Formation Energies Of Intrinsic Defects Research Articles

Formation energy of intrinsic defects in silicon from the Galitskii-Migdal formula

Formation energy of intrinsic defects in silicon from the Galitskii-Migdal formula

Antimony-Bismuth Alloying: The Key to a Major Boost in the Efficiency of Lead-Free Perovskite-Inspired Photovoltaics.

The perovskite-inspired Cu2 AgBiI6 (CABI) material has been gaining increasing momentum as photovoltaic (PV) absorber due to its low toxicity, intrinsic air stability, direct bandgap, and a high absorption coefficient in the range of 105 cm-1 . However, the power conversion efficiency (PCE) of existing CABI-based PVs is still seriously constrained by the presence of both intrinsic and surface defects. Herein, antimony (III) (Sb3+ ) is introduced into the octahedral lattice sites of the CABI structure, leading to CABI-Sb with larger crystalline domains than CABI. The alloying of Sb3+ with bismuth (III) (Bi3+ ) induces changes in the local structural symmetry that dramatically increase the formation energy of intrinsic defects. Light-intensity dependence and electron impedance spectroscopic studies show reduced trap-assisted recombination in the CABI-Sb PV devices. CABI-Sb solar cells feature a nearly 40% PCE enhancement (from 1.31% to 1.82%) with respect to the CABI devices mainly due to improvement in short-circuit current density. This work will promote future compositional design studies to enhance the intrinsic defect tolerance of next-generation wide-bandgap absorbers for high-performance and stable PVs.

A computational study of CaWO4: Raman spectrum, intrinsic defects, and excited state properties

In this study, we present an ab initio investigation of the optical properties of CaWO4. The crystal structure of the bulk material along with its vibrational (in the ground state) and optical properties (in both ground and excited states) were analyzed. Our simulations accurately identified the observed Raman signature of the material. Additionally, we simulated the intrinsic luminescence properties of CaWO4 and identified the charge transfer that occurs during the de-excitation process. Finally, through the use of ab initio techniques, the intrinsic defect formation energies and their corresponding concentrations were determined providing the first instance of such findings for this material.

Energy of formation of intrinsic defects and their clusters in the powellite CaMoO-=SUB=-4-=/SUB=-

Atomistic simulation of powellite crystals has been carried out. A system of parameters of interatomic potentials is proposed for the calculation of point defects in powellite. The formation energies of intrinsic defects and their clusters, which are spatially related combinations of defects, are estimated. The most favorable localization of interstitial calcium and oxygen ions was determined. The formation energies of the Frenkel and Schottky defects are calculated. It was shown that the most energetically preferred in stoichiometric powellite are the oxygen Frenkel defects. Defects arising from deviations from stoichiometry are considered. It is established that the clustering of defects allows to reduce the energy of their formation by 15-20%. Keywords: powellite, tungstates and molybdates, atomistic simulation, intrinsic defects.

First-principles study on the electronic structures and diffusion behaviors of intrinsic defects in BiOCl

Intrinsic defects and hydrogen play an essential role in regulating photocatalytic properties of semiconductor materials. Bismuth oxychloride (BiOCl) is an important ternary bismuth-based photocatalytic material, for its wide bandgap allows band-gap engineering to tailor its properties. In this paper, we use first-principles to study the effects of intrinsic defects and hydrogen on the electronic structure of BiOCl. We studied the intrinsic defect formation energies, defect electronic structures and the diffusion of these defects in BiOCl. We found that the p-type conductivity and band gap variation in BiOCl may be caused by the Fermi level pinning effect and intrinsic defective states around the band edges. Intrinsic defects of Hint, OCl and BiCl show delocalized features which may have a predominant effect for carrier transfer. As the temperature rises, the diffusion of oxygen vacancies in the xy plane goes up most drastically, while the diffusion of interstitial hydrogen in the yz plane changes smoothest and has the fastest speed. Our work offers fundamental insights for explaining the band gap variation, p-type conductivity and the transport of defects in BiOCl and provides basic guidance in regulating the photocatalytic performance of BiOX (X = Cl; Br; I) by defect engineering.

First-principles study of intrinsic point defects and Xe impurities in uranium monocarbide

Based on density functional theory (DFT) calculations, we perform an extensive investigation of intrinsic point defects and Xe impurities in uranium monocarbide (UC). The DFT calculations involve both the conventional generalized gradient approximation (GGA) and the GGA+U approach with the Hubbard parametric term (U), using up to 5×5×5 supercells. GGA calculations for the formation energy of intrinsic defects demonstrate the significant effect of using larger supercells than in previous studies. Results confirm that the ⟨111⟩ and ⟨100⟩ dumbbell interstitials are the most stable interstitial configurations for U and C, respectively. The interstitial mechanisms are favored for self-diffusion of both uranium and carbon and diffusion of Xe under equilibrium conditions. Calculations also reveal that the Xe substitutional defect at the C lattice site tends to adopt an off-site configuration, which can be interpreted as a Xe interstitial–C vacancy complex. We also utilize GGA+U to assess the impact of effective U parameter (Ueff) on the results. Moreover, we introduce a method to estimate the carbon chemical potential by fitting the phase diagram composition data and propose a selection Ueff=1.25 eV based on the experimental Xe diffusion activation energy. With this approach, GGA+U calculations reproduce the available experimental data for the formation energy of the carbon Frenkel pair and can explain the stepwise recovery of intrinsic properties and burst Xe release behavior in UC observed in annealing experiments.

ZnO transparent conducting thin films codoped with anions and cations

In the past two decades, ZnO related materials and devices are widely used for various applications, such as thin-film transistors, photodetectors, light-emitting diodes, and transparent conductors because ZnO has several appealing features like abundant raw materials, environmental friendliness, wide and direct band gap, high electron mobility, and large exciton bonding energy. Especially, ZnO has been recognized as one of the most promising candidates for substituting commercial transparent conductors ITO which is exploited as transparent electrodes in solar cells and flat planar display and transparent heaters etc. For example, aluminum doped ZnO (AZO) transparent conducting films have been commercialized successfully. However, the figure of merits and stability of ZnO transparent conducting films still need to be further improved in order to meet the strict demands of practical large-scale applications. So far, various techniques have been developed to alleviate that contradiction between conductivity and visible light transmittance. In the past few years, a new strategy called anion and cation co-doping in ZnO has been widely implemented to improve the figure of merit, in which IIIA group elements are used as cation dopants and fluorine is used as anion dopants. It is worth noting that F has several advantages as anion dopant in ZnO. First, fluorine has a similar ionic radius with oxygen, which will avoid large lattice distortion. Second, the substitution for oxygen with fluorine only disturbs the valence band of ZnO and leaves the conduction band unaffected, which is beneficial for achieving high electron mobility. In this review, the latest researches of F and boron, or aluminum, or gallium, or indium co-doped ZnO thin films are summarized from both the theoretical and the experimental aspects. Effects of anion and cation co-doping on the optical and electrical properties and thermal stability of ZnO thin films are systematically discussed. Both theoretical and experimental results show that cation and anion co-doping can effectively improve the conductivity and visible light transmittance, no matter ZnO thin films were prepared by physical vapor deposition or synthesized by chemical solution processs. Taking Al and F co-doped ZnO (AFZO) thin films prepared with pulse laser deposition as an example, the mobility of AFZO thin film was apparently higher than that of Al-doped ZnO at the same carrier concentration. The high carrier mobility of AFZO is due to that the F can passivate the surface defects and acceptor-like complex defects in ZnO. In addition, the highly electronegative F makes the AFZO thin films exhibit excellent thermal stability, e.g. maintaining high conductive even after air-annealing at 600°C. These results indicate that anion and cation co-doping can effectively improve the performance of ZnO transparent conductive thin films and broaden their applications. Despite these exciting achievements, challenges still remain in the field of anion and cation co-doped ZnO thin films. For example, the mechanism of the co-doping strategy has not been clarified because the formation energy of intrinsic defects in ZnO is relatively low and the defects configurations for anion and cation co-doping are more complicated than single element doping case. Besides, we are still short of effective techniques and theoretical methods to reveal and simulate the behavior of defect clusters in co-doped ZnO. It is also urgent to establish the industrial standard of ceramic targets for co-doped ZnO to eliminate the discrepancies between different groups. More efforts should also be made in developing new equipment for large-scale manufacturing co-doped ZnO thin films. We hope this paper will provide reference to the readers who are interested in transparent conducting oxides.

Inflexible stoichiometry in bulk pyrite FeS2 as viewed by in situ and high-resolution X-ray diffraction.

Non-stoichiometry is considered to be one of the main problems limiting iron pyrite, FeS2, as a photovoltaic absorber material. Although some historical diffraction experiments have implied a large solubility range of FeS2-δ with δ up to 0.25, the current consensus based on calculated formation energies of intrinsic defects has lent support to line-compound behavior. Here it is shown that pyrite stoichiometry is relatively inflexible in both reductive conditions and in autogenous sulfur partial pressure, which produces samples with precise stoichiometry of FeS2 even at different Fe/S ratios. By properly standardizing in situ gas-flow X-ray diffraction measurements, no significant changes in the lattice parameter of FeS2 can be resolved, which portrays iron pyrite as prone to forming sulfur-deficient compounds, but not intrinsic defects in the manner of NiS2-δ.

Mechanism of Carbon Dioxide and Water Incorporation in Ba2TiO4: A Joint Computational and Experimental Study

CO2 incorporation in solids is attracting considerable interest in a range of energy-related areas. Materials degradation through CO2 incorporation is also a critical problem with some fuel cell materials, particularly for proton conducting ceramic fuel cells. Despite this importance, the fundamental understanding of the mechanism of CO2 incorporation is lacking. Furthermore, the growing use of lower temperature sol gel routes for the design and synthesis of new functional materials may be unwittingly introducing significant residual carbonate and hydroxyl ions into the material, and so studies such as the one reported here investigating the incorporation of carbonate and hydroxyl ions are important, to help explain how this may affect the structure and properties. This study on Ba2TiO4 suggests highly unfavorable intrinsic defect formation energies but comparatively low H2O and CO2 incorporation energies, in accord with experimental findings. Carbonate defects are likely to form in both pristine and undo...

Derivation of enhanced potentials for cerium brannerite and the calculation of lattice and intrinsic defect properties

A new potential has been derived for ceria and used to calculate its lattice and defect properties. The Ce4+⋯O2- potential is obtained via a combination of empirical fitting to crystal structural data and parametric fitting to additional physical properties, while the O2-⋯O2- potential is transferred from earlier publications on UO2 and PuO2. The overall potential is subsequently verified and validated by calculation of elastic and dielectric constants, whose values agree favourably with those measured experimentally. The potential is then employed to calculate intrinsic defect formation energies and predict the most favourable type of intrinsic disorder.

Mechanical alloying-spark plasma sintering synthesis and thermoelectric properties of n-type NiSe2+x semiconductors: analysis of intrinsic defects and phase structures

A series of bulk samples NiSe2+x (x = 0, 0.01, 0.02, 0.06) were prepared by mechanical alloying (MA) and spark plasma sintering (SPS). The effect of excessive selenium content (x) on the phase structure, microstructure and thermoelectric properties was investigated. The formation energy of intrinsic defects in NiSe2 was calculated using first principle theory. The added selenium initially entered into the Se vacancy ( $$V_{Se}^{ \cdot }$$ ) which is resulted from the Se volatilization during the MA and/or SPS process and subsequently into the interstitial sites ( $$Se_{i}^{\prime }$$ ). An improved absolute value of Seebeck coefficient (|α|) 9.27 μVK−1 at 323 K for the NiSe2.06 was reached along with an inversely varied electrical conductivity (σ) as increasing x. A minimum thermal conductivity (κ) value 3.43 Wm−1 K−1 was achieved at 323 K for the NiSe2.01, along with a high ZT value 0.0049 at 323 K for the NiSe2.02 among NiSe2+x system. Our work provides a preliminary approach towards TE performance enhancement in n-type NiSe2 semiconductors via tailoring Se content in a nonstoichiometry, and will certainly be of value for further exploration of high performance NiSe2 semiconductors by either doping or microstructure controlling.

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

A new potential has been derived for plutonium dioxide and used to calculate its lattice and defect properties. The Pu4+⋯O2− potential is obtained via a combination of empirical fitting to crystal structural data and parametric fitting to additional physical properties, while the O2−⋯O2− potential is transferred from an earlier publication on UO2. The overall potential is subsequently verified and validated by calculation of elastic and dielectric constants, whose values agree favourably with those measured experimentally. Calculations are then presented of the intrinsic defect formation energies and predictions of the expected type of intrinsic disorder made.

Lithium transition metal fluorophosphates (Li2CoPO4F and Li2NiPO4F) as cathode materials for lithium ion battery from atomistic simulation

Lithium transition metal fluorophosphates (Li2MPO4F, M: Co and Ni) have been investigated from atomistic simulation. In order to predict the characteristics of these materials as cathode materials for lithium ion batteries, structural property, defect chemistry, and Li+ ion transportation property are characterized. The core–shell model with empirical force fields is employed to reproduce the unit-cell parameters of crystal structure, which are in good agreement with the experimental data. In addition, the formation energies of intrinsic defects (Frenkel and antisite) are determined by energetics calculation. From migration energy calculations, it is found that these flurophosphates have a 3D Li+ ion diffusion network forecasting good Li+ ion conducting performances. Accordingly, we expect that this study provides an atomic scale insight as cathode materials for lithium ion batteries.

First-principles study of possible shallow donors in ZnAl2O4spinel

ZnAl${}_{2}$O${}_{4}$ (gahnite) is a ceramic which is considered a possible transparent conducting oxide (TCO) due to its wide band gap and transparency for UV. Defects play an important role in controlling the conductivity of a TCO material along with the dopant, which is the main source of conductivity in an otherwise insulating oxide. A comprehensive first-principles density functional theory study for point defects in ZnAl${}_{2}$O${}_{4}$ spinel is presented using the Heyd, Scuseria, and Ernzerhof hybrid functional (HSE06) to overcome the band gap problem. We have investigated the formation energies of intrinsic defects which include the Zn, Al, and O vacancy and the antisite defects: Zn at the Al site (Zn${}_{\mathrm{Al}}$) and Al at the Zn site (Al${}_{\mathrm{Zn}}$). The antisite defect Al${}_{\mathrm{Zn}}$ has the lowest formation energy and acts as a shallow donor, indicating possible $n$-type conductivity in ZnAl${}_{2}$O${}_{4}$ spinel by Al doping.

First-principles calculation on the concentration of intrinsic defects in 4H-SiC

Based on the first-principles pseudopotentials and the plane wave energy band method, the supercells of perfect crystal 4H-SiC and those with intrinsic defects VC, VSi, VC—C and VC—Si were calculated. Ignoring the atomic relaxations, the results show that the formation energy of intrinsic defects is ranked, from low to high, as VC, VC—C, VSi to VSi—Si at 0 K. The equilibrium concentration of each intrinsic defect can be deduced from the formation energy of each intrinsic defect. The concentration ranks, from high to low, as VC, VC—C, VSi, VSi—Si, which is in accordance with the ESR and PL results. The stabilizing process of metastable defects VSi converting to VC—C was explained by formation energy.

Atomistic modelling of actinide oxides for nuclear fuel applications

AbstractAtomistic computer simulations were performed for the actinide oxides (UO2, PuO2 and MOX) in the temperature range 300 K‐2000 K, in order to elucidate the physical and defect properties of these nuclear fuel materials. Parameterised cation‐anion Buckingham potentials, coupled to a standard Coulomb function, were used to describe the short range and long range interatomic interactions respectively. Mott‐Littleton methodology was employed for accurate defect modelling and to calculate intrinsic defect formation energies. This paper summarises the key findings of a bulk study into these materials and extends this briefly to consider surface effects and morphologies. In addition to highlighting the thermodynamic stability of aggregated defects over isolated point defects, it is shown that the empirical potentials themselves are thermally stable beyond the standard centreline temperatures experienced within a Light Water Reactor (LWR). Finally, minimum energy pathways (MEP) for oxygen migration in UO2 have been investigated, indicating that at least one of the mechanisms, the <110> vacancy migration, is non‐linear. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Lattice stability and formation energies of intrinsic defects in Mg2Si and Mg2Ge via first principles simulations

We report an ab initio study of the semiconducting Mg2X (with X = Si, Ge) compounds and in particular we analyze the formation energies of the different point defects with the aim of understanding the intrinsic doping mechanisms. We find that the formation energy of Mg2Ge is 50% larger than that of Mg2Si, in agreement with the experimental tendency. From a study of the stability and the electronic properties of the most stable defects, taking into account the growth conditions, we show that the main cause of the n doping in these materials comes from interstitial magnesium defects. Conversely, since other defects acting like acceptors such as Mg vacancies or multivacancies are more stable in Mg2Ge than in Mg2Si, this explains why Mg2Ge can be of n or p type, in contrast to Mg2Si. The finding that the most stable defects are different in Mg2Si and Mg2Ge and depend on the growth conditions is important and must be taken into account in the search for the optimal doping to improve the thermoelectric properties of these materials.

Defect mechanisms in the In2O3(ZnO)k system (k = 3, 5, 7, 9)

The defect chemistry of several compounds in the In2O3(ZnO)k series (k = 3, 5, 7, and 9) was investigated in bulk specimens by analysis of the dependence of their conductivity on the oxygen partial pressure. The resulting Brouwer slopes were inconsistent with a doubly charged oxygen vacancy defect model, and varied with the phase. The k = 3 phase had behavior similar to donor-doped In2O3, and the behavior of the other phases resembled that of donor-doped ZnO. The donor in both cases is proposed to be In occupying Zn sites. First principles calculations of the formation energy of intrinsic defects in this system support the proposed models. The present work expands prior theoretical analysis to include acceptor defects, such as cation vacancies (VZn, VIn) and oxygen interstitials (Oi).

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.