Defects In Transition Metal Oxides Research Articles

Stabilization of the (1 1 1) surface of NiO and CoO by segregation of point defects

Vacancies are ubiquitous point defects in transition metal oxides. Surprisingly, the effect of vacancies on the structural stability of oxide surfaces has been neglected in the last decades. On the other hand, experimentally observed surface structures of oxides are frequently in contrast to theoretical predictions. A typical example is the widely observed (111) surface of the rock-salt-type NiO and CoO, which is predicted to be polar and unstable according to electrostatic arguments. Previous theoretical models proposed to compensate the surface polarity assume a perfect bulk, despite the high density of cation vacancies in these materials. Through first-principles calculations, we show in this work that the cation vacancies have a strong tendency to segregate to the surface, leading to the formation of surfaces with high-proportion of cation vacancies even at low oxygen potentials. Energetically, the segregated surfaces are more stable than the well-known octopolar reconstruction. The role of segregation of point defects in surface behaviors of transition metal oxides may be of significance for understanding their catalysis and electronic properties.

Recent Progress on Defect-rich Transition Metal Oxides and Their Energy-Related Applications.

The applications of many energy-related electrochemical energy conversion and storage devices are changing with each passing day. Oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) are the key steps in the commercial application of these energy conversion/storage equipment. Defect-rich transition metal oxides (TMOs), such as Co, Mn, Fe, Ni, etc., have always been one of the most promising electrocatalysts, which are cheap, easy to obtain, high in catalytic activity and stable during electrocatalysis. In this review, we first introduce the definition, classification, characteristics, and construction of defects. Then, the latest developments of defect-rich TMO electrocatalysts in electrocatalysis and energy conversion storage device is summarized. Furthermore, the relationship between defects and activity and the potential mechanism are also discussed. The defects in defect-rich TMOs can adjust the surface/interface electronic structure of the electrocatalyst, change the adsorption energy of the intermediate product, or increase the intrinsic catalytic activity of active sites, which are beneficial for enhanced catalytic performance. Finally, the current challenges and prospects of defect-rich TMO electrocatalysts are proposed. Therefore, the introduction of defects in TMO will be a potential strategy for the rational design of high-performance electrocatalysts in the future.

(Invited) Structural Stability and Ionic Defects in Transition Metal-Oxides from Diffusion Quantum Monte Carlo

Transition metal-oxides (TM) play a central role in many applications and in the fundamental research for advance materials. The standard computational methods to study these systems are based on Density Functional Theory (DFT) approximations. These approximations, however, are unreliable for a wide range of TM because they are based on model systems. A natural solution to overcome the intrinsic limitations of DFT approximations is to directly solve the many-body problem in TM. Today this is only practical by applying quantum Monte Carlo (QMC) methods. We will show that QMC methods, such as diffusion quantum Monte Carlo (DMC), are now practical to study multiple properties of TM. The application of DMC to study the structural, electronic and ionic defect properties of various TM, including ZnO, LaFeO3 and SrFeO3, will be presented.

Functional Ion Defects in Transition Metal Oxides

Dynamically tuning the concentration and profile of ions and vacancies in transition metal oxides provides a route to control of new functionalities.

Ligand field splittings in core level transitions for transition metal (TM) oxides: Tanabe-Sugano diagrams and (TM) dangling bonds in vacated O-atom defects

Defect states in the forbidden band-gap below the conduction band edge are active as electron traps in nano-grain high-) transition metal (TM) oxides with thickness >0.3 nm, e.g., ZrO2 and HfO2. These oxides have received considerable attention as gate-dielectrics in complementary metal oxide semiconductor (CMOS) devices, and more recently are emerging as candidates for charge storage and memory devices. To provide a theoretical basis for device functionality, ab-initio many-electron theory is combined with X-ray absorption spectroscopy (XAS) to study O K edge and TM core level transitions. These studies identify ligand field splittings (ΔLF) for defect state features,. When compared with those obtained from O-atom and TM-atom core spectroscopic transitions, this provides direct information about defect state sun-nm bonding arrangements. comparisons are made for (i) elemental TiO2 and Ti2O3 with different formal ionic charges, Ti4+ and Ti3+ and for (ii) Magneli Phase alloys, TinO2n-1, n is an integer 9≥n>3, and (TiO2)x(HfO2)1−x alloys. The alloys display multi-valent behavior from (i) different ionic-charge states, (ii} local bond-strain, and (iii) metallic hopping transport. The intrinsic bonding defects in TM oxides are identified as pairs of singly occupied dangling bonds. For 6-fold coordinated Ti-oxides defect excited states in 2nd derivative O K pre-edge spectra are essentially the same as single Ti-atom d2 transitions in Tanabe-Sugano (T-S) diagrams. O-vacated site defects in 8-fold coordinated ZrO2 and HfO2 are described by d8 T-S diagrams. T-S defect state ordering and splittings are functions of the coordination and symmetry of vacated site bordering TM atoms. ΔLF values from the analysis of T-S diagrams indicate medium range order (MRO) extending to 3rd and 4th nearest-neighbor (NN) TM-atoms. Values are different for 6-fold Ti, and 8-fold ZrO2 and HfO2, and scale inversely with differences in respective formal ionic radii. O-vacated site bonding defects in TM nano-grain oxides are qualitatively similar to vacant-site defects in non-crystalline SiO2 and GeO2 for ulta-thin films, < 0.2 nm thick, and yield similar performance in MOSCAPs on Ge substrates heralding applications in aggressively-scale CMOS devices.

Transport through singlet states in resistive memory materials: Magneli-phase, TinO2n−1 for 9 ≥ n &gt; 3, and TiO2-HfO2 alloys

Energy states in the forbidden band-gap below the conduction band edge are active as electron traps in nanograin high dielectric constant (κ) transition metal (TM) oxides, e.g., ZrO2 and HfO2. These TM oxides have received considerable attention for at least 10 yr as gate-dielectrics in complementary metal oxide semiconductor devices. More recently, they are emerging as candidates for charge storage and memory devices. To provide a theoretical basis for device applications, this article combines ab initio many-electron theory and x-ray absorption spectroscopy to study O K edge and TM core level transitions. These studies allow the extraction of ligand field splittings (ΔLF) for defect state features, which can then be compared with those obtained from O and TM core spectroscopic transitions, thereby providing an increased understanding of intrinsic defect bonding arrangements. These comparisons have been made for (i) elemental Ti-oxides TiO2 and Ti2O3 with different formal charge state, Ti4+ and Ti3+, respectively, and for (ii) Ti Magneli-phase alloys, TinO2n−1, n is an integer 9 ≥ n > 3, (TiO2)x(HfO2)1−x alloys. The alloys display multivalent behavior, with additional valence states associated with bond-strain, and metallic hopping transport of electrons through singlet exited states immediately below the conduction band edge. Three significant new results have been highlighted in this article. First based on comparisons with noncrystalline SiO2 and GeO2, the intrinsic defects in TM oxides have been identified as pairs of singly occupied dangling bonds in vacated (empty) O-atom bonding sites. Second, the ordering and symmetries of two-electron features identified in second derivative O K pre-edge spectra have been compared with d2 transitions described by Tanabe–Sugano diagrams. These splitting are dependent on bonding coordination and symmetry of the bordering TM atoms, sixfold octahedral arrangements for Ti, and eightfold arrangements for ZrO2 and HfO2 in cubic and tetragonal phases. ΔLF values obtained from these studies are the core level spectroscopies and defects. For the defect states, there is medium range order that extends to third and fourth nearest-neighbor TM metal–atom correlations. Finally, and equally important, these results establish that bonding defects in TM nanograin oxides and noncrystalline SiO2 and GeO2 are qualitatively similar, each indicative of different values of ΔLF and indicating different levels of partially ionic bonding.

Quantitative theory of the oxygen vacancy and carrier self-trapping in bulk TiO2

Standard approximations of density functional theory often fail for defects in transition metal oxides. Despite its significance in applications and decades of research, the oxygen vacancy in TiO${}_{2}$ is also poorly understood from the theory point of view. The Heyd-Scuseria-Ernzerhof functional (HSE06) provides a total energy that is a linear function of the occupation number (as it should be for the exact functional) also in TiO${}_{2}$. This allows reproduction of the measured infrared absorption, photoluminescence, and thermal ionization data within \ensuremath{\sim}0.1 eV. From our calculations, a consistent and quantitative interpretation of the conflicting experiments emerges. Electron self-trapping in rutile makes the properties of the vacancy concentration dependent. In oxidized samples the vacancies are passivated by native Ti${}^{3+}$/Ti${}^{4+}$ traps. This explains why electrons localized to the vacancy could only be observed after illumination at very low temperature in magnetic resonance experiments. In strongly reduced samples, electrons may stay localized in the vacancy and even the neutral state gives rise to two vertical electronic transitions (at 0.8 and 1.2 eV). The situation is much simpler in anatase, where only holes are self-trapped by O${}^{2\ensuremath{-}}$/O${}^{1\ensuremath{-}}$ transitions. The oxygen vacancy is a shallower donor in anatase than in rutile.

Effects of resistance switching in niobium oxides: Metastable nanochannels

The recent experimental data suggest that lattice defects in transition-metal oxides tend to be aligned into chains, which can promote pairing of charge carriers with subsequent formation of bosonic stripes. In this study, possible relationship between the extraordinary manifestations of electronic properties, observed by different methods in niobium oxides, and the processes of formation of metastable nanochannels from bosonic stripes are discussed.

Spectroscopic studies of O-vacancy defects in transition metal oxides

Dielectrics comprised of nano-crystalline HfO2 in gate stacks with thin SiO2/SiON interfacial transition regions display significant asymmetries with respect to trapping of Si substrate injected holes and electrons. Based on spectroscopic studies, and guided by ab initio theory, electron and hole traps in HfO2 and other transition metal elemental oxides are assigned to O-atom divacancies clustered at internal grain boundaries of nano-crystalline films. Engineering solutions in which grain boundary defects are suppressed include: (i) ultra-thin, <2 nm, HfO2 fims, (ii) chemically phase separated high HfO2 content silicate films, and (iii) non-crystalline Zr/Hf Si oxynitride films.

Quadrupole terms in defect energies in transition metal oxides

Abstract In a previous paper it was shown that, for defects in transition metal oxides, there was evidence for an additional term in the potential energy beyond the usual monopole and dipole terms of the shell model. This appeared to be a quadrupole term in which the open 3d shells of the host ions, as well as any impurities, responded to local electric field gradients. Quantitative estimates are made which confirm our hypothesis and, in CoO, show that the effect is sufficient to bring theory into accord with experimental measurements. Such quadrupole interactions will also be relevant to other systems with host ions or impurities having incomplete electric shells.

Defects in highly nonstoichiometric oxides

A number of important physical and chemical properties of nonstoichiometric transition metal oxides are strongly dependent on the kind and concentration of point defects in the crystalline lattice of these materials. If the concentration of these defects is sufficiently small they do not interact and the defecteed crystal can be treated as ideal solid solution of defects in its crystalline lattice. However, in many cases the concentration of defects in transition metal oxides is high enough for them to interact, what can result in formation of extended defects, i.e. of three-dimensional complexes and defect clusters, or of two-dimensional planar defects. For these cases the point defect model is not applicable and transport processes cannot be described in terms of solid state diffusion theory. An attempt has been made to systematize the current knowledge of extended defects and to formulate some conclusions concerning transport phenomena in crystals containing these defects.

A perfect lattice approach to nonstoichiometry

Although nonstoichiometry implies the presence of defects in a structure and hence a departure from perfect crystallinity, it is sometimes found that the defects are ordered or, at least, show a tendency to order. In these cases, it becomes possible to model the nonstoichiometry by considering a supercell and using perfect lattice techniques. We illustrate the viability of this approach with two examples. Firstly, we show how factors controlling the long-range ordering of extended defects in transition-metal oxides may be elucidated; indeed, we find that an adequate treatment of this phenomenon requires calculations on supercells. Secondly, we discuss how perfect lattice calculations may be used, in conjunction with diffraction data, to examine possible vacancy ordering schemes in the oxidation of magnetite, Fe 3O 4, to maghemite, the defect spinel structured γ-Fe 2O 3.

Quelques aspects de la non-stœchiométrie en cristallochimie minérale

A few aspects of non-stoichiometry in inorganic crystal chemistry. Two aspects of non-stoichiometry are reviewed : non-stoichiometric compounds formed by insertion of M[charged]n+ ions in a transition metal oxide matrix and non-stoichiometry by introducing planar defects in transition metal oxides. The study of several vanadium oxide bronzes shows the importance of the nature of the inserted element in this compounds. Several examples are given to illustrate the possibility of obtaining deviations to the stoichiometry by introducing Wadsley's defects in lattice of the ReO₃ and PdF₃ type.