Ab initio and empirical defect modeling of LaMnO(3±δ) for solid oxide fuel cell cathodes

Sr doped LaMnO(3) is a perovskite widely used for solid oxide fuel cell (SOFC) cathodes. Therefore, there is significant interest in its defect chemistry. However, due to coupling of defect reactions and inadequate constraints of the defect reaction equilibrium constants obtained from thermogravimetry analysis, large discrepancies (up to 4 eV) exist in the literature for defect energetics for Sr doped LaMnO(3). In this work we demonstrate how ab initio energetics and empirical modelling can be combined to develop a defect model for LaMnO(3). Defect formation enthalpies, including concentration dependence due to defect interactions, are extracted from ab initio energies calculated at various defect concentrations. Defect formation entropies for the defect reactions in LaMnO(3) involving O(2-)(solid) ? ?O(2)(gas) + 2e(-) are shown to be accessible through combining the gas phase thermodynamics and simple models for the solid phase vibrational contributions. This simple treatment introduces a useful constraint on fitting defect formation entropies. The predicted defect concentrations from the model show good agreement with experimental oxygen nonstoichiometry vs. P(O(2)) for a wide range of temperatures (T = 873-1473 K), suggesting the effectiveness of the ab initio defect energetics in describing the defect chemistry of LaMnO(3). Further incorporating a temperature dependent charge disproportionation energy within 0.0-0.2 eV, the model is capable of describing both defect chemistry and oxygen tracer diffusivity of LaMnO(3). The model suggests an important role for defect interactions which are typically excluded from LaMnO(3) defect models, and sensitivity of the oxygen defect concentration to the charge disproportionation energy in the high P(O(2)) region. Similar approaches to those used here can be used to model the defect chemistry for other complex oxides.     

Optimization of ionic conductivity in solid electrolytes through dopant-dependent defect cluster analysis

Atomistic simulation based on an energy minimization technique has been carried out to investigate defect clusters of R(2)O(3) (R = La, Pr, Nd, Sm, Gd, Dy, Y, Yb) solid solutions in fluorite CeO(2). Defect clusters composed of up to six oxygen vacancies and twelve accompanied dopant cations have been simulated and compared. The binding energy of defect clusters increases as a function of the cluster size. A highly symmetric dumbbell structure can be formed by six oxygen vacancies, which is considered as a basic building block for larger defect clusters. This is also believed to be a universal vacancy structure in an oxygen-deficient fluorite lattice. Nevertheless, the accurate positions of associated dopants depend on the dopant radius. As a consequence, the correlation between dopant size and oxygen-ion conductivity has been elucidated based on the ordered defect cluster model. This study sheds light on the choice of dopants from a physical perspective, and suggests the possibility of searching for optimal solid electrolyte materials through atomistic simulations.     

In situ evaluation of defect size distribution for supported zeolite membranes

The permporometry measurements are performed with respect to a series of zeolite membranes with different defect sizes, which can be further applied for in situ measurement of the defect size distribution in zeolite membranes. Gas permeation experiments are conducted for CO₂/N₂ gas mixture to test the separation performance of the studied zeolite membranes. By taking into account the “t-layer” on defect walls, a mathematical model and the corresponding procedure are developed so that the defect size distribution in zeolite membranes can be calculated by using the results of permporometry measurements. The defect size distribution and the maximal defect size show a good correlation with the separation performance of CO₂/N₂ gas mixture for zeolite membranes. It is demonstrated that the separation performance of zeolite membranes is mainly determined by large defects. It has been shown that the permporometry-based methodology proposed in this contribution is an effective way for the quality evaluation of zeolite membranes.     

Thermal Destiny of an Ionic Crystal

Recently it has been shown that defect‐defect interactions leading to anomalous rise in defect concentrations and eventually to phase transitions into a disordered state can be successfully described by a quasi defect lattice model [1]. Using this approach the fact is highlighted that the evolution of an ionic crystal from the perfect state at T = 0 K to a weakly defective ideal crystal and eventually via passing a pre‐melting regime to a superionic crystal (or melt) is unavoidable (given sufficient stability) and can be roughly phenomenologically calculated by using a few basic parameters only. Semi‐empirical relations (e. g. Tammann rule) can be quantified in this way.     

Ordered structures of defect clusters in gadolinium-doped ceria

The nano-domain, with short-range ordered structure, has been widely observed in rare-earth-doped ceria. Atomistic simulation has been employed to investigate the ordering structure of the nano-domain, as a result of aggregation and segregation of dopant cations and the associated oxygen vacancies in gadolinium-doped ceria. It is found that the binding energy of defect cluster increases as a function of cluster size, which provides the intrinsic driving force for the defect cluster growth. However, the ordered structures of the defect clusters are different from the chain model as previously reported. Adjacent oxygen vacancies prefer to locate along <110>/2 lattice vector, which results in a unique stable structure (isosceles triangle) formation. Such isosceles triangle structure can act as the smallest unit of cluster growth to form a symmetric dumbbell structure. This unique dumbbell structure is hence considered as a building block for the development of larger defect clusters, leading to nano-domain formation in rare-earth-doped ceria.     

Dimensional stability and defect chemistry of doped lanthanum chromites

Acceptor doped lanthanum chromites are potential interconnect materials to be used in high temperature Solid Oxide Fuel Cells (SOFC). However, instability of these materials when exposed to low oxygen partial pressure causes a volume expansion that can be detrimental to the SOFC performance. The stability of La_0.8Sr_0.2Cr_0.97V_0.03O₃ is determined as function ofpO₂ and temperature by isothermal thermogravimetry and dilatometry. The experimental data are analysed using a simple model for the defect chemistry. The relation between expansion behaviour and change in defect chemistry is discussed using a simple structural model.This work was supported by the Danish Energy Agency and ELSAM under the DK-SOFC programme. Dr. T. R. Armstrong, The Pacific Northwest Laboratory is thanked for valuable discussions.     

Active Sites and Mechanisms for Direct Oxidation of Benzene to Phenol over Carbon Catalysts

The direct oxidation of benzene to phenol with H₂O₂ as the oxidizer, which is regarded as an environmentally friendly process, can be efficiently catalyzed by carbon catalysts. However, the detailed roles of carbon catalysts, especially what is the active site, are still a topic of debate controversy. Herein, we present a fundamental consideration of possible mechanisms for this oxidation reaction by using small molecular model catalysts, Raman spectra, static secondary ion mass spectroscopy (SIMS), DFT calculations, quasi in situ ATR‐IR and UV spectra. Our study indicates that the defects, being favorable for the formation of active oxygen species, are the active sites for this oxidation reaction. Furthermore, one type of active defect, namely the armchair configuration defect was successfully identified.     

Quantum mechanical criteria for choosing appropriate voltage stabilization additives for polyethylene

One of the major application fields for solid dielectric polymers is their use as insulating materials for power cables. Since the electrical aging of the insulating polymeric materials is one of the most important factors affecting the service lifetime of power cables, developing a model which can be used to design materials with an improved resistance to electrical degradation would be highly beneficial. We developed a model for the electrical field within the polymer material contaminated with a sharp conducting defect (a metallic needle) and defined a parameter characterizing the resistance of polymer to electrical treeing. The model was used to analyse data for the electrical degradation of polyethylene stabilized with polycyclic aromatic hydrocarbons. Based on quantum mechanically calculated electron affinities and ionisation potentials of the stabilizer molecules, we discovered that if a molecule is to be considered for voltage stabilization use it has to have a specific combination of the ionisation potential and adiabatic electron affinity. The model allows for a choice of appropriate voltage stabilizers based on theoretical calculations only and can help to facilitate any experimental study for choosing appropriate voltage stabilizer additives.     

Study of Wave Functions and Radiation Transitions in a Rydberg Polar Cluster Using the Continuum Model

A continuum model describing highly excited (Rydberg) electronic states in clusters composed of polar molecules was proposed. On the basis of this model, the wave functions of Rydberg electronic states of clusters were calculated for a wide range of characteristic cluster parameters. These states are not hydrogen-like and can be described using the quantum defect theory. This fact indicates that radiative transitions can be forbidden within certain ranges of cluster parameters. The quantum defects in clusters and the lifetimes of different states were calculated. The possibility of formation of metastable Rydberg states and anomalous spectral characteristics of Rydberg clusters was shown.     

Influence of intrinsic defects on the properties of zinc oxide

The effects of oxygen vacancies and zinc interstitials on the structure and energy of zinc oxide were studied with the semiempirical MO method MSINDO. Cyclic clusters were chosen as model systems. Single and multiple removal of oxygen atoms and zinc interstitials in zinc oxide served to determine the defect formation energy and the band gap. The interaction between two and three oxygen vacancies was investigated. The vacancies cause a decrease of the band gap, which originates from an occupied defect level. This is also found for zinc interstitials under zinc rich conditions. The defect formation energy of such zinc interstitials is found to be lower than that of oxygen vacancies at 0 K but decreases for oxygen vacancies and increases for zinc interstitials with increasing temperature.     

Phoxonic crystals—a new platform for chemical and biochemical sensors

A new sensor platform is based on so-called phoxonic crystals. Phoxonic crystals are structures designed for simultaneous control of photon and phonon propagation and interaction. They are characterized by a periodic spatial modulation of the dielectric constant as well as elastic properties on a common wavelength scale. Multiple scattering of photons and phonons results in a band gap where propagation of both waves is prohibited. The existence of photonic and phononic band gaps opens up opportunities for novel devices and functional materials. The usage of defect modes is an advantageous concept for measurement. The defect also acts as point of measurement. We show theoretically that the properties of the defect mode can be tailored to provide very high sensitivity to optical and acoustic properties of matter confined within a defect cavity or surrounding the defect or being adsorbed at the cavity surface. In this paper, we introduce the sensor platform and analyze the key features of the sensor transduction scheme. Experimental investigations using a macroscopic device support the theoretical findings.     

How Is Oxygen Incorporated into Oxides? A Comprehensive Kinetic Study of a Simple Solid‐State Reaction with SrTiO3 as a Model Material

The kinetics of stoichiometry change of an oxide--a prototype of a simple solid-state reaction and a process of substantial technological relevance--is studied and analyzed in great detail. Oxygen incorporation into strontium titanate was chosen as a model process. The complete reaction can be phenomenologically and mechanistically understood beginning with the surface reaction and ending with the transport in the perovskite. Key elements are a detailed knowledge of the defect chemistry of the perovskite as well as the application of a variety of experimental and theoretical tools, many of them evolving from this study. The importance of the reaction and transport steps for (electro)chemical applications is emphasized.     

Tune the Fluorescence and Electrochemiluminescence of Graphitic Carbon Nitride Nanosheets by Controlling the Defect States

The effects of defect states on the fluorescence (FL) and electrochemiluminescence (ECL) properties of graphite phase carbon nitride (g-CN) are systematically investigated for the first time. The g-CN nanosheets (CNNSs) obtained at different condensation temperatures are used as the study models. It can be found that all the CNNSs have two kinds of defect states, one is originated from the edge of CNNSs (labeled as CN-defect) and the other is attributed to the partially carbonization regions (labeled as C-defect). Both two kinds of defect states substantially affect the luminescent properties of CNNSs. Both the FL and ECL signals of CNNSs contain a band gap emission and two defect emissions. For the FL of CNNSs, decreasing the density of defect states can increase efficiently the FL quantum yield, while increasing the density of defect states can make the FL spectra red shift. For the ECL of CNNSs, increasing the density of CN-defect states and decreasing the density of C-defect states are greatly important to improve the ECL activity. This work provides a deep insight into the FL and ECL mechanisms of g-CN, and is of significance in tuning the FL and ECL properties of g-CN. Also, it will greatly promote the applications of CNNSs based on the FL and ECL properties.     

Molecular structure and properties of click hydrogels with controlled dangling end defect

In this study, controlled amount of dangling ends is introduced to the two series of poly(ethylene glycol)‐based hydrogel networks with three and four crosslinking functionality by using click chemistry. The structure of the gels with regulated defect percentage is confirmed by comparing the results of low‐field NMR characterization and Monte Carlo simulation. The mechanical properties of these gels were characterized by tensile stress–strain behaviors of the gels, and the results are analyzed by Gent model and Mooney–Rivlin model. The shear modulus of the swollen gels is found to be dependent on the functionality of the network, and decreases with the defect percentage. Furthermore, the value of shear modulus well obeys the Phantom model for all the gels with varied percentage of the defects. The maximum extension ratio, obtained from the fitting of Gent model, is also found to be dependent on the functionality of the network, and does not change with the defect percentage, except at very high defect percentage. The value of the maximum extension ratio is between that predicted from Phantom model and the Affine model. This indicates that at the large deformation, the fluctuation of the crosslinking points is suppressed for some extend but still exists. Polymer volume fractions at various defect percentages obtained from prediction of Flory–Rehner model are found to be in well agreement with the swelling experiment. All these results indicate that click chemistry is a powerful method to regulate the network structure and mechanical properties of the gels. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1227–1236     

Contributions on the Properties of Titanates with Ilmenite Structure. IV. The effect of an order-disorder transition on the high temperature electrical conductivity of NiTiO3

NiTiO₃ shows an order-disorder transition from an ordered ilmenite structure to a corundum structure at high temperatures. The transition is followed by a strong increase of the specific electrical conductivity. The conductivity was investigated as a function of temperature and oxygen activity. An order parameter according to common phase transition theories can be used to describe the behaviour of the conductivity in the transition region and vice versa. A model for the defect structure of NiTiO₃ is presented.     

Model of a disclination core in nematics

The core structure of a disclination line of strength 1 in uniaxial nematics is determined by using a space-dependent mean-field approach somehow based on the rod-like molecular model. It is shown that, due to distortion, biaxiality arises at all points of the defect core, except at the centre line where symmetry dictates uniaxiality; the orientational distribution of the rod-like molecules is there 'oblate', however. Although, as is well known, the configuration of a 1 defect is stable only under limiting conditions (as compared with either escaping in the axial direction or splitting into two 1/2 lines), the simple example developed here is indicative of a method which can readily be extended to more realistic, if mathematically more complex, situations such as 1/2 lines, layers close to solid boundaries, etc.     

Efficient sampling of ice structures by electrostatic switching

An electrostatic switching procedure is introduced that enables the systematic generation of high-quality ice configurations at various temperatures. Proton disordered ice Ih configurations were generated for the TIP4P water model at temperatures from 50 to 240 K, for the SPC/Fw water model from 100 to 240 K, and for the DC97 water model at 240 K. The resulting configurations were found to properly sample the canonical ensemble. The dielectric constant of ice Ih was determined from the net dipole fluctuation of the ice configurations. The calculated dielectric constant compares favorably with the study by Rick and Haymet [J. Chem. Phys. 2003, 118, 9291]. However, our method gives smaller error bars, especially at lower temperatures. At temperatures above 200 K, a type of hydrogen-bond defect is identified that cannot be categorized as a D or L type defect.     

On the origin of the lattice constant anomaly in nanocrystalline ceria

The lattice parameter of nanocrystalline ceria films prepared by sputtering was monitored as a function of annealing temperature. Within the temperature range of 150-420 degrees C, an equilibrium with atmospheric oxygen is established within a few hours, whereas grain growth does not occur. On the basis of the experimental results and analysis of literature data, we present a model that posits the formation of a non-uniform grain structure with stoichiometric interiors and oxygen deficient boundaries. This model, based on defect thermodynamics, correctly describes the dependence of the lattice parameter of nanocrystalline ceria on annealing temperature and grain size and can be extended to other materials as well.     

A Model of Simple Liquids

The intuitively clear hole defect model can be used for quantitative estimates and for understanding the properties of liquids. An ideal liquid is defined in analogy with an ideal gas and corrected for real liquids with hole defects. The good agreement permits an application of this approximation process to the understanding of some important material properties. The effective model fills a gap in the area of applied chemistry and of chemical teaching. It also leads to a correction factor for the failure of the theorem of corresponding states. The reasons for the applicability of such a simple model are demonstrated on the basis of the Lennard-Jones potential.     

Role of surface anchoring and geometric confinement on focal conic textures in smectic-A liquid crystals

A high surface area-to-volume ratio in microchannels increases the importance of surface interactions within them. In layered liquids, such as smectic liquid crystals, surface interactions play an important role in the formation of defect textures. We use 8CB liquid crystal, which is in the smectic-A phase at room temperature, as a model layered liquid. PDMS surfaces can be tuned to be hydrophilic or hydrophobic, and due to the nature of liquid crystalline molecules, we show that this results in planar or homeotropic anchoring conditions, respectively. In a confined system, contrary to the bulk, generated defects cannot grow freely. In the present work, we show that the confinement offered by PDMS microchannels along with the capability of creating mixed anchoring conditions within them results in the formation of particular ordered defect textures through increased surface interactions in smectic-A liquid crystals. Our observations imply that microscale confinement is useful for controlling the size, size distribution, and packing structure of microscale defect structures within these materials. In addition, we show that by placing a droplet of smectic-A liquid crystal on a PDMS surface containing microscale parallel cracks, ordered focal conic defects form between two adjacent cracks. The distance between two adjacent cracks dictates the size of the defects. These observations could lead to useful ideas for exploring new technologies for flexible optical devices or displays that utilize smectic-A liquid crystals.