Concentration Of Antisite Defects Research Articles

Synthesis of ultrathin carbon layer-coated LiNiPO4 nanoparticles by solvothermal method

The emerging market of high-power lithium-ion batteries (LIBs) highlights the importance of developing high-voltage cathodes. Lithium nickel phosphate (LiNiPO4) with the highest theoretical discharge potential (∼5.1V) has attracted increasing attention due to its stable olivine structure. In this study, ultrathin carbon layer-coated LiNiPO4 was synthesized using oleylamine-assisted solvothermal method under different solvothermal times, and the effect of solvothermal time (1h–18h) on LiNiPO4 samples is fully investigated. X-ray diffraction (XRD) shows that high-purity LiNiPO4 materials can be prepared after calcination of the solvothermal product under N2, but adding part of H2 to the calcination atmosphere will lead to the generation of impurities in the target product. As the solvothermal time increases, the product will agglomerate and grow, the particle size will increase, and the morphology will change from nanoparticle to rodlike structure. After the high-temperature calcination step, the oleylamine is completely carbonized, and a conductive carbon layer is successfully coated on the LiNiPO4 with the thickness of 2 nm. As the solvothermal time increases, the graphitization degree of oleylamine increases after carbonization. The existence of the carbon layer and the increase in the degree of graphitization are beneficial to improving the electrical conductivity and Li-ion diffusion rate of LiNiPO4. The initial discharge capacity of LNP@C-12h product prepared with 12 h solvothermal time is 35 mA h g−1 at 0.1 C. Rietveld analysis shows that the anti-site defect concentration of the LNP@C material is 1.07 %, and the proportion increases to 2.83 % after several cycles. This work provides a positive contribution to the synthesis and modification of high-voltage LiNiPO4 materials and explains the electrochemical performance degradation of LiNiPO4 from the perspective of increasing the concentration of anti-site defects.

Computational prediction of high thermoelectric performance in As2Se3 by engineering out-of-equilibrium defects.

We employed first-principles calculations to investigate the thermoelectric transport properties of the compound As2Se3. Early experiments and calculations have indicated that these properties are controlled by a kind of native defect called antisites. Our calculations using the linearized Boltzmann transport equation within the relaxation time approximation show good agreement with the experiments for defect concentrations of the order of 1019 cm-3. Based on our total energy calculations, we estimated the equilibrium concentration of antisite defects to be about 1014 cm-3. These results suggest that the large concentration of defects in the experiments is due to kinetic and/or off-stoichiometry effects and in principle it could be lowered, yielding relaxation times similar to those found in other chalcogenide compounds. In this case, for relaxation time higher than 10 fs, we obtained high thermoelectric figures of merit of 3 for the p-type material and 2 for the n-type one.

Defect Synergistic Regulations of Li&Na Co-Doped Flexible Cu2 ZnSn(S,Se)4 Solar Cells Achieving over 10% Certified Efficiency.

Ion doping is an effective strategy for achieving high-performance flexible Cu2 ZnSn(S,Se)4 (CZTSSe) solar cells by defect regulations. Here, a Li&Na co-doped strategy is applied to synergistically regulate defects in CZTSSe bulks. The quality absorbers with the uniformly distributed Li and Na elements are obtained using the solution method, where the acetates (LiAc and NaAc) are as additives. The concentration of the harmful CuZn anti-site defects is decreased by 8.13% after Li incorporation, and that of the benign NaZn defects is increased by 36.91% after Na incorporation. Synergistic Li&Na co-doping enhances the carrier concentration and reduces the interfacial defects concentration by one order of magnitude. As a result, the flexible CZTSSe solar cell achieves a power conversion efficiency (PCE) of 10.53% with certified 10.12%. Because of the high PCE and the homogeneous property, the Li&Na co-doped device is fabricated to a large area (2.38 cm2 ) and obtains 9.41% PCE. The co-doping investigation to synergistically regulate defects provides a new perspective for efficient flexible CZTSSe solar cells.

A comparative study of the amorphization of NiTi-B2 structure by anti-site defects

This paper reports a comparative study of the amorphization of B2 structure of near-equiatomic NiTi and three other equiatomic binary B2 alloy systems, including AlFe, CuZr and FeTi. The study was conducted by means of molecular dynamics simulation on the effects of anti-site defects within the B2 structure. It was found that anti-site defects cause severe random distortions to the B2 structures, lowering their crystalline ordering. Among the four B2 systems studied, equiatomic AlFe and FeTi cannot be fully amorphized by anti-site defects but NiTi and CuZr can become fully amorphous at above a threshold concentration of the defects, as compared to the structure of quenched liquid. It is also found that the threshold anti-site concentration for amorphization decreases with increasing temperature. In comparison between the two, NiTi is easier to be amorphized than CuZr by anti-site defects, both in terms of the threshold anti-site defect concentration required and the temperature range within which it is possible. These findings may help to explain the long-observed phenomenon of the easiness of equiatomic NiTi to become amorphous during cold working and the new observation of third element substitutional diffusion caused amorphization.

High Concentration Intrinsic Defects in MnSb2Te4.

MnSb2Te4 has a similar structure to an emerging material, MnBi2Te4. According to earlier theoretical studies, the formation energy of Mn antisite defects in MnSb2Te4 is negative, suggesting its inherent instability. This is clearly in contrast to the successful synthesis of experimental samples of MnSb2Te4. Here, the growth environment of MnSb2Te4 and the intrinsic defects are correspondingly investigated. We find that the Mn antisite defect is the most stable defect in the system, and a Mn-rich growth environment favors its formation. The thermodynamic equilibrium concentrations of the Mn antisite defects could be as high as 15% under Mn-poor conditions and 31% under Mn-rich conditions. It is also found that Mn antisite defects prefer a uniform distribution. In addition, the Mn antisite defects can modulate the interlayer magnetic coupling in MnSb2Te4, leading to a transition from the ideal antiferromagnetic ground state to a ferromagnetic state. The ferromagnetic coupling effect can be further enhanced by controlling the defect concentration.

Lithium iron phosphate with high-rate capability synthesized through hydrothermal reaction in low Li concentration solution

Lithium iron phosphate (LiFePO4) is one of the most important cathode materials for high-performance lithium-ion batteries in the future due to its high safety, high reversibility, and good repeatability. However, high cost of lithium salt makes it difficult to large scale production in hydrothermal method. Therefore, it is urgent to reduce production costs of LiFePO4 while maintaining the unique advantages of hydrothermal methods. In this work, the lithium iron phosphate powders (LFP-twi) with low Fe-Li anti-site defect concentration and inhibition along the direction [010] was constructed by hydrothermal reaction with low lithium usage. As a result, this allows the material to have an excellent reversible capacity (163.6 mAhg−1 at 0.1 C), ultra-high-rate capability (120 mAhg−1 at 10 C), and noticeable low-temperature performance (130 mAhg−1 at - 20 ℃ and 0.1 C). These findings open a new avenue for the development of high-density current and low-temperature resistant LiFePO4 batteries.

Effect of Magnetic Field and Hydrostatic Pressure on Metamagnetic Isostructural Phase Transition and Multicaloric Response of Fe49Rh51 Alloy

The effect of a high magnetic field up to 12 T and a high hydrostatic pressure up to 12 kbar on the stability of the metamagnetic isostructural phase transition and the multicaloric effect of Fe49Rh51 alloy has been studied. The phase transition temperature shifts under the magnetic field and the hydrostatic pressure on with the rates of dTm/μ0dH = −9.2 K/T and dTm/dP = 3.4 K/kbar, respectively. The magnetocaloric and multicaloric (under two external fields) effects were studied via indirect method using Maxwell relations. The maximum of the entropy change is increasing toward the high temperature region from ∆S~2.5 J/(kg K) at 305 K to ∆S~2.7 J/(kg K) at 344 K under simultaneously applied magnetic field of 0.97 T and hydrostatic pressure of 12 kbar. The obtained results were explained using the first-principle calculations of Gibbs energies and the phonon spectra of the ferromagnetic and the antiferromagnetic phases. Taking into account the low concentration of antisite defects in the calculation cells allows us to reproduce the experimental dTm/dP coefficient.

Quantifying antisite defect concentrations in yttrium aluminum garnet by high-precision density analysis

Quantifying the nature and density of lattice defects in high-quality crystals is often difficult or impossible with commonly employed material characterization techniques. In many cases, even the density of intentionally doped impurities present in a grown crystal must be inferred indirectly using methods such as mapping spatial variations in crystal properties or, when the defects are optically active, interpreting optical absorption with imprecisely known oscillator strengths. In this work we explore gravimetric techniques to obtain absolute defect densities in crystals, a powerful approach often overlooked in modern materials science. A hydrostatic weighing apparatus based on Archimedes’ principle was constructed, tested, and refined to enable high-precision density measurements of small crystal samples. Application of this system is illustrated by determining the density of Yttrium Aluminum Garnet (YAG) relative to a high-purity silicon crystal density reference, minimizing traditional difficulties in correcting for the variations in surface tension and the water and air density due to small changes in ambient temperature, pressure, and solution composition. This method, using a relatively simple apparatus and small samples, is effective in measuring the intrinsic concentration of Y–Al antisite lattice defects in YAG, agreeing with results from far more elaborate high-resolution optical laser spectroscopy at cryogenic temperatures. Furthermore, we quantitatively analyze the effects of several potential sources for error, including natural variations in isotope distributions, residual surface contamination, and ambient temperature or pressure variations. Subsequently, we find that the density analysis method is a promising technique for characterizing new and modified materials being developed for applications ranging from solid-state lasers and scintillators to quantum information.

Short- and Long-Range Cation Disorder in (AgxCu1–x)2ZnSnSe4 Kesterites

Understanding the nature of and controlling the cation disorder in kesterite-based absorber materials remain a crucial challenge for improving their photovoltaic (PV) performances. Herein, the combination of neutron diffraction and synchrotron-based X-ray absorption techniques was implemented to investigate the relationships among cation disorder, defect concentration, overall long-range crystallographic order, and local atomic-scale structure for (AgxCu1–x)2ZnSnSe4 (ACZTSe) material. The joint Rietveld refinement technique was used to directly reveal the effect of cation substitution and quantify the concentration of defects in Ag-alloyed stoichiometric and nonstoichiometric Cu2ZnSnSe4 (CZTSe). The results showed that 10%-Ag-alloyed nonstoichiometric ACZTSe had the lowest concentration of detrimental antisite CuZn defects (∼8 × 1019 defects per cm–3), which was two times lower than pristine and five times lower than the stoichiometric compositions. Moreover, Ag incorporation maintained the concentrations of beneficial Cu vacancies (VCu) and antisite ZnCu defects to >2 × 1020 defects per cm–3. X-ray absorption measurements were performed to verify the degree of disorder through the changes in bond length and coordination number. Therefore, the incorporation of Ag into the CZTSe lattice could control the distribution of antisite defects, in the form of short- and long-range site disorder. This study paves the way to systematically understand and further improve the properties of kesterite-based materials for different energy applications.

Antisite defect manipulation enables the high thermoelectric performance of p-type Bi2-xSbxTe3 alloys for solid-state refrigeration

In thermoelectrics, carrier concentration, carrier mobility, density-of-state effective mass, and lattice thermal conductivity are all intimately associated with native point defects. Taking p-type Bi0.4Sb1.6Te3 polycrystal as an example, the excessive antisite defects results in too high carrier concentration and inferior carrier mobility. Herein the constructive role of antisite defect manipulation for enhancing thermoelectric performance of Bi0.4Sb1.6Te3 is reported. We explored the thermoelectric study of sole Gallium or Indium-doped p-type Bi0.4Sb1.6Te3 in two distinct compositional series: Bi0.4Sb1.6-xGaxTe3 and Bi0.4Sb1.6-yInyTe3, aka the x-series and y-series. In x-series, the fine-tuning of antisite defects is achieved for the first time via the reciprocal variation of electronegativity difference and atomic size difference between cations and anions on the grounds of chemical composition-regulated (χ, r) model. Compared to the dramatic reduction of antisite defect concentration for y-series, the slightly declined antisite defect concentration for x-series contributes to a more optimized carrier concentration and superior material parameter. In addition, the generated multiscale microstructures induced by Ga doping and hot deformation substantially diminish the lattice thermal conductivity through broad wavelength phonon scattering. As a result, a state-of-the-art zT = 1.47 at 350 K is attained in p-type Bi0.4Sb1.59Ga0.01Te3, indicating the potential advantages in solid-state refrigeration field. These results not only attest to the efficacy of native point defect engineering in V2VI3 and other thermoelectric materials, but also bring out the understanding and manipulation of native point defect to a new level.

NAi/Li Antisite Defects in the Li1.2Ni0.2Mn0.6O2 Li-Rich Layered Oxide: A DFT Study

Li-rich layered oxide (LRLO) materials are promising positive-electrode materials for Li-ion batteries. Antisite defects, especially nickel and lithium ions, occur spontaneously in many LRLOs, but their impact on the functional properties in batteries is controversial. Here, we illustrate the analysis of the formation of Li/Ni antisite defects in the layered lattice of the Co-free LRLO Li1.2Mn0.6Ni0.2O2 compound through a combination of density functional theory calculations performed on fully disordered supercells and a thermodynamic model. Our goal was to evaluate the concentration of antisite defects in the trigonal lattice as a function of temperature and shed light on the native disorder in LRLO and how synthesis protocols can promote the antisite defect formation.

Structure and transport behavior of hydrothermally grown phase pure Cu2ZnSn1-xGexS4 (x = 0.0, 0.3) nanoparticles

Phase pure Cu2ZnSnS4 (CZTS) and Cu2ZnSn0.7Ge0.3S4 (CZTGS) kesterite nanoparticles of 10–35 nm size range were fabricated through a low-temperature hydrothermal process. Morphology, structure, and structural phase of the nanoparticles were determined utilizing field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and Raman spectroscopy. Electrical transport behaviors of the pelletized nanostructures were studied in a physical properties measurement system (PPMS) in the 100–320 K temperature range. Obtained results indicate that both the CZTS and CZTGS nanostructures have p-type conductivity, with high room temperature hole concentration and carrier mobility. Substitution of 30% Sn by Ge enhances the electrical conductivity of CZTS about four times. While the room temperature hole mobility in the kesterite nanostructures reduces to about 60%, the hole concentration increases about one order on Ge incorporation. The limited substitution of Sn atoms by Ge does not affect the position of acceptor levels in the electronic band gap of the kesterite nanostructures. However, it increases the concentration of CuZn and ZnCu antisite defects and affects the transition temperature where the electrical transport of kesterite nanostructures changes from lattice scattering controlled to defect scattering controlled conduction.

Optimizing the Ratio of Sn4+ and Sn2+ in Cu2ZnSn(S,Se)4 Precursor Solution via Air Environment for Highly Efficient Solar Cells

The use of different Sn valence states (such as Sn4+ and Sn2+) in the Cu2ZnSn(S,Se)4 (CZTSSe) precursor solution is especially important for the quality of the subsequent growth of the CZTSSe films. The latest study has found that replacing SnCl2·2H2O with anhydrous SnCl4 can remarkably improve the performance of CZTSSe solar cells, but it needs to be operated in the glovebox. Herein, for the precursor solution, SnCl4·5H2O powder is used instead of anhydrous SnCl4 in air environment, and the proportion of Sn4+ and Sn2+ precursor solutions is further systematically studied. When the ratio of Sn4+ to Sn2+ is 1:1, a uniform, compact, and noncracking CZTSSe thin film is obtained, effectively alleviating the interface recombination and reducing the concentration of deep‐level defects. In particular, the concentration of CuZn antisite defects is decreased by an order of magnitude, and the carrier recombination and band tail effect are alleviated. When JSC is maintained, VOC and FF are considerably improved. Finally, CZTSSe thin‐film solar cells are fabricated with an efficiency of over 11%. Herein, the feasibility of controlling the ratio of Sn4+ to Sn2+ in the CZTSSe precursor solution for higher efficiency of CZTSSe thin‐film solar cells is demonstrated.

Effects of Antisite Defect Density on Crystal and Electronic Structures of Monolayer Hexagonal Boron Nitride

The effect of antisite defects and their density on monolayer hexagonal boron nitride is discussed in detail. Different supercell sizes to simulate different defect densities are set up. All structures containing antisites are fully optimized. It is indicated that the high density of antisite defects leads to the instability of the BB bond. The influence of supercell size on lattice structure is also summarized. Like vacancies and dopant atoms, the antisite defects also lead to the appearance of the defect energy band. Different antisite defect densities have different effects on different orbitals. Electron energy‐loss spectroscopy theoretical simulation is conducted on a single atom to analyze the influence of antisite defect density on the electronic structure of a single atom. It is shown that the high density of antisite defects makes the σ* transition of B atoms that are far away from the defect more preferred than π* transition, while it has less influence on the transition of central N atoms of the antisite defect NB because of the influence of neighboring atoms. The concentration of antisite defects plays a key role in manipulating the physical properties of monolayer hexagonal boron nitride and is helpful in expanding potential application scenarios.

Numerical and experimental determination of the thermal conductivity of pristine and substituted Fe2VAl

In this article, we investigate the lattice thermal conductivity in the thermoelectric Fe2VAl Heusler alloy by implementing a Green’s function approach to describe phonon-defect scattering mechanisms in the pristine and substituted compounds. Former calculations for pristine Fe2VAl largely overestimated the lattice thermal conductivity compared to the experimental value (28 W m−1 K−1). By considering a realistic concentration ( ~ 5%) of combined AlV and VAl antisite defects as an extra phonon scattering mechanism, calculated and experimental values could be reconciled. The same theoretical approach was applied to Ta- and Ta + Sn-substituted Fe2VAl. In agreement with experiments it shows that the dual Ta + Sn-substitution is more efficient in decreasing the lattice thermal conductivity of Fe2VAl than the single Ta-substitution. A strong decrease of 70% is theoretically obtained in Fe2V0.96Ta0.04Al0.96Sn0.04 which could thus lead to better global thermoelectric performances.

In Situ Induced Surface Reconstruction of Single-Crystal Lithium-Ion Cathode Toward Effective Interface Compatibility.

LiNixCoyMn1-x-yO2 (x ≥ 0.5) layered oxide materials are generally considered as one of the most prospective candidates for lithium-ion battery (LIBs) cathodes due to their high specific capacity and working voltage. However, surface impurity species substantially degrade the electrochemical performance of LIBs. Herein, surface reconstruction from layered structure to disordered layer and rock-salt coherent region together with a uniform Li2CO3-dominant coating layer is first in situ constructed on the single-crystal LiNi0.5Co0.2Mn0.3O2 (NCM) material by a simple water treatment procedure. The unique surface structure is elucidated by Ar-sputtering-assisted X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (spherical aberration-corrected-scanning transmission electron microscopy (STEM), high-resolution transmission electron microscopy (HRTEM), and TEM). Meanwhile, neutron powder diffraction (NPD) indicates that the antisite defect concentration is mitigated in the treated materials. The modified samples display superior cycle stability with a capacity retention of up to 87.5% at 1C after 300 cycles, a high rate capacity of 151 mAh g-1 at 5C, an elevated temperature (45 °C) cycling property with 80% capacity retention (4.5 V), and improved full-cell performance with 91% after 250 cycles at 1C. Importantly, postmortem examination on the cycled cathodes by time-of-flight secondary-ion mass spectroscopy (TOF-SIMS), XPS, TEM, and X-ray diffractometer (XRD) pattern further demonstrate that these results are mainly attributed to the thin cathode electrolyte interface (CEI) film and low solubility of transition-metal ions. Therefore, this expedition provides an opportunity to construct an effective armor for the interface compatibility and stability of LIBs.

A point defect model for YBa2Cu3O7 from density functional theory

The advent of high temperatures superconductors (HTS) will enable the development of compact fusion reactors capable of delivering large quantities of low carbon energy. However, the inside of a fusion reactor is a hostile environment and bombardment by high energy neutrons will alter the microstructure of constituent materials, including the HTS in the magnets. Key to understanding the evolution of a material’s microstructure when subjected to neutron irradiation is knowledge of the defect population in the starting material. Therefore, this work uses density functional theory (DFT) simulations, combined with simple thermodynamics, to create a point defect model that enables prediction of the types and concentrations of defects present in a model HTS, YBa2Cu3O7, under a range of fabrication conditions. The simulations predict that the defect chemistry of YBa2Cu3O7 is dominated by oxygen defects, predominantly vacancies, in agreement with prior experimental observations. Interestingly, the simulations predict that the exchange of Y and Ba atoms is the second lowest energy defect process in YBa2Cu3O7 after the oxygen Frenkel process. Furthermore, the point defect model shows that any cation non-stoichiometry will also be accommodated via antiste defects rather than either vacancies or interstitials. Overall, these results suggest that future fusion magnets will contain a high concentration of oxygen and cation antisite defects and that these must be considered in future studies of the evolution of HTS materials under irradiation.

Enhancing room-temperature thermoelectric performance of n-type Bi2Te3-based alloys via sulfur alloying

Bismuth-telluride-based alloys are the best thermoelectric materials used in commercial solid-state refrigeration near room temperature. Nevertheless, for n-type polycrystalline alloys, their thermoelectric figure of merit (zT) values at room temperature are often less than 1.0, due to the high electron concentration originating from the donor-like effect induced by the mechanical deformation process. Herein, carrier concentration for better performance near room temperature was optimized through manipulating intrinsic point defects by sulfur alloying. Sulfur alloying significantly decreases antisite defects concentration and suppresses donor-like effect, resulting in optimized carrier concentration and reduced electronic thermal conductivity. The hot deformation process was also applied to improve carrier mobility due to the enhanced texture. As a result, a high zT value of 1 at 300 K and peak zT value of 1.1 at 350 K were obtained for the twice hot-deformed Bi2Te2.7Se0.21S0.09 sample, which verifies sulfur alloying is an effective method to improve thermoelectric performance of n-type polycrystalline Bi2Te3-based alloys near room temperature.

Effects of La doping on the crystal growth, phase stability and scintillation properties of Lu3Al5O12 single crystals

The growth process, crystal composition, optical and scintillation properties of Lu3Al5O12:La (La = 0–0.45%) single crystals were investigated in detail. The thermodynamical stability of the garnet phase formation was studied in the undoped and La doped Lu3Al5O12 single crystals. The crystals were grown from the melt using a micro-pulling down method. Detailed composition analysis reviled the complex multiphase character of the samples. Both garnet and α-Al2O3 phases were detected in the undoped and slightly La-doped samples, while heavily La-doped crystals contained the mixture of α-Al2O3, garnet and perovskites phases. The substitution La for Lu led to the formation of LaLu centers and increased the concentration of LuAl antisite defects. Both types of centers caused a strong luminescence in the deep UV spectral range. Heavy La-doping significantly enhanced the scintillation performance of Lu3Al5O12 crystals.

Mechanical and thermal conductivity properties of BiSbTe nanofilms using molecular dynamics

In this decade, Bi x Sb 2-x Te 3 of Bi 2 Te 3 -based thermoelectric (TE) materials has been proven effective in improving the TE properties and figure of merit (ZT) at room temperature of these materials. In this work, the mechanical and thermal properties of quintuple layers of the Bi x Sb 2-x Te 3 nanofilms are studied by using non-equilibrium molecular dynamics simulation. The effects of different proportions of Sb content on deformation mechanism and thermal conductivity are determined. Results indicate that the mechanical and thermal properties of the Bi x Sb 2-x Te 3 nanofilms are significantly affected by Sb content and differ greatly from those of pure Bi 2 Te 3 nanofilms. Furthermore, the Bi x Sb 2-x Te 3 nanofilms exhibit better tensile strength than pure Bi 2 Te 3 nanofilms. Changes in Sb content strongly affect the concentration of SbTe antisite defects and alter thermal conductivity. The results of this investigation should benefit the potential application of the Bi x Sb 2-x Te 3 nanofilms to TE devices and energy generation. • We investigated the mechanical and thermoelectric properties of Bi x Sb 2-x Te 3 nanofilms with different ratios of Bi:Sb at different temperatures using nonequilibrium molecular dynamics simulation. • The Bi x Sb 2-x Te 3 nanofilms with Sb-rich content increased the concentration of SbTe antisite defects and improved the thermal conductivity. • When x was less than or equal to 0.5, the Bi x Sb 2-x Te 3 nanofilms exhibited the best mechanical and TE properties.