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.     

Defect clustering and local ordering in rare earth co-doped ceria

Defect clustering and local ordering in rare earth co-doped ceria were studied by computer simulation and electron diffraction, respectively. The simulation of electrically neutral defect clusters containing up to four oxygen vacancies revealed that the permutation of different dopant cations in a co-doped cluster could have a significant influence on the binding energy of the cluster. Moreover, the growth of larger clusters (number of oxygen vacancies ≥ 3) could be restrained by a co-doping effect. The selected area electron diffraction study indicated that the restrained growth of larger clusters will further lead to a suppression of the local ordering of oxygen vacancies in co-doped ceria. The correlation between defect clustering, local ordering of oxygen vacancies and ionic conduction in co-doped ceria was discussed.     

Microstructural evolution in a CeO2-Gd2O3 system

Microstructural evolution in a CeO2-Gd2O3 system at atomic and nanoscale levels with increasing Gd concentration has been comprehensively investigated by transmission electron microscopy. When the Gd concentration was increased from 10 to 80 at.%, the phase transformation from ceria with fluorite structure to solid solution with C-type structure was not a sudden change but an evolution in the sequence of clusters, domains, and precipitates with C-type structure in the fluorite-structured matrix. Moreover, the ordering of aggregated Gd cations and oxygen vacancies in these microstructural inhomogeneities developed continuously with increasing Gd concentration. This microstructural evolution can be further described based on the development of defect clusters containing Gd cations and oxygen vacancies.     

Doping and defect association in AZrO(3) (A = Ca, Ba) and LaMO(3) (M = Sc, Ga) perovskite-type ionic conductors

Computer simulation techniques have been used to investigate the defect chemistry of perovskite-structured ionic conductors based upon AZrO(3)(A = Ca, Ba) and LaMO(3)(M = Sc, Ga). Our studies have examined dopant site-selectivity, oxide ion migration and dopant-defect association at the atomic level. The energetics of dopant incorporation in AZrO(3) show strong correlation with ion size. We predict Y(3+) to be one of the most favourable dopants for BaZrO(3) on energetic grounds, which accords with experimental work where this cation is the commonly used acceptor dopant for effective proton conduction. Binding energies for hydroxy-dopant pairs in BaZrO(3) are predicted to be favourable with the magnitude of the association increasing along the series Y < Yb < In < Sc. This suggests that proton mobility would be very sensitive to the type of acceptor dopant ion particularly at higher dopant levels. Oxygen vacancy migration in LaScO(3) is via a curved pathway around the edge of the ScO(6) octahedron. Dopant-vacancy clusters comprised of divalent dopants (Sr, Ca) at the La site have significant binding energies in LaScO(3), but very low energies in LaGaO(3). This points to greater trapping of the oxygen vacancies in doped LaScO(3), perhaps leading to higher activation energies at increasing dopant levels in accord with the available conductivity data.     

An insight into the structure,conductivity and ion dynamics of Sr-Sm co-doped ceria oxygen ion conductors: Effect of defect interaction

In this work, we investigate the structure, conductivity and ion dynamics of mixed di and tri-valent doped Ce_0.8Sm_0.2-xSr_xO_2-δ (x = 0–0.2) oxygen ion conductors. The lattice parameter and root mean square strain are significantly affected by the ionic radius of dopants and their solubility into ceria lattice. Due to the solubility limit of Sr²⁺ ions, SrCeO₃ phase increases with the doping concentration of Sr²⁺. The increase of Sr²⁺ ions into ceria lattice promotes the formation of large defect clusters by expense of formed oxygen vacancies. The coulombic interaction between oxygen vacancies with substituted dopant cations enhances with Sr²⁺ ions due to decrease of the value of dielectric constant of the compositions. The defect interaction significantly affects the conductivity values by means of increase of SrCeO₃ phase and defect clusters. The conductivity values are found to be consistent with the migration and association energy. The scaled spectra of dielectric tangent loss and real part of complex conductivity confirm the temperature and defect interaction independent nature of hoping mechanism in the compositions.     

A computational modelling study of oxygen vacancies at LaCoO3 perovskite surfaces

Atomistic computational modelling of the surface structure of the catalytically-active perovskite LaCoO(3) has been undertaken in order to develop better models of the processes involved during catalytic oxidation processes. In particular, the energetics of creating oxygen ion vacancies at the surface have been investigated for the three low index faces (100), (110) and (111). Two mechanisms for vacancy creation have been considered involving dopant Sr(2+) cations at the La(3+) site and reduction of Co(3+) to Co(2+). For both mechanisms, there is a general tendency that the smaller the cation defect separation, the lower the energy of the cluster, as would be expected from simple electrostatic considerations. In addition, there are clear indications that oxygen vacancies are more easily created at the surface than in the bulk. The results also confirm that the presence of defects strongly influences crystal morphology and surface chemistry. The importance of individual crystal surfaces in catalysis is discussed in terms of the energetics for the creation of oxygen vacancies.     

Disproportionation, dopant incorporation, and defect clustering in Perovskite-structured NdCoO3

Atomistic simulation techniques are used to examine the defect chemistry of perovskite-structured NdCoO(3), a material whose electrochemical properties make it attractive for use in heterogeneous oxidation catalysis, as well as in gas sensors and mixed ionic/electronic conductors. In practice, dopants are added to NdCoO(3) to obtain the desired properties, such as high electrical conductivity and rapid gas adsorption/desorption; thus, a wide range of dopants substituted on both Nd and Co sites are examined. Charge compensation for aliovalent dopants is predicted to occur via formation of oxide ion vacancies; these are understood to be key sites with respect to catalytic and sensor activity. Low activation energies calculated for oxide ion migration are consistent with high oxygen mobilities measured experimentally. Sr and Ca, which occupy Nd sites in the lattice, are found to be the most soluble of the alkaline earth metals, in agreement with experiment. These two dopant ions also have the weakest binding energies for dopant-vacancy cluster formation. Mechanisms of electronic defect formation, critical to the overall transport properties of the material, are also considered. The results suggest that disproportionation of the Co ion to form small polaron species is the most favorable intrinsic defect process. In doped compounds, formation of electronic holes via uptake of oxygen at vacant sites is found to be a low energy process.     

Defect and dopant properties of MgTa2O6

Atomistic computer simulation techniques have been used, for the first time, to reproduce the crystal structure of MgTa₂O₆ and to investigate the defect chemistry and dopant properties of this material. The calculated defect energetics suggest that the concentration of intrinsic atomic defects in this phase is insignificant and that the system is probably stable to both oxidation and reduction. Dopant solution energy versus ion size trends are found for both isovalent and aliovalent dopant incorporation at Mg and Ta sites. Divalent dopants (e.g. Ca, Cu) preferentially occupy the Mg site whereas dopants with higher charge (e.g. Sc, Zr, Nb) are more favorable on the Ta site. High migration activation energies (>2 eV) predict limited ionic conductivity in this material.     

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.     

Defect clusters in wustite,Fe1−xO

A quantum chemical approach based on predominantly covalent “normalized ion energies” has been developed for estimating structures and energies for defect clusters in quenched nonstoichiometric wustite (Fe_1?xO). Small defect clusters of zinc blende structure show special stability over other clusters considered. Of these, either a 13:5 or a 16:7 defect cluster (13 or 16 Fe³⁺ vacancies and 5 or 7 tetrahedral Fe³⁺ interstitials) have the proper structure and composition to account for the observed P′ and P″ phases in wustite.     

Small-defect clusters in chromia-doped rutiles

Bright-field phase contrast electron microscope images of chromia-doped rutiles have been used to study the size and distribution of small defect clusters in specimens of TiO₂, (Ti,Cr)O_1.995, and (Ti,Cr)O_1.985. Comparisons of observed and measured spot contrast densities, and contrast calculations using computer simulations of the defect structures, lead to the conclusion that the smallest clusters observed contained ～32 Cr³⁺ cations. Larger clusters occurred with increasing frequency for higher dopant levels, which is consistent with increased interaction and aggregation of traditional or reconstructed small defects (e.g., Cr³⁺ interstitials or charge-compensated oxygen vacancies).     

Defect and transport properties of the NdCoO3 catalyst and sensor material

The NdCoO₃ perovskite has been investigated using a combination of atomistic simulation and experimental techniques to examine its possible use as an oxidation catalyst and/or sensor material. The sensing properties of NdCoO₃ and Nd_0.8Sr_0.2CoO₃ towards CO have been investigated by employing thin films deposited by means of radio-frequency (RF) magnetron sputtering onto polycrystalline Al₂O₃. The response of the films was monitored by performing four-probe DC-conductivity measurements. The conductivity variation induced by switching between a CO-free atmosphere (air) and a CO-rich one with the same composition of residual gas was recorded and analysed as a function of temperature; results are compared for the two samples. Simulation studies focussed on the dopant, transport and redox properties of the pure material; the results indicate that Sr and Ca on the Nd site are the most soluble dopants and that when divalent dopants are incorporated in the structure, charge compensation occurs via oxygen ion vacancies. The low activation energy for oxygen vacancy migration suggests high oxide-ion mobility through the lattice. Particular attention is paid to the electronic processes because of their importance with respect to practical applications of the material.     

Kinetic lattice Monte Carlo model for oxygen vacancy diffusion in praseodymium doped ceria: Applications to materials design

Kinetic lattice Monte Carlo (KLMC) model is developed for investigating oxygen vacancy diffusion in praseodymium-doped ceria. The current approach uses a database of activation energies for oxygen vacancy migration, calculated using first-principles, for various migration pathways in praseodymium-doped ceria. Since the first-principles calculations revealed significant vacancy-vacancy repulsion, we investigate the importance of that effect by conducting simulations with and without a repulsive interaction. Initially, as dopant concentrations increase, vacancy concentration and thus conductivity increases. However, at higher concentrations, vacancies interfere and repel one another, and dopants trap vacancies, creating a “traffic jam” that decreases conductivity, which is consistent with the experimental findings. The modeled effective activation energy for vacancy migration slightly increased with increasing dopant concentration in qualitative agreement with the experiment. The current methodology comprising a blend of first-principle calculations and KLMC model provides a very powerful fundamental tool for predicting the optimal dopant concentration in ceria related materials.     

Density functional study of the interaction between small Au clusters, Au(n) (n=1-7) and the rutile TiO2 surface. II. Adsorption on a partially reduced surface

We use density functional theory to examine the electronic structure of small Au(n) (n=1-7) clusters, supported on a rutile TiO(2)(110) surface having oxygen vacancies on the surface (a partially reduced surface). Except for the monomer, the binding energy of all Au clusters to the partially reduced surface is larger by approximately 0.25 eV than the binding energy to a stoichiometric surface. The bonding site and the orientation of the cluster are controlled by the shape of the highest occupied molecular orbitals (HOMOs) of the free cluster (free cluster means a gas-phase cluster with the same geometry as the supported one). The bond is strong when the lobes of the HOMOs overlap with those of the high-energy states of the clean oxide surface (i.e., with no gold) that have lobes on the bridging and the in-plane oxygen atoms. In other words, the cluster takes a shape and a location that optimizes the contact of its HOMOs with the oxygen atoms. Fivefold coordinated Ti atoms located at a defect site (5c-Ti(*)) participate in the binding only when a protruding lobe of the singly occupied molecular orbital (for odd n) or the lowest unoccupied molecular orbital (for even n) of the free Au(n) cluster points toward a 5c-Ti(*) atom. The oxygen vacancy influences the binding energy of the clusters (except for Au(1)) only when they are in direct contact with the defect. The desorption energy and the total charge on clusters that are close to, but do not overlap with, the vacancy differ little from the values they have when the cluster is adsorbed on a stoichiometric surface. The behavior of Au(1) is rather remarkable. The atom prefers to bind directly to the vacancy site with a binding energy of 1.81 eV. However, it also makes a strong bond (1.21 eV) with any 5c-Ti atom even if that atom is far from the vacancy site. In contrast, the binding of a Au monomer to the 5c-Ti atom of a surface without vacancies is weak (0.45 eV). The presence of the vacancy activates the 5c-Ti atoms by populating states at the bottom of the conduction band. These states are delocalized and have lobes protruding out of the surface at the location of the 5c-Ti atoms. It is the overlap of these lobes with the highest orbital of the Au atom that is the major reason for the bonding to the 5c-Ti atom, no matter how far the latter is from the vacancy. The energy for breaking an adsorbed cluster into two adsorbed fragments is smaller than the kinetic energy of the mass-selected clusters deposited on the surface in experiments. However, this is not sufficient for breaking the cluster upon impact with the surface, since only a fraction of the available energy will go into the reaction coordinate for breakup.     

CeO2基固溶体氧缺位拉曼光谱表征的研究进展

总结了拉曼光谱表征CeO2基固溶体中氧缺位的研究成果,评述了氧缺位的生成和影响氧缺位浓度观察值的因素,并提出了亟待解决的问题.CeO2基固溶体的拉曼谱图中出现三个重要的特征拉曼峰(465、560、600cm-1),一般分别归属于CeO2的F2g振动模式、氧缺位和MO8型缺陷物种.研究发现氧缺位的产生与掺杂金属离子价态有关,而MO8型缺陷物种的产生与掺杂金属离子半径有关.CeO2基固溶体中氧缺位浓度观察值(AD/AF2g)与样品吸光度和表面富集有关.原位拉曼光谱研究表明:气氛及温度影响CeO2基固溶体的吸光度变化,从而影响拉曼光谱采样深度,导致氧缺位浓度观察值的变化.     

Proton transport in doped yttrium oxide. Monte-Carlo simulation

The Monte-Carlo technique is used to calculate the dependences of the tracer diffusion coefficient D* and conductivity ?? of protons on the temperature and content x of the acceptor dopant in oxides with the structure of distorted fluorite (??bixbyite?? structure). Analysis is carried out for the calcium doped Y₂O₃ oxide as an example. The effect of dopant-generated traps (different energy states for protons) and also the effect of site blocking due to interaction of protons between themselves and with oxygen vacancies are considered. It is shown that mobility of protons decreases significantly as a result of doping already at low x ?? 1 mol %. The behavior of conductivity noticeably depends on the distribution of dopant ions over nonequivalent cation sites in the lattice. The predominant dopant distribution over the sites of the same type accounts for the appearance of a maximum in the dependence of ??(x) when correlations are taken into account.     

Metallic and molecular orbital concepts in XMg8 clusters, X = Be-F

The electronic structure and stability of the XMg(8) clusters (X = Be, B, C, N, O, and F) are studied using first principles theoretical calculations to understand the variation in bonding in heteroatomic clusters which mix simple divalent metals with main group dopants. We examine these progressions with two competing models, the first is a distorted nearly free electron gas model and the second is a molecular orbital picture examining the orbital overlap between the dopant and the cluster. OMg(8) is found to be the most energetically stable cluster due to strong bonding of O with the Mg(8) cluster. BeMg(8) has the largest HOMO-LUMO gap due to strong hybridization between the Mg(8) and the Be dopant states that form a delocalized pool of 18 valence electrons with a closed electronic shell due to crystal field effects. Be, B, and C are best described by the nearly free electron gas model, while N, O, and F are best described through molecular orbital concepts.     

Structure, optical properties and defects in nitride (III-V) nanoscale cage clusters

Density Functional Theory calculations are reported on cage structured BN, AlN, GaN and InN sub- and low nanosize stoichiometric clusters, including two octahedral families of T(d) and T(h) symmetry. The structures and energetics are determined, and we observe that BN clusters in particular show high stability with respect to the bulk phase. The cluster formation energy is demonstrated to include a constant term that we attribute to the curvature energy and the formation of six tetragonal defects. The (BN)(60) onion double-bubble structure was found to be particularly unstable. In contrast, similar or greater stability was found for double and single shell cages for the other nitrides. The optical absorption spectra have been first characterised by the one-electron Kohn-Sham orbital energies for all compounds, after which we concentrated on BN where we employed a recently developed Time Dependent Density Functional Theory approach. The one-electron band gaps do not show a strong and consistent size dependency, in disagreement with the predictions of quantum confinement theory. The density of excited bound states and absorption spectrum have been calculated for four smallest BN clusters within the first ionisation potential cut-off energy. The relative stability of different BN clusters has been further explored by studying principal point defects and their complexes including topological B-N bond rotational defects, vacancies, antisites and interstititials. The latter have the lowest energy of formation.     

Exact, Born-Oppenheimer, and quantum-chemistry-like calculations in helium clusters doped with light molecules: The He2N2(X) system

Helium clusters doped with diatomic molecules, He(N)-BC, have been recently studied by means of a quantum-chemistry-like approach. The model treats He atoms as "electrons" and dopants as "nuclei" in standard electronic structure calculations. Due to the large mass difference between He atoms and electrons, and to the replacement of Coulomb interactions by intermolecular potentials, it is worth assessing up to what extent are the approximations involved in this model, i.e., decoupling of the BC rotation from the He-atom orbital angular momenta and Born-Oppenheimer separation of the BC stretch versus the He motions, accurate enough. These issues have been previously tackled elsewhere for the (4)He(2)-Br(2)(X) system, which contains a heavy dopant [Roncero et al., Int. J. Quantum Chem. 107, 2756 (2007)]. Here, we consider a similar cluster but with a much lighter dopant such as N(2)(X). Although the model does not provide the correct energy levels for the cluster, positions and intensities of the main detectable lines of the vibrotational Raman spectrum at low temperature are accurately reproduced.     

Study of defect sites in Ce1-xMxO2-δ (x = 0.2) solid solutions using Raman spectroscopy

A series of Ce(1-x)M(x)O(2-δ) (M = Gd, Zr, La, Sm, Y, Lu, and Pr) samples were characterized by Raman spectroscopy to investigate the evolution of defect sites (oxygen vacancies and MO(8)-type complex) and their distributions in the samples. It was found that the evolution of oxygen vacancies was due to the different ionic valence state of dopant from that of Ce(4+), while the evolution of the MO(8)-type complex was due to the different ionic radius of dopant from that of Ce(4+). The distributions of defect sites were investigated using 325 and 514 nm excitation laser lines, indicating that the defect sites were surface enriched. Moreover, the increasing ordering level of the sample led to a decline in the concentration of the MO(8)-type complex in the sample but the constant concentration of oxygen vacancies, implying that the metastable MO(8)-type complex species were more disordered compared to the oxygen vacancies.