Antistructure Defects Research Articles

High-Temperature Plasticity of the β-phase in Nearly-Equiatomic Nickel-Titanium Alloys

Plastic behaviour of the β nickel-titanium alloys with nearly-equiatomic compositions, Ti-49.5, 50.0 and 50.5 mol%Ni, has been studied. The alloys were highly ductile due to dislocation glide at temperatures where the β-phase was in equilibrium. The ductility became largest at the lowest temperature in the stability range of the β-phase. The strain-rate vs. flow stress relations of easy-glide deformation in tensile testing and steady-state creep deformation satisfied the power law. Both of the deformations are associated with the dislocation glide which are likely to be controlled by the Peierls mechanism. A yield drop at the onset of plastic flow appeared only in the tensile deformation of Ti-50.5 mol%Ni, but not in Ti-49.5 and 50.0 mol%Ni alloys. Excess nickel atoms would exist as antistructure defects, which might be responsible for the yield point phenomena.

Evidence of Granular Superconductivity in Pr2<-zCezSr2Cu2NbO10

Pr 2-z Ce z Sr2Cu2NbO10 (Pr222Nb10), is widely believed to not superconduct, although its homologues in which Pr is replaced by Nd, Sm, Eu, or Gd do (with Tc ≈ 28 K). On the basis of bulk resistivity, bulk diamagnetism, electron spin resonance, and surface resistance measurements of Pr222Nb10, we conclude that it does exhibit granular superconductivity (as predicted) with a critical temperature in the range 25–28 K, but is not currently a bulk superconductor. Neutron diffraction studies show that our Pr222Nb10 is multi-phase, but does not contain significant amounts of any (known) superconducting phase other than Pr222Nb10. It does contain ~ 23% PrSr defects, but far fewer SrPr impurities (<2%), so that the predicted antistructure defect ( SrPr , PrSr ) is probably not responsible for breaking Cooper pairs and destroying superconductivity, but the important half of it, PrSr , almost certainly is.

Solute diffusion of Al-substituting elements in Ni 3Al and the diffusion mechanism of the minority component

Solute diffusion of Al-substituting elements (Ga, Ge, Ti, and Nb) has been investigated in Ni 3Al single crystals with the composition Ni 75.9Al 24.1 in a wide temperature range. The concentration profiles were determined by secondary ion mass spectrometry (SIMS). The experimental data were analyzed to establish the governing diffusion mechanism in this compound. A model for the minority component diffusion by anti-structure defects has been suggested for the L1 2 structure. It involves both nearest-neighbor jumps of the solute atoms on the Ni sublattice as anti-structure atoms and the formation of so-called anti-structure bridges which correspond to jumps between different sublattices. The model under consideration was shown to agree with the experimental data on the Al-substituting solute diffusion (X=Ga, Ge, Ti, Nb) in Ni 3Al and it allows the observed ratio of the diffusivities D Ni/ D X to be explained, which may be higher and/or lower than unity depending on the solute and temperature.

Point defect behavior in high temperature region in the B2-type intermetallic compound FeAl

The defect structure was investigated for B2-type Fe 1 − c Al c alloys (0.44 < c ≤ 0.51) by in-situ neutron diffraction measurements in the temperature range from room temperature to 1260–1300 K. Resulting point-defect behavior at high temperatures above about 800–1000 K where additional thermal vacancies are created is summarized as two common features: (1) thermal vacancies are formed on both sublattice sites; (2) antistructure atoms tend to return to their own sublattice sites. The second feature may be due to recovery process of excess antistructure atoms into their right sublattice sites. Based on the first feature, the random vacancy distribution (RVD) process was proposed as an essential defect mechanism in FeAl alloys. Further, the defect type (antistructure defect, triple defect or hybrid types) in these alloys was discussed and as a result it was concluded that the extended hybrid type, including the RVD process, is necessary for better understanding of their defect mechanism.

X-Ray study of the nGeTe·mBi2Te3 mixed layered tetradymite-like compounds

Single crystals and powders of the ternary mixed layered compounds Ge3Bi2Te6 (phase A), GeBi2Te4 (phase B) and GeBi4Te7 (phase C) have been investigated by X-ray diffraction analysis. The dependencies of the a and c/N parameters of the hexagonal lattices (N is the number of layers per unit cell) on the cation-to-anion ratio for the ternary phases and the GeTe and Bi2Te3 binary compounds have been analyzed. These dependencies were considered to be very close to linear. Based on those results, the lattice parameters were also calculated for other members of the (GeTe)n(Bi2Te3)m homologous series in the quasi-binary GeTe–Bi2Te3 system for which experimental data were not reported. The GeBi2Te4 and Ge1.5Bi2.5Te5 (D) crystal structures were determined using an automatic diffractometer “Syntex P1̄”. Results indicated that Ge and Bi atoms are statistically distributed in octahedral interstices to form mixed-cation layers. In addition to the observed disorder in the cation sublattice, anti-structure defects were also discovered in these compounds.

Mechanisms of Al Self- and Al-Substituting Solute Diffusion in Ni3Al

ABSTRACTDiffusion of the Al-substituting elements Ga, Ge, Ti and Nb has been investigated in Ni3Al single crystals by the secondary ion mass spectrometry (SIMS). A model for the minority component diffusion by anti-structure defects has been suggested for the Ll2 structure. It involves both nearest neighbor jumps of the solute atoms on the Ni sublattice as anti-structure atoms and the formation of so-called anti-structure bridges which correspond to jumps between different sublattices. The model under consideration was shown to agree with the experimental data on the Al-substituting solute diffusion (X = Ga, Ge, Ti, Nb) in Ni3Al and it allows to explain also the observed ratio of the diffusivities DNi/DX, which may be higher and/or lower than unity in dependence on the solute and temperature.

The Role of Hydrogen in Semi-Insulating Inp

ABSTRACTComplexes of vacancy at indium site with one to four hydrogen atoms and isolated hydrogen or hydrogen dimer and other infrared absorption lines, tentatively be assigned to hydrogen related defects were investigated by FTIR. Hydrogen can passivate imperfections, thereby eliminating detrimental electronic states from the energy bandgap.Incorporated hydrogen can introduce extended defects and generate electrically-active defects. Hydrogen also can acts as an actuator for creating of antistructure defects. Isolated hydrogen related defects(e.g. H12+) may play an important role in the conversion of the annealed wafers from semiconducting to the semi-insulating behavior. H2+ may be a deep donor, whose energy level is very near the iron deep acceptor level in the energy gap.

Theoretical Study of Antistructure Defects in GaAs

Theoretical Study of Antistructure Defects in GaAs

Diffusion by anti-structure defects in non-stoichiometric intermetallic compounds with B2 and structures

Self-diffusion in non-stoichiometric intermetallic compounds with the B2 and structures has been considered. It was shown that the thermal and compositional anti-structure defects substantially affect the diffusivity forming `anti-structural bridges' for a vacancy migration without additional disordering of the intermetallic compound. While this contribution was found to have a percolation nature for both sublattices in the case of the B2 structures, the percolation is to be taken into account only for diffusion of the minor element in intermetallic compounds with the structure. The numerical calculations were carried out for the CoGa triple-defect intermetallic compound. Critical exponents and fractal characteristics of percolation clusters were estimated. Values of the threshold anti-structure defect concentrations are for B2 structures and for the -sublattice of the compound with the structure. Owing to the temperature dependence of defect concentration the percolation threshold is temperature dependent also and, e.g., corresponds to composition at T = 1323 K. The theoretical predictions are compared with experimental data.

Point defect concentrations and hardening in binary B2 intermetallics

Point defect hardening in binary B2 intermetallic compounds with the anti-structure defect structure (FeCo and AuZn) and the triple defect structure (NiAl, FeAl, and CoAl) was investigated. Thermodynamic modeling combined with experimental measurements of lattice parameters and bulk densities were used to establish point defect concentrations as functions of composition and temperature. Microhardness measurements were made on samples of varying compositions and quenching temperatures. Solution hardening rates of vacancies were found to be significantly larger than those of anti-site defects. It was possible to relate the hardening rates of anti-site defects to the magnitude of the lattice dilation. This suggests that the elastic size effect was the primary hardening mechanism. No such correlation was found for vacancies.

Thermoelectric properties of 25%Bi 2Te 3-75%Sb 2Te 3 solid solution prepared by hot-pressing method

The Seebeck coefficient and electrical conductivity of the 25%Bi 2Te 3-75%Sb 2Te 3 solid solution prepared by hot-pressing method were measured and the effects of particle size, oxidation, hot-pressing temperature and time on the Seebeck coefficient of the polycrystalline solid solution were examined in detail. It has been found that the mechanical deformation during pulverization or pressing processes is very important in controlling the Seebeck coefficient. The role of the anion vacancies formed by the mechanical deformation as well as the change of the antistructure defect concentration were discussed. By optimizing particle size and hot-pressing temperature, the figure of merit about 2.9 x 10 −3 K −1 could be obtained for the P-type polycrystalline 25%Bi 2Te 3-75%Sb 2Te 3 solid solutions without excess Te addition.

TeCd antistructural defects in CdTe crystals

Two-temperature annealing with a controlled vapor pressure of tellurium PTe2 is used to study CdTe〈Cl〉 crystals under conditions of high-temperature thermodynamic equilibrium of the crystal with the gas phase (735–940 °C). For low pressures PTe2 (≥Pmin) ClTe+ begins to condense because of the formation of VCd−2 in the crystal. As PTe2 increases, this mechanism of exact self-compensation no longer operates because of the formation of TeCd+ intrinsic antistructural point defects.

Point defect behavior in the B2 type intermetallic compounds CoAl

The B2 type intermetallic compounds consisting of VIII and IIIb component atoms, such as the NiAl and CoAl, exhibit different types of point defects with deviation of alloy composition from the stoichiometry. Thus the determination of detailed defect structure is a very important subject for more clarification of its correlation to various properties in these compounds. Some work focusing on this problem has been recently presented by means of the powder X-ray diffraction (XRD) for the compounds NiAl, CoAl, and FeAl. The results for the former two compounds revealed that other than substantial amounts of Ni(Co) vacancies and antistructure defects (ASD), appreciable amounts of the Al-vacancy and Al-ASD which are neglected in the triple defect model (TRD model) were formed in the Al-rich composition region. It is an interesting problem to compare the defect behavior in NiAl with that in other compounds belonging to the same group: e.g., CoAl and FeAl. The aim of the present paper is thus to determine the defect structure appearing in the B2 type intermetallic compounds CoAl as a function of composition from the XRD and density measurements.

Investigation of deep levels of AsGa and GaAs antistructural defects and InAs and SbGa heteroantistructural defects in GaAs using a 4×4×4 expanded unit cell technique

A procedure is developed for calculation of the electron-energy spectrum of crystals with point defects using a 128-atom periodic cluster and with consideration of local defect symmetry. Electron-energy spectra are calculated for GaAs with AsGa, SbGa, GaAs, and InAs defects. The calculated results for deep levels agree with observations. Deep-level electron-density distribution is discussed as a function of center type. The spinorbital orbital splitting of localized levels is estimated.

Determination of vacancy concentration and defect structure in the B2 type NiAl β-phase alloys

The B2 type NiAl {beta}-phase alloys exhibit a complex type of point defects as the constitutional defect in off-stoichiometric composition regions. This was first established by Bradley and Taylor from the measurements of density and lattice constant. Their proposed defect structure model, the BT model, is that, on the Ni-rich side of stoichiometry the Ni antistructure defects, the Ni-ASD, resulting from substitution of the excess of Ni atoms on the Al-site which is the lattice site assigned to Al atoms, are formed, whereas instead of formation of the Al-ASD, the vacancies on the Ni-site, the Ni-vacancies, are created on the Al-rich side. In this paper which is concerned with the defect structure appearing in the NiAl {beta}-phase, the authors report the results of density measurements by the use of powdered samples instead of bulk material and the results of X-ray analyses performed for these samples based on their own density data.

Quenching by Copper Atoms of the EL2‐induced Luminescence in GaAs

AbstractIt is shown that the introduction of copper atoms into GaAs crystals containing antistructure defects EL2 (isolated arsenic atoms on gallium sites AsGa) leads to a practically complete disappearance of the EL2‐induced luminescence bands peaked at 0.63 and 0.68eV. This effect is connected with the passivation of the EL2 defects (i.e. with the substantial decrease in their concentration) because of their interaction with copper atoms (they become bound by copper atoms) resulting in an appearance of electrically inactive ASGaCUGa complexes.

Observation of the defects induced by hydrogen absorption and desorption in LaNi 5

The types and densities of defects generated in LaNi 5 lattice have been observed using transmission electron microscopy after hydrogen absorption and desorption with increasing hydrogen concentration. Basically two types of dislocations, ripple-like facile dislocations and misfit super dislocations having Burgers vectors of 〈hk1〉 and 〈hk0〉, respectively, were observed. After full absorption and desorption of hydrogen, the generation and dynamic recovery of these dislocations resulted in well-developed dislocation walls and slip band structure. Structural disorders and micro-twins were also occasionally observed. The macroscopic effects such as, the solubility enhancement in α-phase, anisotropic X-ray peak broadening and large stored energy in the activated LaNi 5 could be related to the dislocations and anti-structural defects observed in this study.

A generalized quasi-chemical model for ordered multi-component, multi-sublattice intermetallic compounds with anti-structure defects

A generalized quasi-chemical model was formulated for ordered multi-component, multi-sublattice intermetallic compounds with anti-structure defects. The assumptions made in formulating this model were (i) the atoms are randomly distributed on each of the sublattices, and (ii) the enthalpy term is due entirely to the interaction of the first-nearest neighbor atoms. Analytical equations were derived for the Gibbs energy and the partial Gibbs energies of the component elements. The equilibrium distributions of the atoms on the various sublattices were obtained by the minimization of the Gibbs energy. From the equilibrium distribution of the atoms, the integral and partial thermodynamic quantities of an intermetallic compound can be readily obtained. The generalized thermodynamic equations reduce to the equations reported in the literature for ordered binary intermetallic compounds exhibiting the B2, L10 and L12 structures with anti-structure defects. The generalized equations may be used to describe the thermodynamic properties of any ordered intermetallic compound with anti-structure defects. These equations may be used for calculating phase diagrams and defect concentrations in the lattices.

Composition dependence of site occupancies in the ternary Ll2 compound Al3TiCr

Composition dependence of the site occupancies of constituent atoms was investigated for the ternary Ll 2 compounds Al 3 TiCr (25–28% Ti and 9–14% Cr) by means of powder X-ray diffractometry. In the region of low Ti and low Cr concentration, the ternary element Cr occupied preferentially the Al-site. Further, some antistructure defects (ASD) caused by Al atoms (Al-ASD) and Ti atoms (Ti-ASD) were observed. Total ASD-fraction increased monotonically with increase in Ti-content. On the other hand, in the region of high Ti and high Cr concentration, the existence of Cr atoms substituted on the Ti-site (Cr-ASD) was newly detected. The amount of Ti-ASD and thus the ASD-fraction indicated a rapid increase in this composition region as the result of formation of the Cr-ASD. The reliability of the defect structure determined was examined by supposing two-defect structure models. The relation of the ASD to the mechanical properties in these compounds was also examined.

Defect structure in Al-rich composition region in the β-NiAl intermetallic compound phase

Abstract The defect structure in the B2 type β-NiAl phase was investigated in the Al-rich composition region by means of powder X-ray diffractometry. In the analysis, earlier published density data were referred to for estimation of the total vacancy concentration in the alloy. As a result, the main constitutional defect was found to be the vacancy on the Ni-site (Ni-vacancy), supporting the defect structure model proposed previously by Bradley and Taylor. However, in a region of high Al concentration, some Al atoms substituted on the Ni-site, i.e. anti-structure defects of Al atoms (Al-ASD), as well as some vacancies on the Al-sites (Al-vacancies), were recognized to exist. Their amounts did not change with increasing temperature up to 1150 °C. These defect types of the Al-ASD and Al-vacancy had always been excluded in usual treatments based on the triple defect model. The reliability of the defect structure determined was examined by comparison with the triple defect model. Further, thermodynamic calculations based on the Bragg-Williams approximation were performed in order to reproduce the observations.