Defect Chemistry Research Articles

Exploring the Effect of Cation Vacancies in TiO2: Lithiation Behavior of n-Type and p-Type TiO2.

TiO2 offers several advantages over graphite as an anode material for Li-ion batteries (LIBs) but suffers from low electrical conductivity and Li-diffusion issues. Control over defect chemistry has proven to be an effective strategy to overcome these issues. However, defect engineering has primarily been focused on oxygen vacancies (VO). The role of another intrinsic TiO2 vacancy [i.e., titanium vacancies (VTi)] with regard to the Li+ storage behavior of TiO2 has largely evaded attention. Hence, a comparison of VO- and VTi-defective TiO2 can provide valuable insight into how these two types of defects affect Li+ storage behavior. To eliminate other factors that may also affect the Li+ storage behavior of TiO2, we carefully devised synthesis protocols to prepare TiO2 with either VO (n-TiO2) or VTi (p-TiO2). Both TiO2 materials were verified to have a very similar morphology, surface area, and crystal structure. Although VO provided additional sites that improved the capacity at low C-rates, the benefit obtained from over-lithiation turned out to be detrimental to cycling stability. Unlike VO, VTi could not serve as an additional lithium reservoir but could significantly improve the rate performance of TiO2. More importantly, the presence of VTi prevented over-lithiation, significantly improving the cycling stability of TiO2. We believe that these new insights could help guide the development of high-performance TiO2 for LIB applications.

Advances in ZnO: Manipulation of defects for enhancing their technological potentials

Abstract This review attempts to compile the physics and chemistry of defects in zinc oxide (ZnO), at both, the fundamental and application levels. The defects, either inherent ones or introduced externally, have broadened the ZnO material field in various directions. The ZnO material exhibits many defect-attributed properties leading to broad technological applications: electronic and optoelectronic devices, sensors, optical components, ceramic industry, biomedical, catalysis, lightening, etc. Considering the huge defect-dependent technological scopes, the ZnO material is constantly engineered for various defects, and corresponding functionalities are tailored with respect to particular applications. The functional properties of ZnO are strongly influenced by the defects, and as a result, the defect engineering of the ZnO materials has remained an important motivation in materials science and engineering in terms of localized defects, extended defects, impurities, and surface defects, etc. A detailed characterization of these defects seems to be an essential part of any research area. The correlations of the microstructural characteristics with electrical and optical properties of ZnO are then a natural step for further facilitating an efficient way toward advanced ZnO-based materials and devices. The present review is an effort to shed light on the defects of ZnO, properties, theoretical aspects, and corresponding applications.

Defect Chemistry of Individual Grains with and without Grain Boundaries of Al-Doped Ceria Determined Using Well-Defined Microelectrodes

The deconvolution of grain boundary and grain interior contributions to the total impedance of a polycrystalline material and their influence on its conductivity can be challenging, depending on the specific material. In this study, we present a microelectrode approach which enables the direct characterization of the electrochemical transport processes of individual grains with and without a grain boundary of microcrystalline oxide thin films by using electrochemical impedance spectroscopy. A combination of photolithography and electron-beam lithography was used to prepare well-defined microelectrodes onto a ceria thin film sintered at 1500 °C for 8 h to contact individual grains with and without a grain boundary. Impedance data was collected in the temperature range between 613 and 773 K and in an oxygen partial pressure range between 0.2 and 10–5 bar. The conductivity of single grains increases with increasing oxygen partial pressure accompanied by a decrease of the activation energy from 1.32 to 0.93 eV. In contrast, the conductivity of grains with a grain boundary remains constant in the oxygen partial pressure range investigated. These differences are analyzed quantitatively in terms of the defect chemistry of the grain interior and the near grain boundary region, which differs due to the segregation of impurities at the grain boundaries. The results demonstrate that locally resolved measurements of the impedance of microcrystalline samples are a powerful tool to determine the transport characteristics of individual grains and grain boundaries as well as the equilibrium constants of the underlying defect chemical reactions.

Scalable and tunable Y2O3‐MgO composite for infrared transparency applications

AbstractThe process‐structure‐property correlationships in yttria‐magnesia (YM) composite have been investigated. YM composite was synthesized using commercial powders via ball‐milling route with three different grinding balls (Si3N4, Al2O3, ZrO2) having two different sizes (2 and 5 mm diameter). The alteration in grinding ball material and size produces sintered ceramic having different grain sizes (420–560 nm) and degree of phase mixing homogeneity (0.40–0.70). The contamination induced by the milling ball resulted in changes in Y2O3 and MgO defect chemistry, which influenced the grain growth behavior in the YM composite. The hot‐pressed composite prepared using 2‐mm Si3N4 ball‐milled powders exhibited the finest grain size (420 nm) and better phase mixing homogeneity (0.63). The subsequent impact was seen on transmittance efficiency (71%) over the 3–7‐μm wavelength range, which is ∼85% of the theoretical limit. The findings show that the selection of the right size and type of grinding ball for milling commercial powder is a simple and cost‐effective way for scalable production of YM composite with high transmittance efficiency for infrared windows and dome applications.

Colossal permittivity and ultralow dielectric loss in SrTi1-xNbxO3 ceramics sintered at different atmospheres via defect chemistry improvement

SrTiO3 is recognized as one of the most promising candidate materials for colossal permittivity. However, the origin of colossal permittivity is still not well understood. In this work, SrTi0.998Nb0.002O3 ceramics (SNT) were synthesized via typical solid-state reaction process and sintered in air, N2, and N2–H2 atmospheres (labelled as SNT-Air, SNT-N2 and SNT-H2). The SNT-H2 ceramic achieves a colossal permittivity of 89254 and a dielectric loss of 0.013 at 1 kHz and maintains a colossal permittivity (∼8 × 104) with a low dielectric loss (<0.1) throughout a wide temperature (50 °C–300 °C) and frequency (100 Hz-1 MHz) range. The mechanism of the outstanding properties of SNT-H2 ceramics was investigated by X-ray photoelectron spectroscopy (XPS), impedance spectrum and activation energy analysis. The TiTi′−VO⋅⋅−TiTi′ defect dipole and VO⋅⋅−3TiTi′−NbTi⋅ defect dipole clusters exist in SNT ceramics by sintering in the reducing atmosphere of H2. Otherwise, the grain is semi-conducting, and the grain boundary is insulating in SNT-H2 ceramics by impedance spectrum analysis. In conclusion, the defect dipole and clusters localized in the grain could be the fundamental cause for the ultra-dielectric properties. As a result, The SNT-H2 ceramic exhibits the colossal permittivity with low dielectric loss, as well as good temperature and frequency stability, potentially aiding in the development of colossal materials.

Microwave assisted processing of X8R nanocrystalline BaTiO3 based ceramic capacitors and multilayer devices

BaTiO3 based multilayer ceramic capacitor (MLCC) is an important component in electronic devices. Achieving high dielectric performance, miniaturisation and cost effectiveness are still challenging. Co-sintering ceramic layers with metal electrodes and use of expensive Pt and high-Pd content compositions as electrodes are other key issues. Thus, the major factors that could lead to cost effectiveness of MLCCs are reduction in sintering temperature and using less expensive metal electrodes without compromising dielectric performance and these traits could auger well for the next generation low-cost high performance electroceramic devices – this is the subject matter of the present study.In this work, initially we have fabricated and analysed the dielectric performance of BaTiO3 (BT) ceramics with different BT particle sizes (50 nm, 100 nm and 200 nm). Based on results, 200 nm BaTiO3 was used for further studies due to its superior dielectric performance. Then, to reduce the sintering temperature and improve the dielectric performance of 200 nm BaTiO3 ceramics, Bi2O3 was used as a dopant which acts both as a sintering aid and helps to improve the dielectric performance. A combination of rare earth dopant system (proprietary from the industrial partner Knowles, UK) with Bi2O3 was also added to tune the defect chemistry of BaTiO3 for enhancement in dielectric performance further. A homogeneously mixed BT-based ceramic slurry system (containing the above dopants) with optimum rheology was developed using a non-aqueous medium, dispersant and a binder system. Addition of glass frits for lowering the sintering temperature and its effect on dielectric performance was also investigated on both discs (made using dry pressing of the powders obtained via drying of the slurry) as well as multilayer ceramic capacitors (MLCCs) fabricated through screen printing. Conventional, microwave and hybrid sintering procedures were employed for the densification of the electroceramic devices and these were characterized for density, microstructure, composition and dielectric performance systematically. Conventional sintering resulted in undesired large micron sized surface features which are detrimental to device durability, whereas formation and growth of these features have been demonstrated to be significantly minimised using the rapid microwave assisted sintering procedures for the first time. Further efficient co-sintering of ceramic layers with less expensive (Ag/Pd) alloy has also been accomplished using the microwave methodology. The resultant BaTiO3 based capacitive devices exhibited high dielectric permittivity, superior X8R performance and low dissipation factor, asserting their massive potential for widespread high temperature applications in automotive, sensing and space sectors.

Competing Superior Electronic Structure and Complex Defect Chemistry in Quasi-One-Dimensional Antimony Chalcogenide Photovoltaic Absorbers

Antimony chalcogenides Sb2Se3 and Sb2S3 are an emerging class of quasi-one-dimensional van der Waals bonded materials with rapid progress and great promise for low-cost, high-efficiency, and scalable thin-film photovoltaics. Here using first-principles theoretical calculations, we show that antimony chalcogenides Sb2Se3 and Sb2S3 possess competing superior electronic structure and complex defect chemistry, limiting their photovoltaic performance. The high optical absorption, large Born effective charge, and high dielectric constant lead to low exciton binding energy and small hole effective mass, which ultimately benefit the photovoltaic performance. In contrast, quasi-1D antimony chalcogenides exhibit complex defect chemistry. The cation–anion antisites are the dominating native defects in the Sb rich condition, while in the Sb poor condition the anion–cation antisites are dominant. In both cases, the anion vacancies are also easy to form. These native defects introduce midgap transition levels which act as electron–hole recombination centers, limit the dopability, and lower the open circuit voltage. As a result, Sb2Se3 is a p-type semiconductor in Se rich condition, while Sb2S3 is an intrinsic semiconductor without significant free carrier density. Additionally, due to the unique quasi-1D structures and strong electron–lattice coupling, antimony chalcogenides are able to accommodate larger lattice relaxation than conventional 3D bulk crystals, resulting in negative-U behavior in their defect structures. Therefore, the competing superior electronic structure and complex defect chemistry suggest that, while antimony chalcogenides are excellent photoabsorbers, effort shall be made to suppress the formation of the detrimental native defects to further improve their open-circuit voltage, electronic transport, and, thus, power conversion efficiency.

Ion transport mechanism in anhydrous lithium thiocyanate LiSCN Part I: ionic conductivity and defect chemistry.

This work reports on the ion transport properties and defect chemistry in anhydrous lithium thiocyanate Li(SCN), which is a pseudo-halide Li+ cation conductor. An extensive doping study was conducted, employing magnesium, zinc and cobalt thiocyanate as donor dopants to systematically vary the conductivity and derive a defect model. The investigations are based on impedance measurements and supported by other analytical techniques such as X-ray powder diffraction (XRPD), infrared (IR) spectroscopy, and density functional theory (DFT) calculations. The material was identified as Schottky disordered with lithium vacancies being the majority mobile charge carriers. In the case of Mg2+ as dopant, defect association with lithium vacancies was observed at low temperatures. Despite a comparably low Schottky defect formation enthalpy of (0.6 ± 0.3) eV, the unexpectedly high lithium vacancy migration enthalpy of (0.89 ± 0.08) eV distinguishes Li(SCN) from the chemically related lithium halides. A detailed defect model of Li(SCN) is presented and respective thermodynamic and kinetic data are given. The thiocyanate anion (SCN)- has a significant impact on ion mobility due to its anisotropic structure and bifunctionality in forming both Li-N and Li-S bonds. More details about the impact on ion dynamics at local and global scale, and on the defect chemical analysis of the premelting regime at high temperatures are given in separate publications (Part II and Part III).

Optical band gap and photoluminescence studies of precursor optimized Indium-Gallium Zinc oxide thin films

Spray pyrolysis at an optimal substrate temperature of 400 °C produces thin layers of pure and Indium-Gallium doped Zinc oxide on glass substrates. In the growth of precursor solution, ammonium acetate is used as a stabilizing agent. Polycrystalline films with hexagonal shaped crystalline structures are deposited in all cases. We have investigated the optical band gap and photoluminescence behavior of Indium and Gallium doped Zinc oxide thin films. Powder X-ray diffraction studies show that the deposited thin films are polycrystalline structure, with a hexagonal crystal structure with (002) orientation. In the visible wavelength range, all thin films have a transmittance of around 85% and a strong absorption beginning of about 375 nm, which corresponds to the fundamental absorption edge of 3.3 eV of Indium-Gallium doped ZnO. UV–Vis-NIR results along with tauc’s plot show that energy gap decreases with the increase of Indium-Gallium doping concentration. Photoluminescence results indicate the effect of indium-gallium dopants at of various concentrations plays a significant role in the defect chemistry in the films.

Theoretical insights into interfacial stability and ionic transport of Li2OHBr solid electrolyte for all-solid-state batteries.

Li-rich antiperovskite materials are promising candidates as inorganic solid electrolytes (ISEs) for all-solid-state Li-ion batteries (ASSLIBs). However, the material faces several pressing issues for its application, concerning the phase stability and electrochemical stability of the synthesized material and the Li-ion transport mechanism in it. Herein, we performed first-principles computational studies on the phase stability, interfacial stability, defect chemistry, and electronic/ionic transport properties of Li2OHBr material. The calculation results show that the Li2OHBr is thermodynamically metastable at 0 K and can be synthesized experimentally. This material exhibits a wider intrinsic electrochemical stability window (0.80-3.15 V) compared with sulfide solid electrolytes. Moreover, the Li2OHBr displays significant chemical stability when in contact with typical cathode materials (LiCoO2, LiMn2O4, LiFePO4) and moisture. The dominant defects of Li2OHBr are predicted to be VLi- and Lii +, corresponding to lower Li-ion migration barriers of 0.38 and 0.49 eV, respectively, while the replacement of some of the OH- by F- is shown to be effective in decreasing migration barriers in Li2OHBr. These findings provide a theoretical framework for further designing high performance ISEs.

The proton uptake process in double perovskite triple ionic-electronic conducting oxides for protonic ceramic cells

The proton uptake mechanism in a triple conducting oxide, PrBa0.5Sr0.5Co1.5Fe0.5O5+δ, is comprehensively investigated based on defect chemistry and experimental analyses of mass and conductivity changes under dry and humidified atmospheres.

Effects of Sr(Co, Nb, Ta)O3 addition on the defect structures and electrical properties of ZnO-based varistors

A schematic representation of the defect chemistry in ZCM-SCNT and ZCM-SCO.

Defect-controlled halogenating properties of lanthanide-doped ceria nanozymes.

Marine organisms combat bacterial colonization by biohalogenation of signaling compounds that interfere with bacterial communication. These reactions are catalyzed by haloperoxidase enzymes, whose activity can be emulated by nanoceria using milli- and micromolar concentrations of Br- and H2O2. We show that the haloperoxidase-like activity of nanoceria can greatly be enhanced by Ln substitution in Ce1-xLnxO2-x/2. Non-agglomerated nanosized Ce1-xLnxO2-x/2 (Ln = Pr, Tb, particle size < 10 nm) was prepared mechanochemically from CeCl3 and Na2CO3 followed by short calcination. Lanthanide metals could be incorporated into the CeO2 host without solubility limit, as shown for Tb. The distribution of the Ln3+ defect sites in the CeO2 host structure was analyzed by electron spin resonance spectroscopy. Ce3+ and superoxide O2- species are present at surface sites. Their formation is promoted by increasing dopant concentration. Ce1-xLnxO2-x/2 was prepared in copious amounts by ball-milling. This energy-saving and residue-free method can be upscaled to industrial scale. The surface defect chemistry of Ce1-xLnxO2-x/2 was unravelled by vibrational spectroscopy. It is associated with the mechanochemical preparation and leads to enhanced catalytic activity. Although Ce0.9Pr0.1O1.95 had a lower BET surface area than pure CeO2, its catalytic activity, calibrated by oxidative bromination of phenol red, was much higher because the ζ-potential increased from 15 mV (for CeO2) to 30 mV (for Ce0.9Pr0.1O1.95). This facilitates adsorption of Br- in aqueous conditions and explains the high catalytic activity of the Ln-substituted CeO2. Ce1-xLnxO2-x/2 is an effective and "green" nanoparticle haloperoxidase mimic for antifouling applications, as no chemicals other than the ubiquitous Br- and H2O2 (generated in daylight) are required, and only natural metabolites are released into the environment.

Insertion of metal cations into hybrid organometallic halide perovskite nanocrystals for enhanced stability: eco-friendly synthesis, lattice strain engineering, and defect chemistry studies.

In this work, we developed a facile and environmentally friendly synthesis strategy for large-scale preparation of Cr-doped hybrid organometallic halide perovskite nanocrystals. In the experiment, methylammonium lead bromide, CH3NH3PbBr3, was efficiently doped with Cr3+ cations by eco-friendly method at low temperatures to grow crystals via antisolvent-crystallization. The as-synthesized Cr3+ cation–doped perovskite nanocrystals displayed ∼45.45% decrease in the (100) phase intensity with an enhanced Bragg angle (2θ) of ∼15.01° compared to ∼14.92° of pristine perovskites while retaining their cubic (221/Pm-cm, ICSD no. 00-069-1350) crystalline phase of pristine perovskites. During synthesis, an eco-friendly solvent, ethanol, was utilized as an antisolvent to grow nanometer-sized rod-like crystals. However, Cr3+ cation-doped perovskite nanocrystals display a reduced crystallinity of ∼67% compared to pristine counterpart with ∼75% crystallinity with an improved contact angle of ∼72° against water in thin films. Besides, as-grown perovskite nanocrystals produced crystallite size of ∼48 nm and a full-width-at-half-maximum (FWHM) of ∼0.19° with an enhanced lattice-strain of ∼4.52 × 10−4 with a dislocation-density of ∼4.24 × 1014 lines per m2 compared to pristine perovskite nanocrystals, as extracted from the Williamson–Hall plots. The as-obtained stable perovskite materials might be promising light-harvesting candidates for optoelectronic applications in the future.

Multi-scale chemo-mechanical evolution during crystallization of mixed conducting SrTi0.65Fe0.35O3−δ films and correlation to electrical conductivity

Crystallization of a perovskite mixed conductor is coupled to oxidation, which drives defect chemistry, ion coordination, polyhedra alignment, hierarchical microstructure, and property evolution – including a boost to electrical conductivity.

Photochromism of UV-annealed Fe-doped SrTiO3

High-temperature annealing coupled with above bandgap UV illumination is an emerging approach to manipulate defect chemistries and resultant properties of electroceramics. To explore defect-processing-property relationships in these materials, an advanced multiphysics and multiscale model has been developed, which involves (a) high-fidelity first principles simulations of defect energies, (b) grand canonical thermodynamics of defect equilibria, (c) UV-perturbed defect formation energies from Shockley–Read–Hall generation and recombination, and (d) finite-element analyses of electrostatic potential and defect redistribution. Using this model, bottom-up insights into defect mechanisms associated with the UV-induced brown photochromism of Fe-doped SrTiO3 at high temperatures are provided. It is found that UV illumination leads to dissociation of the FeTi-vO complex and reduction in the oxygen vacancy concentration through exchange with the gas reservoir. Changes to these defect populations cause reionization of the FeTi defect from −1 to 0 charge state to maintain charge neutrality. This collectively gives rise to an increased concentration of FeTi0, which is the source of brown chromism. In addition, this model reproduces the experimentally observed electrical resistance degradation of samples annealed in this manner due to the increasing hole concentration in the material with time. The present model itself offers a route to guide and facilitate future efforts in this field.

Current understanding of ceria surfaces for CO2 reduction in SOECs and future prospects – A review

Ceria-based materials form a very promising class of electrocatalysts for CO2 reduction in solid oxide electrolysis cells (SOECs). On the pathway towards the implementation of ceria into technological electrodes on a large scale, an in-depth atomistic understanding of the reaction mechanism over ceria surfaces is crucial in order to optimize its intrinsic electrocatalytic activity. In this review article, we offer a critical discussion of the phenomena governing the electrochemical reduction of CO2 over ceria surfaces. We focus on the steps that limit the reaction rate and on how these can be accelerated by appropriate tuning of ceria properties and defect chemistry, via the introduction of tensile or compressive isotropic strain in the lattice, the promotion of a specific surface orientation and the control of the amount of acceptor or donor dopants incorporated in the fluorite structure. Our review aims to gather and examine the evidence of the role that these levers play in altering the energy landscape of the reaction, along with their influence on the capability of ceria to suppress carbon deposition during cell operation. Ultimately, we identify the areas that need further investigation and propose new lines of work towards the performance optimization of ceria as highly efficient catalyst for CO2 reduction in SOECs.

Defects and Phase Formation in Non-Stoichiometric LaFeO3: a Combined Theoretical and Experimental Study

The defect chemistry of perovskite compounds is directly related to the stoichiometry and to the valence states of the transition-metal ions. Defect engineering has become increasingly popular as it offers the possibility to influence the catalytic properties of perovskites for applications in energy storage and conversion devices such as solid-oxide fuel- and electrolyzer cells. LaFeO$_3$ (LFO) can be regarded as a base compound of the family of catalytically active perovskites La$_{1-x}$A$_x$Fe$_{1-y}$B$_y$O$_{3-\delta}$, for which the defect chemistry as well as the electronic and ionic conductivity can be tuned by substitution on cationic sites. Combining theoretical and experimental approaches, we explore the suitability for A-site vacancy engineering, namely the feasibility of actively manipulating the valence state of Fe and the concentration of point defects by synthesizing La-deficient LFO. Formation energies and concentrations of point defects were determined as a function of processing conditions by first-principles DFT+U calculations. Based on the results, significant compositional deviations from stoichiometric LFO cannot be expected by providing rich or poor conditions of the oxidic precursor compounds (Fe$_2$O$_3$ and La$_2$O$_3$) in a solid-state processing route. In the experimental part, LFO was synthesized with a targeted La-site deficiency. We analyze the resulting phases by X-ray diffraction and scanning electron microscopy, (scanning) transmission electron microscopy in combination with energy-dispersive X-ray spectroscopy, and electron energy-loss spectrometry. Instead of a variation of the La/Fe ratio, a mixture of the two phases Fe$_2$O$_3$ and LFO was observed, resulting in an invariant charge state of Fe, in line with the theoretical results. We discuss our findings with respect to partly differing assumptions made in previous studies on this material system.

Fundamentals and practical dielectric implications of stoichiometry and chemical design in a high-performance ferroelectric oxide: BaTiO3

The processing science and fundamental understanding of defect chemistry of BaTiO3 is a model example of how material science is used to guide the materials engineering of capacitive devices. The fundamentals are discussed from the phase equilibria, defect chemistry, and the impact on intrinsic properties. We reviewed the phenomenological defect chemistry approaches and considered the importance of doping strategies and the formation of associated point defect complexes. First principles calculations have proven to be a most informative strategy towards understanding these complexes and implications conduction mechanism. The nature of mixed conduction is considered with various dopants in the BaTiO3 and conditions that can arise with different oxygen vacancy concentrations over a wide range of conditions. Defect dynamics are considered experimentally in terms of associations and dissociations kinetics of oxygen vacancies from these various complexes. The deleterious impact on time-dependent properties is reviewed, including time-dependent dielectric breakdown, fatigue, and aging.

Defect chemistry of LaGaO3 doped with divalent cations

Solid oxide fuel cells are highly efficient energy conversion devices and can produce electrical energy from a variety of fuels. One of the main challenges is to decrease its operating temperatures, since the high temperatures currently required for satisfactory oxide-ion conduction at the electrolyte decrease the lifetime of the device. LaGaO3-based materials possess high conductivities in lower temperatures when compared to traditional fluorite-structured materials. In this study, density functional theory calculations were performed to investigate the potential effects of divalent dopants in the ionic conductivity of the perovskite. The most suitable dopants were determined, with steric and electronic effects being found to play a role in how easily these dopants are accepted into the lattice. The distortion of the doped perovskite local structure was observed to be an indicator of the tendency of vacancies to be trapped around dopants, possibly hindering ionic conductivity.