Aluminum Vacancies Research Articles

Relaxations and Interfacial Water Ordering at the Corundum (110) Surface

In situ high resolution specular X-ray reflectivity measurements were used to examine relaxations and interfacial water ordering occurring at the corundum (110)-water interface. Sample preparation affected the resulting surface structure. Annealing in air at 1373 K produced a reconstructed surface formed through an apparently ordered aluminum vacancy. The effect of the reconstruction on in-plane periodicity was not determined. The remaining aluminum sites on the surface maintain full coordination by oxygen and the surface was coated with a layer of physically adsorbed water. Ordering of water further from the surface was not observed. Acid etching of this surface and preparing a surface through annealing at 723 K both produced an unreconstructed surface with identical relaxations and water ordering. Relaxations were confined primarily to the top ∼4 A of the surface and were dominated by an increased distribution width of the fully occupied surface aluminum site and outward relaxation of the oxygen surface...

Multiscale modeling of interaction of alane clusters on Al(111) surfaces: A reactive force field and infrared absorption spectroscopy approach

We have used reactive force field (ReaxFF) to investigate the mechanism of interaction of alanes on Al(111) surface. Our simulations show that, on the Al(111) surface, alanes oligomerize into larger alanes. In addition, from our simulations, adsorption of atomic hydrogen on Al(111) surface leads to the formation of alanes via H-induced etching of aluminum atoms from the surface. The alanes then agglomerate at the step edges forming stringlike conformations. The identification of these stringlike intermediates as a precursor to the bulk hydride phase allows us to explain the loss of resolution in surface IR experiments with increasing hydrogen coverage on single crystal Al(111) surface. This is in excellent agreement with the experimental works of Go et al. [E. Go, K. Thuermer, and J. E. Reutt-Robey, Surf. Sci. 437, 377 (1999)]. The mobility of alanes molecules has been studied using molecular dynamics and it is found that the migration energy barrier of Al(2)H(6) is 2.99 kcal/mol while the prefactor is D(0)=2.82 x 10(-3) cm(2)/s. We further investigated the interaction between an alane and an aluminum vacancy using classical molecular dynamics simulations. We found that a vacancy acts as a trap for alane, and eventually fractionates/annihilates it. These results show that ReaxFF can be used, in conjunction with ab initio methods, to study complex reactions on surfaces at both ambient and elevated temperature conditions.

The origin of 2.78 eV emission and yellow coloration in bulk AlN substrates

The yellow color of bulk AlN crystals was found to be caused by the optical absorption of light with wavelengths shorter than that of yellow. This yellow impurity limits UV transparency and hence restricts the applications of AlN substrates for deep UV optoelectronic devices. Here, the optical properties of AlN epilayers, polycrystalline AlN, and bulk AlN single crystals have been investigated using photoluminescence (PL) spectroscopy to address the origin of this yellow appearance. An emission band with a linewidth of ∼0.3 eV (at 10 K) was observed at ∼2.78 eV. We propose that the origin of the yellow color in bulk AlN is due to a band-to-impurity absorption involving the excitation of electrons from the valence band to the doubly negative charged state, (VAl2−), of isolated aluminum vacancies, (VAl)3−/2− described by VAl2−+hν=VAl3−+h+. In such a context, the reverse process is responsible for the 2.78 eV PL emission.

A HYBRID, QUANTUM-CLASSICAL APPROACH FOR THE COMPUTATION OF DISLOCATION PROPERTIES IN REAL MATERIALS: METHOD, LIMITATIONS AND APPLICATIONS

In this work we introduce a hybrid ab initio-classical simulation methodology designed to incorporate the chemistry into the description of phenomena that, intrinsically, require very large systems to be properly described. This hybrid approach allows us to conduct large-scale atomistic simulations with a simple classical potential (embedded atom method (EAM), for instance) while simultaneously using a more accurate ab initio approach for critical embedded regions. The coupling is made through shared atomic shells where the two atomistic modeling approaches are relaxed in an iterative, self-consistent manner. The magnitude of the incompatibility forces arising in the shared shell is analyzed, and possible terminations for the embedded region are discussed, as a way to reduce such forces. As a test case, the formation energy of a single vacancy in aluminum at different distances from an edge dislocation is studied. Results obtained using the hybrid approach are compared to those obtained using classical methods alone, and the range of validity for the classical approach is evaluated.

Role of the defect core in energetics of vacancies

Electronic structure calculations at macroscopic scales are employed to investigate the crucial role of a defect core in the energetics of vacancies in aluminium. We find that vacancy core energy is significantly influenced by the state of deformation at the vacancy core, especially volumetric strains. Insights from the core electronic structure and computed displacement fields show that this dependence on volumetric strains is closely related to the changing nature of the core structure under volumetric deformations. These results are in sharp contrast to mechanics descriptions based on elastic interactions that often consider defect core energies as an inconsequential constant. Calculations suggest that the variation in core energies with changing macroscopic deformations is quantitatively more significant than the corresponding variation in relaxation energies associated with elastic fields. Upon studying the influence of various macroscopic deformations, which include volumetric, uniaxial, biaxial and shear deformations, on the formation energies of vacancies, we show that volumetric deformations play a dominant role in governing the energetics of these defects. Further, by plotting formation energies of vacancies and di-vacancies against the volumetric strain corresponding to any macroscopic deformation, we find that all variations in the formation energies collapse on to a universal curve. This suggests a universal role of volumetric strains in the energetics of vacancies. Implications of these results in the context of dynamic failure in metals through shock-induced spalling are analysed.

Color center of YAlO 3 with cation vacancies

High quality pure YAlO 3 crystal with dimension of Φ 30 × 50 mm 2 was grown by Czochralski technique. UV irradiation and air annealing bring additional absorption in the region 200–800 nm. The absorption spectra of perfect YAlO 3 and YAlO 3 containing cation vacancy (aluminium vacancy and yttrium vacancy) were calculated using density functional theory code CASTEP. Comparison of the simulated absorption spectra with the experimental absorption spectra of YAlO 3 after UV irradiation and air annealing treatments shows that cation vacancies are responsible for part of the coloration on YAlO 3 crystal.

Scattering theory of defects in solids: Theory and application to the atomic vacancy

The application of scattering theory to calculate properties of defects in solids is reviewed. Expressions for the change in density of states and in the charge density are obtained. Recent applications of this method to atomic vacancies in silicon and aluminum are described. An appendix contains an expression for the determinantal function on the basis of a nonorthogonal set of localized orbitals.

Point defects in aluminum: Single vacancy

The results of a first-principles calculation for the formation energy of a single vacancy in aluminum are reported. Solid-state scattering theory is modified to use localized Gaussian orbitals (instead of Wannier functions) and applied to evaluate the one-electron contribution. Energy bands and wavefunctions, obtained from a self-consistent LCGO band calculation, are used to evaluate the Green's function matrix. The defect potential consists of that due to the absence of a neutral atom, including the lattice relaxation. It is further made partially self-consistent by including the electronic dielectric function for screening.

First Principles Calculations of Vacancy Formation Energies in Σ13 Pyramidal Twin Grain Boundary of α-Al<SUB>2</SUB>O<SUB>3</SUB>

Defect energetics in Σ13 pyramidal twin grain boundary (GB) of Al 2 O 3 was investigated by a first principles projector-augmented wave method. It was found that the vacancy formation energy depends on the atomic site and the defect energetics at the GB is similar to that in the bulk Al 2 O 3 , namely the oxygen vacancy shows much higher formation energy than the aluminum vacancy and the Schottky defect is the most preferable species in a wide range of atmospheres. By analyzing the atomic structures of the GB in detail, it was found that the defect energetics at the GB is closely related to the structural distortions, such as strains and dangling-bonds in the vicinity of the GB.

Densification Behavior of Ti-Doped Polycrystalline Alumina in a Nitrogen-Hydrogen Atmosphere

The densification behavior during sintering of 0.1 mol% TiO 2 -doped Al 2 O 3 was measured in either an air or N 2 + 5%H 2 gas atmosphere at the sintering temperature of 1573―1673 K. The grain boundary diffusivity was evaluated from the densification rate. High-resolution transmission electron microscopy (HRTEM) and nano-probe energy-dispersive X-ray spectroscopy (EDS) analyses revealed that the doped Ti cations segregate in the vicinity of the grain boundaries in the A1 2 0 3 . An electron energy loss spectroscopy (EELS) investigation indicated that the valence state of Ti in the Al 2 O 3 sintered in the reducing atmosphere was close to +3. The grain boundary diffusivity in undoped A1 2 0 3 was insensitive to the atmosphere, but was enhanced by the grain boundary segregation of Ti 4+ . The grain boundary diffusivity of alumina in the reducing atmosphere was, however, retarded by the Ti 3+ -doping. The retarded diffusivity by Ti 3+ -doping must be related to the lack of aluminum vacancies and the large ionicity of Ti-O compared to Al-O.

Role of Macroscopic Deformations in Energetics of Vacancies in Aluminum

Electronic structure calculations on million-atom samples are employed to investigate the effect of macroscopic deformations on energetics of vacancies in aluminum. We find that volumetric strain associated with a deformation largely governs the formation energies of monovacancies and divacancies. The calculations suggest that nucleation of these defects is increasingly favorable under volumetric expansion, so much to the point that they become thermodynamically favorable under large positive volumetric strains. On the contrary, on an average, existing vacancies are found to bind preferentially under compressive volumetric strains. Shear deformations did not affect the formation energies of vacancies, but strongly influenced the 110 divacancy binding energies, causing them to orient under energetically preferential directions.

Growth strains and creep in thermally grown alumina: Oxide growth mechanisms

In situ measurements of growth strains and creep relaxation in α-Al2O3 films, isothermally grown on β-NiAl alloys at 1100 °C, are reported and analyzed. Samples containing the reactive element Zr, and Zr-free samples, are examined. For Zr-free samples, steady state growth strains are compressive, whereas the growth strains are tensile when the reactive element (RE) is added to the alloy. This behavior is attributed to the counterflow of oxygen and aluminum interstitials, and to simultaneous counterflow of oxygen and aluminum vacancies, all moving through the grain boundaries. Cross diffusing oxygen and aluminum interstitials may merge and combine within the film, forming new oxide along grain boundary walls, a mechanism that leads to an in-plane compressive stress. Cross diffusing oxygen and aluminum vacancies will also merge and combine within the film; in this case material is removed from grain boundary walls, a mechanism that leads to an in-plane tensile stress. When no RE is present, the interstitial mechanism dominates and the resultant stress is compressive. Consistent with the “dynamic segregation model,” the RE slows the outdiffusion of Al interstitials permitting the tensile mechanism to dominate. This interpretation invokes the unconventional view that oxygen and aluminum interstitials and vacancies, created in and driven by the strong chemical gradient, all participate meaningfully in the scale growth process. Grain boundary diffusion measurements were obtained from low stress creep data, interpreted using the Coble model of grain boundary diffusion. Reported diffusion measurements of oxygen through grain boundaries of α-Al2O3, which are known to be inconsistent with oxide scale growth, are critically examined. A simple picture, a “balanced defect model,” emerges that is consistent with the dynamic segregation model, observed growth stresses and their dependence on the presence of a reactive element, sequential oxidation experiments, and our best knowledge about grain boundary diffusion coefficients.

Microwave Synthesis of an Aluminum Fluoride Hydrate with Cationic Vacancies: Structure, Thermal Stability, and Acidic Properties

Aluminum fluoride hydrate was synthesized by a microwave hydrothermal process. The structure derives from the ReO3 type structure, that is, the high temperature structure of α-AlF3. The cubic symmetry adopted by this compound arises from water molecules, stabilized as ligand surrounding Al3+ cations which induces cationic vacancies as revealed by Rietveld refinement. The following chemical formula Al0.82◻0.18F2.46(H2O)0.54 is supported by chemical analysis and TGA measurements. This represents the first example of aluminum vacancy compound in the Al-based fluorides chemistry. High field 27Al NMR spectroscopy enabled identification and quantification of the following species: AlF6 and AlF6−x(H2O)x with x = 1, 2, 3, and showed that vacancies are mainly surrounded by water molecules but also by a low content of fluoride ions as also evidenced by 19F NMR. The hydrogen bonding network, which takes place in the vicinity of the cationic vacancies, was characterized by FTIR and 1H NMR spectroscopies. A 2:1 comple...

First Principles Study of Aluminium Vacancy in Wurtzite Aluminium Nitride

We report that the aluminium vacancy in wurtzite AlN brings about two impurity levels e and a2 in the band gap, not just one single t2 level. The aluminium vacancy carries a magnetic moment of lμB in the ground state. The molecule orbit of the aluminium vacancy becomes rather than . The calculation is carried out by using the CASTEP code. The intrinsic symmetry of wurtzite AlN is the driving force for this spin splitting. Finally the symmetry of wurtzite AlN results in an anti-ferromagnetic coupling between the aluminium vacancies, as is predicted. Our findings are helpful to gain a more through understanding of the structural and spin property of aluminium vacancy in wurtzite AlN.

Ab initio modeling of the formation and migration of monovacancies in Ti 2AlC

We performed ab initio calculations for monovacancy formation and migration in Ti 2 AlC. Carbon and aluminum vacancies have almost equally low formation energies, respectively, at (Ti- and Al-rich) and (Ti- and C-rich) growth conditions, wherein both defects exhibit a high equilibrium concentration and structural tolerance to large off-stoichiometry in Ti 2 AlC. In contrast, V Ti has the highest formation energy at all possible conditions. The intrinsic migration energies of various vacancies are determined to be in the sequence E m ( V Al ) < E m ( V Ti ) < E m ( V C ).

Intrinsic point defects in aluminum antimonide

Calculations within density functional theory on the basis of the local density approximation are carried out to study the properties of intrinsic point defects in aluminum antimonide. Special care is taken to address finite-size effects, band gap error, and symmetry reduction in the defect structures. The correction of the band gap is based on a set of GW calculations. The most important defects are identified to be the aluminum interstitial $Al_{i,Al}^{1+}$, the antimony antisites $Sb_{Al}^0$ and $Sb_{Al}^{1+}$, and the aluminum vacancy $V_{Al}^{3-}$. The intrinsic defect and charge carrier concentrations in the impurity-free material are calculated by self-consistently solving the charge neutrality equation. The impurity-free material is found to be n-type conducting at finite temperatures.

26Al tracer diffusion in titanium doped single crystalline α-Al 2O 3

Simultaneous 18O and 26Al tracer diffusion experiments were performed on titanium doped single crystalline α-Al 2O 3. The simultaneous tracer diffusion of 18O and 26Al clearly demonstrates that the aluminium diffusivity in Ti doped alumina is orders of magnitude higher than the oxygen diffusivity. The enhanced aluminium diffusivity is caused by an increase of the concentration of mobile Al vacancies. The reasonably good agreement between earlier conductivity measurements and our transport measurements indicates in accordance with theoretical work that the migration enthalpy of aluminium vacancies is (3.8 ± 0.1) eV.

Energetics of Aluminum Vacancies and Incorporation of Foreign Trivalent Ions in γ‐Al2O3: An Atomistic Simulation Study

Atomistic simulation methods based on pair‐wise interatomic potentials and energy minimization have been applied to elucidate the energetics of cation vacancies and the incorporation of 13 trivalent M3+ cations (Cr3+, Ga3+, Fe3+, Lu3+, Yb3+, Er3+, Y3+, Tb3+, Gd3+, Eu3+, Sm3+, Nd3+, La3+) in γ‐Al2O3. Calculations have been carried out using Al64O96 defect spinel supercells containing eight aluminum vacancies. The lowest energy configurations correspond to a random distribution of tetrahedral and octahedral vacancies. The energy gain in comparison with exclusive tetrahedral or octahedral vacancies is rather small (0.03 and 0.09 eV/Al2O3, respectively). Unit cell volume, density, and lattice properties of optimized structures are in good agreement with the experimental values or the results of high‐quality density functional theory calculations. The trends observed for the solution energy of the M2O3 oxides in the supercell with minimum energy indicate the preferential incorporation of the foreign ions at the tetrahedral site and an increase of the solubility of M2O3 in the defect spinel in comparison with α‐Al2O3. Configurations with the lowest energy have negative solution energies and, consequently, incorporation of trivalent ions can improve the thermodynamic stability of γ‐Al2O3 in comparison with α‐Al2O3 and increase the γ→α transition temperature.

Microstructural Modification of Metallic Materials by Electromagnetic Processing and the Theoretical Interpretation

The recrystallization behaviors of cold rolled aluminum alloys in electric field up to 400kV/mm and the phase transformation processes of proeutectoid steels under magnetic field up to 14 Tesla have been experimentally examined. It has been found that both the electric field and the magnetic field have influence on the evolution of texture and microstructure characteristics. During the recrystallization annealing under the electric field of the cold-rolled 3104 aluminum alloy sheets, the electric field postpones the recovery and recrystallization progress. First principle calculation was performed to study the electric structures of aluminum atoms and vacancies. It shows that vacancies that are helpful for recovery are electrically negative. As the sample worked as anode during electric field annealing, it was covered with positive surface charges that attract the electronegative vacancies in the vicinity of the free surface and annihilate them. In this way, the recovery and then the recrystallization are postponed. The magnetic field applied changes the precipitation sequence of transitional carbides during low temperature tempering that makes the relatively high-temperature monoclinic χ-Fe5C2 carbide precipitated without following the usual precipitation sequence, i.e. by skipping the precipitation of the usual orthorhombic η-Fe2C carbide. To reveal the working mechanism of this phenomenon, first principle calculations were performed to study the formation energies of the two iron-carbide systems and their electronic and magnetic structures and properties. Calculation results show that η-Fe2C has lower formation energy, which is proved by the formation sequence observed during the usual low temperature tempering process. However, χ-Fe5C2 has the higher magnetic moment, which enhances the stability under the magnetic field through magnetization. Therefore, under the magnetic field its precipitation tendency is increased.

Quantitative chemical analysis of vacancy‐solute complexes in metallic solid solutions by coincidence Doppler broadening spectroscopy

AbstractLow background measurements of 1D momentum spectra of the positron annihilation radiation by the coincidence Doppler broadening (CDB) method are currently used to obtain the quantitative chemical analysis of the environment of vacancy‐like defects in supersaturated solid solution of metals. Fractional concentrations are obtained by fitting the CDB spectra with linear combination of reference spectra. Examples regarding Al‐based alloys are presented to show the quality of the fitting and the consistency of the CDB analysis with positron lifetime measurements. The appropriate choice of the references is considered a key factor for reducing systematic errors. Nevertheless the method still awaits a theoretical validation. Preliminary first‐principles calculations regarding Cu‐decorated vacancies in aluminium are presented. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)