Formation Of Antisite Defects Research Articles

A comparative first-principles study on the physical properties of Gd2Zr2O7 weberite and pyrochlore

A comparative analysis of the physical properties of Gd2Zr2O7 weberite and pyrochlore is conducted using first-principles methods. The structural characteristics of Gd2Zr2O7 pyrochlore and weberite are examined at the atomic site, local coordination, and lattice parameter levels. The findings from ab initio molecular dynamics simulations and experimental data confirm the existence and stability of the Gd2Zr2O7 weberite structure at 300 K. The formation of cation antisite defects is calculated to be more facile in the weberite lattice compared to pyrochlore. The formation energy of vacancy defects is strongly correlated to the distinct defect configurations. The calculations further highlight that Gd2Zr2O7 weberite exhibits mechanical properties comparable to pyrochlore. The insulating nature, chemical bonding characteristics, and charge states of individual atoms in weberite and pyrochlore are elucidated through analysis of the partial density of states and Bader charges.

Effect of Starting Powder Particle Size on the Thermoelectric Properties of Hot-Pressed Bi0.3Sb1.7Te3 Alloys.

P-type Bi0.3Sb1.7Te3 polycrystalline pellets were fabricated using different methods: melting and mechanical alloying, followed by hot-press sintering. The effect of starting powder particle size on the thermoelectric properties was investigated in samples prepared using powders of different particle sizes (with micro- and/or nano-scale dimensions). A peak ZT (350 K) of ~1.13 was recorded for hot-pressed samples prepared from mechanical alloyed powder. Moreover, hot-pressed samples prepared from ≤45 μm powder exhibited similar ZT (~1.1). These high ZT values are attributed both to the presence of high-density grain boundaries, which reduced the lattice thermal conductivity, as well as the formation of antisite defects during milling and grinding, which resulted in lower carrier concentrations and higher Seebeck coefficient values. In addition, Bi0.3Sb1.7Te3 bulk nanocomposites were fabricated in an attempt to further reduce the lattice thermal conductivity. Surprisingly, however, the lattice thermal conductivity showed an unexpected increasing trend in nanocomposite samples. This surprising observation can be attributed to a possible overestimation of the lattice thermal conductivity component by using the conventional Wiedemann-Franz law to estimate the electronic thermal conductivity component, which is known to occur in nanocomposite materials with significant grain boundary electrical resistance.

Anion optimization for bifunctional surface passivation in perovskite solar cells.

Pseudo-halide (PH) anion engineering has emerged as a surface passivation strategy of interest for perovskite-based optoelectronics; but until now, PH anions have led to insufficient defect passivation and thus to undesired deep impurity states. The size of the chemical space of PH anions (>106 molecules) has so far limited attempts to explore the full family of candidate molecules. We created a machine learning workflow to speed up the discovery process using full-density functional theory calculations for training the model. The physics-informed machine learning model allowed us to pinpoint promising molecules with a head group that prevents lattice distortion and anti-site defect formation, and a tail group optimized for strong attachment to the surface. We identified 15 potential bifunctional PH anions with the ability to passivate both donors and acceptors, and through experimentation, discovered that sodium thioglycolate was the most effective passivant. This strategy resulted in a power-conversion efficiency of 24.56% with a high open-circuit voltage of 1.19 volts (24.04% National Renewable Energy Lab-certified quasi-steady-state) in inverted perovskite solar cells. Encapsulated devices maintained 96% of their initial power-conversion energy during 900 hours of one-sun operation at the maximum power point.

A Comprehensive Review on the Recent Strategy of Cation Substitution in CZTS(Se) Thin Films to Achieve Highly Efficient Kesterite Solar Cells

Kesterite Cu2ZnSnS4 (CZTS) and selenized CZTS (CZTSSe) are the most potential replacements of Cu(InxGa1−x)Se2 as absorber layers in thin‐film solar cells, aligning with modern green energy policies. In spite of a reported power conversion efficiency (η) of nearly 14.9%, their efficiency still lags much behind their predecessors like cadmium telluride and CIGS. Major obstacles hindering the performance of CZTS‐based thin‐film solar cells pertain to the formation of CuZn and ZnCu antisite defects, along with high density of 2CuZn + SnZn donor defects, which adversely affects open‐circuit voltage and η. To combat these challenges, researchers have explored cation substitution by incorporating alternative isovalent atoms, such as substituting Ag/Li for Cu, Mn/Cd for Zn, and Ge/Ti for Sn, as a potent strategy to inhibit severe potential fluctuations and undesirable tail states caused by Cu–Zn disorder and Sn‐related defects. Recently single‐cation substitution in CZTS (or CZTSSe) has garnered considerable attention, while dual‐cation incorporation has emerged as an intriguing avenue to enhance device performance, although in its nascent stage of research and development. This review discusses recent progress on cation substitution to suppress antisite defects in CZTS (or CZTSSe), leading to high‐efficiency thin‐film solar cells.

Restraining formation of Fe-Li anti-site defects via in-situ surface construction on single-crystal LiFePO4 cathode for extending fast charging lifespan

The widespread utilization of olivine LiFePO4 is impeded by its inadequate electrochemical characteristics, primarily caused by the inherently low diffusion coefficient of Li+ ions. This deficiency is further exacerbated by the presence of Fe-Li anti-site defects, which significantly deteriorate the diffusion coefficient during prolonged cycling periods. Herein, a highly efficient and cost-effective method is employed to fabricate single-crystal LiFePO4 possessing minimal Fe-Li anti-site defects. The Fe-O bond is reinforced by the formation of a uniform and stable carbon layer through the introduction of acetylene. This reinforcement effectively hinders the migration of Fe ions to the 4a site, which is associated with the Li site (known as LiFe defects). The validity of this phenomenon has been confirmed through theoretical calculations. The uniform carbon layer on LiFePO4 surface not only facilitates the migration of Li+ ions without obstruction but also significantly enhances the pseudo-capacitance, leading to remarkable preservation of capacity even at a high discharge rate of 10 C, with a retention rate of 80.86 % after undergoing 1500 cycles. The present study signifies a notable advancement in the acquisition of uniformly carbon-coated single-crystal cathode materials, thereby presenting a novel methodology for augmenting their electrochemical efficacy.

A theoretical investigation to boost the efficiency of CZTS solar cells using SCAPS-1D

Cu2ZnSnS4 (CZTS) is predicted to be an efficient absorber than CIGS due to its cost effective, earth inabundant constituents. But, it reached a maximum experimental efficiency of 12.6% in 2013 and it remains the same for the past decade. This is due to the various crystallographic factors that limit the performance of CZTS like antisite defect formation, unfavourable band alignment with n-type CdS, etc. The major objective of this article is to identify potential strategies through which CZTS research can be pursued further using Solar Cell Capacitance Simulator (SCAPS) 1D simulator. First, it deals with understanding the difference between CZTS and its derivatives Cu2ZnSnSe4 (CZTSe) and Cu2ZnSn(SxSe1−x)4 (CZTSSe). Then, in pursuit of finding an alternative to n-CdS, the performance of various n-type semiconductors is investigated. Later, bandgap-graded structures using exponential and linear gradient laws are constructed and the performance of such structures is evaluated. Cation-substituted CZTS structures like (Ag1−xCux)2ZnSn(SxSe1−x)4(ACZTSSe) and Cu2Cd1−xZnxSn(SxSe1−x)4 (CCZTSSe) are discussed and the performance characteristics of such devices are also studied.

Third element diffusion induced amorphization of NiTi in a NiTi-Nb nanocomposite

The paper reports a unique and rare phenomenon observed in a NiTi-Nb nanowire in-situ composite system. The phenomenon is the amorphization of the B2 NiTi matrix caused by the diffusion of Nb through the B2 lattice. The diffusion of Nb through the NiTi matrix is driven by the effect of Oswald ripening associated with the fragmentation and granulation of Nb nanowires at elevated temperatures and the amorphization of the NiTi matrix is attributed to the effect of Ni-Ti atomic swapping, i.e., the formation of anti-site defects in the B2 structure, caused by the substitutional diffusion of a third element, Nb in this case. The phenomenon is observed by transmission electron microscopy examination and x-ray diffraction measurement and the hypothesis of explanation is supported by molecular dynamic simulation studies.

Mitigation of Power Factor Degradation in Bi2Te3 Thin Films

For thin-film thermoelectric modules with electrodes deposited on the sides of the thermoelectric material, the diffusion of atoms from the electrode affects the module’s performance. Long-term aging is crucial when the module is for practical applications. In this study, Bi2Te3 thin films with a highly preferred orientation were fabricated by a co-sputtering deposition method. Cu was used as the electrode because of its high electrical conductivity; it migrates through the Bi2Te3 film via surface and grain boundary diffusion. The diffusing Cu segregates at grain boundaries and forms a Cu2–xTe intermetallic compound on the sample surface. Contact resistivity at the interface between the Cu electrode and Bi2Te3 thin films substantially increases with aging time. Long-term aging unavoidably degrades the power factor (PF) of the pristine Bi2Te3 since n-type Bi2Te3 converts into p-type by the formation of antisite defects. The study shows that the diffusing Cu from the electrodes of the Cu/Bi2Te3/Cu module significantly mitigates the degradation of the PF after long-term aging.

Phase transformation and radiation resistance of B-site high entropy pyrochlores

Two high entropy pyrochlores (HEPs), Gd2(Ti0.2Zr0.2Sn0.2Hf0.2Ta0.2)2O7 and Gd2(Ti0.2Zr0.2Sn0.2Hf0.2Nb0.2)2O7 have been synthesized and irradiated with 800 keV Kr2+ ions and examined by in situ transmission electron microscopy (TEM). The irradiation-induced order-to-disorder phase transformation from pyrochlore to the fluorite structure in these two HEPs and Gd2Sn2O7 was observed by using selected area electron diffraction (SAED), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and energy dispersive spectroscopy (EDS) mapping. The cation antisite defect (CAD) formation energy and the electron localization function (ELF) were calculated using density functional theory (DFT). Both the experimental results and calculated results suggest that the HEPs exhibit a higher resistance to phase transformation and amorphization than Gd2Sn2O7.

Ag Alloying in Cu2−yAgyBa(Ge,Sn)Se4 Films and Photovoltaic Devices

Trigonal Cu2BaGe1−xSnxSe4 (CBGTSe) has recently gained interest as a potential photovoltaic absorber to target mitigation of antisite defect formation in Cu2ZnSn(S,Se)4. This study examines partial substitution of Cu by Ag as a potential approach to tune the properties of Ag‐incorporated CBGTSe in the following aspects: 1) phase stability and crystal structure as a function of Ag content; 2) film morphology and grain structure; 3) charge carrier properties; 4) band positions; and 5) charge carrier kinetics and recombination. Up to 20% of Cu can be substituted by Ag in CBGTSe, while above 20% a phase mixture appears. Increasing Ag content induces larger average grain size and reduced hole carrier densities. In contrast, photoelectron spectroscopy and photoluminescence measurements reveal negligible impact of Ag substitution on ionization potential (≈5.4 eV) and electron affinity (≈3.7 eV). Also, Ag content offers negligible impact on carrier lifetimes (few ns). Consistent with these fundamental properties, solar cells based on two different Ag/(Ag + Cu) ratios (≈0% and ≈20%) show comparable power conversion efficiencies (≈2.7–2.8%). These results indicate that CBGTSe films and solar cells may be less sensitive to Ag substitution compared to Cu2ZnSn(S,Se)4, at least at the current level of absorber and device optimization.

UV/blue/green converted efficient red-NIR photoluminescence in Cr incorporated MgAl2O4 nanocrystals: Site selective emission tailored through cation inversion and intrinsic defects

The UV/visible activated near-infrared (NIR) phosphors have many applications in solid state lighting, night vision devices and bio-imaging. The early research reported the red-NIR emitting phosphors doped with Cr3+ centers upon visible light excitation. Here, in this work the intense red-NIR emission and color tuning is achieved for broad excitation range (UV/blue/green) through Cr dopant induced defect centers and cation inversion in MgCr Al2O4 ( = 0.5, 1, 3, 5 and 10 mol%) nanocrystals. The Cr3+ doped MgAl2O4 nanocrystals were synthesized by combustion method through stoichiometric substitution of Mg by Cr, while most of the Cr3+ ions occupied the octahedral sites of spinel host with the formation of antisite defects, Cr3+ clusters, magnesium and oxygen vacancies. These defect centers were probed through Rietveld refinement, photoluminescence (PL), x-ray photoelectron and nuclear magnetic resonance spectra analyses. At UV excitation, the intrinsic defects played an interesting role in exhibiting the blue-violet emission attributed to host lattice defects and red-NIR emission attributed to strong/weak ligand field octahedral Cr3+ sites, via charge transfer to Cr3+ ions. The PL spectra evinced the enhanced red-NIR emission intensity upon 266 nm excitation than upon blue and green light excitation. Further, the weak ligand field site emission is found to be dominating with increase in doping concentration. Thus, Cr doped MgAl2O4 nanocrystals showed their potency of exhibiting the intense red-NIR emission and color tuning (from red purple to bluish purple and then to red color) upon UV/blue/green excitation.

Широкополосная ЭПР-спектроскопия кристалла Lu-=SUB=-3-=/SUB=-Al-=SUB=-5-=/SUB=-O-=SUB=-12-=/SUB=-:Tb-=SUP=-3+-=/SUP=-

In a single crystal of lutetium aluminum garnet (Lu3Al5O12, LuAG) in the frequency range of 37–210 GHz, at a temperature of 4.2 K, the EPR spectra of non-Kramers Tb3+ impurity ions were recorded. The measurement results indicate that Tb3+ ions replace Y3+ ions in the dodecahedral position with local D2 symmetry. The value of the g factor, the hyperfine structure constant, and the energy interval between the ground and the first excited nondegenerate sublevels of the ground multiplet are determined. Weak satellite signals were also registered, the origin of which is associated with the formation of antisite defects in the environment of paramagnetic Tb3+ centers.

Wide-band EPR spectroscopy of a Lu-=SUB=-3-=/SUB=-Al-=SUB=-5-=/SUB=-O-=SUB=-12-=/SUB=-:Tb-=SUP=-3+-=/SUP=- crystal

In a single crystal of lutetium aluminum garnet (Lu3Al5O12, LuAG) in the frequency range of 37-210 GHz, at a temperature of 4.2 K, the EPR spectra of non-Kramers Tb3+ impurity ions were recorded. The measurement results indicate that Tb3+ ions replace Y3+ ions in the dodecahedral position with local D2 symmetry. The value of the g factor, the hyperfine structure constant, and the energy interval between the ground and the first excited nondegenerate sublevels of the ground multiplet are determined. Weak satellite signals were also registered, the origin of which is associated with the formation of antisite defects in the environment of paramagnetic Tb3+ centers. Keywords: hyperfine interaction, rare earth ions, spin Hamiltonian, non-Kramers ion.

The impact of Mn nonstoichiometry on the oxygen mass transport properties of La0.8Sr0.2Mn y O3±δ thin films

Oxygen mass transport in perovskite oxides is relevant for a variety of energy and information technologies. In oxide thin films, cation nonstoichiometry is often found but its impact on the oxygen transport properties is not well understood. Here, we used oxygen isotope exchange depth profile technique coupled with secondary ion mass spectrometry to study oxygen mass transport and the defect compensation mechanism of Mn-deficient La0.8Sr0.2Mn y O3±δ epitaxial thin films. Oxygen diffusivity and surface exchange coefficients were observed to be consistent with literature measurements and to be independent on the degree of Mn deficiency in the layers. Defect chemistry modeling, together with a collection of different experimental techniques, suggests that the Mn-deficiency is mainly compensated by the formation of antisite defects. The results highlight the importance of antisite defects in perovskite thin films for mitigating cationic nonstoichiometry effects on oxygen mass transport properties.

DFT+U study and in-situ TEM investigation of high-entropy titanate pyrochlore (Lu0.25Y0.25Eu0.25Gd0.25)2Ti2O7

High-entropy pyrochlore has become a promising immobilization matrix due to its ability to immobilize multiple nuclides especially for high-level radioactive waste. This work adopts first principle method to analyze the formation possibility of (Lu0.25Y0.25Eu0.25Gd0.25)2Ti2O7 (HTP-1). By calculating the Gibbs free energy of possible chemical reactions, we predict that the driving force to synthesize HTP-1 is greater than the driving force to form single-component pyrochlore at temperatures above 1225 K. The characterization results show that the HTP-1 sample is successfully prepared by the solid-state reaction at the predicted temperature. To investigate the radiation resistance of HTP-1, the sample is irradiated by 800 keV Kr2+ ions, and the microstructure evolution is characterized by in-situ TEM. HTP-1 sample achieves complete amorphous at 0.26 dpa, indicating that its radiation resistance is between Eu2Ti2O7 and Y2Ti2O7, which is proved by the calculation results of xO48f and antisite defect formation energy.

Defect mediated structural, optical, electrical and mechanical properties of mechano-synthesized PbTe nanostructure as a superior thermoelectric material: Correlation among electrical, mechanical and optical properties

This work reports on the formation of anti-site defects in mechanically alloyed PbTe samples and their anomalous effects on the optical and electrical properties of the sample. PbTe nanoparticles have been synthesised by mechanical alloying of the elemental Pb and Te powders mixed in stoichiometric ratio under an inert (Ar) atmosphere for different time durations. The structural and microstructural characterisations have been carried out by analysing X-ray diffraction (XRD) patterns, field-emission scanning electron microscope (FESEM), and high-resolution transmission electron microscope (HRTEM) images. The elemental composition has been revealed from the energy-dispersive X-Ray (EDX) spectrum. The phase pure cubic PbTe is formed within just 30 min of milling. The Rietveld analysis of XRD patterns, FESEM, and HRTEM images revealed the crystallite size reduction and the increase in r.m.s. lattice strain with the increase in milling durations. Inclusions of more and more anti-site defects of type [PbTe] with increasing milling time are reflected in the expansion of the cubic lattice parameter with milling durations. Fourier-transform infrared (FTIR) spectrum reveals that PbTe is a narrow bandgap semiconductor, and the bandgap shows a redshift with increased milling time and reduced crystallite size. This anomalous behaviour of the optical bandgap with reduced particle size is well explained by Urbach tailing due to increased defects in the more extended milled sample. The positions of the conduction and valence bands in the milled PbTe samples are determined for different milling times. The DC conductivity study also shows anomalous behaviour; electrical conductivity increases with particle size reduction in more extended milling time samples. It is resolved by considering that the carrier concentration increases with a longer milling time due to induced anti-site defects while milling. It results in bandgap reduction with milling durations and shows the predominant effect of electronic conductivity over ionic conductivity. The conduction mechanism is explained as per the Pertritz model, and the dependence of activation energies on particle size and temperature has been explained in detail. The Vickers hardness of the milled samples is determined, and its variation with grain size is explained with the Hall-Petch relation. The improved electrical conductivity with increased milling is also manifested in the measured hardness values.

Phase transformation and amorphization resistance in high-entropy MAX phase M2SnC (M = Ti, V, Nb, Zr, Hf) under in-situ ion irradiation

Chemical complexity significantly affects structures and properties in materials, such as high-entropy alloys and oxides. In this study, we firstly studied the radiation effects in high-entropy MAX phases, M2SnC (M=Ti, V, Nb, Zr, Hf), irradiated by 800 keV Kr2+ ions coupling with an in-situ transmission electron microscopy. Phase transformation of the initial hexagonal phase to intermediate γ phase and amorphization was observed during irradiation in both Ti2SnC and (TiVNbZrHf)2SnC using selected area electron diffraction (SAED) and high-resolution TEM (HRTEM) imaging. By comparing the structural evolution in these two materials under the same irradiation condition, the high-entropy MAX phase exhibits better tolerance to irradiation-induced phase transformation and amorphization than Ti2SnC. The roles of chemical complexity on the susceptibilities of these materials to structural evolution were elucidated by ab initio calculations. The M-Sn (M = Ti, V, Nb, Zr, Hf) antisite defect formation energy in the (TiVNbZrHf)2SnC is lower than that in Ti2SnC due to the chemical complexity. Thus, (TiVNbZrHf)2SnC is prone to accommodate more point defects and maintain the lattice structure during irradiation. This study provides a comprehensive understanding of structural evolution in high-entropy MAX phases and proposes a new approach to searching MAX phases with outstanding radiation tolerance.

Controlling the Defects of Cs2AgBiBr6 by Varied Precursor Compositions

The amount and type of defects in Cs2AgBiBr6 are controlled by varying the Ag/Bi ratio of the precursor solutions in two different synthesis routes, that is, slow solution cooling crystallization and fast microwave‐assisted hydrothermal synthesis. The correlation between the Ag/Bi ratio in the precursor solution and defect formation in the crystals is studied by band broadening analysis in Raman spectroscopy, the estimated Urbach energy in UV–vis spectroscopy, and thermogravimetric analysis. Ag‐rich precursors are found to prevent the formation of the secondary‐phase Cs3Bi2Br9, but at the same time induced the formation of Br vacancies and antisite defects. Time‐resolved photoluminescence measurements reveal that the formation of beneficial defects such as Br vacancies causes to trap the charge carriers, thus avoiding the recombination of charge carriers and leading to a longer carrier lifetime. Herein, the findings provide a guidance to decrease the defect densities and can be applied to the fabrication of Pb‐free solar cells based on Cs2AgBiBr6.

Synthesis of N-type Bi2Te2.7Se0.3 Compounds through Oxide-Reduction Process and Related Thermoelectric Transport Properties

We present a study on the synthesis of Bi2Te2.7Se0.3 powders through an oxide reduction process and also on the effect of the oxide reduction process on the thermoelectric transport properties of the resulting Bi2Te2.7Se0.3 compounds. Starting materials of Bi2O3, TeO2 and SeO2 were mechanical milled for homogeneous mixing, and then oxidized to prepare complex oxide powders. The complex oxide powders were reduced at different reduction temperatures under hydrogen atmosphere, and a single phase of Bi2Te2.7Se0.3 could be achieved in the reduced powder, with shorter reduction times as the reduction temperature was increased. The shape of the synthesized powder particles changed from granules to flakes with increasing reduction temperature and longer reduction time. Bi2Te2.7Se0.3 powders with different shapes were consolidated by spark plasma sintering (SPS). The SPSed Bi2Te2.7Se0.3 compounds exhibited anisotropic n-type thermoelectric transport properties along the vertical and parallel planes with respect to the pressing direction of the SPS. The degree of anisotropy in the thermoelectric properties was quite consistent with the degree of anisotropy in the microstructure, which originated from the anisotropic shapes in the Bi2Te2.7Se0.3 powders depending on the reduction conditions. Detailed thermoelectric transport properties of the n-type Bi2Te2.7Se0.3 compounds were interpreted in terms of anti-site defect formation and their structural anisotropy.

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.