Composition dependent intrinsic defect structures in ASnO_3(A=Ca,Sr,Ba)

Perovskite stannates are promising semiconductors that are widely used in optoelectronic devices.Here,the composition dependent intrinsic point defects of stannate perovskites ASnO_3(A=Ca,Sr,Ba) are studied by first-principles calculations.The prefe rences of de fects under stoichiometric and nonstoichiometric conditions are unsealed,meanwhile the charge states of each intrinsic defect varying with the change of electron Fermi energy are clarified.For stoichiometric BaSnO_3 and SrSnO_3,the Schottky defect complexes are predicted as the most stable defect structure,while the antisite defect complexes are the most stable one in CaSnO_3.In nonstoichiometric ASnO_3,excessive AO is beneficial to the formation of oxygen vacancies and A-Sn antisite defects in all ASnO_3;while the Ca interstitial is another major defect existing in CaSnO_3.In the case of SnO_2 excess,the predominant de fects are the Sn-A antisite defects and A vacancies.Since the functionality of these perovskite oxides is closely related to the types and concentrations of their point defects,the present results are expected to guide the future experiments to optimize the function of the perovskite oxides by tailoring the intrinsic defects through controlling the composition of AO and SnO_2.     

Thermodynamics and kinetics of point defects in manganous sulphide

The defect equilibria and diffusion kinetics of point defects and manganese ions in manganese sulphide have been discussed with the background of available literature data. It has been shown that the behaviour of defects over the whole phase field of this compound can be satisfactorily described in terms of point defect thermodynamics. Over the major part of the homogeneity range, doubly ionized cation vacancies and electron holes are the dominating defects, manganous sulphide being a metal deficit p-type semiconductor (Mn_1–y S). Near the Mn/MnS phase boundary, on the other hand, doubly ionized interstitial cations and quasi-free electrons prevail, the sulphide being thus a metal excess n-type semiconductor (Mn_1+y S). The enthalpy and entropy of cation vacancy formation in Mn_1–y S and those of interstitial cations in Mn_1+y S, have been calculated. Both near and at the stoichiometric composition, the defect structure of manganous sulphide is more complex and strongly dependent on temperature. Below about 1100 K, intrinsic Frenkel electronic disorder dominates. In agreement with the defect model it has been shown that the self-diffusion of cations in Mn_1–y S proceeds by a simple vacancy mechanism via doubly ionized cation vacancies, randomly distributed in the crystal lattice. The activation enthalpy and entropy of this process have been calculated. On the other hand, diffusion of manganese ions in Mn_1+y S proceeds by an interstitial mechanism, in which the cation enters from its normal lattice site into a neighbouring interstitial position. In agreement with theoretical predictions, the activation enthalpy of this process has been found to be considerably higher than that of the vacancy diffusion. It has been demonstrated that the minimum rate of manganese self-diffusion at a given temperature corresponds very well to the stoichiometric composition of the sulphide, which is again in agreement with the defect model.     

Magnetic and Electronic Properties of Point Defects in ZrO2

Using the Korringa–Kohn–Rostoker Coherent Potential Approximation (KKR-CPA) method in connection with the Generalized Gradient Approximation (GGA), we study the magnetic and electronic properties of different point defects in cubic ZrO₂. In particular, we discuss the zirconium interstitial (Zr_i), zirconium antisite (Zr_O), zirconium vacancy (V_Zr), oxygen interstitial (O_i), oxygen antisite (O_Zr), and oxygen vacancy (V_O) defects. It has been shown that oxygen vacancy and zirconium interstitial (V_O, Zr_i) are n-type, while the other point defects are p-type. The magnetic moments are observed only in the oxygen interstitial and antisite (O_i, O_Zr) cases. The corresponding ferromagnetic states are more stable than the spin–glass states. It has been found that the mechanism responsible of such stabilities is the double exchange. Based on Mean Field Approximation (MFA), the Curie temperature (T _C ) is estimated. Moreover, it has been found that the O_i and O_Zr defects provide half-metallic properties being the responsible for ferromagnetism.     

The Sluggish Diffusion of Cations in CeO2 Probed through Molecular Dynamics and Metadynamics Simulations

Cation diffusion in fluorite-structured CeO₂, though far slower than anion diffusion, is an important, high-temperature process because it governs diverse fabrication and degradation phenomena. Herein, cation diffusion is studied by means of classical molecular dynamics and metadynamics simulations. Three different mechanisms are examined: migration involving an isolated cerium vacancy, migration involving a cerium vacancy in a defect associate with an oxygen vacancy, and migration involving a cation divacancy. For each mechanism, defect diffusion coefficients are calculated as a function of temperature, from which the respective activation enthalpy of defect migration is obtained. Through comparisons with experimental cation diffusion data (specifically, of the absolute magnitude of the cation diffusivity as well as its activation enthalpy), it is concluded that cation diffusion takes place predominantly neither by isolated vacancies nor by cation vacancy–oxygen vacancy associates but by cation divacancies.     

Chemical activity of oxygen atoms in the magnetron sputter-deposited ZnO films during film growth

The role of oxygen atoms in the growth of magnetron sputter-deposited ZnO films was studied in a deposition and post-deposition study in which the deposition of a several-nanometer-thick ZnO layer altered with an exposure to an O2/Ar mixed plasma, i.e., a layer-by-layer (LbL) technique. The film crystallization was promoted by suppressing the oxygen vacancy and interstitial defects by adjusting the exposure conditions of the O2/Ar plasma. These findings suggest that the chemical potential of the oxygen atom influences the film crystallization and the electronic state. The diffusion and effusion of oxygen atoms at the growing surface have an effect similar to that of thermal annealing, promoted film crystallization and the creation and the annihilation of oxygen- and zinc-related defects. The role of oxygen atoms reaching the growing film surface is discussed in terms of chemical annealing and a possible oxygen diffusion mechanism is proposed.     

Point defects in ZnO: an approach from first principles

Abstract

Recent first-principles studies of point defects in ZnO are reviewed with a focus on native defects. Key properties of defects, such as formation energies, donor and acceptor levels, optical transition energies, migration energies and atomic and electronic structure, have been evaluated using various approaches including the local density approximation (LDA) and generalized gradient approximation (GGA) to DFT, LDA+U/GGA+U, hybrid Hartree–Fock density functionals, sX and GW approximation. Results significantly depend on the approximation to exchange correlation, the simulation models for defects and the post-processes to correct shortcomings of the approximation and models. The choice of a proper approach is, therefore, crucial for reliable theoretical predictions. First-principles studies have provided an insight into the energetics and atomic and electronic structures of native point defects and impurities and defect-induced properties of ZnO. Native defects that are relevant to the n-type conductivity and the non-stoichiometry toward the O-deficient side in reduced ZnO have been debated. It is suggested that the O vacancy is responsible for the non-stoichiometry because of its low formation energy under O-poor chemical potential conditions. However, the O vacancy is a very deep donor and cannot be a major source of carrier electrons. The Zn interstitial and anti-site are shallow donors, but these defects are unlikely to form at a high concentration in n-type ZnO under thermal equilibrium. Therefore, the n-type conductivity is attributed to other sources such as residual impurities including H impurities with several atomic configurations, a metastable shallow donor state of the O vacancy, and defect complexes involving the Zn interstitial. Among the native acceptor-type defects, the Zn vacancy is dominant. It is a deep acceptor and cannot produce a high concentration of holes. The O interstitial and anti-site are high in formation energy and/or are electrically inactive and, hence, are unlikely to play essential roles in electrical properties. Overall defect energetics suggests a preference for the native donor-type defects over acceptor-type defects in ZnO. The O vacancy, Zn interstitial and Zn anti-site have very low formation energies when the Fermi level is low. Therefore, these defects are expected to be sources of a strong hole compensation in p-type ZnO. For the n-type doping, the compensation of carrier electrons by the native acceptor-type defects can be mostly suppressed when O-poor chemical potential conditions, i.e. low O partial pressure conditions, are chosen during crystal growth and/or doping.     

Chemically-biased diffusion and segregation impede void growth in irradiated Ni-Fe alloys

Recent irradiations of Ni-Fe concentrated solid solution alloys have demonstrated significant improvement of radiation performance. This improvement is attributed to redistribution of the alloying elements near sinks of point defects (voids, dislocations) due to chemically-biased atomic diffusion, where vacancies have preference to migrate via Fe atoms and interstitials via Ni atoms. In Ni-Fe, all sinks are enriched by Ni atoms, which strongly affects further interactions of radiation-produced mobile defects with voids and dislocations, hence void growth and dislocation climb. Ni-decorated sinks interact stronger with interstitial atoms than vacancies, which enhances dislocation loops growth. At the same time, Ni segregation creates Fe-enriched “channels” for vacancy migration out of the damage region to agglomerate in the outer regions, inaccessible to interstitial atoms. Strong effect of chemically-biased diffusion is supported by transmission electron microscope characterization and calls for special attention in designing alloys with desired properties through tuning defect mobilities.     

第一原理研究LuPO4中氧空位的性质

磷酸结构的晶体在掺杂二价阳离子后容易形成产生焦磷酸结构(P₂O₇) ^4-, 这种含有焦磷酸结构的氧化物材料十分适合做质子导体、燃料电池、气体传感器以及陶瓷膜等。本文利用第一性原理研究了LuPO₄晶体中氧空位的结构性质, 结果显示当氧空位带二价正电时, 会引发氧空位周围原子奇特的畸变, 形成焦磷酸结构。为了解释这种结构畸变的机理, 本文利用过渡态搜索计算了结构变化过程中势能面的变化情况, 正一价氧空位形成焦磷酸 结构需要越过2.4 eV的势垒, 而正二价氧空位形成焦磷酸结构则不需要越过任何势垒, 因此很容易形成焦磷酸 结构。最后给出氧空位不同带电态的晶格结构、电子态密度以及电荷密度分布等基本物理性质, 氧空位处于正二价态结构下, 氧空位附近的P原子与O原子成键, 又由于O原子有较强的电负性, P的s轨道电子向O的p轨道转 移。P的s、p轨道在禁带中出现了与总态密度对应的缺陷能级, 结果表明带正二价氧空位的晶体性质发生了明显变化。     

The stage of crystal lattice, containing the complexes of vacancies and plane defects

A computer simulation of plane and point defects has been executed in FeAl alloys in the pair interatomic potential approximation. The lattice relaxation was executed first for unit plane defect (first stage) and then for complex of plane and point defects as a whole. It has revealed the existence of preferable vacancy positions near plane defects of different orientations. It has shown that the stabilization of plane defects is possible not only on account of segregation of vacancies near them, but making more active of simple odd diffusion acts in nearness from defect.     

Structure of point defects in B2 Fe-Al alloys: An atomistic study by semi-empirical simulation

We present the first results of a simulation study of point-defect properties in B2 ordered Fe-Al alloys (34% < x_Al < 52%). After examining the T = 0 K energetics of simple defects, we turn to complexes and discuss the usual hypothesis of independent elementary defects. Almost all complex defects involving an Al vacancy and an Al antisite atom in nearest neighbour position are shown to be unstable, confirming that Al vacancies are rare in these alloys. Small deviations from stoichiometry induce a change in the nature of defects: whereas on the Al-rich side, isolated Al antisite atoms dominate, Fe antisite atoms have a strong trend towards clustering on the Fe-rich side, leading to the formation of local DO₃ order.     

First-principles modeling of lattice defects: advancing our insight into the structure-properties relationship of ice

We discuss a number of examples that demonstrate the value of computational modeling as a complementary approach in the physics and chemistry of ice I _h , where real-life experiments often do not give direct access to the desired information or whose interpretation typically requires uncontrollable assumptions. Specifically, we discuss two cases in which, guided by experimental insight, density-functional-theory-based first-principles methods are applied to study the properties of lattice defects and their relationship to ice I _h s macroscopic behavior. First, we address a question involving molecular point defects, examining the energetics of formation of the molecular vacancy and a number of different molecular interstitial configurations. The results indicate that, as suggested by earlier experiments, a configuration involving bonding to the surrounding hydrogen-bond network is the preferred interstitial structure in ice I _h . The second example involves the application of modeling to elucidate on the microscopic origin of the experimental observation that a specific type of ice defect is effectively immobile while others are not. Inspired by previous suggestions that this defect type may be held trapped at other defect sites and our finding that the bound configuration is the preferred interstitial configuration in ice I _h , we use first-principles modeling to examine the binding energetics of the specific ice defect to the molecular vacancy and interstitial. The results suggest a preferential binding of the immobile defect to the molecular interstitial, possibly explaining its experimentally observed inactivity.     

Low-temperature sintering mechanism on uranium dioxide

Based on a point defect model, the mechanism of low-temperature sintering of uranium dioxide was studied in this paper. The diffusion coefficient of uranium in UO_{2 + x}, sintering temperature and densification equation in low-temperature sintering were analyzed by both the point defect model and low-temperature sintering experiments. The results showed that the diffusion activation energy of uranium in over-stoichiometric UO_{2 + x} was lowered by 3.0 ev than that in stoichiometric UO₂. And the diffusion coefficient of uranium in UO_{2 + x} was proportional to x². In addition, the theoretical low temperature sintering temperature was calculated to be in the range of 1089–1151 °C, which indicated that it was necessary to maintain proper over-stoichiometric oxygen for low-temperature sintering process. Moreover, the calculation results by the point defect model matched perfectly with the experiment results, PDM might be a good model to describe the relationships between defects concentration and atmosphere composition.     

B2－NiAl力学性质Cr合金化效应的第一原理计算

采用第一原理赝势平面渡方法和基于虚拟晶体势函数近似（VCA）,计算了Cr合金化（浓度x〈1．0at％）时完整与缺陷B2-NiAl晶体的弹性性质,并采用弹性常数G44、Cauchy压力参数（C12-C44）、杨氏模量E、剪切模量G及其与体模量B0的比值G／B0等,表征和评判了Cr合金化浓度X对NiAl金属间化合物延性与硬度的影响。结果表明：Cr合金化浓度在0～1at％范围内均对NiAl晶体的硬度有明显影响。无论点缺陷存在与否,Cr合金化均可使B2-NiAl晶体的硬度大幅提高。Cr合金化浓度在0at％～0．5at％区间,有利于无缺陷NiAl晶体材料延性的提高,且以0．05at％的合金化浓度为最好。Ni空位或Ni反位降低B2-NiAl晶体的本征延性。Cr合金化浓度在x〈1at％时,Ni空位的NiAl晶体增塑效应并不明显,而对于Ni反位的缺陷NiAl晶体,Cr合金化浓度在0．9at％附近存在比较明显的增塑效应。     

Roles of oxygen defects in copper oxide superconductors

Oxygen vacancy and interstitial defects can have a profound effect on the superconducting properties of copper oxide compounds. Recent work on compounds such as La₂CuO_4+x and HgBa₂CuO_4+x has provided new insight into the role of interstitial oxygen defects as a doping mechanism. The number of carriers created by each interstitial defect depends on the local defect structure. Studies of (La, Sr, Ca)₃Cu₂O_6+x with various metal compositions and metalsite ordering show that interstitial oxygen defects that form between the CuO₂ layers in this structure systematically lowerT _c and eventually destroy superconductivity. Conversely, oxygen vacancies in the CuO₂ planes have surprisingly little effect at concentrations below 3%. The infinite-layer compounds, ACuO₂, where A=La, Sr, Ca, Nd, etc., in solid-solution combinations, could offer a similar environment for the formation of interstitial oxygen defects between the CuO₂ planes, allowing interstitial oxygen defects to contribute to the doping of these compounds. However, neutron diffraction experiments on Sr_0.9La_0.1CuO₂ (T _c = 42 K) have not found any interstitial oxygen.     

Phase evolution and mechanical properties of novel nanocrystalline Y2(TiZrHfMoV)2O7 high entropy pyrochlore

High entropy pyrochlores(HEP)are potential candidates as dispersoids in the oxide dispersed strength-ened steels or alloys,which can be used in nuclear reactors and supercritical boilers.For the first time,HEP oxides Y2(TiZrHfMoV)2O7 were synthesized with Y2Ti2O7 as a base structure with the B site(Ti)substituted with five cations through reverse co-precipitation technique in the nanocrystalline form at lowest synthesis temperature.The synthesis parameters for Y2(TiZrHfMoV)2O7(5C)and other derived compositions(five compositions of four cationic systems with each cation eliminated at B site from 5C)are optimised to obtain lower crystallite and particle sizes.5C has a smaller crystallite size(27 nm)than other single-phase compositions.The cation's influence,oxidation state,and oxygen vacancy in the phase formation were analysed through XPS.The single-phase HEPs are consolidated through spark plasma sin-tering.Y2(TiZrHfMo)2O7(4C-V)shows the highest hardness among the compositions reported so far due to its finer grain size,and Y2(TiHfMoV)2O7(4C-Zr)has a higher Young's modulus compared to other single-phase composition due to its higher degree of order in the structure.     

Phase-field simulation of irradiated metals: Part I: Void kinetics

We present a phase-field model of void formation and evolution in irradiated metals by spatially and temporally evolving vacancy and self-interstitial concentration fields. By incorporating a coupled set of Cahn–Hilliard and Allen–Cahn equations, the model captures the processes of point defect generation and recombination, annihilation of defects at sinks, as well as void nucleation and growth in the presence of grain boundaries. Illustrative results are presented that characterize the rate of void growth or shrinkage due to supersaturated vacancy or interstitial concentrations, void nucleation and growth kinetics due to cascade-induced defect production, as well as void denuded and peak zones adjacent to grain boundaries.     

Monte Carlo simulation on the cation diffusion via vacancies in simple spinels

The correlation factors of cations and vacancies for diffusion in simple spinels were calculated by using the Monte Carlo simulation technique. Since the predominant point defects are cationic vacancies at high temperature and especially at high oxygen partial pressures in many spinels, the cations are considered to move primarily via cationic vacancies in this study. Several input parameters including the lattice size, the number of jumps per cation, etc., were optimized to calculate the correlation factors. Then the correlation factors of cations moving via vacancies in simple spinels were calculated in different types of exchange jumps, i.e., diffusion that occurs on tetrahedral or octahedral or both cationic sites. Results were obtained with satisfactory accuracy. The influence of the vacancy concentration on the correlation factors was also analyzed.     

Theoretical insights into nitrogen oxide activation on halogen defect-rich{001}facets of bismuth oxyhalide

Surface vacancies,serving as the activation centers for surface-adsorbed species,have been widely applied in catalysts to improve their activity and selectivity.In the case of ternary compound semicon-ductors,there is some controversy about exposed atoms and surface defects.Two-dimensional layered BiOCl is an important photocatalyst,which has had numerous studies focused on its oxygen vacancy(Ov)and bismuth vacancy(Biv).It has been realized that its(001)surface can consist of exposed halogen atoms rather than oxygen atoms,which thus needs a new explanation for its surface defect engineering mechanism.Using first-principles calculations,the activation behavior of NOx(NO2,NO,N2O)at a chlo-rine vacancy(Clv)on the BiOCl(001)surface is systematically studied.It is found that after introducing Clv on BiOCl(001)surfaces,NOx molecules all show excellent activities with longer chemical bonds by capturing electrons from the catalyst.Our work furnishes fundamental insight into the activation of small molecules on defect-rich surfaces of ternary compound catalysts.     

First-principles study of point defects and Si site preference in Al3Ti

The formation energies of various point defects in Al₃Ti(D0₂₂) have been calculated by using first-principles calculation. The effects of vacancies on Si site preference were examined to better understand Si substitution behavior in Al₃Ti. The results show that, under Al-rich condition, the formation energy of antisite Al_Ti is the lowest than those of other point defects, and Ti vacancy is more preferred than Al vacancy. But Si prefers to occupy Al vacancy compared with Ti vacancy which causes a finite solubility of Si in Al₃Ti system. The calculation is instructive for the further improvements of process of Si removal.     

Chemical activity of oxygen atoms in the magnetron sputter-deposited ZnO films

The role of oxygen atoms in the growth of magnetron sputter-deposited ZnO films was studied by alternating the deposition of a several-nanometer-thick ZnO layer and an O₂/Ar mixed plasma exposure, i.e., a layer-by-layer (LbL) technique. The film crystallization was promoted by suppressing the oxygen vacancy and interstitial defects by adjusting the exposure conditions of the O₂/Ar plasma. These findings suggest that the chemical potential of the oxygen atom influences the film crystallization and the electronic state. The diffusion and effusion of oxygen atoms at the growing surface have an effect similar to that of thermal annealing, promoted film crystallization and the creation and the annihilation of oxygen- and zinc-related defects. The role of oxygen atoms reaching at the growing film surface is discussed in terms of chemical annealing and a possible oxygen diffusion mechanism is proposed.