Formation Energies Of Intrinsic Defects Research Articles

Defect energetics in LaAlO3polymorphs: A first-principles study

Formation energies of intrinsic defects in rhombohedral and cubic LaAlO${}_{3}$ were calculated using a first-principles projector augmented wave method based on the density functional theory. Defect energetics of single-atom vacancies, partial and full Schottky defects, and antisite cationic defects were investigated. Finite-size cell interactions in charged defects were corrected by calculating with various supercell sizes up to 480 atoms, and band-gap correction was also performed for neutral oxygen vacancy. It was found that the defect formation behaviors of both rhombohedral and cubic LaAlO${}_{3}$ are very similar to each other, that is, Schottky-type defects are preferably formed under all atmospheres and that the formation of neutral oxygen vacancies requires much more energy compared with other perovskite oxides.

Structure, Defect Chemistry, and Lithium Transport Pathway of Lithium Transition Metal Pyrophosphates (Li2MP2O7, M: Mn, Fe, and Co): Atomistic Simulation Study

Lithium transition metal pyrophosphate materials (Li2MP2O7, M: Mn, Fe, and Co) have been proposed as promising novel cathode materials for lithium ion batteries. Using atomistic simulation with empirical potential parameters, which has been validated on various cathode materials by Islam et al. [Phil. Trans. R. Soc. A2010, 368, 3255–3267], these new pyrophosphates are investigated to elucidate structure, defect chemistry, and Li+ ion transport pathway. The core–shell model with empirical force fields reproduces the experimental unit-cell parameters, and formation energies of intrinsic defects (Frenkel and antisite) are calculated. From migration energy calculation, it is found that the pyrophosphates without partial occupation have a 2D Li+ ion pathway. Meanwhile, under the condition of partial occupancies of Li and transition metal atoms, the diffusion pathway of Li+ ions is a 3D network.

Ab initio study of defect properties in YPO 4

Ab initio methods based on density functional theory have been used to calculate the formation energies of intrinsic defects, including vacancies, interstitials, antisites and Frenkel pairs in YPO 4 under the O-rich and Y 2O 3-rich, and the O-rich and Y-rich conditions. The larger size of the yttrium atom may give rise to higher formation energy of the phosphorus antisite defect. In general, the formation energies of anion interstitials are much smaller than those of cation interstitials for both conditions considered. It is of greatly interest to find that the relative stabilities among the same types of interstitials are independent of the reference states. The most stable configuration for oxygen interstitials is an O–O split interstitial near the T a site, while the most stable configuration for cation interstitials is a tetrahedral interstitial near the T a site. The cation split interstitials are unfavorable in YPO 4, with much higher formation energies. Furthermore, the properties of Frenkel pairs are compared with those calculated using empirical potentials. The results reveal that both ab initio and empirical potential calculations show a similar trend in the formation energies of Frenkel pairs, but the formation energies obtained by empirical potentials are much larger than those calculated by ab initio method.

Native Defects in ZnO Nanowires: Atomic Relaxations, Relative Stability, and Defect Healing with Organic Acids

The geometry and formation energies of intrinsic defects in ZnO nanowires have been investigated with the self-consistent-charge density functional tight-binding method (SCC-DFTB). We have calculated the atomic relaxations and formation energies for neutral and charged defects using an empirical potential alignment procedure. Oxygen vacancies and zinc interstitials have been found the lowest energy defects under Zn-rich conditions, with oxygen interstitials and zinc vacancies favored under O-rich conditions. Most defects have been found favorable on the surface of the wire; except to the charged oxygen interstitials, which prefer inner positions because of a better charge accommodation observed there. Besides, we have also investigated the interaction between oxygen vacancies and carboxylic acids, demonstrating that an oxidation−reduction reaction may explain the suppression of green luminescence bands experimentally observed for organic-coated ZnO nanowires.

Derivation of enhanced potentials for uranium dioxide and the calculation of lattice and intrinsic defect properties

Previously reported forms of the cation–anion Buckingham potential provide a significantly greater contribution than the repulsive Coulombic component at short-range thus predicting an unphysical attraction between the pair of ions. A detailed reappraisal of the computer modelling of uranium dioxide (UO 2) employing atomistic simulation techniques is presented. An improved set of interatomic potentials is derived in order to describe the lattice correctly under conditions subsequent to radiation damage with the creation of Frenkel pair defects. Novel methodology is employed in the derivation of potentials ensuring applicability over the entire region of interest. The cation–anion potential is obtained via a combination of empirical fitting to crystal structural data and parametric fitting to additional physical properties. These potentials are subsequently verified and validated by calculation of additional bulk lattice properties, whose values agree favourably with those measured experimentally. Atomistic computer simulation techniques are then used to investigate the defect properties of UO 2. The theoretical techniques are based upon efficient energy minimization procedures and Mott–Littleton methodology for accurate defect modelling and employed to calculate intrinsic defect formation energies and enable predictions of the expected type of intrinsic disorder to be made.

Intrinsic defects in ZnO calculated by screened exchange and hybrid density functionals

The formation energies of intrinsic defects in ZnO are calculated by a family of screened exchange and hybrid density functionals, which include different fractions of Fock exchange and range separation in the hybrids. All functionals improve on local-density methods and agree remarkably well for formation energies of neutral vacancies but show significant variations for the energy of charge transition levels in the gap. This result highlights that a correct prediction of the band gap by a functional does not guarantee a high accuracy for the defect levels. Hybrid functionals obtain the correct localization of trapped hole states at the Zn vacancy.

First Principles Study on Intrinsic Vacancies in Cubic and Orthorhombic CaTiO<SUB>3</SUB>

The structural relaxations and the formation energies of intrinsic defects in cubic and orthorhombic CaTiO 3 were investigated by a first principles projector-augmented wave method. It was found that cations and oxygen vacancies in both phases cause extra levels near the valence band maximum and the conduction band minimum, respectively, and the Ti-vacancy induced level in orthorhombic CaTi0 3 is closer to the valence band maximum than that in cubic CaTiO 3 . Among the neutral defect species, including neutral isolated vacancy, partial Schottky, and full Schottky, it was found that the V Ca 2― + V O 2+ and V O 0 are the most preferable defect species for orthorhombic CaTiO 3 under reduction and oxidization conditions, respectively, whereas the V Ca 2― + V O 2+ partial Schottky is always stable in any atmosphere in cubic CaTi0 3 . As compared to cubic CaTi0 3 , it was found that orthorhombic CaTi0 3 shows higher defect formation energies.

Ab initio study on intrinsic defect properties of germanium nitride considered for gate dielectric

First-principles calculations based on density-functional theory and local-density approximation were carried out to investigate intrinsic defect properties in β-Ge3N4. It was found that nitrogen vacancies are the main source of intrinsic defects in Ge3N4 due to their low formation energy. The N vacancies might become charge trapping centers since they induce energy levels near the Ge conduction band edge and in the middle of the Ge3N4 band gap. The formation energy of intrinsic defects in Ge3N4 is sensitive to N chemical potential, and N-rich ambient is favorable to reduce the concentration of N vacancies.

First-principles study on the formation energies of intrinsic defects in LiNbO 3

First-principles plane-wave ultrasoft pseudopotential method within local density approach (LDA) has been used to study three possible vacancy-defect models for non-stoichiometric lithium niobate (LiNbO 3): (1) the oxygen-vacancy model V O 2 + + 2 V Li - , (2) the niobium-vacancy model 5 Nb Li 4 + + 4 V Nb 5 - , and (3) the lithium-vacancy model 4 V Li - + Nb Li 4 + . The corresponding formation energies are obtained via energy minimization of a supercell. In Nb-rich environment, the calculated defect formation energies, both under oxidation and reduction conditions, show little effect on the intrinsic defect structures. We find that the lithium vacancy model 4 V Li - + Nb Li 4 + has the most stable configuration in the non-stoichiometric lithium niobate crystals. Our calculations also show that the formation of any type of neutral defects and Frenkel pairs in a Nb-rich environment is difficult.

Modelling intrinsic defects and transport mechanisms in the bismuth germanate crystalline system

In the present work, computer modelling techniques based on energy minimisation were used to study the defect chemistry and physics for the Bi4Ge3O12 scintillator phase. A potential set was derived by fitting to the structure of this material together with the Bi12GeO20 phase and this potential set was then used to: i- calculate the bulk properties of both Bi4Ge3O12 and Bi12GeO20 phases; ii- study the relative stability of the phases; iii- calculate formation energies of intrinsic defects and non-stoichiometry in BGO; and iv- investigate charge transport mechanisms, comparing them to experimental results obtained via Impedance Spectroscopy. The potential parameter set reproduces the lattice parameters of the 4-3-12 and the 12-1-20 phases to within 0.3% and the available elastic and dielectric constants within reasonable accuracy. The main intrinsic disorder in the 4-3-12 phase is predicted to be the Bi/Ge anti-site. Impedance Spectroscopy measurements for the Bi4Ge3O12 phase were also carried out, and the results indicates that the charge transport mechanism would be related to a process with a distribution in activation energies, with mean value of about (1.41 ± 0.04) eV. The simulations provided an interpretation of those results picturing a model that explain the origin of such distribution. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of extended defects on the properties of intrinsic and extrinsic point defects in silicon

We investigated the interaction of intrinsic and extrinsic point defects with stacking faults in silicon. The calculations were carried out using ab initio total energy methods. The results show that the formation energies of intrinsic defects and impurities (P, As, and Al) are lower at the stacking fault as compared to the respective defects in crystalline environment. Therefore, stacking faults should have a large concentration of defects, and they should play an important role on the mechanisms of dislocation motion.

The defect chemistry of sapphire (α-Al2O3)

The point defect chemistry in sapphire (α-Al2O3) has been investigated using traditional mass action calculations. These required a consistent set of defect formation and cluster binding energies, which were provided by atomistic simulation calculations. The defect reactions studied include those associated with intrinsic defects and the aliovalent impurities Mg2+ and Ti4+. Both perfect co-doping and doping that assumes an excess of Mg2+ or Ti4+ ions were considered. In all cases the intrinsic defect concentrations are dramatically affected by dopant ions, even at the p.p.m. level. This is a consequence of the high intrinsic defect formation energies. Thus, even in unintentionally doped sapphire, it is suggested that the point defect chemistry will be controlled by background trace impurity ions.

A calculation of the formation energies of intrinsic defects near grain boundaries in NiO

Abstract The formation energies of intrinsic defects near coincidence pin boundaries in NiO have been calculated using computer simulation techniques. At each type of interface ionic sites with defect formation energies lower than the corresponding bulk values were found. Therefore the equilibrium concentration of all defects will be enhanced clone to the boundary. However, the concentration of singly charged vacancies is enhanced more than the concentration of the compensating holes. Therefore a net negative charge density is produced in the boundary region. This is compensated by a positive apace-charge layer. For the example of e (211)/[011] interface at 1000 K, the calculations predict an increase in the vacancy concentration of about 40 over the bulk value in the boundary region. Thin will contribute towards an increased diffusion coefficient.