Oscillatory kinetics at solid/solid phase boundaries in ionic crystals

The transfer of matter and charge across interfaces between two solids is related to defect relaxation in the regions near the interface. A transfer rate which exceeds the rate of defect relaxation may lead to degradation of the interface, causing a feedback effect for the transfer process itself. As a consequence, non-linear phenomena (dissipative structures) like periodic oscillations of the interfacial properties can occur under conditions far from equilibrium. Possible mechanisms and experimental examples are discussed

Nonstoichiometry and reactivity of lithium solid electrolytes for solid state batteries

Ideal solid electrolytes are ion-conducting phases with negligible nonstoichiometry – as any nonstoichiometry is usually related to unwanted changes in (electronic) transport properties. In solid-state batteries the solid electrolyte experiences extreme chemical boundary conditions, from very high chemical potential of Li at the anode side to very low chemical potential at the cathode side. Depending on the electrodes, the range of chemical potential can span more than 4 V (expressed as OCV). Under these conditions, the redox chemistry and the nonstoichiometry of solid electrolytes becomes decisive for the function of the batteries itself. Please click Additional Files below to see the full abstract

The magnetoresistance of homogeneous and heterogeneous silver-rich silver selenide

The magnetoresistance (MR) effect of the low-temperature phase of silver selenide (-Ag2 + Se) is measured as a function of composition. Very small composition variations in the order of = 10–6 are achieved by coulometric titration and can be performed simultaneously during the MR measurement. A homogeneous Ag2 + Se shows an ordinary magnetoresistance (OMR) effect, which can be well described by the two-band model. For silver selenide with a heterogenous silver excess, we found quite a different MR behavior. Up to a minor silver excess of 1×10–4 10–2) shows again an OMR effect

Structure formation and electrical properties of thin films: The Ce-Ti-O system

Recently Nagarajan et al. found a new type of chemically driven insulator-metal transition in non-stoichiometric gallium oxide films (GaOx) [1]. Leichtweiß et al. [2] showed a similar behavior for non-stoichiometric titania (TiO1.6) films. Both oxides disproportionate during heating and relax into the thermodynamically stable phases. A similar approach has been applied to ternary oxides like ITO [3]. Davila et al. produced a nanocomposite and highly oxygen deficient ITO film. Due to its non-stoichiometry the thin film separates directly into a stoichiometric matrix (In1.8Sn0.2O3) which contains metallic clusters (In1.8Sn0.2). This can be explained by the absence of stable suboxides in the system In-Sn-O. Figure 1 highlights the general experimental approach: Depositing metastable and non-stoichiometric oxide thin films followed by a heating step makes it possible to study the reaction pathways during relaxation into the thermodynamically stable state by various in and ex situ techniques. Please click Additional Files below to see the full abstract

La1-xSrxMnO3±δ as a nonstoichiometric model system for the catalysis of oxygen evolution reaction

The main issue of electrochemical water splitting is the search for suitable catalyst materials for the kinetically hindered oxygen evolution reaction (OER) at the anode. The state-of-art precious metal catalysts suffer from their high price and insufficient long term stability in the common electrolytes. Binary or multinary transition metal oxides in alkaline medium are an cost-efficient alternative, since they can achieve both lower overvoltages as well as better long-term stability than precious metal oxides. One of the best-studied materials for SOFC applications is the perovskite system La1-xSrxMnO3±δ (LSMO). Due to its general nonstoichiometry and the large variety of possible defect species it is a perfect model system for studying the influence of defect chemistry on the catalytic activity in OER. We have systematically investigated LSMO as a nonstoichiometric model catalyst system for OER in alkaline media. Nanocrystalline powders over the whole composition range have been prepared by a sol-gel based auto combustion method. XPS and XRD analysis verified the presence of pure phase materials with a continuous change of manganese oxidation state from Mn3+ to Mn4+. Measurements in an electrochemical RDE setup showed a clear trend in catalytic activity in OER with the highest values at medium La/Sr compositions. An equivalent trend could also be observed in the electrical conductivity of the powders, leading to the assumption of a higher polaron hopping probability at medium La/Sr compositions. Additional annealing of pristine powder samples in oxidizing and reducing atmospheres caused a further change in manganese oxidation state and ongoing electrochemical measurements should reveal whether the defined adjustment of the nonstoichiometry will lead to an improvement of catalytic activity compared to the untreated catalysts

Influence of synthesis parameters on crystallization behavior and ionic conductivity of the Li4_{4}PS4_{4}I solid electrolyte

Superionic solid electrolytes are key to the development of advanced solid-state Li batteries. In recent years, various materials have been discovered, with ionic conductivities approaching or even exceeding those of carbonate-based liquid electrolytes used in high-performance Li-ion batteries. Among the different classes of inorganic solid electrolytes under study, lithium thiophosphates are one of the most promising due to their high Li-ion conductivity at room temperature and mechanical softness. Here, we report about the effect of synthesis parameters on the crystallization behavior and charge-transport properties of Li$_{4}$PS$_{4}$I. We show that thermally induced crystallization of Li$_{4}$PS$_{4}$I (P4/nmm), starting from the glassy phase 1.5Li$_{2}$S–0.5P$_{2}$S$_{5}$–LiI, adversely affects the material’s conductivity. However, both conductivity and crystallization temperature can be significantly increased by applying pressure during the preparation

Chemische Triebkräfte : Von der Verbrennung zum Herzschlag

Was hat eine einfache chemische Verbrennung mit dem Herzschlag zu tun? Diese Frage soll im Rahmen einer physikalisch-chemischen Experimentalvorlesung beantwortet werden. Dabei fügen sich anscheinend weit voneinander entfernte Phänomene in ein klar strukturiertes Bild. Es wird deutlich, dass der menschliche Körper aus thermodynamischer Sicht eine biologische \u27Wärmekraftmaschine\u27 ist, deren Wirkungsgrad höher als der Wirkungsgrad mechanische Wärmekraftmaschinen (\u27Dampfmaschinen\u27). Spröde anmutende thermodynamische Sätze füllen sich mit Leben - die Begriffe \u27Energie, Wärme, Entropie\u27 werden anschaulich. Anhand einfacher Demonstrationsexperimente wird ein Bogen über 250 Jahre Forschung im Bereich der chemischen Energiewandlung gespannt

Novel anion conductors - conductivity, thermodynamic stability and hydration of anion-substituted mayenite-type cage compounds C(12)A(7):X (X = O, OH, Cl, F, CN, S, N)

Mayenite (Ca12Al14O33) is a highly interesting functional material not only in view of its unique crystal structure as a cage compound but also for its variety of possible applications. Its ability to incorporate foreign ions into the cage structure opens the possibility to create new types of solid electrolytes and even electrides. Therefore, the conductivity of various anion substituted mayenites was measured as a function of temperature. Due to controversial reports on the stability of mayenite under specific thermodynamic conditions (dry, wet, reducing, and high temperature), a comprehensive study on the stability was performed. Mayenite is clearly not stable under dry conditions (ppm H2O < 100) at temperatures above 1050 degrees C, and thus, the mayenite phase vanishes from the calcium aluminate phase diagram below a minimum humidity. Two decomposition reactions were observed and are described in detail. To get further insight into the mechanism of hydration of mayenite, the conductivity was measured as a function of water vapour pressure in a range of -5 <= lg[pH(2)O/bar] <= -1.6 at temperatures ranging from 1000 degrees C <= theta <= 1200 degrees C. The hydration isotherms are described with high acuracy by the underlying point defect model, which is confirmed in a wide range of water vapour pressure