Intrinsic Defects in LiMn2O4: First-Principles Calculations.

Sodium manganese oxide Na0.44MnO2 (NMO) in an open structure with large tunnels is of great interest for sodium-ion battery cathode materials due to its high electrode voltage and capacity. However, its practical application is limited by poor rate performance, which can be tuned and improved by controlling point defects. We herein present a comprehensive study of intrinsic point defects in NMO using density functional theory (DFT) calculations in combination with thermodynamics. Using the DFT+U approach, we determine the formation energies of elementary defects and defect complexes depending on the sets of atomic chemical potentials, corresponding to a certain thermodynamic condition for the synthesis of stable NMO. Sodium interstitials are found to have the lowest formation energies in the relevant ranges of temperature and pressure. Other intrinsic point defects such as oxygen vacancies, sodium vacancies and manganese antisites can also be formed with proper formation energies and have an impact on the cathode performance. Compared to the perfect system, oxygen vacancies lower the electrode voltage, whereas manganese vacancies and antisites increase the voltage. We find that most point defects and defect complexes improve the sodium ion diffusivity, highlighting a proper control of defect formation for enhancing the performance of sodium-ion batteries.

First-principles plane-wave pseudopotential calculations were performed to study electronic structures, structural relaxation, and energetics of intrinsic vacancies and interstitials in ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}.$ In the presence of the intrinsic point defects, extra levels appeared in the band gap. Considering various charge states for the intrinsic point defects, it was found that each point defect is most stable in its fully ionized state. From the formation energies of individual point defects, Schottky, O Frenkel, and Al Frenkel energies were also evaluated and were compared with previous results by experiment and static lattice calculations. Although previous static lattice calculations showed different relative stabilities of Schottky and Frenkel formation, depending on the choice of interatomic potentials, our calculations revealed that the relative values of formation energies are in the order of $\mathrm{Schottky}l\mathrm{Al}$ $\mathrm{Frenkel}l\mathrm{O}$ Frenkel, which is in good agreement with experimental data.

In the paper, we study how the geometry structure and the growing ambience and the Fermi level of several intrinsic point defects effect the defect formation energy of the rutile and anatase TiO2, which type of point defect will be formed, and how to predict the experimental condition of point defect from theory. The key problem is how to calculat the defect formation energy with charge, and correcte the calculation results. The results show the defect type and the defect concentration are related to the nonequilibrium growth condition. In general, under the O-rich condition, VTi will form spontaneously, and under the Ti-rich condition, Ti4+ and VO easily appear in Schottky defects.

The properties of nanowires are distinctively different from those of bulk materials because they are affected by quantum confinement effect and large surface-to-volume ratios. A fundamental understanding of the intrinsic properties of nanowires, in terms of their structure, electronic and optical properties as well as a study of the nanowire surface is required to develop them for future device applications. The morphology and properties of the nanowires are strongly dependent on the synthesis process and growth parameters. Key to realizing the full potential of nanowires is the ability to synthesize uniform, defect free nanowires. Any deviation from stoichiometry leads to imperfections within the nanowire crystal lattice and affects its properties. Efforts to optimize growth conditions are spearheaded by investigations to identify and control native point defects inherent in semiconductors. Structural characterization using electron microscopy is usually the first step towards identifying the quality of the nanowire in terms of composition, morphology and defects like twinning and stacking faults. Optical characterization tools like Photoluminescence (PL) spectroscopy is useful for extracting information on band-to-band transition and other sub-band electronic transitions, thus providing information on any deviations from stoichiometry. However, it cannot provide a comprehensive view of every aspect of the optical properties of nanowires. Hence, complementary experiments like time-resolved PL, pump probe measurements, cathodoluminescence, absorption and reflection spectroscopy are often used. In this chapter, we will present a brief review of some of the techniques used in identifying and characterizing defects in semiconductor nanowires. Studies drawn primarily from the author’s work, addressing the role of intrinsic point defects in nanowires and discussion of strategies for determining the type, energy and concentration of defects in nanowires will be presented in this chapter. The presence of low energy luminescence bands in un-intentionally doped nanowires is evidence that intrinsic point defects (IPDs) play a significant role in the emission spectrum. IPDs in a compound semiconductor are vacancies, interstitials and antisites. In bulk semiconductors, IPDs are often studied by Optical Detection of Magnetic Resonance (ODMR) and Electron Paramagnetic Resonance (EPR) studies. However, the use of such studies on nanowires is only recent (Mal et al., 2010). It is important to understand the behavior of IPDs for the successful application of any semiconductor, since they control, directly or indirectly, doping, compensation, minority carrier lifetime, and luminescence efficiency. Semiconductor doping, in particular, can be adversely affected by IPDs, by causing self-compensation. For example, IPDs which act as donors may compensate the deliberately introduced acceptors. In group II-VI semiconductors like ZnO, specific native defects have long been believed to play 13

Point and small cluster defects in magnesium aluminate spinel have been studiedfrom a first principles viewpoint. Typical point defects that occur during collisioncascade simulations are cation anti-site defects, which have a small formationenergy and are very stable, O and Mg split interstitials and vacancies. IsolatedAl interstitials were found to be energetically unfavourable but could occur aspart of a split Mg–Al pair or as a three atom–three vacancy Al ‘ring’ defect,previously observed in collision cascades using empirical potentials. The structure andenergetics of the defects were investigated using density functional theory (DFT)and the results compared to simulations using empirical fixed charge potentials.Each point defect was studied in a variety of supercell sizes in order to ensureconvergence. It was found that empirical potential simulations significantly overestimateformation energies, but that the type and relative stability of the defects are wellpredicted by the empirical potentials both for point defects and small defect clusters.

Intrinsic and extrinsic defects play a major role in determining solar cell efficiencies of lead halide hybrid perovskite absorbers. Here, we present the results of first-principles computations performed on MAPbBr3 (MA = methylammonium) to study the energetics and defect levels of intrinsic point defects, namely vacancy, self-interstitial and anti-site, and extrinsic Pb-substitution defects. While vacancies are the lowest formation energy intrinsic defects and create shallow transition levels, a number of extrinsic defects can have comparable formation energies under desirable chemical potential conditions. therefore, carrier concentrations may be tunable with these substituents. Further, some extrinsic defects create deeper transition levels which can potentially be exploited to enhance solar cell efficiencies via sub-gap absorption.

Half-metallic full-Heusler alloys represent a promising class of materials for spintronicapplications. However, (i) intrinsic point defects in Heusler compounds can be detrimentalwith respect to their predicted 100% spin polarization at the Fermi level and (ii) whenjoined to mainstream semiconductors the presence of interface states—whichdestroys half-metallicity—can degrade their performance. Here, we present anoverview of recent first-principles calculations performed to explore both theseissues. In particular, we focus on ab initio FLAPW calculations performed forCo2MnGe and Co2MnSi in the presence of intrinsic defects (such as stoichiometric atomic swaps as well asnon-stoichiometric antisites) and when interfaced with GaAs and Ge. Our findings showthat Mn antisites, due to their low formation energies, can easily occur, in excellentconsistency with experimental observations, and that they do not destroy half-metallicity.On the other hand, Co antisites, which also show a modest formation energy, give rise todefect states at the Fermi level. As for the [001]-ordered interfaces, we show that the stronghybridization in proximity to the junction gives rise to rather broad interfacestates that locally destroy half-metallicity. However, the bulk gaps (both in theminority spin channel for the Heusler alloy and for both spin channels in thesemiconducting side) are fully recovered within a few layers away from the junction.

Modifications to alloy chemistry are often used to tailor the intrinsic flow behavior of structural materials. Models of solution strengthening, high-temperature yield stress and creep must relate the effects of chemistry to the mechanisms which influence these material properties. In ordered alloys, additional information regarding the crystallographic site occupancy of substitutional solid solutions is required. The energy of intrinsic and substitutional point defects in ${L1}_{0}$ TiAl are calculated within a first principles, local density functional theory framework. We calculate the relaxed structures and energies of vacancies, antisites, and substitutional defects using a plane-wave pseudopotential method. The results of these total energy calculations are incorporated into a simple thermodynamic model in order to determine the density of point defects as a function of temperature and stoichiometry. Defect densities are presented as a function of alloy chemistry and temperature for binary TiAl and ternary additions (Si, Nb, Mo, Ta, and W). Defect formation energies are calculated using the derived chemical potentials of each atomic species for several alloy compositions. The predicted site selection of the solutes are in excellent agreement with recent x-ray emission experiments using a quantitative statistical method for atom location by channelling enhanced microanalysis.

Laser processing of monocrystalline silicon has become an important tool for a wide range of applications. Here, we use microsecond spot laser melting as a model experiment to investigate the generation of crystal defects and residual stress. Using Micro-Raman spectroscopy, defect etching, and transmission electron microscopy, we find no dislocations in the recrystallized volume for cooling rates exceeding |dT/dt|=2×107 K/s, and the samples remain free of residual stress. For cooling rates less than |dT/dt|=2×107 K/s, however, the experiments show a sharp transition to a defective microstructure that is rich in dislocations and residual stress. Moreover, transmission electron microscopy indicates dislocation loops, stacking-fault tetrahedra, and voids within the recrystallized volume, thereby indicating supersaturation of intrinsic point defects during recrystallization. Complementing photoluminescence spectroscopy indicates even three regimes with decreasing cooling rate. Spectra of regime 1 do not contain any defect related spectral lines. In regime 2, spectral lines appear related to point defect clusters. In regime 3, the spectral lines related to point defect clusters vanish, but dislocation-related ones appear. We propose a quantitative model explaining the transition from dislocation-free to dislocation-rich recrystallization by means of the interaction between intrinsic point defects and dislocations.

Point defects are thermodynamically stabilized in all crystalline materials, with increased densities negatively impacting the properties and performance of transition metal dichalcogenides (TMDs). While recent point defect reduction methods have led to considerable improvements in the optoelectronic properties of TMDs, there is a clear need for theoretical work to establish the lower limit of defect densities, as represented by thermal equilibrium. To that end, an abinitio and thermodynamic analysis of the equilibrium densities of intrinsic point defects in MoSe2 and WSe2 is presented. The intrinsic defect formation energies at the limits of the selenium and metal-rich regimes are determined by density functional theory (DFT) and then augmented with elemental chemical potential functions to determine temperature- and pressure-dependent formation energies. Equilibrium defect densities are determined for MSe, SeM, vM, and vSe, where M and v, respectively, represent the metal and the vacancy, as a function of synthesis temperature and pressure. The effects of vibrational free energy contributions and treatment of the DFT exchange-correlation potential are found to be non-negligible. Calculated equilibrium densities are several orders of magnitude below reported defect densities in TMDs made by chemical vapor deposition, chemical vapor transport, and flux methods, thereby establishing that current synthesis methods are either kinetically limited or impurity dominated.

Defects and material processing in compound semiconductors

Formation of intrinsic point defect in yttrium aluminum garnet ( YAG ) is comprehensively investigated using first‐principles calculations. We showed that the defect formation energies of intrinsic point defects closely depend on the possible values of chemical potentials of Y , Al , and O . Both Y Al16(a) anti‐site defect, oxygen vacancy V O , and interstitial O i might be the preferred defect species when chemical potentials of Y , Al , and O vary in different areas. The O ‐related defects are also important intrinsic point defects in YAG and deserve to be further examined. The oxygen self‐diffusion are investigated and we found that energy barrier of oxygen diffusion is enhanced and decreased by neighboring Y Al16(a) and Al Y anti‐site defects, respectively. The results are used to explain the mechanism of experimental observations that Al 2 O 3 excess speeds up oxygen diffusion and that Y 2 O 3 excess suppresses oxygen diffusion in YAG .

The formation and migration energies for various point defects, including vacancies and self-interstitials, in aluminium are systematically reinvestigated using the supercell approximation in the framework of orbital-free density functional theory. In particular, the finite-size effects and the accuracy of various kinetic energy density functionals are examined. It is demonstrated that as the supercell size increases, the finite-size errors asymptotically decrease as . Notably, the formation energies of self-interstitials converge much more slowly than that of vacancies. With carefully chosen kinetic energy density functionals, the calculated results agree quite well with the available experimental data and those obtained through Kohn–Sham density functional theory, which has an exact kinetic term.

ABSTRACTThe formation energies of intrinsic point defects and the interaction energies of possible defect pairs in NiA1 are calculated from first principles within an order-N, locally self-consistent Green's function method in conjunction with the multipole electrostatic corrections to the atomic sphere approximation. The theory correctly reproduces the ground-state properties of the off-stoichiometric NiAl alloys. The constitutional defects (antisite Ni atoms in Ni-rich and Ni vacancies in Al-rich NiAl) are shown to form ordered structures in the ground state, in which the defects of the same kind tend to avoid each other at the shortest separation distance on their sublattice. A mean-field theory is applied to calculate the equilibrium concentrations of thermal defects. The statistics of thermal defects is interpreted in terms of dominant composition-conserving complex defects which are shown to be triple defects in Ni-rich and nearly stoichiometric NiAl. In the Al-rich region a novel thermal excitation dominates where two constitutional Ni vacancies are replaced by one antisite Al atom. The number of vacancies, as well as the total number of point defects decrease with temperature in Al-rich NiAl. The boundary between the two regions is treated analytically. The vacancy concentration exhibits a minimum in its temperature dependence at the boundary. Similar analysis is applied to study constitutional and thermal defects in Ni3Al. The calculated effective vacancy formation energy in Ni3Al as a function of concentration is in excellent agreement with recent experimental data.

Titanium alloys find extensive use in the aerospace and biomedical industries due to a unique combination of strength, density, and corrosion resistance. Decades of mostly experimental research has led to a large body of knowledge of the processing-microstructure-properties linkages. But much of the existing understanding of point defects that play a significant role in the mechanical properties of titanium is based on semi-empirical rules. In this work, we present the results of a detailed self-consistent first-principles study that was developed to determine formation energies of intrinsic point defects including vacancies, self-interstitials, and extrinsic point defects, such as, interstitial and substitutional impurities/dopants. We find that most elements, regardless of size, prefer substitutional positions, but highly electronegative elements, such as C, N, O, F, S, and Cl, some of which are common impurities in Ti, occupy interstitial positions.