Crystal-Orientation-Dependent Oxygen Exchange Kinetics on Mixed Conducting Thin-Film Surfaces Investigated by In Situ Studies.

The oxygen exchange kinetics and the surface chemistry of epitaxially grown, dense La0.6Sr0.4CoO3-δ (LSC) thin films in three different orientations, (001), (110), and (111), were investigated by means of in situ impedance spectroscopy during pulsed laser deposition (i-PLD) and near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS). i-PLD measurements showed that pristine LSC surfaces exhibit very fast surface exchange kinetics but revealed no significant differences between the specific orientations. However, as soon as the surfaces were in contact with acidic, gaseous impurities, such as S-containing compounds in nominally pure measurement atmospheres, NAP-XPS measurements revealed that the (001) orientation is substantially more susceptible to the formation of sulfate adsorbates and a concomitant performance decrease. This result is further substantiated by a stronger increase of the work function on (001)-oriented LSC surfaces upon sulfate adsorbate formation and by a faster performance degradation of these surfaces in ex situ measurement setups. This phenomenon has potentially gone unnoticed in the discussion of the interplay between the crystal orientation and the oxygen exchange kinetics and might have far-reaching implications for real solid oxide cell electrodes, where porous materials exhibit a wide variety of differently oriented and reconstructed surfaces.

Improving and degrading the oxygen exchange kinetics of La0.6Sr0.4CoO3-δ by Sr decoration.

Minimizing the overpotential at the air electrode of solid oxide fuel cells (SOFC) is one of the key challenges regarding a broad applicability of this technology. Next to novel materials and geometry optimization, surface modification is a promising and flexible method to alter the oxygen exchange kinetics at SOFC cathode surfaces. Despite extensive research, the mechanism behind the effect of surface decorations is still under debate. Moreover, for Sr decoration, previous studies yielded conflicting results, reporting either a beneficial or a detrimental impact on the oxygen exchange kinetics. In this contribution, in situ impedance spectroscopy during pulsed laser deposition was used to investigate the effect of Sr containing decorations under different deposition conditions. Depending on deposition temperature and interactions with the gas phase, opposing effects of Sr decoration were found. In combination with near-ambient pressure X-ray photoelectron spectroscopy and non-ambient X-ray diffractometry, it was possible to trace this phenomenon back to different chemical environments of the surface Sr. At high temperatures, Sr is deposited as SrO, which can have a beneficial effect on the oxygen exchange kinetics. At low temperatures, SrCO3 adsorbates are formed from trace amounts of CO2 in the measurement atmosphere, causing a decrease of the oxygen exchange rate. These results are in excellent agreement with the concept of surface acidity as a descriptor for the effect of surface decorations, providing further insight into the oxygen exchange kinetics on SOFC cathode surfaces and its degradation. In addition, this study shows that Sr segregation itself initially does not lead to performance degradation but that segregated SrO readily reacts with acidic compounds, reducing the catalytic capability of mixed conducting oxides.

Synthesis and characterization of cobalt SCS pincer complexes.

The synthesis and characterization of two Co(II) complexes stabilized by a tridentate SCS pincer ligand are described. Paramagnetic [Co(κ3-SCSCH2-Et)2] and [Co(κ3-SCSCH2-tBu)(κ2-SCSCH2-tBu)] were obtained via transmetalation protocol from CoBr2 and S(C-Br)SCH2-R (R = Et, tBu). Oxidation of the latter with [Cp2Fe]PF6 affords the diamagnetic 18 VE complex [Co(κ3-SCSCH2-tBu)2]PF6. X-ray structures and DFT calculations are presented. Supplementary Information: The online version contains supplementary material available at 10.1007/s00706-022-02949-1.

Insights into the Correlation between Residual Stresses, Phase Transformation, and Wettability of Femtosecond Laser-Irradiated Ductile Iron.

Despite numerous studies on the wettability behavior of ductile iron after ultrafast laser structuring, the correlation between the phase change due to the interaction with an intense pulse and wettability is not yet well understood. In the present work, phase transformations of ductile iron substrates after femtosecond laser irradiation are investigated and correlated with the wettability behavior. Laser parameters such as fluence (F), cumulative fluence (CH), number of pulses (N), and scan speed were varied to produce hierarchical structures with different morphologies and phase concentrations. Our outcomes indicated that substrates with higher concentrations of austenite in the absence of hierarchical structures have a superhydrophilic nature despite being stored in an ambient atmosphere for several days and the application of a vacuum process. In addition, we measured the concomitant residual stresses after laser irradiation using the X-ray diffraction (XRD) method and established a relationship with the doses of CH and induced micro/nanostructures. Transmission electron microscopy (TEM) revealed that laser-structured surfaces are covered with oxides; moreover, phase transformation occurs at the near-subsurface layer.

Polytypism in mcalpineite: a study of natural and synthetic Cu3TeO6.

Synthetic and naturally occurring forms of tricopper orthotellurate, CuII3TeVIO6 (the mineral mcalpineite) have been investigated by 3D electron diffraction (3D ED), X-ray powder diffraction (XRPD), Raman and infrared (IR) spectroscopic measurements. As a result of the diffraction analyses, CuII3TeVIO6 is shown to occur in two polytypes. The higher-symmetric CuII3TeVIO6-1C polytype is cubic, space group Ia3, with a = 9.537 (1) Šand V = 867.4 (3) Å3 as reported in previous studies. The 1C polytype is a well characterized structure consisting of alternating layers of CuIIO6 octahedra and both CuIIO6 and TeVIO6 octahedra in a patchwork arrangement. The structure of the lower-symmetric orthorhombic CuII3TeVIO6-2O polytype was determined for the first time in this study by 3D ED and verified by Rietveld refinement. The 2O polytype crystallizes in space group Pcca, with a = 9.745 (3) Å, b = 9.749 (2) Å, c = 9.771 (2) Šand V = 928.3 (4) Å3. High-precision XRPD data were also collected on CuII3TeVIO6-2O to verify the lower-symmetric structure by performing a Rietveld refinement. The resultant structure is identical to that determined by 3D ED, with unit-cell parameters a = 9.56157 (19) Å, b = 9.55853 (11) Å, c = 9.62891 (15) Šand V = 880.03 (2) Å3. The lower symmetry of the 2O polytype is a consequence of a different cation ordering arrangement, which involves the movement of every second CuIIO6 and TeVIO6 octahedral layer by (1/4, 1/4, 0), leading to an offset of TeVIO6 and CuIIO6 octahedra in every second layer giving an ABAB* stacking arrangement. Syntheses of CuII3TeVIO6 showed that low-temperature (473 K) hydrothermal conditions generally produce the 2O polytype. XRPD measurements in combination with Raman spectroscopic analysis showed that most natural mcalpineite is the orthorhombic 2O polytype. Both XRPD and Raman spectroscopy measurements may be used to differentiate between the two polytypes of CuII3TeVIO6. In Raman spectroscopy, CuII3TeVIO6-1C has a single strong band around 730 cm-1, whereas CuII3TeVIO6-2O shows a broad double maximum with bands centred around 692 and 742 cm-1.

Cation non-stoichiometry in Fe:SrTiO3 thin films and its effect on the electrical conductivity.

The interplay of structure, composition and electrical conductivity was investigated for Fe-doped SrTiO3 thin films prepared by pulsed laser deposition. Structural information was obtained by reciprocal space mapping while solution-based inductively-coupled plasma optical emission spectroscopy and positron annihilation lifetime spectroscopy were employed to reveal the cation composition and the predominant point defects of the thin films, respectively. A severe cation non-stoichiometry with Sr vacancies was found in films deposited from stoichiometric targets. The across plane electrical conductivity of such epitaxial films was studied in the temperature range of 250-720 °C by impedance spectroscopy. This revealed a pseudo-intrinsic electronic conductivity despite the substantial Fe acceptor doping, i.e. conductivities being several orders of magnitude lower than expected. Variation of PLD deposition parameters causes some changes of the cation stoichiometry, but the films still have conductivities much lower than expected. Targets with significant Sr excess (in the range of several percent) were employed to improve the cation stoichiometry in the films. The use of 7% Sr-excess targets resulted in near-stoichiometric films with conductivities close to the stoichiometric bulk counterpart. The measurements show that a fine-tuning of the film stoichiometry is required in order to obtain acceptor doped SrTiO3 thin films with bulk-like properties. One can conclude that, although reciprocal space maps give a first hint whether or not cation non-stoichiometry is present, conductivity measurements are more appropriate for assessing SrTiO3 film quality in terms of cation stoichiometry.

CuSO4/[Cu(NH3)4]SO4-Composite Thermochemical Energy Storage Materials.

The thermochemical energy-storage material couple CuSO4/[Cu(NH3)4]SO4 combines full reversibility, application in a medium temperature interval (<350 °C), and fast liberation of stored heat. During reaction with ammonia, a large change in the sulfate solid-state structure occurs, resulting in a 2.6-fold expansion of the bulk material due to NH3 uptake. In order to limit this volume work, as well as enhance the thermal conductivity of the solid material, several composites of anhydrous CuSO4 with inorganic inert support materials were prepared and characterized with regard to their energy storage density, reversibility of the storage reaction, thermal conductivity, and particle morphology. The best thermochemical energy storage properties were obtained for a 10:1 CuSO4-sepiolite composite, combining an attractive energy storage density with slightly improved thermal conductivity and decreased bulk volume work compared to the pure salt.

Bifunctional Fe(II) spin crossover-complexes based on ω-(1H-tetrazol-1-yl) carboxylic acids.

To increase the supramolecular cooperativity in Fe(ii) spin crossover materials based on N1-substituted tetrazoles, a series of ω-(1H-tetrazol-1-yl) carboxylic acids with chain-lengths of C2-C4 were synthesized. Structural characterization confirmed the formation of a strong hydrogen-bond network, responsible for enhanced cooperativity in the materials and thus largely complete spin-state transitions for the ligands with chain lenghts of C2 and C4. To complement the structural and magnetic investigation, electronic spectroscopy was used to investigate the spin-state transition. An initial attempt to utilize the bifunctional coordination ability of the ω-(1H-tetrazol-1-yl) carboxylic acids for preparation of mixed-metallic 3d-4f coordination polymers resulted in a novel one-dimensional gadolinium-oxo chain system with the ω-(1H-tetrazol-1-yl) carboxylic acid acting as µ2-η2:η1 chelating-bridging ligand.

Cycle Stability and Hydration Behavior of Magnesium Oxide and Its Dependence on the Precursor-Related Particle Morphology.

Thermochemical energy storage is considered as an auspicious method for the recycling of medium-temperature waste heat. The reaction couple Mg(OH)2⁻MgO is intensely investigated for this purpose, suffering so far from limited cycle stability. To overcome this issue, Mg(OH)2, MgCO3, and MgC2O4·2H2O were compared as precursor materials for MgO production. Depending on the precursor, the particle morphology of the resulting MgO changes, resulting in different hydration behavior and cycle stability. Agglomeration of the material during cyclization was identified as main reason for the decreased reactivity. Immersion of the spent material in liquid H2O decomposes the agglomerates restoring the initial reactivity of the material, thus serving as a regeneration step.

Cooperativity in spin crossover materials as ligand's responsibility - investigations of the Fe(ii) - 1,3-bis((1H-tetrazol-1-yl)methyl)bicyclo[1.1.1]pentane system.

Criteria for a technologically relevant spin crossover (SCO) material include temperature and abruptness. A series of Fe(ii) - 1,3-bis((1H-tetrazol-1-yl)methyl)bicyclo[1.1.1]pentane SCO complexes with various anions (BF4-, ClO4-, and PF6-) designed using a structure-property based concept is reported. All complexes feature abrupt SCO-behavior with T1/2 between 170 K and 187 K. These materials demonstrate that without stabilizing the effects of incorporated solvents or a hydrogen bond-network, the observed cooperativity during high-spin-low-spin transition is anion independent and originates only from the rigidity and internal strain of the propellane-moiety in the ligand. Spectroscopy and structural investigations of these materials are supported by quantum chemical calculations.

A Modified Synthetic Pathway for the Synthesis of so far Inaccessible N1-Functionalized Tetrazole Ligands - Synthesis and Characterization of the 1D Chain-Type Spin Crossover Compound [Fe(3ditz)3](BF4)2.

A modified phase-transfer-catalyst-assisted synthetic pathway was developed that widens the pool of accessible 1-substituted tetrazoles, which are possible ligands for iron(II) spin-crossover compounds. Within the family of α,ω-bis(tetrazol-1-yl)alkanes, a series of ligands and their respective iron(II) spin-crossover compounds were synthesized and structurally and spectroscopically characterized in the past. The classical route to prepare these ligands is based on the respective amino-precursors. Hence the pool of accessible compounds is limited by the commercial or synthetical availability of α,ω-diaminoalkanes. Furthermore, the concomitant transformation to the tetrazole moieties turns out to be easier for diamino-alkanes with an even number of carbon atoms than for those with an odd number. In line with this observation, the shortest odd-numbered homologues such as 1,1-bis(tetrazol-1-yl)methane (1ditz) and 1,3-bis(tetrazol-1-yl)propane (3ditz) were inaccessible so far. In this paper, we report the successful preparation and characterisation of the classically inaccessible 1,3-bis(tetrazol-1-yl)propane (3ditz) and of its spin-crossover complex [Fe(3ditz)3](BF4)2, which features an abrupt and almost complete spin transition at T[Formula: see text] = 159 K. The single-crystal X-ray structure of the low-spin and the high-spin species is presented. The magnetic data are supported by variable-temperature IR, UV/Vis/NIR, and 57Fe Mössbauer spectra.