteristics. Moreover, CTL-mediated hole transport is significantly faster than acceptor dopant diffusion, modulating solute drag and grain boundary kinetics. Finally, the model predicts grain boundary properties dependent on both thermal history and boundary type, with slow and fast boundaries exhibiting distinct behaviors. This framework links defect chemistry, Fermi level, CTLs, and grain boundary kinetics, providing new insights for designing oxide ceramics with tailored properties.

cond-mat.mtrl-sci↗

arXiv2025

Anomalous Sodium Insertion in Highly Oriented Graphite: Thermodynamics, Kinetics and Evidence for Two-Sided Intercalation

The difficult intercalation of sodium (Na) into graphite is studied by systematic and long-time investigations (of up to 2 years) using highly oriented pyrolytic graphite (HOPG). By studying chemical insertion of solid, liquid and gaseous Na at low and high temperatures (LT, HT) as well as using electrochemical insertion at 25 degree Celsius into uncoated and coated HOPG, it became clear that insertion equilibrium requires HT. On decreasing chemical intercalation temperature from HT (500 degree Celsius) to LT (25 degree Celsius), thermodynamic control was found to change to diffusion control and finally to interfacial control. For the electrochemical insertion, coating (TiO2) proved advisable (to avoid co-intercalation) and efficient in reducing the interfacial resistance. Measured saturation values were found to be not higher than about 1 mol %. Towards room temperature higher equilibrium values cannot be excluded but would in view of the very low driving force kinetically be very difficult to reach. The reversible cell voltage of the saturated composition (versus alkali metal) is distinctly lower than for the analogous cells using lithium (Li) or potassium (K). Detailed transmission electron microscopy (TEM) studies reveal the unexpected fact that at HT Na predominantly enters HOPG in the form of two-sided intercalation sandwiching carbon layers (bilayers), while at LT more highly aggregated layers appear to a comparable degree, accompanied with the formation of higher-dimensional crystal imperfections. The reasons for this peculiar feature and the non-monotonic thermodynamics in the sequence Li-Na-K-Rb-Cs are discussed not only from an energetic but also from an entropic point of view.

cond-mat.mtrl-sci↗

arXiv2024

A defect-chemistry-informed phase-field model of grain growth in oxide ceramics: application to Fe-doped SrTiO3

Dopants can significantly affect the properties of oxide ceramics through their impact on the property-determined microstructure characteristics such as grain boundary (GB) segregation, space charge layer formation in the GB vicinity, and the grain growth deviating from normal patterns. To support the rational design of oxide ceramics, we propose a defect-chemistry-informed phase-field grain growth model to simulate the microstructure evolution of oxide ceramics. It fully respects the defect-chemistry theory by accounting for the distinct segregation energies and available site densities of charged point defects (oxygen vacancies and acceptor dopants) in both the grain interior and boundaries, and it considers the competing kinetics of defect diffusion and GB movement. The proposed phase-field model is benchmarked against well-known bicrystal models, including the Mott-Schottky and Gouy-Chapman models. Various simulation results are presented to reveal the effect of different defect-chemistry parameters on the space charge layer formation and key microstructural aspects. In particular, simulation results confirm that the solute drag effect alone can lead to skewed grain size distribution that do not follow the log-normal distribution, without any contribution from grain misorientation and other anisotropy. Interestingly, simulations also demonstrate that grain boundary potentials can vary substantially: GBs of larger grains tend to have lower potentials than those of smaller grains. Such heterogeneous GB potential distribution may inspire a new material optimization strategy through microstructure design. This study provides a comprehensive framework for defect-chemistry-consistent investigations of microstructure evolution in polycrystalline oxide ceramics, offering fundamental insights into microscopic processes during critical manufacturing stages.

cond-mat.mtrl-sci↗

arXiv2017

Microscopic Dynamics of Li$^{+}$ in Rutile TiO$_{2}$ Revealed by $^{8}$Li $β$-detected NMR

We report measurements of the dynamics of isolated $^{8}$Li$^{+}$ in single crystal rutile TiO$_{2}$ using $β$-detected NMR. From spin-lattice relaxation and motional narrowing, we find two sets of thermally activated dynamics: one below 100 K; and one at higher temperatures. At low temperature, the activation barrier is $26.8(6)$ meV with prefactor $1.23(5) \times 10^{10}$ s$^{-1}$. We suggest this is unrelated to Li$^{+}$ motion, and rather is a consequence of electron polarons in the vicinity of the implanted $^{8}$Li$^{+}$ that are known to become mobile in this temperature range. Above 100 K, Li$^{+}$ undergoes long-range diffusion as an isolated uncomplexed cation, characterized by an activation energy and prefactor of $0.32(2)$ eV and $1.0(5) \times 10^{16}$ s$^{-1}$, in agreement with macroscopic diffusion measurements. These results in the dilute limit from a microscopic probe indicate that Li$^{+}$ concentration does not limit the diffusivity even up to high concentrations, but that some key ingredient is missing in the calculations of the migration barrier. The anomalous prefactors provide further insight into both Li$^{+}$ and polaron motion.

cond-mat.mtrl-sci↗