Georg Roth

Bio: Georg Roth is an academic researcher from RWTH Aachen University. The author has contributed to research in topics: Crystal structure & Neutron diffraction. The author has an hindex of 31, co-authored 168 publications receiving 3379 citations.

Quantitative phase analysis of bentonites by the rietveld method

Abstract: Thirty six bentonite samples from 16 different locations were examined in order to demonstrate the applicability of a new Rietveld description approach for quantitative phase analysis. X-ray diffraction patterns of the bulk material were obtained and analyzed by the Rietveld method. The samples contain up to ten different minerals, with dioctahedral smectite as the major component. A model for turbostratic disorder of smectites was formulated inside a structure-description file of the Rietveld program BGMN. The quality of the refinements was checked using an internal standard mineral (10.0 or 20.0 wt.% corundum) and by cross-checking results with X-ray fluorescence (XRF) data. The corundum content was reproduced with only small deviations from the nominal values. A comparison of the chemical composition obtained by XRF and the composition as re-calculated from quantitative Rietveld results shows a satisfactory agreement, although X-ray amorphous components such as volcanic glasses were not considered. As a result of this study, the Rietveld method combined with the new structure model for turbostratic disorder has proven to be a suitable method for routine quantitative analysis of bentonites with smectites as the dominant clay minerals.

Growth of thick YBa2Cu3O7−x single crystals from Al2O3 crucibles

Abstract: The growth of large, isometric and free-standing single crystals of YBa 2 Cu 3 O 7− x using a CuOBaO self-flux is reported. Within an optimum concentration range in the phase diagram centered around 4 at% Y, 30 at% Ba and 66 at% Cu, isometric crystals with aspect ratios between 1–10 and sizes of 5×5×2 mm 3 were grown. Outside this range of favourable flux compositions thin platelets up to 8×8 mm 2 with aspect ratios ≫ 10 were obtained. The thickness of the isometric crystals is essentially determined by the cooling rate: 500μm, 1 mm and 2 mm thick crystals were obtained for cooling rates of 0.5, 0.3 and 0.1° C/h, respectively. For cooling rates 30° C/h a dendritic non equilibrium morphology with a zigzag-like habit of the edge of the (001) face can be observed. Aluminum impurities arising from the Al 2 O 3 crucible have a strong influence on the crystal growth: they considerably reduce the soak time and the viscosity of the melt. The presence of Al also favours the formation of yellow-greenish needles of a new quaternary phase, YBa 3 Al 2 O 7.5 .

Ferromagnetic order in epitaxially strained La Co O 3 thin films

Abstract: $\mathrm{La}\mathrm{Co}{\mathrm{O}}_{3}$ films grown epitaxially on ⟨001⟩ oriented ${(\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3})}_{0.3}{({\mathrm{Sr}}_{2}\mathrm{Al}\mathrm{Ta}{\mathrm{O}}_{6})}_{0.7}$ substrates by pulsed laser deposition exhibit ferromagnetic ordering below a critical temperature, ${T}_{c}$, of $85\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Polycrystalline films of $\mathrm{La}\mathrm{Co}{\mathrm{O}}_{3}$ prepared in the same way did not show ferromagnetic order down to $T\ensuremath{\approx}5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and their temperature dependent susceptibility was identical to that of bulk $\mathrm{La}\mathrm{Co}{\mathrm{O}}_{3}$. The ferromagnetism in epitaxial films is not simply a property of the surface region, rather it extends over the complete film thickness, as shown by the linear increase of the saturated magnetic moment with increasing film thickness. We discuss this surprising result in terms of epitaxial tensile strain via the properly chosen substrate inducing ferromagnetic order.

Description of X-ray powder pattern of turbostratically disordered layer structures with a Rietveld compatible approach

Abstract: We address the problem of the quantitative de- scription of X-ray powder pattern of turbostratically disor- dered layer compounds. The Debye formula is used, which allows the aperiodic description of any arrangement of atoms. With the extension of Yang and Frindt (1996) for the ideal turbostratic case, these calculations are used to gener- ate reference data that are subsequently treated by the Riet- veld method. We are able to show that the case of uncorre- lated turbostratic disorder can be modelled equally well in a periodic supercell approach with a single layer in the super- cell that is suitable for the Rietveld technique. A brief intro- duction of this new model was given as an oral contribution at EUROCLAY 2003 (Ufer et al., 2003). The fundamental principles are described in this article because of its com- plexity. The applicability of this approach to real systems is demonstrated for smectite and corundum mixtures.

NaV2O5 as a Quarter-Filled Ladder Compound

Abstract: A new x-ray diffraction study of the one-dimensional spin-Peierls compound ${\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{-}\mathrm{Na}{\mathrm{V}}_{2}{\mathrm{O}}_{5}$ reveals a centrosymmetric $(\mathrm{Pmmn})$ crystal structure with one type of V site, contrary to the previously postulated noncentrosymmetric ${P2}_{1}\mathrm{mn}$ structure with two types of V sites ( ${V}^{+4}$ and ${\mathrm{V}}^{+5}$). Density-functional calculations indicate that $\mathrm{Na}{\mathrm{V}}_{2}{\mathrm{O}}_{5}$ is a quarter-filled ladder compound with the spins carried by V-O-V molecular orbitals on the rungs of the ladder. Estimates of the charge-transfer gap and the exchange coupling agree well with experiment and explain the insulating behavior of $\mathrm{Na}{\mathrm{V}}_{2}{\mathrm{O}}_{5}$ and its magnetic properties.

Toward the rational design of non-precious transition metal oxides for oxygen electrocatalysis

Abstract: In this Review, we discuss the state-of-the-art understanding of non-precious transition metal oxides that catalyze the oxygen reduction and evolution reactions. Understanding and mastering the kinetics of oxygen electrocatalysis is instrumental to making use of photosynthesis, advancing solar fuels, fuel cells, electrolyzers, and metal–air batteries. We first present key insights, assumptions and limitations of well-known activity descriptors and reaction mechanisms in the past four decades. The turnover frequency of crystalline oxides as promising catalysts is also put into perspective with amorphous oxides and photosystem II. Particular attention is paid to electronic structure parameters that can potentially govern the adsorbate binding strength and thus provide simple rationales and design principles to predict new catalyst chemistries with enhanced activity. We share new perspective synthesizing mechanism and electronic descriptors developed from both molecular orbital and solid state band structure principles. We conclude with an outlook on the opportunities in future research within this rapidly developing field.

Quantum Phase Transitions

Abstract: We give a general introduction to quantum phase transitions in strongly-correlated electron systems. These transitions which occur at zero temperature when a non-thermal parameter $g$ like pressure, chemical composition or magnetic field is tuned to a critical value are characterized by a dynamic exponent $z$ related to the energy and length scales $\Delta$ and $\xi$. Simple arguments based on an expansion to first order in the effective interaction allow to define an upper-critical dimension $D_{C}=4$ (where $D=d+z$ and $d$ is the spatial dimension) below which mean-field description is no longer valid. We emphasize the role of pertubative renormalization group (RG) approaches and self-consistent renormalized spin fluctuation (SCR-SF) theories to understand the quantum-classical crossover in the vicinity of the quantum critical point with generalization to the Kondo effect in heavy-fermion systems. Finally we quote some recent inelastic neutron scattering experiments performed on heavy-fermions which lead to unusual scaling law in $\omega /T$ for the dynamical spin susceptibility revealing critical local modes beyond the itinerant magnetism scheme and mention new attempts to describe this local quantum critical point.

Geological controls on the methane storage capacity in organic-rich shales

Abstract: High-pressure methane sorption isotherms were measured on selected Paleozoic and Mesozoic organic-rich shales, considered as shale gas targets in Europe. The samples include the Upper Cambrian–Lower Ordovician Alum Shale, Carboniferous (Mississippian–Pennsylvanian) shales and Lower Toarcian Posidonia Shale. In addition, samples from producing shale gas formations in the USA (Barnett, Haynesville and Eagle Ford) were studied for comparison. Excess sorption measurements were performed over an extended range of pressures (up to 25 MPa) and temperatures (up to 150 °C) on dry samples and at 38 °C on moisture-equilibrated samples to study the effect of organic matter content (TOC), maturity, mineralogy and moisture content on the methane sorption capacity. Additionally, water isotherms were measured at 24 °C and at relative humidities (RH) from 8 to 97%. A 3-parameter ( n L , p L , ρ a ) excess sorption function based on the Langmuir equation for absolute sorption was used to fit the measured methane sorption isotherms. The water sorption isotherms were parameterized by the Guggenheim–Anderson–de Boer (GAB) function. In both cases, excellent fits to the measured data were achieved. The methane sorption capacities of the dry shales show a positive correlation with TOC but significant deviations from this trend exist for individual samples. The TOC-normalized sorption capacities correlate positively with maturity in terms of Vitrinite Reflectance (VR r ) up to a certain value of VR r (~ 2.5%) above which an opposite trend is observed. No correlation was observed between the clay content and the TOC-normalized sorption capacity to methane, indicating that clay minerals do not significantly contribute to methane sorption in these organic-rich shales. The shape of the excess isotherms changes systematically with temperature and maturity. The Langmuir pressure ( p L ) increases exponentially with temperature and follows a negative power-law trend with maturity. Compared to dry samples, the sorption capacity in moisture-equilibrated samples (at 97% RH) is reduced by 40 to 60%. No difference is observed between 97% and 75% RH, indicating that the critical moisture content is at or below 75% RH. The monolayer sorption capacities for water obtained from the GAB fit are 0.5 to 3 times those for methane, derived from the Langmuir fit. There is a weak positive correlation between the methane and the water sorption capacity, suggesting that methane and water molecules share some of the sorption sites and these reside partly within the organic matter.

Unwinding of a skyrmion lattice by magnetic monopoles.

Abstract: Skyrmion crystals are regular arrangements of magnetic whirls that exist in a wide range of chiral magnets. Because of their topology, they cannot be created or destroyed by smooth rearrangements of the direction of the local magnetization. Using magnetic force microscopy, we tracked the destruction of the skyrmion lattice on the surface of a bulk crystal of Fe(1-x)Co(x)Si (x = 0.5). Our study revealed that skyrmions vanish by a coalescence, forming elongated structures. Numerical simulations showed that changes of topology are controlled by singular magnetic point defects. They can be viewed as quantized magnetic monopoles and antimonopoles, which provide sources and sinks of one flux quantum of emergent magnetic flux, respectively.