Charged Point Defects Research Articles

Origin of Overpotential-Dependent Surface Dipole at CeO2–x/Gas Interface During Electrochemical Oxygen Insertion Reactions

Ion insertion at the interfaces of batteries, fuel cells, and catalysts constitutes an important class of technologically relevant, charge-transfer reactions. However, the molecular nature of charge separation at the adsorbate/solid interface remains elusive. It has been hypothesized that electrostatic dipoles at the adsorbate/solid interface could result from adsorption-induced charge redistribution, preferential segregation of charged point defects in the solid, and/or intrinsic dipoles of adsorbates. Using operando ambient-pressure X-ray photoelectron spectroscopy, we elucidate the coupling between electrostatics and adsorbate chemistry on the surface of CeO2–x, an excellent electrocatalyst and a model system for studying oxygen-ion insertion reactions. Three adsorbate chemistries were studied—OH–/CeO2–x (polar adsorbate), CO32–/CeO2–x (nonpolar adsorbate), and Ar/CeO2–x (no adsorbate)—under several hundred mTorr of gas pressure relevant to electrochemical H2/CO oxidation and H2O/CO2 reduction. By inte...

Point defect engineering of high temperature piezoelectric BiScO3-PbTiO3 for enhanced voltage response

BiScO3-PbTiO3 is the most promising system among high sensitivity piezoelectric BiMO3-PbTiO3 perovskite solid solutions with high Curie temperature, which are under extensive investigation for expanding the operation temperature of state of the art Pb(Zr,Ti)O3 (PZT) up to 400°C. The viability of these alternative materials requires the development of specific point defect engineering that allow a range of piezoelectric ceramics comparable to commercial PZTs to be obtained, optimized for the different applications. A distinctive feature of BiMO3-PbTiO3 systems is the simultaneous presence of both Bi3+ and Pb2+ at the A-site of the perovskite. This enables the possibility of introducing charged point defects without incorporating new chemical species, just by defining an A-site non–stoichiometry. In this work, we present a comprehensive study of the effects of Bi substitution for Pb, along with the formulation of Pb vacancies for charge compensation. Results indicate an overall lattice stiffening that yields reduced polarizability and compliance, and dominates over a limited enhancement of the ferroelectric domain wall dynamics, so as a largely enhanced voltage response is obtained. Specifically, BiScO3-PbTiO3 with the A-site non-stoichiometry is shown to be very suitable as the piezoelectric component of magnetoelectric composites for magnetic field sensing.

Structure and Properties of the Amorphous Lipon Electrolyte By First-Principles Simulations

Amorphous materials belonging to the family of Lithium Phosphorus Oxynitrides (LiPON) are increasingly popular solid electrolytes for thin-film Li-ion batteries. RF-sputtering of a lithium orthophosphate target in a nitrogen plasma leads to the deposition of amorphous glasses. Rationalization of the transport properties rests on a valid structural model for these glassy structures, whose simulation represents a main challenge from a computational point of view also because of their non-trivial compositions. In close cooperation with experimental partners, we assessed the structural, electronic and transport properties of an amorphous member of the LiPON family with non-trivial composition and cross-linking. The issue of structure prediction for a given composition is a longstanding problem even for the simplest crystalline solids. The problem is further complicated in the case of an amorphous material, as the potential energy landscape that describes the glassy region exhibits a large number of minima of varying depths. We circumvented the problem by using an evolutionary algorithm to find a stable structure for a realistic, composition provided by experiments and subjecting it to ab-initio simulated annealing to create disorder. In this work we present the results of Density Functional Theory calculations. After characterizing the structural and electronic properties of our material, we addressed the issue of ionic conductivity by calculating the defect formation energies of neutral and charged point defects with respect to two different Li reservoirs: a typical anode, metallic lithium, and a typical cathode, lithium cobalt oxide (LCO). We found that charged defects dominate over neutral ones, as expected of an ionic conductor. Li+ interstitials dominate by far over vacancies, both when the lithium reservoir is metallic lithium and LCO. The formation of charged interstitials when LiPON is brought into contact with LCO has already been observed by our experimental partners. Furthermore, at the interface with metallic lithium the formation of neutral interstitials is a competitive process that results in the chemical reduction of LiPON and the disruption of the network. The occurrence of side reactions at the interface between LiPON and lithium has been observed and quantified by means of X-ray photoemission spectroscopy by our experimental partners. Following up on this result, we proceeded to investigate the structural and electronic properties of the interface between LiPON and metallic lithium with a special focus on the occurrence of side reactions. We conclude by showing preliminary studies of the interface between LiPON and selected LCO terminations suggested by available experimental results.

Effect of charge on point defect size misfits from ab initio: Aliovalently doped [formula omitted

We demonstrate a computationally efficient method to calculate size misfits of point defects in aliovalently doped SrTiO3 using density functional theory. The size misfits of neutral and charged single point defects are calculated and linearly combined to estimate the effective size misfits of multiple point defects in aliovalently doped SrTiO3. The charge state has a demonstrable effect on point defect size misfits for both isolated and multiple defects. We attribute this to the different charge localization between the defective systems. The extra electron or hole for the charged isolated point defects compensates the difference of charge localization between the neutral point defects and aliovalently doped system. The sum of size misfits of charged isolated point defects accurately estimates size misfits for aliovalently doped SrTiO3, without charge correction which was possibly required due to the additional charge state of point defect.

Symmetry breaking in hexagonal and cubic polymorphs of BaTiO3

BaTiO3 appears in cubic and hexagonal variants, both of which are centrosymmetric. Samples of cubic BaTiO3 are known to exhibit breaking of the centric symmetry locally and globally. It has been proposed that the local symmetry breaking originates in polar regions, the precursors of the ferroelectric phase. Origins of the macroscopic symmetry breaking, which are not well understood, have been previously tentatively correlated with inhomogeneities in the samples, such as strain gradients that may align or redistribute objects such as charged point defects or polar regions making material macroscopically polar. No such data are available for BaTiO3 with hexagonal symmetry. We compare dielectric, elastic, and pyroelectric properties of the two materials in polycrystalline form. In contrast to cubic BaTiO3, hexagonal BaTiO3 does not exhibit macroscopic pyroelectric response at room temperature. This is consistent with apparent absence of polar regions in the hexagonal material, and the fact that in hexagonal BaTiO3, strain rather than polarization is the order parameter for the phase transition into ferroelectric-ferroelastic phase. The thermally stimulated currents measured in hexagonal and cubic BaTiO3, however, show that both materials exhibit noncentric macroscopic symmetry. This result supports the idea that extrinsic factors, such as strain gradients, which are apparently common for both materials, may break the macroscopic symmetry, which may then lead to alignment and redistribution of polar regions or charged defects.

The Band Structure of Polycrystalline Al2O3 and Its Influence on Transport Phenomena

The electronic (band) structure of polycrystalline Al2O3, in particular the density of near‐band edge grain‐boundary localized states, plays a significant role in a host of high‐temperature phenomena, including sintering, high‐temperature creep, oxygen permeability in dense “dry” Al2O3 ceramics, and Al2O3 scale formation on Al2O3 scale‐forming alloys. All these phenomena involve creation or annihilation of charged point defects (vacancies and/or interstitials) at grain boundaries and interfaces, and must of necessity involve electrons and holes. Thus, the density of states associated with grain boundaries in Al2O3 assume great importance, and has been calculated using DFT for both nominally undoped and Y‐doped Σ7 bi‐crystal boundaries. These quantum mechanical calculations must be taken into account when considering why Y2O3 segregation to Al2O3 grain boundaries is so effective in enhancing high‐temperature creep resistance of polycrystalline Al2O3, and in understanding the reactive element effect in Al2O3 scale‐forming alloys. Finally, a case will be made that grain‐boundary diffusion is mediated by the migration of a class of grain‐boundary ledge defects called disconnections, which are characterized by a step height h and a Burgers vector b.

Effects of Defect on Ferroelectric Stability in PbTiO3 Thin Films

ABSTRACTEffects of defect on ferroelectric stability in PbTiO3 (PTO) thin films have been investigated using molecular dynamics with first-principles effective Hamiltonian. In this paper, oxygen vacancy (Vo) has been considered to study the hysteresis loop, spontaneous polarization as a function of film thickness. Vo has been modeled as a charged point defect. Density functional theory (DFT) calculations are performed to determine the Vo-induced localized fields (both mechanical and electrical) and the calculated DFT results are used as inputs to molecular dynamics simulations in a large system. The strain field induced by the Vo is calculated by DFT calculations and fitted by the continuum strain modeling, and the electrostatic field is given by the superposition of the Vo-induced field and the external field. Vo significantly reduces the spontaneous polarization and increases the critical thickness.

Suppression of the antiferroelectric phase during polarization cycling of an induced ferroelectric phase

The ceramic Pb0.99Nb0.02[(Zr0.57Sn0.43)0.92Ti0.08]0.98O3 can exist in either an antiferroelectric or a ferroelectric phase at room temperature, depending on the thermal and electrical history. The antiferroelectric phase can be partially recovered from the induced ferroelectric phase when the applied field reverses polarity. Therefore, polarization cycling of the ferroelectric phase in the ceramic under bipolar fields at room temperature is accompanied with repeated phase transitions. In this letter, the stability of the recovered antiferroelectric phase upon electrical cycling of the ceramic is investigated. Ex-situ X-ray diffraction reveals that bipolar cycling suppresses the antiferroelectric phase; this is indirectly supported by piezoelectric coefficient d33 measurements. It is speculated that the accumulated charged point defects during polarization cycling stabilize the polar ferroelectric phase. The findings presented are important to the fundamental studies of electric fatigue and field-induced phase transitions in ferroelectrics.

Silicon germanium interdiffusion in SiGe device fabrication: A calibrated TCAD model

SiGe alloys are used in many types of electronic devices. For upcoming FinFET technologies, there is interest in the use of SiGe heterostructures in the complete mole fraction range (0–100% Ge atoms). For high Ge mole fraction and small device dimensions, interdiffusion of Si and Ge atoms can lead to critical changes in the distribution of Si and Ge atoms, impacting channel strain and carrier mobility. A process model for Si–Ge interdiffusion has been developed which covers the needs for actual device fabrication processes. Interdiffusion is understood to be correlated with the diffusion of vacancies, self-interstitials, and dopant-defect pairs. We consider the impact of non-equilibrium point defect concentrations (such as interstitial super-saturation after ion implantation), the impact of compressive or tensile strain, and a twofold impact of doping: first, doping determines the local electron concentration and thereby the abundance of charged point defects, and second, the diffusion of dopant-defect pairs can contribute to the interdiffusion of Si and Ge atoms. The model covers the full range of possible Ge mole fractions (0–100%). For undoped SiGe, it has been calibrated against a broad range of published experimental data for radio-tracer diffusion in SiGe and for interdiffusion in biaxially strained SiGe heterostructures grown on Si, stress-relaxed SiGe, or Ge. The extensions for doped SiGe are presented with a reference to few interdiffusion data for heavily doped SiGe heterostructures. Finally, the model has been consistently integrated into a set of models for the continuum process simulation of point defects and dopants in pure Si, SiGe, and pure Ge, in Sentaurus Process.

Pressure, relaxation volume, and elastic interactions in charged simulation cells

The ab initio calculation of charged supercells within density-functional theory is a necessary step to access several important properties of matter. The relaxation volume of charged point defects or the partial molar volume of ions in solution are two such examples. However, the total energy and therefore the pressure of charged systems is not uniquely defined when periodic boundary conditions are employed. This problem is tightly related to the origin of the electrostatic potential in periodic systems. This effect can be easily observed by modifying the electrostatic convention or modifying the local ionic potential details. We propose an approach to uniquely define the pressures in charged supercells with the use of the absolute deformation potentials. Only with such a definition could the ab initio calculations provide meaningful values for the relaxation volumes and for the elastic interactions for charged defects in semiconductors or ions in solution. The proposed scheme allows one to calculate sensible data even when charge neutrality is not enforced, thus going beyond the classical force-field-based approaches.

Self‐Regulation Mechanism for Charged Point Defects in Hybrid Halide Perovskites

AbstractHybrid halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) exhibit unusually low free‐carrier concentrations despite being processed at low‐temperatures from solution. We demonstrate, through quantum mechanical calculations, that an origin of this phenomenon is a prevalence of ionic over electronic disorder in stoichiometric materials. Schottky defect formation provides a mechanism to self‐regulate the concentration of charge carriers through ionic compensation of charged point defects. The equilibrium charged vacancy concentration is predicted to exceed 0.4 % at room temperature. This behavior, which goes against established defect conventions for inorganic semiconductors, has implications for photovoltaic performance.

Self-regulation mechanism for charged point defects in hybrid halide perovskites.

Hybrid halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) exhibit unusually low free-carrier concentrations despite being processed at low-temperatures from solution. We demonstrate, through quantum mechanical calculations, that an origin of this phenomenon is a prevalence of ionic over electronic disorder in stoichiometric materials. Schottky defect formation provides a mechanism to self-regulate the concentration of charge carriers through ionic compensation of charged point defects. The equilibrium charged vacancy concentration is predicted to exceed 0.4 % at room temperature. This behavior, which goes against established defect conventions for inorganic semiconductors, has implications for photovoltaic performance.

Anticorrelation between surface and subsurface point defects and the impact on the redox chemistry of TiO2(110).

By using a combination of scanning tunneling microscopy (STM), density functional theory (DFT), and secondary-ion mass spectroscopy (SIMS), we explored the interplay and relative impact of surface versus subsurface defects on the surface chemistry of rutile TiO2 . STM results show that surface O vacancies (VO ) are virtually absent in the vicinity of positively charged subsurface point defects. This observation is consistent with DFT calculations of the impact of subsurface defect proximity on VO formation energy. To monitor the influence of such lateral anticorrelation on surface redox chemistry, a test reaction of the dissociative adsorption of O2 was employed and was observed to be suppressed around them. DFT results attribute this to a perceived absence of intrinsic (Ti), and likely extrinsic interstitials in the nearest subsurface layer beneath inhibited areas. We also postulate that the entire nearest subsurface region could be devoid of any charged point defects, whereas prevalent surface defects (VO ) are largely responsible for mediation of the redox chemistry at the reduced TiO2 (110).

Charged Point Defects in the Flatland: Accurate Formation Energy Calculations in Two-Dimensional Materials

Technologically important semiconductors rely on impurities of foreign atoms. Researchers successfully model charged defects in two-dimensional semiconductors and insulators by assuming a specific dielectric constant profile.

Investigation of the Nature and Mechanism of Resistive Switching in TiO2-x

The spatial redistribution of the charged point defects under direct-current (DC) biasing can have significant implications for electroceramic device performance and lifetime [1,2]. The transport behavior of point defects is regulated by the boundary conditions of the electrodes, which can block ion transfer across the interface to varying degrees. When the electrodes are impermeable to mass transport, there will be an accumulation of point defects in the near-electrode region that can lead to significant modifications in the local electronic carrier concentrations at the interface. Such defect redistribution is responsible for the long-term increases in leakage current in many capacitor devices. On the other hand, this same phenomena can lead to resistive switching behavior via modification of the interface Schottky barrier at the reverse-biased cathode [1-4].

DFT + U investigation of charged point defects and clusters in UO2

We present a physically justified formalism for the calculation of point defects and cluster formation energies in UO2. The accessible ranges of chemical potentials of the two components U and O are calculated using the U-O experimental phase diagram and a constraint on the formation energies of vacancies. We then apply this formalism to the DFT + U investigation of the point defects and cluster defects in this material (including charged ones). The most stable charge states obtained for these defects near stoichiometry are consistent with a strongly ionic system. Calculations predict similarly low formation energies for V and I in hyperstoichiometric UO2. In stoichiometric UO2, V and I have the same formation energy in the middle of the gap and in hypostoichiometric UO2, V is the most stable defect.

Broadening of paramagnetic resonance lines by charged point defects in neodymium-doped scheelites

We study paramagnetic resonance linewidth in a series of CaWO4 and CaMoO4 crystals with different concentrations of neodymium ions (0.0031–0.81 at %). Experimental data are interpreted in the framework of the statistical theory of line broadening by charged point defects. In our calculations, three different contributions are singled out: arising from the local electric fields, electric field gradients and magnetic fields of the nearby point defects. The interaction parameters are determined from the spectroscopic data available for Nd:CaWO4 crystal. Direct calculations of the linewidth are performed for different crystal orientations with respect to external magnetic field. We conclude that major contribution to the broadening comes from the interactions with random electric fields produced by neodymium and charge compensator ions.

Broadening of Paramagnetic Resonance Lines by Charged Point Defects in Neodymium-Doped Scheelites

Broadening of Paramagnetic Resonance Lines by Charged Point Defects in Neodymium-Doped Scheelites

Systematic tuning of the conduction mechanisms in ferroelectric thin films

We have investigated the macroscopic and microscopic properties of large sets of Ba0.7Sr0.3TiO3 thin films including several substitution rates of manganese. Thanks to a high degree of control of the processing parameters at each stage we have been able to find a link between the dc leakage current and the low and high frequency dielectric permittivity and losses. We supplemented these macroscopic observations with in depth investigations of the defect states through x-ray photoelectron spectroscopy. We found that both the leakage current and the extrinsic dielectric parameters arise from a large density of charged point defects related to oxygen vacancies. At the outer surfaces of the films, the density of such charged defects is so high that it can raise the Fermi level to close to the conduction band. Such degradation of the films’ performance can be relieved by appropriate manganese substitution for the titanium host ions. Such doping is able to move back the Fermi level to close to the center of the bandgap thus changing the conduction process from interfacial Schottky to bulk Poole Frenkel and decreasing the extrinsic losses. This beneficial effect was already inferred in ceramics and thin films but we have established a clear link between the macroscopic parameters and the microscopic defect state. This model can be transferred to many high permittivity oxides.

Atomistic-to-continuum multiscale modeling with long-range electrostatic interactions in ionic solids

We present a multiscale atomistic-to-continuum method for ionic crystals with defects. Defects often play a central role in ionic and electronic solids, not only to limit reliability, but more importantly to enable the functionalities that make these materials of critical importance. Examples include solid electrolytes that conduct current through the motion of charged point defects, and complex oxide ferroelectrics that display multifunctionality through the motion of domain wall defects. Therefore, it is important to understand the structure of defects and their response to electrical and mechanical fields. A central hurdle, however, is that interactions in ionic solids include both short-range atomic interactions as well as long-range electrostatic interactions. Existing atomistic-to-continuum multiscale methods, such as the Quasicontinuum method, are applicable only when the atomic interactions are short-range. In addition, empirical reductions of quantum mechanics to density functional models are unable to capture key phenomena of interest in these materials.To address this open problem, we develop a multiscale atomistic method to coarse-grain the long-range electrical interactions in ionic crystals with defects. In these settings, the charge density is rapidly varying, but in an almost-periodic manner. The key idea is to use the polarization density field as a multiscale mediator that enables efficient coarse-graining by exploiting the almost-periodic nature of the variation. In regions far from the defect, where the crystal is close-to-perfect, the polarization field serves as a proxy that enables us to avoid accounting for the details of the charge variation. We combine this approach for long-range electrostatics with the standard Quasicontinuum method for short-range interactions to achieve an efficient multiscale atomistic-to-continuum method. As a side note, we examine an important issue that is critical to our method, namely the dependence of the computed polarization field on the choice of unit cell. Potentially, this is fatal to our coarse-graining scheme; however, we show that consistently accounting for boundary charges leaves the continuum electrostatic fields invariant to choice of unit cell.