Theoretical calculations of spin-Hamiltonian parameters and defect structures for Cu2+ in trigonally-distorted tetrahedral sites of ZnO and GaN crystals

The spin-Hamiltonian (SH) parameters (g factors g _//, g ⊥ and hyperfine structure constants ⁶³ A _//, ⁶³ A ⊥, ⁶⁵ A _//, ⁶⁵ A ⊥) for Cu²⁺ ions in the trigonally-distorted tetrahedral sites of ZnO and GaN crystals are calculated from a complete diagonalization (of energy matrix) method (CDM) based on a two spin-orbit parameter model for d ⁹ ions in trigonal symmetry. In the method, the Zeeman and hyperfine interaction terms are added to the Hamiltonian in the conventional CDM. The calculated results are in good agreement with the experimental values. The calculated SH parameters are also compared with those using the traditional diagonalization method or perturbation method only within the ² T ₂ term. It appears that, for exact calculations of SH parameters of d ⁹ ions in trigonal tetrahedral clusters in crystals, the present CDM is preferable to the traditional diagonalization method or perturbation method within the ² T ₂ term. The local structures of Cu²⁺ centers (which differ from the corresponding structure in the host crystal) in ZnO : Cu²⁺ and GaN : Cu²⁺ are obtained from the calculations. The results are discussed.     

Spin Hamiltonian parameters and local structures for tetragonal and orthorhombic Ir2+ centers in AgCl

The perturbation formulae of the spin Hamiltonian parameters (the anisotropic g factors, hyperfine structure constants and superhyperfine parameters) are established for a 5d⁷ ion in an orthorhombically elongated octahedron based on the cluster approach. These formulae are applied to the theoretical studies of the EPR spectra and the local structures for the tetragonal and orthorhombic Ir²⁺ centers in AgCl. For the tetragonal Ir²⁺ center, the uncompensated substitutional [IrCl₆]⁴ cluster is found to experience a relative elongation of about 0.08 Å along the C ₄ axis due to the Jahn–Teller effect. For the orthorhombic center, the ligand octahedron also suffers Jahn–Teller elongation (by about 0.08 Å) along the [001] (or Z) axis. Meanwhile, the ligand Cl intervening in the impurity Ir²⁺ and the next nearest neighbor silver vacancy V_Ag along the [100] (or X) axis may undergo an inward displacement of 0.004 Å towards the center of the octahedron due to electrostatic repulsion of the V_Ag. The calculated spin Hamiltonian parameters based on the above local structures show good agreement with experimental data for both centers.     

Mesoscale strain measurement in deformed crystals: A comparison of X-ray microdiffraction with electron backscatter diffraction

Mapping of residual stresses at the mesoscale is increasingly practical thanks to technological developments in electron backscatter diffraction (EBSD) and X-ray microdiffraction using high brilliance synchrotron sources. An analysis is presented of a Cu single crystal deformed in compression to about 10% macroscopic strain. Local orientation measurements were made on sectioned and polished specimens using EBSD and X-ray microdiffraction. In broad strokes, the results are similar to each other with orientations being observed that are on the order of 5° misoriented from that of the original crystallite. At the fine scale it is apparent that the X-ray technique can distinguish features in the structure that are much finer in detail than those observed using EBSD even though the spatial resolution of EBSD is superior to that of X-ray diffraction by approximately two orders of magnitude. The results are explained by the sensitivity of the EBSD technique to the specimen surface condition. Dislocation dynamics simulations show that there is a relaxation of the dislocation structure near the free surface of the specimen that extends approximately 650 Å into the specimen. The high spatial resolution of the EBSD technique is detrimental in this respect as the information volume extends only 200 Å or so into the specimen. The X-rays probe a volume on the order of 2 µm in diameter, thus measuring the structure that is relatively unaffected by the near-surface relaxation.     

Correlation between the negative magnetoresistance effect and magnon excitations in single-crystalline CuCr1.6V0.4Se4

Structural, electrical and magnetic measurements, as well as electron spin resonance (ESR) spectra, were used to characterise the single-crystalline CuCr_1.6V_0.4Se₄ spinel and study the correlation between the negative magnetoresistance effect and magnon excitations. We established the ferromagnetic order below the Curie temperature T _C ≈ 193 K, a p-type semiconducting behaviour, the ESR change from paramagnetic to ferromagnetic resonance at T _C, a large ESR linewidth value and its temperature dependence in the paramagnetic region. Electrical studies revealed negative magnetoresistance, which can be enhanced with increasing magnetic field and decreasing temperature, while a detailed thermopower analysis showed magnon excitations at low temperatures. Spin–phonon coupling is explained within the framework of a complex model of paramagnetic relaxation processes as a several-stage relaxation process in which the V³⁺ ions, the exchange subsystem and conduction electron subsystem act as the intermediate reservoirs.     

Effect of Fe on the glass-forming ability,structure and devitrification behavior of Zr–Cu–Al bulk glass-forming alloys

We report on the glass-forming ability and devitrification behavior of Zr₆₀Cu₃₀Al₁₀, Zr₆₀Cu₂₅Al₁₀Fe₅ and Zr_62.5Cu_22.5Al₁₀Fe₅ bulk glass-forming alloys on heating. The effect of Fe addition on the structure of Zr–Al–Cu alloys is also discussed. Crystallization kinetics and structural changes in the glassy alloys were studied using X-ray diffraction, transmission electron microscopy, differential scanning and isothermal calorimetry methods. The results indicate that good glass-formers, such as Zr_62.5Cu_22.5Al₁₀Fe₅, are located somewhat beyond the equilibrium eutectic point. Possible phase separation in the supercooled liquid on heating and electron beam-induced in situ crystallization are observed and discussed.     

Study of defect evolution by TEM with in situ ion irradiation and coordinated modeling

The paper describes a novel transmission electron microscopy (TEM) experiment with in situ ion irradiation designed to improve and validate a computer model. TEM thin foils of molybdenum were irradiated in situ by 1?MeV Kr ions up to ～0.045 displacements per atom (dpa) at 80°C at three dose rates ?5?×?10^?6, 5?×?10^?5, and 5?×?10^?4?dpa/s – at the Argonne IVEM-Tandem Facility. The low-dose experiments produced visible defect structure in dislocation loops, allowing accurate, quantitative measurements of defect number density and size distribution. Weak beam dark-field plane-view images were used to obtain defect density and size distribution as functions of foil thickness, dose, and dose rate. Diffraction contrast electron tomography was performed to image defect clusters through the foil thickness and measure their depth distribution. A spatially dependent cluster dynamic model was developed explicitly to model the damage by 1?MeV Kr ion irradiation in an Mo thin foil with temporal and spatial dependence of defect distribution. The set of quantitative data of visible defects was used to improve and validate the computer model. It was shown that the thin foil thickness is an important variable in determining the defect distribution. This additional spatial dimension allowed direct comparison between the model and experiments of defect structures. The defect loss to the surfaces in an irradiated thin foil was modeled successfully. TEM with in situ ion irradiation of Mo thin foils was also explicitly designed to compare with neutron irradiation data of the identical material that will be used to validate the model developed for thin foils.     

Concurrent formation of two different type precipitation-free zones during the initial stage of homogenization

Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy were used to study the microstructure evolution of Al–Cu–Mg alloy during the initial stage of homogenization. It was found that two types of precipitation-free zones (PFZs) can form concurrently: one near grain boundaries and the other at the grain centres. Depth profile analyses of solute concentrations and dislocation-loop distributions strongly suggested that the formations of the two type of PFZs are different, due solely and exclusively to solute and vacancy depletion, respectively. A mechanism model was proposed to explain the concurrent formation of the two different type of PFZs during the initial stage of homogenization.     

Dislocation content of geometrically necessary boundaries aligned with slip planes in rolled aluminium

Previous studies have revealed that dislocation structures in metals with medium-to-high stacking fault energy, depend on the grain orientation and therefore on the slip systems. In the present work, the dislocations in eight slip-plane-aligned geometrically necessary boundaries (GNBs) in three grains of near 45° ND rotated cube orientation in lightly rolled pure aluminium are characterized in great detail using transmission electron microscopy. Dislocations with all six Burgers vectors of the ½?1?1?0? type expected for fcc crystals were observed but dislocations from the four slip systems expected active dominate. The dislocations predicted inactive are primarily attributed to dislocation reactions in the boundary. Two main types of dislocation networks in the boundaries were identified: (1) a hexagonal network of the three dislocations in the slip plane with which the boundary was aligned; two of these come from the active slip systems, the third is attributed to dislocation reactions (2) a network of three dislocations from both of the active slip planes; two of these react to form Lomer locks. The results indicate a systematic boundary formation process for the GNBs. Redundant dislocations are not observed in significant densities.     

TEM investigation of the formation mechanism of deformation twins in Fe–Mn–Si–Al TWIP steels

The microstructure of a Fe–Mn–Si–Al twinning-induced plasticity (TWIP) steel exhibiting remarkable work hardening rate under uniaxial tensile deformation was investigated using transmission electron microscopy to uncover the mechanism(s) controlling the nucleation and growth of the mechanically induced twins. The results show that the stair-rod cross-slip deviation mechanism is necessary for the formation of the twins, while large extrinsic stacking faults homogenously distributed within the grains could act as preferential sources for the activation of the deviation process. The influence of such features on the thickness and strength of the twins and the resulting mechanical behaviour is discussed and compared to similar works recently performed on Fe–Mn–C TWIP steels.