Formation Of Antisite Defects Research Articles

Engineering Solar Cell Absorbers by Exploring the Band Alignment and Defect Disparity: The Case of Cu‐ and Ag‐Based Kesterite Compounds

The development of kesterite Cu2ZnSn(S,Se)4 thin‐film solar cells is currently hindered by the large deficit of open‐circuit voltage (Voc), which results from the easy formation of CuZn antisite acceptor defects. Suppressing the formation of CuZn defects, especially near the absorber/buffer interface, is thus critical for the further improvement of kesterite solar cells. In this paper, it is shown that there is a large disparity between the defects in Cu‐ and Ag‐based kesterite semiconductors, i.e., the CuZn or CuCd acceptor defects have high concentration and are the dominant defects in Cu2ZnSn(S,Se)4 or Cu2CdSnS4, but the AgZn acceptor has only a low concentration and the dominant defects are donors in Ag2ZnSnS4. Therefore, the Cu‐based kesterites always show p‐type conductivity, while the Ag‐based kesterites show either intrinsic or weak n‐type conductivity. Based on this defect disparity and calculated band alignment, it is proposed that the Voc limit of the kesterite solar cells can be overcome by alloying Cu2ZnSn(S,Se)4 with Ag2ZnSn(S,Se)4, and the composition‐graded (Cu,Ag)2ZnSn(S,Se)4 alloys should be ideal light‐absorber materials for achieving higher efficiency kesterite solar cells.

Energetics and electronic structure of UAl4 with point defects

UAl4 ideal and defect structures were studied within the framework of the Density Functional Theory. The structural and magnetic ordering of UAl4 in paramagnetic, ferromagnetic, and antiferromagnetic states have been investigated, within the collinear and non-collinear spin approximation, using the GGA model, as embedded in the program package VASP. An antiferromagnetic (01¯1) layer structure with spins aligned to the [010] direction was found to be energetically preferred. The analysis of density of states and bonding charge density point out that the bonding mechanism consists primarily of band mixing between the U 5f and Al 3p states. Supercells were built from UAl4 unit cells with the established magnetic structure. For those supercells we calculated the energy of formation of vacancies and antisite defects taking into account the existence of three distinct aluminum sites. Point defect formation energies, local lattice relaxations, as well as the defect induced magnetic ordering and electronic density redistribution, are discussed. It is shown that antiferromagnetism is locally broken. Al antisites and U antisites in Al 4e Wyckoff positions are the constitutional point defects in Al-rich and U-rich oI20 UAl4, respectively. In this way we have presented here the first set of data which makes it possible to discuss and quantify the point defects concentrations in the experimental composition range for existence of this uranium aluminide.

Sodiation Kinetics of Metal Oxide Conversion Electrodes: A Comparative Study with Lithiation.

The development of sodium ion batteries (NIBs) can provide an alternative to lithium ion batteries (LIBs) for sustainable, low-cost energy storage. However, due to the larger size and higher m/e ratio of the sodium ion compared to lithium, sodiation reactions of candidate electrodes are expected to differ in significant ways from the corresponding lithium ones. In this work, we investigated the sodiation mechanism of a typical transition metal-oxide, NiO, through a set of correlated techniques, including electrochemical and synchrotron studies, real-time electron microscopy observation, and ab initio molecular dynamics (MD) simulations. We found that a crystalline Na2O reaction layer that was formed at the beginning of sodiation plays an important role in blocking the further transport of sodium ions. In addition, sodiation in NiO exhibits a "shrinking-core" mode that results from a layer-by-layer reaction, as identified by ab initio MD simulations. For lithiation, however, the formation of Li antisite defects significantly distorts the local NiO lattice that facilitates Li insertion, thus enhancing the overall reaction rate. These observations delineate the mechanistic difference between sodiation and lithiation in metal-oxide conversion materials. More importantly, our findings identify the importance of understanding the role of reaction layers on the functioning of electrodes and thus provide critical insights into further optimizing NIB materials through surface engineering.

Probing Electrochemically-Induced Structural Changes and Defects Affecting Li-Ion Intercalation and De-Intercalation in High Capacity Orthosilicate Cathodes

Li-ion batteries (LIBs) have already made tremendous progress supplying power for multiple devices used in our daily life. Now LIBs are gradually entering the plug-hybrid and fully electric vehicles. Safety and high specific capacity enabling long range driving between charges are among industry’s priorities in this domain. One cathode material group that potentially can provide solutions to these priorities is lithium metal silicates. Orthosilicates, Li2 M SiO4, where M = Fe, Mn, Co, Ni, are characterized by a theoretical specific capacity that is twice that of LiFePO4, namely 340 vs. 170 mAh/g1 ,2. In this presentation, we report on the synthesis and electrochemical/structural evaluation of different phase (monoclinic and orthorhombic) Li2(Fe,Mn)SiO4 materials using a combination of organic-assisted hydrothermal and reducing annealing treatment steps. During the electrochemical process, we find that the Li2FeSiO4 cathode demonstrates rate- and phase- dependent capacity changes upon lithiation/delithiation3. We have probed these changes by conducting both in-situ/postmortem synchrotron XRD/XANES and first-principle calculations of the structure/phase stabilities and Li ion diffusion properties of monoclinic Li2FeSiO4 orthosilicate. It is revealed that formation of Li-Fe antisite defects play a key role in destabilizing the orthosilicate structure and “catalyzing” the monoclinic-to-orthorhombic transition. Furthermore the charge compensation mechanism that is essential in achieving more than one Li extraction is discussed accordingly as well as the Li-ion diffusion properties are analyzed. The rich insight obtained from these Li2FeSiO4 studies is invaluable as we move towards the design and testing of stable orthosilicate structures with fully reversible 2-Li intercalation capacity for high-energy next generation LIB cathodes.

Defect formation energies in A2B2O7 pyrochlores

We performed ab initio calculations of the cation antisite and anion Frenkel pair defect formation energies for a series of pyrochlores. The obtained energies correlate well with the stability field of pyrochlore. We found that in the compounds that tend to crystallize as a disordered fluorite, the anion Frenkel pair defect formation energy is negative. This correlation suggests that the low energy cost for defect accumulation is one of the driving forces behind the order–disorder transition in pyrochlores.

Effect of Doping with Substituent Bi Atoms on the Electrical Transport Properties of a Bi0.4Sb1.6Te3 Film Fabricated by Molecular Beam Epitaxy

A highly crystalline Bi0.4Sb1.6Te3 film was fabricated on a Si substrate by molecular beam epitaxy (MBE) at a substrate temperature of 280°C. On the basis of study of x-ray diffraction patterns and high-resolution transmission electron microscopy lattice fringes it was inferred that Bi atoms were successfully incorporated into Sb lattice sites, forming substituent Bi impurities. Reduction of the carrier concentration was ascribed to the increased resistance to formation of antisite defects when Sb was substituted by Bi. The reduced mobility was a result of enhanced grain boundary scattering and attraction by substituent Bi atoms. Analysis of temperature-dependent electrical transport properties revealed that introduction of Bi atoms resulted in deeper energy level impurities in the Bi0.4Sb1.6Te3 film and higher activation energy (43.2 meV) than the normal value at room temperature, leading to semiconductor characteristics of the film in the temperature range −50°C to 150°C.

Revealing Electrochemically Induced Antisite Defects in LiCoPO4: Evolution upon Cycling

This article aims to reveal the formation of antisite defects that are induced in LiCoPO4 crystals upon electrochemical charge/discharge cycles. This is achieved using Cs-corrected high-angle annular dark-field scanning transmission electron microscopy that allows their direct visualization. By comparison with simulated images, their evolution is discussed and their quantification performed. In a sample free of defects, a disordering caused by the exchange between lithium and cobalt atoms is progressively created. It is the first time that evidence of antisite defect creation in an olivine-type compound upon electrochemical cycling has been reported. Their formation is shown to occur during the charging process. While they are heterogeneously distributed after the first charge/discharge cycle because of their concentration, such exchange defects appear to be more homogeneously dispersed in the lattice when their amount is much larger after the 30th charge/discharge cycle. This article provides new insight...

Studies on sulfur doping and figure of merit in vapor grown Sb2Te3 platelet crystals

A series of p-type Sb2Te3−xSx (x=0, 0.1, 0.2, 0.3 and 0.4) samples for each sulfur composition was vapor deposited to explore the crystal growth mechanism of platelet morphology and to understand their potential use for thermoelectric devices. Structural characterization performed by X-ray diffraction exhibited consistent rhombohedral unit cell parameters for all the antimony telluride crystals with D53d space group symmetry. Scanning electron microscopy (SEM) and energy dispersive analysis by X-rays (EDAX) were employed for morphological and compositional studies. The Seebeck coefficient S (⊥c) of Sb2Te3−xSx crystals for sulfur composition, x=0.3 was remarkably improved, yielding a figure of merit (ZT)=0.54, which is ~2.5 fold rise compared to the melt grown Sb2Te3 crystals. The atomic force microscopic (AFM) imprints revealed that, with increasing content of dopant, distribution of etch pits and microhardness increased in sulfur doped crystals. By optimizing sulfur doping, the antisite (AS) defect formation can be suppressed for enhancing ZT and the results obtained enable to design novel thermoelectric materials for application in power generation and refrigeration.

Antisites in III-V semiconductors: Density functional theory calculations

Density functional based simulation, corrected for finite size effects, is used to investigate systematically the formation of antisite defects in III-V semiconductors (III = Al, Ga, and In and V = P, As, and Sb). Different charge states are modelled as a function of the Fermi level and under different growth conditions. The formation energies of group III antisites (IIIVq) decrease with increasing covalent radius of the group V atom though not group III radius, whereas group V antisites (VIIIq) show a consistent decrease in formation energies with increase in group III and group V covalent radii. In general, IIIVq defects dominate under III-rich conditions and VIIIq under V-rich conditions. Comparison with equivalent vacancy formation energy simulations shows that while antisite concentrations are always dominant under stoichiometric conditions, modest variation in growth or doping conditions can lead to a significantly higher concentration of vacancies.

Investigation of combinatorial coevaporated thin film Cu2ZnSnS4 (II): Beneficial cation arrangement in Cu-rich growth

Cu2ZnSn(S,Se)4 (CZTSSe) is an earth-abundant semiconductor with potential for economical photovoltaic power generation at terawatt scales. In this work, we use Raman scattering to investigate phase coexistence in combinatorial CZTS thin films grown at 325 or 470 °C. The surface of the samples grown at 325 °C is rough except for a prominent specularly reflective band near and along the ZnS-Cu2SnS3 (CTS) tie line in the Cu-Zn-Sn-S quaternary phase diagram. All structurally incoherent secondary phases (SnS2, CuS) exist only as surface phases or are embedded as separate grains, whereas the structurally coherent secondary phase CTS coexists with CZTS in the dense underlying film. In films grown at 325 °C, which are kinetically trapped by the low growth temperature, a change is observed in Cu and Sn site occupancy, evidenced by the shift from cubic-CTS in the Cu-rich region (Cu/Sn > 2) to more tetragonal-CTS in the Sn-rich region (Cu/Sn < 2). For CZTS samples grown at 470 °C, CTS is not observed and regions grown under excess Sn flux are more disordered than Cu-rich regions evidenced by broader CZTS A mode peaks. Therefore, increasing Sn chemical potential results in more CZTS lattice disorder, suggesting, with other evidence, the formation of Sn antisite defects. In contrast, the CZTS A mode breadth is insensitive to Zn richness suggesting that excess Zn does not induce significant disorder within the CZTS lattice. We postulate that initially growing CZTS films Cu-rich (Cu/Sn > 2) results in higher cation ordering meaning fewer antisite defects.

Thermoelectric properties of Cu2Ga4Te7 based compounds with Zn substitution for Cu and Ga

We present two series of Cu2Ga4Te7‐based compounds with Zn substituting for Cu (Cu‐poor) and Ga (Ga‐poor), and have observed enhanced thermoelectric performance. Generally, the Seebeck coefficient (α) and lattice thermal conductivity (κL) increase and electrical conductivity (σ) decreases with the Zn content increasing. The variation in electrical properties might be mainly due to the formation of antisite defects ( and ), the acceptors. The defects increase the local density of states (DOS) near the Fermi level and effective mass, leading to the increase in α, while on the other hand, they widen the bandgap (Eg) leading to the degradation of the electrical conductivity. The gradual enhancement in κL is attributed to the reduced phonon scattering resulting from the decrease in vacancy concentration. As a result, we attained the maximum ZT value of 0.46 for Cu‐CGT and 0.47 for Ga‐CGT at ∼770 K with a proper Zn content, which is about 20–22% higher than that of intrinsic Cu2Ga4Te7.

Experimental approach and atomistic simulations to investigate the radiation tolerance of complex oxides: Application to the amorphization of pyrochlores

Both experimental approach and atomistic simulations are performed in order to investigate the influence of the composition of pyrochlores on their radiation tolerance. Therefore, Gd2Ti2O7 and Gd2Zr2O7 were irradiated with 4MeV Au and 92MeV Xe ions in order to study the structural changes induced by low and high-energy irradiations. XRD results show that, for both irradiations, the structural modifications are strongly dependent on the sample composition: Gd2Ti2O7 is readily amorphized, whereas Gd2Zr2O7 is transformed into a radiation-resistant anion-deficient fluorite structure. Using atomistic simulations with new interatomic potentials derived from the SMTB-Q model, the lattice properties and the defect formation energies were calculated in Gd2Ti2O7 and Gd2Zr2O7. Calculations show that titanates have a more covalent character than zirconates. Moreover, in Gd2Ti2O7 the formation of cation antisite defects leads to strong local distortions around Ti-defects and to a decrease of the Ti coordination number, which are not observed in Gd2Zr2O7. Thus, the radiation resistance is related to the defect stability: the accumulation of structural distortions around Ti-defects could drive the Gd2Ti2O7 amorphization induced by irradiation.

Non-stoichiometry effect and disorder in Cu2ZnSnS4 thin films obtained by flash evaporation: Raman scattering investigation

The cation disorder in Cu2ZnSnS4 thin films grown by flash evaporation of ZnS, CuS and SnS binary compounds has been studied by Raman spectroscopy. Process parameters such as the substrate temperature during the evaporation and the Ar pressure in the post-thermal treatment determined the samples’ composition and Raman spectra. As a measure of cation disorder, the half-width and relative intensity of the Raman band peaking at 331–332cm−1 is analysed. Comparison of the spectra for different samples of known composition showed that the relative intensity of the 331cm−1 defect peak correlates with the previously reported theoretical prediction about enhancement of antisite defect formation in Cu2ZnSnS4 under “Cu-poor, Zn-rich” conditions. For “Cu-rich, Zn-poor” films, further experimental confirmation was obtained of the previously detected effect of the enhancement of cation disorder under intense optical excitation.

The structural transitions of Ti3AlC2 induced by ion irradiation

The structural transitions of Ti3AlC2 induced by ion irradiation were investigated over a wide fluence range by transmission electron microscopy. No amorphization occurs even at the highest dose of 31dpa, indicating a great tolerance to irradiation-induced amorphization. Dynamic electron diffraction simulations and high-resolution observations indicate that the nanolamellar structure of Ti3AlC2 is readily destroyed through the formation of antisite defects and a phase transformation from α-Ti3AlC2 to β-Ti3AlC2 occurs at 2.61dpa. A great number of stacking faults in basal planes are formed with increasing fluence, leading to the formation of nano Ti3AlC2 grains with different stacking sequences at 10.45dpa. Serious structural damage and polygonization are observed at the highest dose of 31dpa. Due to the similar structural transition process with some complex oxides (pyrochlore and murataite), it is assumed that the great irradiation tolerance of Ti3AlC2 results from the low formation energy of antisite defects. These findings first clarify the structural transition mechanism of Ti3AlC2 under ion irradiation and its relationship with irradiation tolerance, which is of vital importance in understanding the irradiation response of MAX phases and provides a clue in searching for materials with higher irradiation tolerance from MAX phases.

Defect chemistry of phospho-olivine nanoparticles synthesized by a microwave-assisted solvothermal process

Nanocrystalline LiFePO 4 powders synthesized by a microwave-assisted solvothermal (MW-ST) process have been structurally characterized with a combination of high resolution powder neutron diffraction, synchrotron X-ray diffraction, and aberration-corrected HAADF STEM imaging. A significant level of defects has been found in the samples prepared at 255 and 275 °C. These temperatures are significantly higher than what has previously been suggested to be the maximum temperature for defect formation in LiFePO 4 , so the presence of defects is likely related to the rapid MW-ST synthesis involving a short reaction time (∼5 min). A defect model has been tentatively proposed, though it has been shown that powder diffraction data alone cannot conclusively determine the precise defect distribution in LiFePO 4 samples. The model is consistent with other literature reports on nanopowders synthesized at low temperatures, in which the unit cell volume is significantly reduced relative to defect-free, micron-sized LiFePO 4 powders. Temperature-dependent antisite defect formation has been observed in nanocrystalline LiFePO 4 powders synthesized by a microwave solvothermal process, using high resolution diffraction and STEM imaging. • LiFePO 4 nanopowders synthesized by a microwave-assisted solvothermal process. • Defects directly observed by aberration-corrected HAADF STEM imaging. • Antisite defects present from synthesis at 255 and 275 °C. • Defects present from higher temperature synthesis than previously reported. • Powder diffraction data have been analyzed in detail for various defect models.

Effect of ball milling time on the thermoelectric properties of p-type (Bi,Sb)2Te3

P-type (Bi,Sb)2Te3 compounds were fabricated with high energy ball-milled powder and their thermoelectric properties were investigated as a function of the ball milling time. The fine grain sizes were confirmed with the X-ray analysis and the microscopy images in the samples fabricated with powders from long ball milling time. The values of the lattice thermal conductivity of the samples at high temperature show smaller values for long milling time (>24h) than those for short time, resulted from the finer grain sizes of the samples. As well as the microstructural change, the Seebeck coefficient and the electrical resistivity of the compounds were significantly altered with the ball milling time. The carrier concentration was dramatically increased in the samples after 24h milling, which was attributed to the formation of the antisite defects introduced by the accumulated thermal energy. The formation of antisite defects may be also promoted by the unintentional Fe doping from jars and balls. The highest value of ZT=1.14 was achieved at 323K for the samples with 10h. The temperature where the value of ZT was maximized and the values of ZT was varied with the ball milling time, which implies that the ball milling time should be carefully optimized for the suitable application of this compounds.

Thermodynamic stability of Co–Al–W L12γ′

Co-based superalloys in the Co–Al–W system exhibit coherent L12 Co3(Al,W) γ′ precipitates in an face-centered cubic Co γ matrix, analogous to Ni3Al in Ni-based systems. Unlike Ni3Al, however, experimental observations of Co3(Al,W) suggest that it is not a stable phase at 1173K. Here, we perform an extensive series of density functional theory (DFT) calculations of the γ′ Co3(Al,W) phase stability, including point defect energetics and finite-temperature contributions. We first confirm and extend previous DFT calculations of the metastability of L12 Co3(Al0.5W0.5) γ′ at 0K with respect to hexagonal close-packed Co, B2 CoAl and D019 Co3W using the special quasi-random structure (SQS) approach to describe the Al/W solid solution, employing several exchange/correlation functionals, structures with varying degrees of disorder, and newly developed larger SQSs. We expand the validity of this conclusion by considering the formation of antisite and vacancy point defects, predicting defect formation energies similar in magnitude to Ni3Al. However, in contrast to the Ni3Al system, we find that substituting Co on Al sites is thermodynamically favorable at 0K, consistent with experimental observation of Co excess and Al deficiency in γ′ with respect to the Co3(Al0.5W0.5) composition. Lastly, we consider vibrational, electronic and magnetic contributions to the free energy, finding that they promote the stability of γ′, making the phase thermodynamically competitive with the convex hull at elevated temperature. Surprisingly, this is due to the relatively small finite-temperature contributions of one of the γ′ decomposition products, B2 CoAl, effectively destabilizing the Co, CoAl and Co3W three-phase mixture, thus stabilizing the γ′ phase.

Ferrimagnetism, antiferromagnetism, and magnetic frustration in La2−xSrxCuRuO6(0⩽x⩽1)

We studied structural and magnetic properties of a series of insulating double perovskite compounds, La${}_{2\ensuremath{-}x}$Sr${}_{x}$CuRuO${}_{6}$ (0 $\ensuremath{\leqslant}$ $x$ $\ensuremath{\leqslant}$ 1), representing doping via $A$-site substitution. The end members La${}_{2}$CuRuO${}_{6}$ and LaSrCuRuO${}_{6}$ form in monoclinic structure while the intermediate Sr doped compounds stabilize in triclinic structure. The Cu and Ru ions sit on alternate $B$ sites of the perovskite lattice with $\ensuremath{\sim}15%$ antisite defects in the undoped sample while the Sr-doped samples show a tendency to higher ordering at $B$ sites. The undoped ($x$ $=$ 0) compound shows a ferrimagnetic-like behavior at low temperatures. In surprising contrast to the usual expectation of an enhancement of ferromagnetic interaction on doping, an antiferromagnetic-like ground state is realized for all doped samples ($x$ $>$ 0). Heat capacity measurements indicate the absence of any long-range magnetic order in any of these compounds. The magnetic relaxation and memory effects observed in all compounds suggest glassy dynamical properties associated with magnetic disorder and frustration. We show that the observed magnetic properties are dominated by the competition between the nearest-neighbor Ru-O-Cu 180${}^{\ensuremath{\circ}}$ superexchange interaction and the next-nearest-neighbor Ru-O-O-Ru 90${}^{\ensuremath{\circ}}$ superexchange interaction as well as by the formation of antisite defects with interchanged Cu and Ru positions. Our calculated exchange interaction parameters from first principles calculations for $x$ $=$ 0 and $x$ $=$ 1 support this interpretation.

Seebeck and Figure of Merit Enhancement in Nanostructured Antimony Telluride by Antisite Defect Suppression through Sulfur Doping

Antimony telluride has a low thermoelectric figure of merit (ZT < ∼0.3) because of a low Seebeck coefficient α arising from high degenerate hole concentrations generated by antimony antisite defects. Here, we mitigate this key problem by suppressing antisite defect formation using subatomic percent sulfur doping. The resultant 10-25% higher α in bulk nanocrystalline antimony telluride leads to ZT ∼ 0.95 at 423 K, which is superior to the best non-nanostructured antimony telluride alloys. Density functional theory calculations indicate that sulfur increases the antisite formation activation energy and presage further improvements leading to ZT ∼ 2 through optimized doping. Our findings are promising for designing novel thermoelectric materials for refrigeration, waste heat recovery, and solar thermal applications.

The effect of native point defect thermodynamics on off-stoichiometry in β-Mg17Al12

The mechanical strength of Mg–Al–Zn alloys can be affected by a fine spatial dispersion of β-Mg17Al12 precipitates in the Mg matrix. In an effort to understand the phase stability and the unusual asymmetric off-stoichiometry observed in β-Mg17Al12, we have performed a series of first-principles density functional theory (DFT) calculations of bulk and defect properties of Mg17Al12. Specifically, we consider native point defects (i.e. vacancies and anti-sites) in all four sublattices of Mg17Al12, i.e. 2a, 8c, 24g (Mg) and 24g (Al). The T=0K static energies of defect Mg17Al12 supercells indicate that anti-site defects are energetically favored over vacancies, and the lowest anti-site defect formation energies are in 24g sites for both AlMg and MgAl. These Al-rich and Mg-rich anti-site defect formation energies are similar in magnitude, and thus do not explain the asymmetric off-stoichiometry of Mg17Al12. We also investigate the effect of atomic vibrations via DFT phonon calculations on native point defect free energies of Mg17Al12 and combine these entropic contributions with the point defect formation energies to evaluate the thermodynamics of off-stoichiometry in this phase. We find that the formation of the AlMg anti-site is not strongly stabilized by vibrational entropy. Thus, we conclude that the observed asymmetry in the off-stoichiometry of the β-Mg17Al12 phase in the Mg–Al phase diagram is not explained by simple native point defect thermodynamics, and must involve a more complicated defect formation mechanism, such as multi-defect clustering.