Formation Of Antisite Defects Research Articles

Two-Dimensional Electron Gas at the Spinel/Perovskite Interface: Suppression of Polar Catastrophe by an Ultrathin Layer of Interfacial Defects.

Two-dimensional electron gas (2DEG) at the interface between two insulating perovskite oxides has attracted much interest for both fundamental physics and potential applications. Here, we report the discovery of a new 2DEG formed at the interface between spinel MgAl2O4 and perovskite SrTiO3. Transport measurements, electron microscopy imaging, and first-principles calculations reveal that the interfacial 2DEG is closely related to the symmetry breaking at the MgAl2O4/SrTiO3 interface. The critical film thickness for the insulator-to-metal transition is approximately 32 Å, which is twice as thick as that reported on the widely studied LaAlO3/SrTiO3 system. Scanning transmission electron microscopy imaging indicates the formation of interfacial Ti-Al antisite defects with a thickness of ∼4 Å. First-principles density functional theory calculations indicate that the coexistence of the antisite defects and surface oxygen vacancies may explain the formation of interfacial 2DEG as well as the observed critical film thickness. The discovery of 2DEG at the spinel/perovskite interface introduces a new material platform for designing oxide interfaces with desired characteristics.

Implementation of Na diffusion layer at Cu2ZnSnSe4/Mo interface for flexible thin film solar cell fabricated on Ti foil by solid state selenization

A Cu2ZnSnSe4 (CZTSe) photovoltaic absorber thin films were prepared using a 2-step selenization process on a Ti substrate including a Na precursor layer and a Na-free Ti substrate, and the effect of Na on the solar cell performance was compared. A CZTSe flexible solar cell fabricated on a Ti foil substrate exhibited an efficiency of 3.06%, which was less than half that of a solar cell fabricated on a soda lime glass substrate. This was attributed to the absence of Na and severe Zn crowding near the back contact. By depositing a 100‐nm-thick sodalime glass thin film on a Ti substrate to supply Na, the efficiency increased up to 5.59%. In the Na-doped CZTSe absorber layer grown on the Ti substrate, the back crowding of Zn was eliminated and the upper part of the absorption layer was converted to a Zn-rich environment, which prevented the formation of CuZn antisite defects.

Efficient Sb2Se3/CdS planar heterojunction solar cells in substrate configuration with (hk0) oriented Sb2Se3 thin films

Antimony selenide (Sb2Se3) based solar cell technology has experienced rapid development with demonstrated cell efficiency reaching ̴ 9.2% for devices in substrate configuration, hence motivating more intense research investigations. Though the effect of crystallographic orientation in this non-cubic material on device performance is now well understood, the influence of composition and intrinsic defects remains debatable. In this work we describe the fabrication and device characteristics of Sb2Se3 solar cells designed in the substrate configuration (SLG/Mo/Sb2Se3/CdS/i-ZnO/ITO). Notably, Sb2Se3 absorber layers with a predominant (hk0) orientation were deposited in a single step by e-beam evaporation of pre synthesized bulk source material. As grown precursor Sb2Se3 thin films were subjected to reactive thermal annealing treatment in the presence of Se source at different temperatures for enhancing their crystalline quality and balancing their stoichiometry. Analysis of the completed solar cells indicated improved efficiencies post reactive thermal annealing process, with the best performing devices exhibiting a power conversion efficiency (η) of ~4.34% for an absorber annealed at a temperature of 350 °C. The improved efficiency is ascribed to the observed changes in chemical composition of the absorber layer and the possible formation of related beneficial antisite defects.

Thermoelectric Transport Properties of N-type Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub> Compound Prepared by Oxide-Reduction Process

We report the thermoelectric transport properties of n-type Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub> compounds prepared by oxide-reduction process. The oxide-reduction process is a recently developed method that can synthesize Bi2Te3-based compounds using only oxide starting materials, which is advantageous in terms of the diversity of the starting materials and cost efficiency. In this study, starting materials of Bi<sub>2</sub>O<sub>3</sub>,TeO<sub>2</sub> and SeO<sub>2</sub> were ball-milled for homogenous mixing, and then oxidized to prepare mixed oxide powders. The mixed oxide powders were reduced at different reduction temperatures (573, 603, 633 and 663 K) for 7 h under hydrogen atmosphere. A single phase of Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub> could be achieved when the reduction temperature was higher than 603 K, and the shape of the particles changed from granules to flakes with increasing reduction temperature. Furthermore, the formation of antisite defects of Bi<sub>Se</sub> was promoted by increasing reduction temperature, which strongly affected the electrical and thermal transport properties of the Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub> compounds, which were prepared by spark plasma sintering of the reduced powders. The anisotropic microstructures in the sintered bodies were significantly influenced by the shape of the reduced powders, which was also controlled by the reduction temperature. The thermoelectric transport properties were characterized at room temperature, and the detailed effects of the reduction temperature on these properties are discussed in terms of the anisotropic microstructure and antisite defects.

К моделям для оценки пострадиационного дефектного состояния -фазы

Processes that take place in the precipitates of -phase under irradiation with fast neutrons are topical and draw attention when one searches for ways of improving radiation resistance of structural reactor steels. A special feature of these processes is that the formation of vacancies and interstices proceeds at the background of a disordering of the initial crystal lattice, which mani-fests itself in the formation of antisite defects. X-ray and neutron diffraction techniques are efficient tools of studying changes that occur in the post-radiation structural state of such systems. However, interpretation of the experimental results calls for more complicated models for the description of structural effects exerted by accumulation of radiation defects in the material than those developed before for elementary metals. Several models are proposed in this paper.

Solution phase fabrication of photoactive Cu2BaSnS4 thin films for solar energy harvesting

The quaternary Cu2BaSnS4 (CBTS) is a new class of compound-semiconducting material, recently, emerging as one of the promising materials in the field of photovoltaics and solar water splitting applications. The difference in the electronic properties between the cations in the CBTS structure discourages the antisite defect formation and associated band tailing seen in kesterite Cu2ZnSn(S,Se)4 (CZTSSe) absorber. The direct bandgap, strong optical absorption coefficient (> 104 cm−1), and usage of earth-abundant and non-toxic elements for the growth of thin film make it alternative to kesterite CZTSSe. In this work, we have developed CBTS thin film by a simple and cost-effective successive ionic layer adsorption and reaction (SILAR) method with a varied thickness on soda–lime glass and molybdenum-coated soda–lime glass. The optical, structural, and morphological properties were investigated, and photoresponse of the material has been characterized. Raman spectroscopy results show two vibrational peaks with a distinct peak at 341 cm−1 and also XRD showing a dominant peak for (014) plane confirming the formation of pure CBTS phase. The electrical characterization of the solution-deposited CBTS films reveals the photo-absorbing properties of the materials and predicts the promising application of these materials in the photovoltaic devices.

First-principles study of antisite defects in perovskite stannates

The perovskite stannates BaSnO3 and SrSnO3 are being actively explored for applications as transparent conductors, in power or high-frequency electronics, and as channel materials in epitaxial integration with functional perovskites. Realizing these applications requires controlled n-type doping, i.e., avoiding the formation of compensating acceptor-type defects. Here, we use density-functional theory to examine the formation of cation antisite defects. Our results indicate that antisites are not a problem in BaSnO3; however, in SrSnO3, SrSn antisites may act as compensating centers.

Challenges in Sr2FeMoO6−δ Thin Film Deposition

This work reviews some fundamental issues that are relevant for the fabrication of stable‐phase strontium ferromolybdate thin films. The main challenges for strontium ferromolybdate thin film deposition arise from the sensitivity of the material's magnetic properties to point defect formation: i) Antisite defect formation and oxygen nonstoichiometry should be avoided by precise composition control during film manufacturing; ii) a highly ordered state of the correct phase and B‐site cation valence will be obtained only in a very narrow window of growth conditions; iii) to avoid additional antisite disorder with decreasing synthesis temperature, the effective temperature at the film surface should be increased by an energy flux to the growing film surface. Since thin film deposition is nonequilibrium in nature, the review starts with the consideration of equilibrium phase stability. Cation and oxygen stoichiometries are analyzed with regard to their effect on key magnetic properties. Film strain formed due to thermal and lattice mismatch is of great concern since it influences the choice of the substrate. Finally, thin film deposition techniques are valued for their benefits in strontium ferromolybdate thin film technology.

Effects of Ni and Cu Antisite Substitution on the Phase Stability of CuGa2 from Liquid Ga/Cu–Ni Interfacial Reaction

Ga and Ga-based alloys have received significant attention for applications in the liquid state and also for their potential as a bonding material in microelectronic assemblies. This study investigates the phase stability of the CuGa2 phase as a product of the interfacial reaction between liquid Ga and Cu-10Ni substrates at room temperature. In the binary Ga-Cu system, CuGa2 is decomposed into liquid Ga and Cu9Ga4 as the temperature increases to around 260 °C, which prevents the widespread application of this alloy. In contrast to CuGa2 grown from a pure Cu substrate, CuGa2 from the Cu-10Ni substrate shows an increase in the decomposition temperature during heating from 25 to 300 °C. According to our first-principle calculations, there is only a minor difference in the total free energy between Ni solute at the Cu sublattice and the Ga sublattice in the tetragonal CuGa2 crystal structure. This result indicates that both of the sublattices can accommodate the dilute Ni solute with comparable probability. Regardless of the sublattice where the Ni impurities are located, the presence of diluted Ni in the matrix stabilizes the CuGa2 system by inducing some localized Ni 3d states at energy levels near the Fermi level. It is also shown that the formation of Cu antisite defects, which also stabilizes CuGa2, is preferable if the CuGa2 matrix is grown on a Ni-containing substrate.

Spontaneous cationic ordering in chemical-solution-grown La2CoMnO6 double perovskite thin films

Double perovskite oxides are of interest because of their electric, magnetic, and elastic properties; however, these properties are strongly dependent on the ordered arrangement of cations in the double perovskite structure. Therefore, many efforts have been made to improve the level of cationic ordering to obtain optimal properties while suppressing antisite defect formation. Here, epitaxial double perovskite La2CoMnO6 thin films were grown on top of (001)-STO oriented substrates by a polymer-assisted deposition chemical solution approach. Confirmation of the achievement of full Co/Mn cationic ordering was found by scanning transmission electron microscopy (STEM) measurements; EELS maps indicated the ordered occupancy of B–B′ sites by Co/Mn cations. As a result, optimal magnetic properties (Msat ≈ 6 µB/f.u. and Tc ≈ 230 K) are obtained. We show that the slow growth rates that occur close to thermodynamic equilibrium conditions in chemical solution methods represent an advantageous alternative to physical deposition methods for the preparation of oxide thin films in which complex cationic ordering is involved.

N–N bond cleavage and ring expansion at the surface of exchange and substitutional antisite defective boron nitride nanotubes by boron cluster: A density functional theory study

Functionalisation of nitrogen–nitrogen bonds of antisite defective boron nitride nanotubes (BNNTs), including exchange antisite defect which is produced by the rotation of BN bond, and substitutional antisite defect which is formed by substitution of an N with B, is investigated through their interaction with a $$\mathrm{B}^{-}_{6}$$ cluster. The smaller defect formation energies for the substitutional antisite defects indicate that the substitution of an N atom with B atom is easier than rotation of a BN bond. The formation of antisite defects at the edge or near the edges is more favourable than that in the middle of the tubes. When complexation between double ring $$\mathrm{B}^{-}_{6}$$ and nitrogen–nitrogen bonds of antisite defective BNNTs occurs, two-fold coordination, double ring configuration of boron cluster and N–N bond cleavage are seen. In the most stable complex, the $$\mathrm{B}^{-}_{6}$$ pulls apart the B–N bond and becomes an integral part of the tube by expanding the hexagonal BN ring, while in the other BNNT-B6 clusters, double ring $$\mathrm{B}^{-}_{6}$$ acts as a bridge at the top of the decagon. Functionalisation of N–N bonds at the edge or near the edges is more favourable than that in the middle of tubes.

Surface Energy and Reconstruction of Li and Na Based Transition Metal Oxides: A Computational Study

Interfaces between cathode particles and electrolyte have significant influence on performance and longevity of Li-ion batteries. Therefore, for further battery improvement deep understanding of cathode-electrolyte interface structure and its relationship with electrochemical processes inside the battery is strongly required. During the last few years, more and more studies are focused on the interfaces and surfaces of cathode particles, especially for layered transition metal oxides (TMO). For example, it was established that TMO surfaces experience reconstruction from layered to spinel and cubic structures, considerably affecting charge transfer through cathode/electrolyte interface and electrolyte and cathode stability [1]. Though initially reconstruction was considered mostly as a detrimental effect, resulting in increased barriers for Li insertion and cathode degradation, new studies show that reconstructed layer can serve as a natural protection from oxygen loss, preventing decomposition of electrolyte and insertion of water in cathode structure [2]. Though it is evident from experimental studies that the reconstructed layer resembles spinel and rock-salt structures, the exact ordering of atoms is controversial. More importantly, the governing factors for reconstruction are still not fully clear: in some cases, reconstruction occurs during synthesis and in others it proceeds under exposure to electrolyte and/or during cycling. Finally, surface reconstruction remains almost unexplored in promising Na-based TMO cathodes, where its role can be highly important, serving as a protective layer from water insertion [2]. In this work, we develop an algorithm for finding reconstructed structures of surface for transition metal oxides. The algorithm is based on the Monte-Carlo method used for a random swap of alkali atoms, transition atoms, and vacancies, while the density functional theory is used for atomic optimization after each Monte-Carlo step and accurate calculation of energies. The Metropolis method allows to take into account the influence of temperature and predict kinetically controlled surface structures. The algorithm is implemented in our SIMAN package for high-throughput materials modelling and available for free use [3]. Employing the developed algorithm, we consider several oxide compounds, such as LiNiO2, NaNiO2 and NaMnO2 used for pragmatic applications for rechargeable Li-ion and Na-ion batteries. We found that in LiNiO2 and NaNiO2 the formation of surface and subsurface anti-site defects is much easier than that in the bulk region. By making simulation at real synthesis temperatures, we observed kinetically controlled formation of spinel layers for both LiNiO2 and NaNiO2. Under oxidative conditions the transformation of the LiNiO2 surface to a rock-salt structure is thermodynamically favorable. The obtained results explain experimentally observed surface reconstruction to spinel and rock-salt structures in layered oxides. The undergoing study will identify the reasons that control the type of reconstruction depending on the chemical composition and synthesis conditions, providing useful guidelines for improving cathode materials.

Antisite Defects in Ge–Te and Ge–As–Te Semiconductor Glasses

The formation of antisite defects in Ge20Te80 and Ge15As4Te81 vitreous alloys in the form of tin atoms in tellurium sites and tellurium atoms in germanium sites is shown by emission Mossbauer spectroscopy with the 119mmSn(119mSn), 119mTe(119mSn), 125Sn(125Te), and 125mTe(125Te) isotopes. It is shown that the isovalent substitution of germanium atoms by tin atoms does not vary the symmetry of the local surrounding of germanium sites, while tin and tellurium atoms reconstruct their local surrounding in sites unnatural for them.

Temperature Driven Unusual Reversible p‐ to n‐Type Conduction Switching in Bi2Te2.7Se0.3

Bismuth telluride based alloys are electronic semiconductors, which exhibit n‐ or p‐type conduction due to the formation of Te vacancies or antisite defects, i.e., substitution of Bi on Te site or vice versa. Here, it is demonstrated that the temperature dependent Seebeck coefficient of Bi2Te2.7Se0.3 exhibits a reversible change in conduction from p‐ to n‐type at temperatures >487 K without exhibiting any structural transformation. The detailed characterization revealed that conversion of BiTe/Se antisite defects into Te vacancies is responsible for the p–n transition. The observed p–n transition makes Bi2Te2.7Se0.3 an ideal candidate for temperature controlled electronic switches.

Hydrogen Solubility in Pd3Ag Phases from First-Principles Calculation

First-principles calculation was used to systematically investigate hydrogen solubility in Pd3Ag phases. It was found that the solubility of hydrogen in Pd3Ag phases was much greater than in face-centered cubic (FCC) Pd, suggesting that Ag atoms enhanced hydrogen solubility with respect to FCC Pd. In addition, the present calculation also revealed that the anti-site defect formation enthalpies of Pd3Ag were close to zero, and the values of vacancy were positive and large, which indicated that Pd3Ag distributed compactly. In the process of hydrogen separation, anti-site defects decreased the hydrogen solubility in the Pd3Ag phases, i.e., the ordered Pd3Ag phases bestowed excellent properties of H selectivity. The results presented not only explore the fundamental properties of Pd3Ag phases and their various potential applications, but also agree with experimental observations reported in the literature.

Theoretical investigation of the cation antisite defect in layer-structured cathode materials for Li-ion batteries.

The formation of cation antisite defects in layer-structured LiTMO2 cathode materials for Li-ion batteries is investigated, where TM represents Ni or Co. Energetically possible arrangements of cation antisite defects are searched by a bottom-up method in which cations are added to a frame supercell that contains some vacant cation lattice points, using a particle swarm optimization algorithm combined with a density functional theory method. The equilibrium concentration of antisite defects is quantified through a calculation model in which an ensemble crystal system is assembled by statistically combining the constructed supercell units. The behavior of antisite defect formation in LiNiO2 and LiCoO2 is compared, and the effect of Co or Mn introduction to LiNiO2 is examined. The underlying mechanism is investigated in terms of the covalent and ionic bonding characteristics, TM oxidation state, and structural changes in the cation sites, based on analyses of the crystal orbital Hamilton population, partial charge, and bond lengths. It is concluded that the structural distortion of the cation sites and the superexchange interaction involved with cation mixing are the two main factors determining the concentration of cation antisite defects in layer-structured cathode materials.

Антиструктурные дефекты в полупроводниковых стеклах Ge-Te и Ge-As-Te

AbstractThe formation of antisite defects in Ge_20Te_80 and Ge_15As_4Te_81 vitreous alloys in the form of tin atoms in tellurium sites and tellurium atoms in germanium sites is shown by emission Mössbauer spectroscopy with the ^119 mm Sn(^119 m Sn), ^119 m Te(^119 m Sn), ^125Sn(^125Te), and ^125 m Te(^125Te) isotopes. It is shown that the isovalent substitution of germanium atoms by tin atoms does not vary the symmetry of the local surrounding of germanium sites, while tin and tellurium atoms reconstruct their local surrounding in sites unnatural for them.

Nanoscale charge transport and local surface potential distribution to probe defect passivation in Ag doped Cu2ZnSnS4 absorbing layer

The performance of earth abundant Cu2ZnSnS4 (CZTS) material is limited by high deficit of open circuit voltage (VOC) which is mainly due to the easy formation of CuZn antisite defects. Suppression of CuZn defects is thus inevitably required for further developments in CZTS based solar cells. We studied systematic increase of Ag doping in CZTS thin film and investigated the nanoscale electrical properties using Kelvin probe force microscopy and current sensing atomic force microscopy (CAFM) to probe CuZn defects. Crystallographic analysis indicated the successful partial substitution of Cu+ ions by large size Ag+ ions. The considerable decrease in grain boundary potential from 66.50 ± 5.44 mV to 13.50 ± 2.61 mV with Ag doping, suggesting the substantial decrease in CuZn defects. Consequently, CAFM measurement confirms the remarkable increment in minority carrier current with Ag doping and their local mobility in CZTS layer. Finally, the lower persistent photoconductivity and fast decay response of photogenerated carriers for Ag doped CZTS photodetector further validate our results. This study provides a fresh approach of controlling deleterious CuZn defects in CZTS by tuning Ag content that may guide researchers to develop next generation high-performance CZTS based solar cells.

On the presence of antisite defect in monoclinic Li2FeSiO4 – A combined X-Ray diffraction and DFT study

Li2FeSiO4 material, which was prepared by a solid state method, crystallized as monoclinic P21/n polymorph. X-ray diffraction analysis with Rietveld structural refinement indicates specific occupation of Li2 crystallographic site by Fe2+ cation in the amount of 6 atom percents as a result of an antisite defect formation. The exclusive occupation of Li2 position, out of two crystallographic positions Li1 and Li2, by Fe2+ was discussed in relation to the differences that exist in the crystal environment of these positions and further investigated by DFT calculations. It was confirmed that Fe-Li2 substitution is energetically favorable compared to both Fe-Li1 substitution and the pristine crystal. In addition, changes of lattice geometry upon antisite defect formation were analyzed, and the obtained result is discussed in light of various factors (electronic, geometrical and enthropic) that contribute to the overall stability of the system.

Microscopic Phase-field Simulation for the Influence of Aging Process on the Precipitation Process of Ni75Al15Ti10 Alloy

The precipitation process of Ni75Al15Ti10 alloy was simulated by a microscopic phase-field kinetics model. The microscopic morphology evolution was studied, which indicates that the precipitation process can be divided into two stages: the first is the transformation from L10 phase to L12 phase, and the second is the formation of the stoichiometric L12 phase. The effect of single aging temperature on the microstructure, atomic occupation probability and atomic diffusion was investigated. The result shows that the shape of γ′ precipitates transforms from irregular shape to regular cuboid and the degree of orientation growth increases with increasing aging temperature. But the higher the aging temperature, the lower the occupation probability and the order degree. Meanwhile, the path of diffusion of Al atom is from the interface of the γ′ phase to the internal, and the diffusion of Ti atom is the opposite. Furthermore, we studied the dual aging to improve occupation probability of γ′ precipitates and to discuss their influence on the γ′ phase. Dual aging can not only obtain a stable γ′ phase with regular shape, but also increase the occupation probability of Al and Ti and inhibit the formation of anti-site defects.