Point Defect Configurations Research Articles

Anisotropic diffusion in symmetric tilt grain boundaries in bcc iron: A DFT study

Diffusion analysis at the atomic scale in three orthogonal directions for both low- and high-energy symmetric tilt grain boundaries (GBs) in bcc iron is presented. Using DFT, we report (i) the energetically preferable configurations of point defects in the GBs (112) and (210) as well as in surrounding atomic layers; (ii) the activation barriers for vacancy and self-interstitial hopping in the GB plane and from the GBs to a nearest atomic layer; (iii) the attempt frequencies in each diffusion direction. The obtained values are refined by taking into account the temperature dependence of magnetization. It has been shown that interstitial and combined interstitial-vacancy mechanisms agree with the experimental data on self-diffusion in bcc iron. The temperature dependencies of the diffusion coefficient are obtained as Arrhenius law taking into account transition between ferro- and paramagnetic states. They provide insight into transport processes at relatively low temperatures which can be important for practical applications.

Identifying native point defect configurations in α-alumina

Intimately intertwined atomic and electronic structures of point defects govern diffusion-limited corrosion and underpin the operation of optoelectronic devices. For some materials, complex energy landscapes containing metastable defect configurations challenge first-principles modeling efforts. Here, we thoroughly reevaluate native point defect geometries for the illustrative case of α-Al2O3 by comparing three methods for sampling candidate geometries in density functional theory calculations: displacing atoms near a naively placed defect, initializing interstitials at high-symmetry points of a Voronoi decomposition, and Bayesian optimization. We find symmetry-breaking distortions for oxygen vacancies in some charge states, and we identify several distinct oxygen split-interstitial geometries that help explain literature discrepancies involving this defect. We also report a surprising and, to our knowledge, previously unknown trigonal geometry favored by aluminum interstitials in some charge states. These new configurations may have transformative impacts on our understanding of defect migration pathways in aluminum-oxide scales protecting metal alloys from corrosion. Overall, the Voronoi scheme appears most effective for sampling candidate interstitial sites because it always succeeded in finding the lowest-energy geometry identified in this study, although no approach found every metastable configuration. Finally, we show that the position of defect levels within the band gap can depend strongly on the defect geometry, underscoring the need to conduct careful searches for ground-state geometries in defect calculations.

The role of native point defects and donor impurities in the electrical properties of ZnSb2O4: a hybrid density-functional study.

The ceramic material zinc antimony oxide ZnSb2O4 has promising electrical and magnetic properties, making it suitable for various applications such as electrochemical and energy storage. However, the effects of point defects and impurities on its electrical properties have never been revealed. Here, we employ hybrid density-functional calculations to investigate the energetics and electronic properties of native point defects and donor impurities in ZnSb2O4. The energetically favorable configurations of native point defects under selected growth conditions (O-rich and O-poor) are identified based on the calculated formation energies. The study finds no shallow donor and shallow acceptor defects with low formation energies. Still, the oxygen vacancy (VO) has the lowest formation energy among the donor-type defects under O-rich and O-poor conditions. However, it acts as a very deep acceptor, making it unlikely to provide free electron carriers to the conduction band. Moreover, electron carriers are likely to be compensated by the formation of zinc vacancies (VZn) and the zinc substituted for antimony (ZnSb), which behave as dominant acceptors. Our analysis of the charge neutrality conditions estimates that the Fermi level of undoped ZnSb2O4 would be pinned in a range that is 2.60 eV to 3.12 eV above the VBM for O-rich to O-poor growth conditions, respectively, suggesting that this material is semi-insulating. The possibility of enhancing free electron carriers by doping with Al, Ga, In, and F impurities is also investigated. Our results, however, indicate that high n-type conductivity is hindered by self-compensation in which the impurities additionally act as electron killers. Our results suggest that other impurity candidates and approaches may need to be considered to efficiently dope this material into n-type. Overall, this work paves the way for point defect engineering in this class of ternary oxides.

Numerical investigation of dislocation climb under stress and irradiation

We investigate the influence of elastic properties of point defects on dislocation climb under stress and irradiation. For this purpose, elastic dipole tensors and diaelastic polarizabilities are evaluated in aluminum for vacancies and self-interstitial atoms in their stable and saddle configurations, using density functional theory calculations. These parameters are introduced in an object kinetic Monte-Carlo code and a continuous diffusion model to estimate the stress dependence of dislocation climb, using a dipole of straight dislocations. We show that both parameters have an influence on absorption of point defects under stress, in agreement with previous analytical models. However, the effect of dipole tensor is found only 5 times larger than polarizability, whereas models predict a factor up to 30. In addition, including polarizability reverses the stress angular dependence when a uniaxial stress is applied orthogonal to the dislocation line, so in general polarizability cannot be ignored for simulations under applied stress. Further comparison with analytical models shows that they give a good description of angular dependence, provided saddle point configuration of point defects is not too anisotropic. For vacancies, which are strongly anisotropic in their saddle configuration, models fail to reproduce quantitatively lattice effects on stress angular dependence observed in simulations. Calculations show that dislocation climb velocity under irradiation is expected to be the highest if the stress is approximately orthogonal to the dislocation line, especially along the Burgers vector, and the lowest if the stress is applied close to the 〈100〉 direction with the largest projection on the dislocation line.

Common defects in diamond lattices as instances of the general T ⊗ (e+t2) Jahn-Teller effect

Finding the ground-state configuration of point defects in semiconductors is in general a high-dimensional optimization problem. However, their adiabatic potential energy surfaces (APES) can be described with few effective coordinates by exploiting symmetry arguments within the Jahn-Teller (JT) theoretical framework. In this paper, we propose a general framework, that combines group theory and ab initio calculations, to build a full picture of common defects in diamond lattices (vacancies and substitutional impurities) as instances of the general T $\ensuremath{\bigotimes}\phantom{\rule{4pt}{0ex}}(e+{t}_{2})$ APES. Starting from the original tetrahedral symmetry, we consider all possible JT symmetries and we numerically investigate the shape of the APES by targeting the minimum energy path between equivalent JT distortions taking as examples the vacancy (V) and the metallic impurities Pd, Pt, and Au in crystalline silicon.

Density functional theory calculations of the thermodynamic and kinetic properties of point defects in β-U

Density functional theory (DFT) calculations of the thermodynamic and kinetic properties of point defects in the β phase of uranium are reported. Defect energies and entropies were calculated using 2 × 2 × 2 supercells and the Generalized Gradient Approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) for the exchange-correlation potential. Due to computational cost, calculations of the vibrational properties governing entropies were performed by only displacing atoms within (roughly) the 3rd or 4th nearest neighbor shell of the defect, which implicitly assumes that atoms beyond this distance are unaffected by the defect. Migration barriers were estimated by nudged elastic band (NEB) calculations. The low symmetry of the β -U phase (the unit cell is tetragonal and contains 30 atoms) results in many point defect configurations and even more migration pathways. A connectivity map, starting from the most stable point defects, was developed in order to identify the rate-limiting step controlling the net diffusion rate in each crystallographic direction. The uranium self-diffusivity tensor was calculated by combining the defect formation energies , entropies, migration barriers and attempt frequencies. The fastest diffusion rate was determined to be a vacancy mechanism in the z crystallographic direction. The predicted uranium self-diffusivity for this mechanism agrees well with available experimental data. The diffusion mechanisms and rates identified in this study will inform fuel performance models of swelling and gas evolution.

Thermo-mechanical properties of nitrogenated holey graphene (C2N): A comparison of machine-learning-based and classical interatomic potentials

Thermal and mechanical properties of two-dimensional nanomaterials are commonly studied by calculating force constants using the density functional theory (DFT) and classical molecular dynamics (MD) simulations. Although DFT simulations offer accurate estimations, the computational cost is high. On the other hand, MD simulations strongly depend on the accuracy of interatomic potentials. Here, we investigate thermal conductivity and elastic modulus of nitrogenated holey graphene (C2N) using passively fitted machine-learning interatomic potentials (MLIPs), which depend on computationally inexpensive ab-initio molecular dynamics trajectories. Thermal conductivity of C2N is investigated via MLIP-based non-equilibrium molecular dynamics simulations (NEMD). At room temperature, the lattice thermal conductivity of 85.5 ± 3 W/m-K and effective phonon mean free path of 36.7 ± 1 nm are found. By carrying out uniaxial tension simulations, the elastic modulus, ultimate strength, and fractural strain of C2N are predicted to be 390 ± 3 GPa, 42 ± 2 GPa, and 0.29 ± 0.01, respectively. It is shown that the passively fitted MLIPs can be employed as an efficient interatomic potential to obtain the thermal conductivity and elastic modulus of C2N utilizing classical MD simulations. Moreover, the possibility of employing MLIPs to simulate C2N with point defects has been investigated. By training MLIP with point defect configurations, the mechanical properties of defective structures were studied. Although using the MLIP is more costly than classical interatomic potentials, it could efficiently predict the thermal and mechanical properties of 2D nanostructures.

Isostructural phase transition by point defect reorganization in the binary type-I clathrate Ba7.5Si45

Competition between microscopic point defect (vacancy and interstitial) configurations is inherent to crystalline phases of increased structural complexity. Phase transitions that preserve symmetry between them belong to a specific class of isostructural transitions. Type-I silicon clathrates are representatives of such structurally complex crystalline phases showing an intriguing structural transition at high pressure associated with an abrupt reduction of volume with no indication for any breakage of symmetry. Using isothermal high-pressure X-ray diffraction performed on a single crystal of the simplest representative type-I silicon clathrates, binary Ba8Si46, we confirm the isostructural character of the transition and identify the associated mechanism. A detailed analysis of the atomic structural parameters across the transition in combination with ab initio studies allow us to pinpoint a microscopic mechanism driven by a rearrangement of point defects initially present in the structure. An analysis based on the Landau theory gives a coherent description of the experimental observations. A discussion on the analogy between this transformation and liquid-liquid transitions is proposed.

Mechanisms of carrier lifetime enhancement and conductivity-type switching on hydrogen-incorporated arsenic-doped BaSi2

A comparative experimental and theoretical study of the role of H incorporation in As-doped BaSi2 films has been carried out based on the experimental results that an optimal time of H treatment for the increase in photoresponsivity and carrier lifetime was in the range of 1 – 20 min. Adequate theoretical representation of the decay curves in the framework of the model for non-radiative processes accounted for various trap-related recombination mechanisms to estimate the trap concentration to be in the range of 1.9 × 1013 to 1.7 × 1014 cm−3. Additionally, the extended theoretical ab initio quantum-chemical simulation of the electronic structure of the studied systems was performed. It was revealed that interstitial As atoms can mostly provide trap states in the gap while H atoms neutralize such traps. The experimentally observed unexpected switching in conductivity from n-type to p-type and vice versa in As-doped BaSi2 with H incorporation was explained to specific configurations of point defects (an As impurity with a H atom in different positions and various interatomic As-H distances) which affect the position of states in the gap.

Stability of native point defects in α-Al2O3 under aqueous electrochemical conditions

The formation energies of native point defects in crystalline α-Al2O3 were investigated by combining first principles-based methods and theoretical models in a grand canonical framework. For defect formation reactions in this framework, the chemical potentials of chemical species and electrons can be constrained by the conditions of aqueous electrochemical systems, where liquid water is thermodynamically stable. Activation relaxation technique (ART) simulations using an empirical interatomic potential were implemented to discover candidates for stable configurations of point defects. Density functional theory (DFT) calculations were then used to confirm the accurate energetics of the candidate defect configurations. The results show that, except Al vacancies, the most stable defect configurations are generated by simply adding/removing atoms at particular high-symmetry sites. We also investigated the stability of these point defects as a function of the chemical potentials of both electron and oxygen. The results reveal that, at the conditions of thermodynamic stability for liquid water in aqueous electrochemical systems, Al vacancies as the most stable point defects in α-Al2O3 can be generated in exothermic defect formation reactions with negative formation energies. These thermodynamic tendencies provide critical insights into the nature of passive films formed under aqueous electrochemical conditions, particularly explaining of the formation of amorphous structures of passive alumina.

Isostructural Phase Transition by Point Defect Reorganization in the Binary Type-I Clathrate Ba <sub>7.5</sub>Si <sub>45</sub>

A competition of point defect configurations is inherent to crystalline phases of increased structural complexity. Symmetry preserving phase transitions between them belong to the special class of isostructural transitions. Type-I silicon clathrates are crystalline complex phases whose unit cell containing a fifties of atoms consists of a 3D covalent silicon framework of face-sharing polyhedral cages encapsulating guest cations. At high pressure, an intriguing structural transition is associated to an abrupt volume reduction with no indication for any symmetry breaking. By means of isothermal high-pressure X-ray diffraction performed on single crystal of the simplest representative type-I silicon clathrates, the binary Ba8Si46, we confirm the isostructural character of the transition and identify the associated mechanism. A detailed analysis of the atomic structural parameters across the transition in combination with ab initio studies allow us to pinpoint a microscopic mechanism driven by a defect rearrangement initially present in the structure. An analysis based on the Landau theory allows to give a coherent description of the experimental observations. A discussion on the analogy between this transformation and liquid-liquid transitions is proposed.

Fingerprinting the vibrational signatures of dopants and defects in a fully random alloy: An ab initio case study of Si, Se, and vacancies in In0.5Ga0.5As

Correct identification of local configurations of dopants and point defects in random alloys poses a challenge to both computational modeling and experimental characterization methods. In this paper, we propose and implement a computationally efficient approach to address this problem. Combining special quasirandom structures, virtual crystal approximation, and real-space lattice static Green’s functions, we are able to calculate, at moderate computational cost, the local phonon density of states (LPDOSs) of impurities in a random alloy crystal for system sizes, surpassing the capabilities of a conventional, cubic-scaling, density functional theory. We validate this method by showing that our LPDOS predictions of substitutional silicon in GaAs and InAs are in excellent agreement with the experimental data. For the case study, we investigate a variety of local configurations of Si and Se substitutional dopants and cation vacancies in quasirandom In0.5Ga0.5As alloys. In all cases, the impurity LPDOS in a random alloy exhibits qualitatively different signatures from those in the pure binary compounds GaAs and InAs. Specifically, they are characterized by a wide continuous band (rather than narrow discrete peaks) of vibrational modes at frequencies typically higher than the bulk modes, a sign of coupling between localized vibrations of the impurity and those of its random neighboring host atoms. The accuracy and computational cost of this approach open a way to the simulation of impurities in random structures on a large scale and the prediction of vibrational signatures of alloys with defects.

Estimation of the spatial distribution of Frenkel defects in NiFe2O4 by simulation of HAADF-STEM images.

Accurate determination of the atomic spatial configuration of Frenkel defects is important for understanding the mechanism and fully utilizing these defects to optimize the material properties. In this study, aberration-corrected scanning transmission electron microscopy (STEM) was used to identify the Fe vacancies and Fe Frenkel defect pairs, which have not been previously investigated, in NiFe2O4 (NFO). The spatial distribution of these point defects is determined by comparing the experimental and simulated images, where the experimental image intensities are consistent with the calculated image intensities. We confirmed the stabilities of the observed point defect configurations and calculated their electronic structures using density functional theory. A comprehensive understanding of the relationship between the Frenkel defect spatial configurations and electronic properties is obtained, which provides an alternative method to regulate the NFO performance.

The damage and healing of quartz under radiation at high temperatures

The effect of radiation on silica has attracted significant attention due to its potential application where radiation exists. However, the nature of the atomistic defects of quartz formed during the radiation-induced damage at high temperatures has not been fully elucidated. Molecular dynamics is adopted to investigate the damage-healing behavior of the quartz at high temperatures in this research. The ensuing diffusion and recovery of point defects in the irradiated specimen at high temperatures are simulated. The high temperature reduces the number of point defects. At 1400 K, all point defects are removed, while only some point defects are erased at 700 K. It is illustrated that the over-coordinated atoms are the main source of point defects at 300 K and 700 K. The temperature influences the configuration of point defects. The healing variable of the molecular dynamics model is subsequently developed to describe the healing process. The healing mechanisms of the irradiated specimen at high temperatures are summarized by analyzing the trajectories of atoms in detail. These results are helpful to understand the nature of radiation damage in quartz at high temperatures.

XANES study of vanadium and nitrogen dopants in photocatalytic TiO2 thin films.

We report an X-ray absorption near edge structure (XANES) study of vanadium (V) and nitrogen (N) dopants in anatase TiO2 thin films deposited by radio-frequency magnetron sputtering. Measurements at the Ti K and V K edges were combined with soft X-ray experiments at the Ti L2,3, O K and N K edges. Full potential ab initio spectral simulations of the V, O and N K-edges were carried out for different possible configurations of substitutional and interstitial dopant-related point defects in the anatase structure. The comparison between experiments and simulations demonstrates that V occupies substitutional cationic sites (replacing Ti) irrespective of the film structure and dopant concentration (up to 4.5 at%). On the other hand, N is found both in substitutional anionic sites (replacing O) and as N2 dimers within TiO2 interstices. The dopants' local structures are discussed with reference to the enhanced optical absorption and photocatalytic activity achieved by (co)doping.

Identification of Si-vacancy related room-temperature qubits in 4H silicon carbide

Identification of microscopic configuration of point defects acting as quantum bits is a key step in the advance of quantum information processing and sensing. Among the numerous candidates, silicon vacancy related centers in silicon carbide (SiC) have shown remarkable properties owing to their particular spin-3/2 ground and excited states. Although, these centers were observed decades ago, still two competing models, the isolated negatively charged silicon vacancy and the complex of negatively charged silicon vacancy and neutral carbon vacancy [Phys. Rev. Lett.\ \textbf{115}, 247602 (2015)] are argued as an origin. By means of high precision first principles calculations and high resolution electron spin resonance measurements, we here unambiguously identify the Si-vacancy related qubits in hexagonal SiC as isolated negatively charged silicon vacancies. Moreover, we identify the Si-vacancy qubit configurations that provide room temperature optical readout.

A DFT study of intrinsic point defects in monolayer MoSe2

This study is a computational investigation of the electronic structure of the eight most-frequently observed intrinsic point-defect configurations in monolayer Molybdenum diselenide (m-MoSe2); analyzed using the Amsterdam Density Functional (ADF) BAND package. Pristine m-MoSe2 is an intrinsic semiconductor with a direct band gap of 1.44 eV. MoSe2 is defect-sensitive due to the similar orbital character of the Valence Band Maximum (VBM) and Conduction Band Minimum (CBM), with deep states induced in the structure by the defects. These states can be attributed solely to the metal d and chalcogen p states, which spring enhanced photoluminescence, making MoSe2 a potential candidate for optoelectronic applications. Band-gap narrowing is proportional to the number of chalcogen vacancies. All defect configurations cause shifting of the Fermi-level, with metal vacancies shifting the semiconducting character of pristine m-MoSe2 to metallic. Only the antisite defect configuration of MoSe2 and Mo-vacancies at a large distance could introduce spin in the structure, with spin attributed to the metal d and chalcogen p states. These findings suggest the possible application of m-MoSe2 for fabricating DMS by defect engineering.

Gamma-rays and heat-treatment conversions of point defects in massive rose quartz from the Borborema Pegmatite Province, Northeast Brazil

An extensive characterization of trace elements and point defects in rose quartz from the Borborema Pegmatite Province (BPP) in the northeast of Brazil was carried out by complementary spectroscopic methods. The aim here was to document the change in the configuration of point defects into the quartz lattice induced by heat-treatment and ionizing radiation. The samples were extracted from the core of two granitic rare element (REL) pegmatites, Taboa (Carnauba dos Dantas, RN) and Alto do Feio (Pedra Lavrada, PB). The contents of Al, P, Ti, Ni, Fe, Ge, Li, Be, B and K were measured by laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Polished plates were heat-treated at 500 and 1000 °C and then irradiated with 50 kGy of γ rays. Point defects were characterized by optical (UV–Vis), infrared (IR), and electron paramagnetic resonance (EPR) spectroscopies. In the as-received condition, [AlO4/H]0 centers, Li- and B-dependent OH defects were observed. Point defects related to Al and Li species were significantly affected by heat-treatment at 1000 °C and/or γ radiation. Paramagnetic centers such as [AlO4]0, [GeO4/Li]0, [TiO4/Li]0 and [O2 3−/Li]0 were created by the diffusion of Li+ ions from their original diamagnetic centers related to substitutional Al3+ and OH-species. The smoky color developed after irradiation and the signal intensities of the paramagnetic centers were independent from the original rose color grade. The samples from the Taboa (TB) pegmatite showed the highest concentration of Al, Ti, Fe and Li elements as well as the highest signal intensities for [AlO4]0, [AlO4/H]0, [GeO4/Li]0 and [TiO4/Li]0 centers. Although TB also showed the higher concentration of B element, the intensity of the 3597 cm−1 IR band related to [BO4/H]0 centers was higher for Alto do Feio (AF) samples. This result suggests that the uptake of B into the quartz core of each pegmatite took place through different mechanisms. It was concluded that the change in the point defect configuration was essentially governed by the motion of Li species whose incorporation into the quartz lattice is closely related to Al concentration.

Point defects and magnetic properties of neutron irradiated MgO single crystal

(100)-oriented MgO single crystals were irradiated to introduce point defects with different neutron doses ranging from 1.0×1016 to 1.0×1020 cm-2. The point defect configurations were studied with X-ray diffuse scattering and UV-Vis absorption spectra. The isointensity profiles of X-ray diffuse scattering caused by the cubic and double-force point defects in MgO were theoretically calculated based on the Huang scattering theory. The magnetic properties at different temperature were measured with superconducting quantum interference device (SQUID). The reciprocal space mappings (RSMs) of irradiated MgO revealed notable diffuse scattering. The UV-Vis spectra indicated the presence of O Frenkel defects in irradiated MgO. Neutron-irradiated MgO was diamagnetic at room temperature and became ferromagnetic at low temperature due to O Frenkel defects induced by neutron-irradiation.

Influence of γ-ray irradiation on the point defect configurations and magnetic properties of MgO single crystals

Influence of γ-ray irradiation on the point defect configurations and magnetic properties of MgO single crystals