Formation Of Antisite Defects Research Articles

Role of Antisite Disorder on Preamorphization Swelling in Titanate Pyrochlores

Ion irradiation experiments and atomistic simulations were used to demonstrate that irradiation-induced lattice swelling in a complex oxide, Lu2Ti2O7, is due initially to the formation of cation antisite defects. X-ray diffraction revealed that cation antisite formation correlates directly with lattice swelling and indicates that the volume per antisite pair is approximately 12 Å3. First principles calculations revealed that lattice swelling is best explained by cation antisite defects. Temperature accelerated dynamics simulations indicate that cation Frenkel defects are metastable and decay to form antisite defects.

Disorder trapping during crystallization of theB2-ordered NiAl compound

Using molecular dynamics simulations, disorder trapping associated with solidification is studied for the (100), (110), and (111) growth directions in the B2 NiAl ordered alloy compound. At the high interface velocities studied we observe pronounced disorder and defect trapping, i.e., the formation of antisite defects and vacancies in the crystal at higher than equilibrium concentrations upon rapid solidification. The vacancies are located primarily on the Ni sublattice and the majority of antisite defects are Ni atoms on the Al sublattice, while the concentration of Al on the Ni sublattice is negligibly small. The defect concentration is found to increase in an approximately linear relationship with increasing the interface velocity. Further there is no significant anisotropy in the defect concentrations for different interface orientations. Our results suggest that the currently available models of disorder trapping should be extended to include both antisite defects and vacancies.

Hydrothermally synthesized LiFePO4 crystals with enhanced electrochemical properties: simultaneous suppression of crystal growth along [010] and antisite defect formation

Improving electrochemical properties of hydrothermally synthesized LiFePO(4) powders is of immense technological significance and has been a subject of much scientific inquiry for many years. As reported previously, reversing the feeding sequence of starting materials and/or introducing ethylene glycol (EG) could significantly improve the electrochemical performance of hydrothermally synthesized LiFePO(4). However, the mechanism remains unclear. Here, we report a systematic study to understand the mechanism from viewpoints of crystal growth and defect concentration control. Combining the results of experimental and theoretical investigations, the improvement in electrochemical performance is attributed to simultaneous suppression of crystal growth along the [010] direction and reduced defect concentration of the antisite. The reduction in antisite defects is readily monitored by significant red shift of the infrared (IR) absorption band around 1000 cm(-1) which is assigned to the symmetric stretching P-O vibration of the PO(4) tetrahedron, as indicated by theoretical calculation. With this knowledge in mind, an output as high as 450 g L(-1) (autoclave volume), and an enhanced specific discharge capacity of 165 A h kg(-1) (close to the theoretical unity of 170 A h kg(-1)) at 0.1 C are achieved.

Electrical properties of PbTe single crystals with excess tellurium

The effects of excess (up to 0.1 at %) Te atoms and heat treatment at 473 and 573 K for 120 h on the conductivity σ, thermopower α, and Hall coefficient R of PbTe single crystals are studied. It is shown that excess Te atoms and annealing strongly affect the values and character of the temperature dependences of these parameters and the signs of α and R at low temperatures, which is caused by the acceptor effect of these atoms and the formation of antisite defects due to localization of Te in vacancies of the lead sublattice upon annealing.

Li Ion Diffusion Mechanisms in LiFePO4: An ab Initio Molecular Dynamics Study

The mechanisms for thermal (self) diffusion of Li ions in fully lithiated LiFePO(4) have been investigated with spin polarized ab initio molecular dynamics calculations. The effect of electron correlation is taken into account with the GGA+U formalism. It was found that Li ion diffusion is not a continuous process but through a series of jumps from one site to another. A dominant process is the hopping between neighboring Li sites around the PO(4) groups, which results in a zigzag pathway along the crystallographic b-axis. This observation is in agreement with a recent neutron diffraction experiment. A second process involves the collaborative movements of the Fe ions leading to the formation of antisite defects and promotes Li diffusion across the Li ion channels. The finding of the second mechanism demonstrates the benefit of ab initio molecular dynamics simulation in sampling diffusion pathways that may not be anticipated.

In Situ Hydrothermal Synthesis of LiFePO4 Studied by Synchrotron X-ray Diffraction

The development of high capacity, safe lithium battery materials requires new tools to better understand how reaction conditions affect nucleation and crystallization, particle size, morphology, and defects. We present a general approach for studying the synthesis of Li battery electrode materials in real time. The formation of LiFePO4 was investigated by time-resolved in situ synchrotron X-ray diffraction under hydrothermal conditions, and the reaction kinetics were determined by changes of the Bragg reflections. We provide the first evidence in support of a dissolution–reprecipitation process for the formation of LiFePO4, which occurs at temperatures as low as 105 °C and appears to be a three-dimensional diffusion-controlled process. Lattice parameters and their evolution were monitored in situ, as well as the formation of antisite defects and their subsequent elimination under various synthesis conditions. The ability to characterize and tailor synthesis reactions in situ is essential for rapid optimiz...

In SituObservation of Antisite Defect Formation during Crystal Growth

Insitu x-ray diffraction (XRD) coupled with molecular dynamics (MD) simulations have been used to quantify antisite defect trapping during crystallization. Rietveld refinement of the XRD data revealed a marked lattice distortion which involves an a axis expansion and a c axis contraction of the stable C11b phase. The observed lattice response is proportional in magnitude to the growth rate, suggesting that the behavior is associated with the kinetic trapping of lattice defects. MD simulations demonstrate that this lattice response is due to incorporation of 1% to 2% antisite defects during growth.

Optical properties of GaSb(001)‐c(2 × 6): The role of surface antisite defects

AbstractWe consider the formation of surface antisite defects on a previously proposed model for the GaSb(001)‐c(2 × 6) surface. Based on ab initio total energy calculations, we show how these defects stabilize the otherwise metallic surface and how their formation is driven by the excess charge associated with the Sb‐rich surface conditions. The surface‐sensitive optical technique of reflectance anisotropy spectroscopy is shown to be crucial for detecting the defects, and computation of spectra yields a good agreement with experiment when defects are included in the surface reconstruction.

Spontaneous Formation of Surface Antisite Defects in the Stabilization of the Sb-Rich GaSb(001) Surface

This Letter solves the long-standing puzzle [Phys. Rev. Lett. 79, 693 (1997)] of why GaSb(001) apparently violates the electron counting rule (ECR) in forming a reconstruction featuring long Sb-dimer chains, rather than the c(4x4) reconstruction found in all other arsenide and antimonide III-V compounds in the V-rich regime. We find that an alternative strategy, that in fact satisfies the ECR, is followed by the Sb-rich GaSb(001) surface, whereby long Sb-dimer chains are stabilized by randomly distributed subsurface Ga antisite defects. The excess of surface Sb drives the defect formation that in turn stabilizes the surface in a metastable phase. The transition to the c(4x4) reconstruction, where the ECR is instead satisfied through missing dimers, is therefore inhibited. Our conclusions are supported by ab initio simulations of experimental reflectance anisotropy spectra.

Constitutional and thermal defects in B8 2–SnTi 2

Vacancy and antisite defect formation energies in B8 2–SnTi 2 are calculated by an ab initio approach. Three sublattices are introduced to account for the B8 2 structure. A statistical model based on a mean-field approximation is developed in the canonical ensemble. The defect concentrations are calculated as function of temperature and deviation from stoichiometry. For stoichiometric B8 2–SnTi 2 alloys, the dominant thermal defects are composed of one antisite Ti atom and three Ti vacancies. In the Sn-rich B8 2–SnTi 2, the constitutional defects are Ti vacancies; the thermal defect below 1000 K is an interbranch where Ti vacancies are replaced by Sn antisites; at high temperatures, it is a four point-defect comprising one Ti antisite and three Ti vacancies. In the Ti-rich B8 2–SnTi 2, the constitutional defects are antisite Ti atoms and the thermal defect is a four point defect comprising one Ti antisite and three Ti vacancies. The effective defect formation enthalpies are derived at low temperature. The Gibbs energy as well as the Sn and Ti chemical potentials in B8 2–SnTi 2 phase are obtained as function of composition for various temperatures. The extension of the one-phase domain of B8 2–SnTi 2 in the Sn–Ti phase diagram is discussed.

Cation diffusion in magnesium aluminate spinel

The mechanisms by which Mg(2+) and Al(3+) ions are transported through the MgAl(2)O(4) spinel lattice are investigated using atomic scale computer simulation. Both vacancy and interstitial cation processes are considered. Stable vacancies can be generated on either the magnesium or aluminium sublattices but the Mg(2+) and Al(3+) cation interstitials are most stable when located in split form with another Mg(2+) ion about a vacant Mg(2+) site. The pathways for diffusion of defects both via vacancy and interstitial mechanisms are analysed in detail with calculation of the energy barriers and the associated exponential prefactors. The results show that vacancies can be exchanged between the two sublattices resulting in the formation of antisite defects (though these processes have a high activation energy); that the Mg(2+) ions are more mobile than the Al(3+) ions and that the preferred mechanism for Al(3+) ion diffusion is via a vacancy mechanism on the magnesium sublattice. Although the calculated values of the prefactors can differ in size by an order of magnitude, in this system it is the relative size of the energy barriers that dominate the diffusion rates. (C) 2008 Elsevier B.V. All rights reserved.

Structural ordering and antisite defect formation in double perovskites

We formulate an effective model for B-B′ site ordering in double perovskite materials A2BB′O6. Even within the simple framework of lattice-gas type models, we are able to address several experimentally observed issues including nonmonotonic dependence of the degree of order on annealing temperature, and the rapid decrease of order upon overdoping with either B or B′ species. We also study ordering in the “ternary” compounds A2BB′1−yB″yO6. Although our emphasis is on the double perovskites, our results are easily generalizable to a wide variety of binary and ternary alloys.

Constitutional and thermal defects in D0 19-SnTi 3

Vacancy and antisite defect formation energies in D0 19-SnTi 3 are calculated by an ab initio approach. Based on a statistical-thermodynamic model, the defect concentrations are calculated as function of temperature and deviation from stoichiometry. The constitutional defects are antisite Sn atoms and antisite Ti atoms in Sn-rich and Ti-rich SnTi 3, respectively. The dominant thermal defects are composed of two antisites for stoichiometric alloys. For Sn-rich D0 19-SnTi 3 alloys, the thermal defect is an interbranch where one antisite Sn atom is replaced by four Ti vacancies. For Ti-rich D0 19-SnTi 3 alloys, the thermal defect is a five point defect comprising one antisite Ti and four Ti vacancies. The effective defect formation enthalpies are derived for different concentration regions of D0 19-SnTi 3. The Gibbs energy as well as the Sn and Ti chemical potentials in the D0 19-SnTi 3 are obtained as function of composition for various temperatures. The extension of the one phase domain of D0 19-SnTi 3 in the Sn–Ti phase diagram is discussed.

P-type Doping and Light Emitting Junction Formation in ZnO

The transport properties of as-deposited and rapid thermal annealed phosphorus-doped ZnO films grown by pulsed laser deposition are reported. As-grown ZnO:P samples show n-type characteristics, presumably due to the formation of anti-site PZn defects. Rapid thermal annealing yields a carrier type conversion from n- to p-type for the ZnO:P films grown at ~ 700ºC; samples grown at substantially lower or higher temperatures tend to remain n-type even after thermal annealing process. The properties and behavior of the n-to-p conversion is most consistent with the formation of PZn-2VZn as the active acceptor state. Research efforts have included the formation and characterization of ZnO pn junctions on c-plane sapphire. Using the p-type ZnO:P films, we have realized near band edge light emission from ZnO:P/ZnMgO/ZnO/ZnMgO/ZnO:Ga epitaxial heterostructures grown on c-plane sapphire.

Diffusion of Re and Ru in the γ′ Phase of Ni Based Alloys

The diffusion of Re and Ru in the � 0 -Ni3Al phase has been investigated at a range of temperatures 1423–1523 K by using the pseudo-binary diffusion couples. It was found that the concentration gradient of Re appeared to promote the uphill diffusion of Al, whereas that of Ru did not promote the Al uphill diffusion. The cross interdiffusion coefficients of ~ D Ni and ~ D Ni were independent of the matrix ordering since they were comparable qualitatively between those in the � 0 -Ni3Al and in the � -Ni phases. The tracer diffusion coefficients for Re and Ru in Ni3Al were estimated by extrapolation and the obtained activation energies were consistent with the site preferences of the elements reported in the literature. Further, the results of this work suggest that the diffusion of Ru in Ni3Al is mostly governed by the diffusion in Ni sublattice and the diffusion of Re is mainly controlled by the formation of anti-site defects in the Ni sublattice with negligible contribution of the anti-site bridge mechanism. [doi:10.2320/matertrans.MRA2007632]

An Ab Initio-based Approach to the Behavior of Ga Atoms on the GaAs Surfaces

Adsorption-desorption behavior of Ga on GaAs surfaces such as (001) and (111)B is systematically investigated using our ab initio-based approach, which incorporates chemical potentials in the gas phase of Ga atom or As molecules as a function of temperature and their beam equivalent pressure. Surface phase diagram calculations of the (001)-c(4×4) reveal that the c(4×4)α consisting of Ga-As dimers independently appears on As2 and As4 in the gas phase, whereas the c(4×4)β with As dimers is destabilized under As4. On the basis of this finding, surface phase transition between c(4×4) and (2×4)β2 is discussed in terms of 0.5 monolayer of Ga predeposition. Furthermore, surface phase diagram calculations of the (111)B-(2×2) imply that Ga adsorption induces As trimer desorption on the (2×2) surface. This suggests that Ga adatom acts as a self-surfactant atom to maintain layer-by-layer growth of GaAs without anti-site defect formation.

Behavior of rapid thermal annealed ZnO:P films grown by pulsed laser deposition

The transport properties of as-deposited and rapid thermal annealed phosphorus-doped ZnO films grown by pulsed laser deposition are reported. As-grown ZnO:P samples showed n-type characteristics, presumably due to the formation of antisite PZn defects. Rapid thermal annealing yielded a carrier-type conversion from n- to p-type for the ZnO:P films grown at ∼700 °C; samples grown at substantially lower or higher temperatures tended to remain n-type even after the thermal annealing process. The properties and behavior of the n-to-p conversion are most consistent with the formation of PZn-2VZn as the active acceptor state. Variable magnetic field Hall measurements confirmed the p-type behavior. Phosphorus doping concentrations in the range of 0.5−1.0 at. % were considered, with evidence for P segregation in the higher phosphorus concentrations.

Variation in lattice parameters of 6H-SiC irradiated to extremely low doses

Irradiation of 6H-SiC single crystals was performed using 4MeV H+ ions at 340 and 210K. The changes in lattice parameters in the basal plane and along the c axes were measured as a function of dose using high-resolution x-ray diffraction. The c-axis lattice parameter increases monotonically with the increasing dose, while a-axis lattice parameter decreases at extremely low doses. An initial volumetric contraction of the unit cell is observed. The decrease in the a parameter may originate from the irradiation-induced vacancies and the possible formation of antisite defects that cause the lattice structure on the basal plane to shrink.

Effects of structural disordering in the manganite La0.825Ba0.175MnO3

Neutron and X-ray diffraction have been used to study the structural state of polycrystalline samples of the perovskite-like manganite La0.825Ba0.175MnO3 with different concentrations of antisite defects produced by fast-neutron irradiation. It has been established that the concentration of antisite defects depends linearly on the fraction of the sample volume affected by cascades of atom-atom displacements, reaching at high fluences a value equal to 0.0875, which is “limiting” for a given composition. The formation of antisite defects is accompanied by the appearance of significant static uncorrelated ion displacements, which for oxygen ions become greater than their tilting displacements. It has been shown that in this case, the tilting mode ϕ, which forms the rhombohedral structure of the manganite, is not destructed completely, although the magnitude of correlated tilting displacements decreases noticeably.

Study of the point defect, Re site, and Re cluster in Re embedded Ni3Al using the embedded atom method

An analytical embedded atom method model of Ni-Al-Re system is constructed on the basis of the physical properties of pure constituents Ni, Al, and Re. The characteristic properties of Ni3Al, including equilibrium lattice constant, elastic constants, cohesive energy, vacancy formation energy, and antisite defect formation energy, are calculated. The types of point defects in Ni3Al are discussed, and it is shown that the point defect is the antisite defect in the nonstoichiometric case. Furthermore, the occupation site of Re in Ni3Al is analyzed, the clustering of Re in Ni3Al and Ni are also studied. The calculation results show that the occupational site of Re is the Al site, and that the growth tendency of Re cluster becomes weak when the size of Re cluster reaches to 11?. The present calculations are in agreement with the experiment and the theoretical results obtained by other authors.