Defect Chemical Model Research Articles

Field-Driven Solid-State Defect Control of Bilayer Switching Devices.

We develop a framework for controlling and investigating reversible ionic transfer between two solid metal oxides layers by examining field-driven changes in electrical properties of the thin film bilayer oxide system Pr0.1Ce0.9O2/La1.85Ce0.15CuO4 (PCO/LCCO). We show that we can reversibly redistribute oxygen ions by applied voltage in a highly controlled and reversible fashion near ambient temperatures over large oxygen ion activity limits, which, for the first time, is directly interpretable by defect chemical models. This allowed us to determine how defect concentrations in each layer systematically varied with voltage and the subsequent impact on each film's conductance. These results showcase the relevance and applicability of defect chemical models, traditionally considered only at elevated temperatures, to the development of bilayer devices of importance to neuromorphic memory applications. This allows for a more systematic approach for studying and understanding the solid-solid exchange process in electrochemically controlled microelectronic devices. Moreover, our work sets the foundation for the development of large-area field-driven defect-controlled bilayer switching devices with potential application to a broad array of functionally modulated devices.

Defect Chemistry of Spinel Cathode Materials-A Case Study of Epitaxial LiMn2O4 Thin Films.

Spinels of the general formula Li2-δM2O4 are an essential class of cathode materials for Li-ion batteries, and their optimization in terms of electrode potential, accessible capacity, and charge/discharge kinetics relies on an accurate understanding of the underlying solid-state mass and charge transport processes. In this work, we report a comprehensive impedance study of sputter-deposited epitaxial Li2-δMn2O4 thin films as a function of state-of-charge for almost the entire tetrahedral-site regime (1 ≤ δ ≤ 1.9) and provide a complete set of electrochemical properties, consisting of the charge-transfer resistance, ionic conductivity, volume-specific chemical capacitance, and chemical diffusivity. The obtained properties vary by up to three orders of magnitude and provide essential insights into the point defect concentration dependences of the overall electrode potential. We introduce a defect chemical model based on simple concentration dependences of the Li chemical potential, considering the tetrahedral and octahedral lattice site restrictions defined by the spinel crystal structure. The proposed model is in excellent qualitative and quantitative agreement with the experimental data, excluding the two-phase regime around 4.15 V. It can easily be adapted for other transition metal stoichiometries and doping states and is thus applicable to the defect chemical analysis of all spinel-type cathode materials.

Defect Chemistry and Mixed Conduction in Lithium Lanthanum Titanate During the Transition from Electrolyte to Anode Material

Lithium lanthanum titanate Li0.29+δ La0.57TiO3 (LLTO) is a promising material in Li ion battery application, due to its ambient stability and high ionic conductivity. When it is subjected to a high Li chemical potential, additional Li ions intercalate into vacant A sites, which is balanced by the reduction of Ti4+ ions to Ti3+. At this point, LLTO becomes a mixed ion and electron conductor, which means that it undergoes a transition from an electrolyte to a high rate capable electrode material in the potential range below ca 1.7 V vs Li metal. However, the exact voltage of the transition from electrolyte to the electrode, as well as the electronic conductivity of reduced LLTO were still unknown. Here, we investigate the thermodynamics of lithium insertion as well as ion and electron conductivity of reduced LLTO by employing a galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS). We can show that LLTO gradually changes from electrolyte material to a mixed conductor, with an ion transference number that depends on the Li chemical potential. Lastly, we present a defect chemical model that fits excellently to the U(δ) curves and the conductivity data.

Resistive, Temperature-Independent Metal Oxide Gas Sensor for Detecting the Oxygen Stoichiometry (Air-Fuel Ratio) of Lean Engine Exhaust Gases.

This study presents a resistive sensor concept based on Barium Iron Tantalate (BFT) to measure the oxygen stoichiometry in exhaust gases of combustion processes. The BFT sensor film was deposited on the substrate by the Powder Aerosol Deposition (PAD) method. In initial laboratory experiments, the sensitivity to pO2 in the gas phase was analyzed. The results agree with the defect chemical model of BFT materials that suggests the formation of holes h• by filling oxygen vacancies VO•• in the lattice at higher oxygen partial pressures pO2. The sensor signal was found to be sufficiently accurate and to have low time constants with changing oxygen stoichiometry. Further investigations on reproducibility and cross-sensitivities to typical exhaust gas species (CO2, H2O, CO, NO, …) confirmed a robust sensor signal that was hardly affected by other gas components. The sensor concept was also tested in real engine exhausts for the first time. The experimental data showed that the air-fuel ratio can be monitored by measuring the resistance of the sensor element, including partial and full-load operation modes. Furthermore, no signs of inactivation or aging during the test cycles were observed for the sensor film. Overall, a promising first data set was obtained in engine exhausts and therefore the BFT system is a possible cost-effective alternative concept to existing commercial sensors in the future. Moreover, the integration of other sensitive films for multi-gas sensor purposes might be an attractive field for future studies.

Lithium ion transport in water-containing Li(SCN) over a wide compositional range: From water doping to hydration

This work explores the Li+ cation transport mechanisms in the system Li(SCN) · x H2O, complementary to our previous studies on crystal structures and hydration properties of lithium thiocyanate Li(SCN). Anhydrous Li(SCN) is a Schottky defective material with lithium vacancies VLi' being the dominant mobile defect. Two water incorporation mechanisms are identified: (i) At very low concentrations H2O acts as a dopant and forms H2OSCN• defects, constituting a substitutive incorporation mechanism. Doping with water not only increases the concentration of VLi' in Li(SCN), but also the vacancy mobility, making H2O-doping a special case. A complete quantitative defect chemical model is derived, including enthalpies and entropies of defect formation, association, and migration. (ii) Higher water activities induce chemical and structural changes in Li(SCN) · x H2O, i.e. hydration by reactive incorporation occurs. Single-phases as well as composites are investigated for their Li+ ion transport mechanisms. In case of x ≤ 1, defect associates determine the conductivity resulting in exceptionally high activation enthalpies. Further hydration to 1 < x < 2 creates composites of solid mono- and dihydrates, in which the low-melting diyhdrate can form percolating pathways and increase the conductivity to values as high as 7 × 10−4 S/cm at room temperature. Further increase of x leads to complete liquefaction. The entire range of x in the Li(SCN) – H2O system was studied regarding ion transport properties, identifying the individual mechanisms and temperature dependences. The results of this work deepen the understanding of hydration effects on solid ion conductors in solid state ionics, and help to elucidate transport phenomena in salt – solvent systems.

Resistive Switching of Perovskite‐Type Oxides Using the Hebb–Wagner Polarization Method

Resistive switching, the change between a high‐resistive OFF state and a low‐resistive ON state, is well known for thin film oxides sandwiched between two ion‐blocking electrodes. Herein, the possibility for resistive switching in perovskite‐type oxides using the Hebb–Wagner polarization setup that uses an ion‐blocking and a reversible electrode is investigated. The resistive switching behavior is simulated numerically in terms of a defect chemical and transport model that describes the polarization of the model system SrTiO3 between two different electrodes by applying an ac voltage. Corresponding experiments are also performed and the experimental results are compared with the simulation results. It is shown that the Hebb–Wagner setup allows the bulk resistive state to change not only for thin films but also for large sample thicknesses using low maximum voltage values, without the need for initially high voltages to form filaments. The effect of other parameters and phenomena on the switching behavior, like maximum voltage, surface exchange coefficient, sample thickness, and a Schottky contact between an electrode and the semiconducting oxide, is also determined. The experimental results show a high agreement with the numerical simulations, demonstrating that the Hebb–Wagner polarization setup enables bulk resistive switching of perovskite‐type oxides.

The influence of step potentials upon defect equilibria and defect concentrations inside solid oxide fuel cells

The defect chemical models with semi-empirical equilibrium constants are widely adopted to determine defect concentrations inside solid oxide materials. However, the equilibrium constants are tested under uniform atmosphere, which can not be directly applied to working fuel cells. Step potentials are formed at the electrode/electrolyte interfaces of working fuel cells, resulting from the completely different atmosphere between anode and cathode. In this work, the physically meaningful potential framework of working fuel cells and the strong relationship between charged defects and step potentials are established. One part of the Gibbs energy of electrode reactions is converted into electrical energy and stored in the step potentials. The accurate equilibrium constants of SOFC electrode reactions are corrected by considering the step potentials.

Broad Applicability of Electrochemical Impedance Spectroscopy to the Measurement of Oxygen Nonstoichiometry in Mixed Ion and Electron Conductors.

Oxygen nonstoichiometry is a fundamental feature of mixed ion and electron conductors (MIECs). In this work, a general electrochemical method for determining nonstoichiometry in thin film MIECs, via measurement of the chemical capacitance, is demonstrated using ceria and ceria-zirconia (Ce0.8Zr0.2O2-δ) as representative materials. A.C. impedance data are collected from both materials at high temperature (750-900 °C) under reducing conditions with oxygen partial pressure (pO2) in the range 10-13 to 10-20 atm. Additional measurements of ceria-zirconia films are made under relatively oxidizing conditions with pO2 in the range 0.2 to 10-4 atm and temperatures of 800-900 °C. Under reducing conditions, the impedance spectra are described by a simple circuit in which a resistor is in series with a resistor and capacitor in parallel, and thickness-dependent measurements are used to resolve the capacitance into interfacial and chemical terms. Under more oxidizing conditions, the impedance spectra (of Ce0.8Zr0.2O2-δ) reveal an additional diffusional feature, which enables determination of the ionic resistance of the film in addition to the capacitance, and hence the transport properties. A generalized mathematical formalism is presented for recovering the nonstoichiometry from the chemical capacitance, without recourse to defect chemical models. The ceria nonstoichiometry values are in good agreement with literature values determined by thermogravimetric measurements but display considerably less scatter and are collected on considerably shorter time scales. The thermodynamic analysis of Ce0.8Zr0.2O2-δ corroborates earlier findings that introduction of Zr into ceria enhances its reducibility.

Analysis of defect chemistry and microstructural effects of non-stoichiometric ceria by the high-temperature microwave cavity perturbation method

The defect chemistry of ceria has been studied using a microwave resonator. The dielectric properties of powder samples were investigated up to 600 °C and at different oxygen partial pressures pO2 (10−26 – 0.2 bar) and evaluated using the microwave cavity perturbation theory. For the ceria powder, an activation of the oxygen incorporation and release kinetics was observed from 480 °C onwards. The data suggest that the increased losses in non-stoichiometric ceria originate from the small polaron hopping mechanism, as known from literature. In addition, an increase in material polarization due to the formation of oxygen vacancies was experimentally observed. A comparison with sintered, coarse-grained samples of the same material and with literature data also demonstrated the importance of the microstructure for the defect chemistry of ceria. For the powder (ca. 80 nm grain size), significantly higher electronic conductivities (assumedly due to the higher concentration of small polarons) and lower activation energies were detected at 600 °C. On the other hand, lower conductivities, controlled by the concentration of acceptor cations in the material were reported for the sintered samples (1.1 μm) at this temperature. The findings are well in-line with existing defect chemical models of ceria. This study presents an analysis of the microwave behavior of ceria and provides insights into intrinsic and extrinsic mechanisms of oxygen exchange in ceria. The findings also contribute to a better understanding of microwave-based state diagnosis of three-way catalysts in automotive applications.

Electrochemically controlled defect chemistry: From oxygen excess to deficiency

Fundamental understanding of materials properties requires the study and development of defect chemical reactions and models. In-depth analysis of the defect chemistry of metal oxides, however, has generally been limited to the oxygen deficient regime. Here, we present a defect chemical study covering both oxygen deficient and excess regimes using donor-doped layered cuprate (La1.85Ce0.15CuO4+δ) thin films over a wide range of oxygen activity levels. Unlike perovskite- or fluorite-structured oxides, layered cuprates can readily accommodate oxygen interstitials as well as vacancies. The scope of the defect chemical study was extended to include highly oxidizing conditions equivalent to many orders of magnitude higher oxygen activities than ambient pressure. This was achieved by electrochemical pumping of oxygen through an ionically conducting yttria-stabilized zirconia (YSZ) substrate with oxygen nonstoichiometry values derived from in situ chemical capacitance measurements between 500°C and 650°C. The defect chemical model was correlated with in-plane conductivity at different oxygen pumping levels in the temperature regime of 275~350°C. Thermodynamic parameters - thermal band gap, enthalpy of reduction and anion Frenkel-pair formation - were derived from a defect chemical analysis. The approach taken here can be extended to other layered oxides and opens up the possibility of accessing much broader oxygen activity limits and thereby an extended range of defect regimes and neighboring phases.

Electric transport properties of rare earth doped YbxCa1-xMnO3 ceramics (part III: Point defect chemistry)

A defect chemical model based on charge neutrality and laws of mass action is proposed to clarify the details of the chemistry of point defects for donor-doped YbxCa1-xMnO3. DC-conductivity measurements were carried out in a wide range of partial pressure of oxygen p(O2) ≈ 10−1 down to 10-19 MPa at 750⁰C for the first time without disintegrating the ceramic sample through reduction. A comparison of the experimental observations and the theoretical defect chemical models clearly shows the possibilities for controlling charge carriers in dependence of partial pressure of oxygen p(O2) and dopant concentration. The origin of a plateau state, of a drastic decrease in conductivity in the intermediate and reduction p(O2) regimes are figured out, respectively. In addition, the kind and concentration of the electronic and ionic majority charge carriers are determined and formulated according to the proposed defect chemical model. Furthermore, phase transitions were studied in a wide range of p(O2) at elevated temperatures

The effect of Fe‐acceptor doping on the electrical properties of Na1/2Bi1/2TiO3 and 0.94 (Na1/2Bi1/2)TiO3–0.06 BaTiO3

AbstractNa1/2Bi1/2TiO3 (NBT) based ceramics are amongst the most promising lead‐free ferroelectric materials. It was expected that the defect chemistry and the effect of doping of NBT would be similar to that observed for lead based materials, however, acceptor doping does not lead to ferroelectric hardening. Instead, high oxygen ionic conductivity is induced. Nevertheless, for solid solutions with BaTiO3 (BT), which are more relevant with respect to ferroelectric applications, such a drastic change of electrical properties has not been observed so far. To rationalize the difference in defect chemistry between NBT and its solid solution 94(Na1/2Bi1/2TiO3)–0.06 BaTiO3 (NBT–6BT) compositions with different concentrations of Fe‐dopant were investigated. The study illustrates that the materials exhibit very similar behavior to NBT, and extraordinarily high oxygen ionic conductivity could also be induced in NBT–6BT. The key difference between NBT–6BT and NBT is the range of the dependence of ionic conductivity with dopant concentration. Previous studies of NBT–6BT have not reached sufficiently high dopant concentrations to observe high conductivity. In consequence, the same defect chemical model can be applied to both NBT and its solid solutions. This will help to rationalize the effect of doping on ferroelectric properties of NBT‐ceramics and defect chemistry related degradation and fatigue.

Oxygen partial pressure dependency of the electrical conductivity of aerosol deposited alumina films between 650 °C and 900 °C

The dependence of the electrical conductivity on the oxygen partial pressure of alumina films produced by the Aerosol Deposition Method (ADM) was investigated. The samples were contacted in a three-electrode setup according to DIN IEC 60093:1993-12 for high insulating materials and the conductivity was measured as a function of the oxygen partial pressure between 650 °C and 900 °C. The conductivity increases with the oxygen partial pressure and thus shows p-type behavior. Constant final values are reached even at these low temperatures. In the double logarithmic representation, a slope of approx. 3/16 can be determined. Following a defect chemical approach for M2O3 (M represents a metal) and the assumption of Schottky disorder, a defect chemical model can be established.

Transport and Charge Carrier Chemistry in Lithium Oxide

Lithium oxide (Li2O) is a highly relevant material for battery applications, and as a binary antifluorite compound of first-row elements, it is equally interesting for basic science. This work investigates the behavior of ionic and electronic charge carriers in Li2O. The predominantly ionic conductivity is shown to be well-explained by a defect chemical model based on Frenkel disorder, vacancy migration, and vacancy-dopant association. The enthalpies and entropies of these three processes are derived, and good agreement is seen to isostructural Li2S, SrF2, and BaF2. An upper bound is determined for the electronic conductivity of Li2O, which is very low. These results provide more reliable thermodynamic and kinetic parameters for future rigorous treatments of Li2O in batteries. For example, even under favorable doping conditions, the ionic conductivity of bulk crystalline Li2O (with no higher-dimensional defects or interfacial effects) is multiple orders of magnitude too slow to account for the resistance of typical solid-electrolyte interface (SEI) layers.

Transport and Charge Carrier Chemistry in Lithium Sulfide

AbstractLithium sulfide is a functional material of great importance for battery research, since it is the discharge product in Li–S cathodes and a frequent component of anode passivation layers. In both cases, transport of charge carriers in Li2S is critical for performance. The exploration of charge carrier chemistry in such a simple binary compound is also of fundamental scientific interest. For that purpose, impedance spectroscopy and electromotive force measurements are performed over a broad range of temperatures and doping conditions. The results indicate predominant ion conduction and can be quantitatively explained by a defect chemical model based on Frenkel disorder and vacancy‐dopant association. Mobilities and migration barriers for both vacancy and interstitial defects are deduced. The thermodynamic and kinetic parameters derived for Li+ transport in antifluorite Li2S show remarkable agreement with the analogous parameters for F− transport in fluorite compounds such as BaF2, thereby improving the structural understanding of charge carrier chemistry in such compounds. An application of these results to passivation layers in solid state batteries is also discussed.

Charge carrier chemistry in methylammonium lead iodide

Both electronic and ionic transport properties are key issues in the performance of perovskite solar cells. In order to understand the electrical behavior of organic-inorganic halide perovskites, and the possibilities to influence them, the elucidation of their equilibrium charge carrier chemistry as a function of stoichiometry and dopant concentration is a necessary and fundamental prerequisite. We provide here insight into the point defect chemistry of methylammonium lead iodide, whereby the decisive carriers are identified and their concentrations discussed as a function of stoichiometry (iodine partial pressure) and dopant content (oxygen partial pressure or Na addition). The experimental results indicate how ionic conductivity, which can be attributed to iodine vacancies, and electronic conductivity, which can be attributed to electron holes (or conduction electrons under reducing conditions), can be significantly and systematically altered by such treatments. Experimental results are discussed in the context of simple defect chemical models.

Electro-chemo-mechanical studies of perovskite-structured mixed ionic-electronic conducting SrSn1-xFexO3-x/2+δ Part III: Thermal and chemical expansion

The thermal and chemical expansion of a potential solid oxide fuel cell (SOFC) cathode material SrSn0.65Fe0.35O3–0.35/2+δ (SSF35) were investigated to assess its thermo-chemo-mechanical stability at SOFC operating temperatures and to establish the correlation between defect concentrations (oxygen vacancies and electrons) and chemical expansion with the aid of the defect chemical model reported in part I of this study. Thermochemical expansion was measured as a function of temperature and oxygen partial pressure. The chemical expansion of SSF35 showed a strong correlation with changes in oxygen nonstoichiometry associated with changes in Fe valence state. Coefficients of both chemical (CCE) and thermal (CTE) expansion were calculated and found to be smaller than that of the closely related mixed conducting perovskite oxide SrTi0.65Fe0.35O3–0.35/2+δ (STF35). The thermal expansion coefficient of SSF was found to be close to that of YSZ (most popular solid oxide electrolyte), which makes SSF35 more attractive in terms of overall thermo-chemical stability. The chemical expansion of SSF35 showed decreasing CCE with increasing temperature and decreasing CTE with increasing oxygen deficiency, both opposite to the trends observed for STF35. Distortion in symmetry from the cubic structure seems to be responsible for the smaller coefficients and increasing asymmetry with expansion seems accountable for opposite trends of CCE and CTE compared to the STF counterpart.

On the defect chemistry of BaFe0.89Al0.01Ta0.1O3‐δ, a material for temperature independent resistive and thermoelectric oxygen sensors

The Seebeck-coefficient (aka thermopower) is a less wide-spread parameter to determine constants for defect chemical models of semiconductor oxides. It is a measure for the charge carrier concentration. In contrast to the often investigated electrical conductivity, it has the benefit of being independent of the material geometry. BaFe0.7Ta0.3O3‐δ is known as a temperature-independent conductometric oxygen sensor material with perovskite crystal structure. The present work considers the Seebeck-coefficient as well as the electrical conductivity of bulk and thick-film BaFe0.89Al0.01Ta0.1O3‐ δ (BFAT10). Seebeck-coefficient and electrical conductivity were measured simultaneously between 600 and 950°C under varying oxygen partial pressures from 10−24 to 1bar. BFAT10 thick films have been successfully deposited by the novel Aerosol Deposition Method (ADM) at room temperature on a special transducer. The electrical conductivity of thick-film sensors shows almost no temperature dependency but depends strongly on the oxygen partial pressure in the temperature range from 600 to 800°C. An n-to-p-type transition was observed in the investigated oxygen partial pressure and temperature range. The defect parameters were derived from the experimental results and an initial defect model for BFAT10 is discussed. A hopping-type conduction mechanism is assumed due to the very low charge carrier mobility.

Design of Oxygen Vacancy Configuration for Memristive Systems.

Oxide-based valence-change memristors are promising nonvolatile memories for future electronics that operate on valence-change reactions to modulate their electrical resistance. The memristance is associated with the movement of oxygen ionic carriers through oxygen vacancies at high electric field strength via structural defect modifications that are still poorly understood. This study employs a Ce1-xGdxO2-y solid solution model to probe the role of oxygen vacancies either set as "free" or as "immobile and clustered" for the resistive switching performance. The experiments, together with the defect chemical model, show that when the vacancies are set as "free", a maximum in memristance is found for 20 mol % of GdO1.5 doping, which clearly coincides with the maximum in ionic conductivity. In contrast, for higher gadolinia concentration, the oxide exhibits only minor memristance, which originates from the decrease in structural symmetry, leading to the formation of "immobile" oxygen defect clusters, thereby reducing the density of mobile ionic carriers available for resistive switching. The research demonstrates guidelines for engineering of the oxide's solid solution series to set the configuration of its oxygen vacancy defects and their mobility to tune the resistive switching for nonvolatile memory and logic applications.

Electronic conduction mechanism and defect chemical model of LaNi0.4Fe0.6O3 − δ

In order to elucidate the electronic conduction mechanism and defect chemical model of LaNi0.4Fe0.6O3−δ at high temperatures, electrical conductivity (σ), Seebeck coefficient (S), and oxygen vacancy concentration (δ) of LaNi0.4Fe0.6O3−δ were measured as a function of oxygen partial pressure (p(O2)) and temperature. Relatively large σ and small positive S values are observed, which indicates the contribution of ionic conduction is negligibly small to thermoelectric power and the major electronic carrier is a hole. From the analysis of σ and S, it is expected that the LaNi0.4Fe0.6O3−δ has small polaron hopping conduction mechanism where an electron is localized on the Fe. The slope of δ vs p(O2) shows a minimum value near the stoichiometric composition and the δ increases as p(O2) reduces and temperature increases. The relationship between δ vs. log p(O2) is analyzed by a localized electron model and the behavior of the oxygen nonstoichiometry of LaNi0.4Fe0.6O3−δ can be explained.