First-principles investigation of thermal point defects in B2 NiAl

The equilibrium concentrations of thermal defects are calculated in the mean-field approximation for the configurational entropy. The stable configurations of point defects in NiAl are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 1630–1636 (September 1999)     

Magnetism in FeAl,CoAl, CoGa and NiAl

Isothermal magnetization vs field data from 1.6 to 300°K for Ni_0.48Al_0.52, Ni_0.50Al_0.50, Co_0.48Al_0.52 and Co_0.50Al_0.50 can be described in terms of contributions from magnetic clusters, from “antistructure” transition metal atoms on Al sites, and from a field- and temperature-independent susceptibility. The moment of antistructure Ni is 0.3μ_B in both nickel alloys. In both cobalt aluminides as well as in Co_0.47Ga_0.53, the antistructure Co moment is 1μ_B. The moment of antistructure Fe in Fe_0.49Al_0.51 is 2.2μ_B.     

Thermodynamics and point defects in the B2 intermetallic phase PdIn

The X-ray and bulk densities of PdIn alloys have been determined at ambient temperature on samples annealed at 1273 and 1373 K and quenched in water-ice mixtures. From these measurements the vacancy concentrations in this intermetallic phase have been obtained as a function of In concentration at these two temperatures. In addition, a generalized thermodynamic model is presented which considers the existence of antisite and vacancy defects on both sublattices without any dilute solution approximations. This model uses three energy parameters, whose values have been obtained for PdIn using the measured vacancy concentrations and the chemical potentials of In. This model is able to describe all the available vacancy data and is suitable for calculating these values at compositions and temperatures when such data are not available. In addition, the model permits the calculation of the concentrations of the minority point defects. It is shown that dilute-solution approximations, which have invariably been used in this kind of modelling, are unsatisfactory. The present results and their analysis confirm that PdIn is a near-triple-defect type of intermetallic and not a near-Schottky type, as has been claimed recently.     

Equilibrium point defects in TiAl studied by PAC

Site fractions of dilute¹¹¹In/¹¹¹Cd probe atoms in different lattice locations in quenched, equi-atomic TiAl are measured using perturbed angular correlations of gamma rays (PAC). The vacancy-free substitutional site fraction, observed via its nuclear quadrupole interaction, is observed to decrease with increasing quenching temperature, reflecting an increase in the thermal defect concentration. Assuming that the thermal defect is the Schottky defect, the law of mass action, and a binding energy of 0.2 eV between vacancies and the In probe, the experimental temperature dependence is analysed to yield a formation enthalpy of 4.7(4) eV and entropy of 25(4)k _B for the Schottky defect. The vacancy concentration on one sublattice is given in terms of these parameters by the expression [v]=exp(S _F/2k _B) exp(–E _F/2k _B T), and is found, for example, to have the value 1.4% at 1350°C, a large value.     

点缺陷对 L12-Al3Li金属间化合物热力学性能的影响

运用基于第一性原理的平面波赝势法研究了L12-Al3Li金属间化合物中Li原子空位和Al原子反位缺陷对Al3Li热力学性能的影响,结果表明：Al反位缺陷易与周围原子形成局域共价键,使晶体体积增大,而Li空位缺陷却减小了晶体体积。Li空位缺陷使L12-Al3Li的硬度增加,延展性降低,德拜温度值升高。Al反位缺陷降低了晶体的硬度,增加了延展性,降低了德拜温度值。在德拜温度以下,Li空位缺陷减小了L12-Al3Li的热容, 而Al反位缺陷使晶体热容增大。晶格畸变对L12-Al3Li晶体的热力学性能有重要影响     

点缺陷对L1_2-Al_3Li金属间化合物热力学性能的影响

运用基于第一性原理的平面波赝势法研究了L12-Al3Li金属间化合物中Li原子空位和Al原子反位缺陷对Al3Li热力学性能的影响,结果表明:Al反位缺陷易与周围原子形成局域共价键,使晶体体积增大,而Li空位缺陷却减小了晶体体积.Li空位缺陷使L12-Al3Li的硬度增加,延展性降低,德拜温度值升高.Al反位缺陷降低了晶体的硬度,增加了延展性,降低了德拜温度值.在德拜温度以下,Li空位缺陷减小了L12-Al3Li的热容,而Al反位缺陷使晶体热容增大.晶格畸变对L12-Al3Li晶体的热力学性能有重要影响.     

First-principles investigation of the effects of B impurities on the mechanical properties of NiAl intermetallics

We have investigated the effects of B impurities on the structure and mechanical properties of NiAl intermetallics by using a first-principles pseudopotential total-energy method, based on the density functional theory with a generalized gradient approximation. We found that the impurity B atoms can either replace Ni atoms or Al atoms or both, depending on the surrounding environment. We demonstrated that the presence of B will cause an increase in brittleness and a decrease in the ductility of NiAl for the...     

Surface structure and energy of B2 type intermetallic compound NiAl

The surface structure and energies for 22 surfaces of NiAl, an ordered intermetallic compound of B2 structure, have been studied by using embedded atom method. The results show that, for alternating Ni and Al surfaces with odd numbers of the sum of their three Miller indices, the energy difference between the Ni terminated surface and Al terminated surface increase linearly with increasing the interlayer distance. So from surface energy minimization, the Al terminated surface is favorable for each alternating Ni and Al surface. This is in agreement with experimental results. However, the energy of the (1 1 0) surface belonged to the other kind of the surface consisted of stoichiometric atomic layers and with even numbers of the sum of their three Miller indices, is the lowest in all two kinds of the surfaces. Therefore the (1 1 0) texture of NiAl appears mostly in the experiments.     

The changes in the electronic structure of B2 FeAl alloy with a Fe antisite and absorbed hydrogen

The electronic structure and bonding in a B2 FeAl alloy with and without hydrogen interaction with a Fe antisite were computed using a density functional theoretical method. The hydrogen absorption turns out to be a favorable process. The hydrogen was found close to an octahedral site where one of its Al capped is replaced by a Fe antisite. The Fe–H distance is of 1.45 Å same as the Al–H distance.The density of states (DOS) curves show several peaks below the d metal band which is made up mostly of hydrogen based states (>50% H_1s) while the metal contribution in this region includes mainly s and p orbitals.An electron transfer of nearby 0.21e⁻ comes from the metal to the H. The overlap population values reveal metal–metal bond breaking, the intermetallic bond being the most affected. The H bond mainly with the Al atom and the reported Fe–H overlap population is much lower than that corresponding to FePd alloys and BCC Fe. The changes in the overlap population show the Fe–Al bond is weakened nearly 41.5% after H absorption, while the Fe–Fe bond is only weakened 34.5%. H also develops a stronger bond with the Al atoms. The main bond is developed with Al being twice stronger than Fe–H.     

Equilibrium configurations and energetics of point defects in two-dimensional colloidal crystals

We demonstrate a novel method of introducing point defects (mono- and divacancies) in a confined monolayer colloidal crystal by manipulating individual particles with optical tweezers. Digital video microscopy is used to study defect dynamics in real space and time. We verify the numerical predictions that the stable configurations of the defects have reduced symmetry compared to the triangular lattice and discover that in addition they are characterized by distinct topological arrangements of the particles in the defect core. Surprisingly, point defects are thermally excited into separated dislocations, from which we extract the dislocation pair potential.     

Point defects in NiAl near the equiatomic composition

Perturbed γ-γ angular correlation spectroscopy was applied to study the structure and properties of point defects near¹¹¹In probes in quenched and annealed NiAl samples with five compositions in the range c_Ni=48–52%. Quadrupole interaction signals are analysed in terms of near-neighbor Ni-vacancy and nextnear-neighbor Ni antisite atom defects. Large mono- and divacancy concentrations were observed in all samples after quenching. For c_Ni<50%, the vacancy defects could not be annealed out (structural defects). For C_Ni>50%, large quenched-in vacancy concentrations annealed out near 500°C. For c_Ni=50%, most vacancies annealed out near 500°C, but about 35% of the¹¹¹In probes retained a trapped monovacancy. This is attributed to a very low value of the formation enthalpy of a vacancy next to the In probe, estimated to be 0.16(2) eV. A lesser annelaing stage detected at 300°C is tentatively attributed to diffusion of Al-vacancies.     

Equilibrium concentration of point defects in crystalline4He at O K

We calculate the concentrations of vacancies and intersitials in the ground state of a Bose solid which models⁴He. Because ground-state boson wave functions are nodeless, their probability densities correspond to classical Boltzmann factors, and properties of Bose solids, such as the concentration of vacancies and interstitials, can be calculated using classical statistical mechanics. We model the ground-state wave function of⁴He with the product (Jastrow) form that corresponds to a classical 1/r ^b pair potential, and use a quasiharmonic approximation to calculate the concentrations of vacancies and interstitials in an fcc lattice with this potential. We find that the fractional concentration of vacancies at the melting point is 1.60×10^–5 for 1/r ⁹ and 6.36×10^–6 for 1/r ⁶, while the interstitial fractional concentrations are 1.32×10^–3 and 1.08×10^–5, respectively; the defect concentrations decrease by 7–16 orders of magnitude when the crystal density increases by 50%. At the same density, and with the same 1/r ⁹ potential, the concentration of vacancies in an hcp lattice is essentially the same as in an fcc lattice, but the interstitial concentration is much lower, apparently because the fcc lattice contains a more favorable split-interstitial site than does hcp. Therefore, our fcc vacancy results should be directly relevant for (hcp)⁴He, providing what we think is a lower bound on the vacancy concentration, while the interstitial concentration in⁴He is probably much lower than our results.     

Electronic and magnetic states in metallic compound—II: Electron transport and magnetic susceptibility in NiAl and FeAl

Electron transport and magnetic properties were investigated in NiAl and FeAl alloys in the vicinity of exact stoichiometry. The results as well as those of other workers were compared with recent augmented-plane-wave band calculations for NiAl, and semiquantitative agreement was found. With respect to impurity states in NiAl, it was concluded that transport anomalies, though definitely magnetic in origin, were not due to localized-magnetic moments on excess Ni atoms on Al sites. It was suggested that the anomalies may be due to subtle structural defects or such defects in combination with one of several intrinsic (non-impurity or defect) mechanisms. In the FeAl alloys, it was concluded that excess Fe atoms on Al sites (Fe_x atoms) carry a moment in the paramagnetic state of 7·8 μ_B. Negative magneto-resistance but no resistance minima effects were observed. The data, similar in several respects to those for dilute Rh(Fe) alloys, were interpreted in terms of antiferromagnetically coupled Fe_x atoms which lead to magnetic transitions of a spin-glass or mictomagnetic nature, that is, having no long-range order.     

Point-defect concentrations in B2 NiAl alloys under pressure

The pressure dependence of point-defect concentrations is calculated on the basis of two recently published models. It is demonstrated that the effective formation volume of vacancies shows asymmetrical character about stoichiometry. For Ni-rich and stoichiometric compositions, the concentration of Ni vacancies versus pressure can be described by a simple exponential function. The concentration of Ni antisite defects is independent of pressure. For Al-rich compositions, the concentration of Ni vacancies shows a more complex behaviour. At low pressures the vacancy concentration is independent of pressure, the effective formation volume is zero, while above a critical pressure the vacancy concentration becomes pressure dependent. This can be explained by the annihilation reaction of Ni vacancies. The pressure of complete replacement depends on the stoichiometry and has proved to be very sensitive to the input parameters used in the different models. The changes expected in diffusion activation energy and activation volume with composition are discussed on the basis of calculated temperature and pressure dependence of Ni vacancy concentration, assuming vacancy-mediated diffusion. The pressure effect on the boundary that separates regions in which triple defects or interbranch defects dominate was also investigated. It was observed that pressure expands the region in which Al interbranch defects dominate.     

Ab initio calculations of the geometry and electronic structure of point defects in ferroelectrics with a perovskite structure

Ab initio calculations of the optimized geometry and the electronic structure of lattice defects in incipient perovskite ferroelectrics SrTiO₃ and KTaO₃ are performed in the framework of the density functional theory. The results are presented for the Li⁺ impurity ion at the A site in the KTaO₃ and SrTiO₃ ferroelectrics; the Mn²⁺, Cd²⁺, Ca²⁺, Mg²⁺, and Zn²⁺ ions at the A site and the Mn⁴⁺ and Mg²⁺ ions at the B site in the SrTiO₃ compound; and the MN _Ti ²⁺ -V _O and Mg _Ti ²⁺ -V _O complexes in the SrTiO₃ ferroelectric. The results are obtained by the cluster method with allowance made for the structural relaxation initiated by the defect and, for nonisovalent substitutional impurities, with due regard for the charge and spin states of the defect. It is established that the Ca _Sr ²⁺ , Cd _Sr ²⁺ , Mn _Ti ⁴⁺ , and Mg _Ti ²⁺ ions have a stable central position, whereas the Li _K ⁺ ion in the KTaO₃ compound and the Li _Sr ⁺ , Mn _Sr ²⁺ , and Zn _Sr ²⁺ defects in the SrTiO₃ ferroelectric are off-center ions. The shape of the multiminimum adiabatic potential and the parameters of dielectric relaxators (activation barrier, dipole moment) for polar defects are obtained. The electronic impurity levels are determined for the Li _Sr ⁺ and Mg _Ti ²⁺ neutral defects.     

Diffusion and point defects in Cu2O

The diffusion of⁶⁴Cu in Cu₂O has been measured by a serial-sectioning technique as a function of temperature (700–1153°C) and oxygen partial pressure (10^?6?8 × 10^2&#x0304; atm). The oxygen-partial-pressure and temperature dependencies of D¹_Cu suggest that both neutral and singly charged copper vacancies contribute to cation self-diffusion. A defect model involving both neutral and singly charged copper vacancies, electron holes, and singly charged oxygen-interstitial ions is developed and fit to the tracer-diffusion data, the electrical-conductivity data (Maluenda et al.), and the stoichiometry data (O'Keeffe and Moore). The resulting defect data are quantitatively consistent with the chemical-diffusion data (Maluenda et al.) and with a correlation factor $\text{f}{}_{\mathrm{v}}\text{=}\text{2}\text{3}$. The defect data are also quantitatively consistent with the high-temperature oxidation studies of copper metal (Iguchi et al.) and with the copper vacancy being 10 times more mobile than the oxygen-interstitial ion at 10002C.     

Magnetic properties and a combined recursive-CPA calculation of pseudobinary intermetallics: Application to Hf(FeAl)2

The pseudobinary compounds Hf(Fe_1−xAl_x)₂, 0.15×0.30, show a pure C14 phase. Since the structure is stabilized in the presence of impurities, a combined recursive method — extended CPA is applied. The local densities of states for C14 are obtained within the recursive method and then the CPA describes the disorder. The changes with x of the local densities, and Fe magnetic moment are estimated.     

Vacancies effect on the mechanical properties in B2 FeAl intermetallic by the first-principles study

ABSTRACT

The geometric structures, electronic and mechanical properties of the high vacancy concentration intermetallic FeAl (experimental value: 3.3 at.% at 1451?K) were investigated by first-principles calculations based on density functional theory. The FeAl structures of different vacancy concentration with minimum energy were addressed, which shows that vacancies of iron (V_Fe) are more favourable and tend to gather together. For mechanical properties, both Young's modulus and elastic constants show an overall downward trend as vacancy concentration increases, but increase abnormally with the vacancy concentration ranging from 3.7 at.% to 5.6 at.%. All can be explained by the strength of Al–Fe bond, in other words, the Al–Fe interaction. Interestingly enough, intermetallic FeAl shows a transfer from the brittle manner to ductile manner, which also behaves as an important feature of FeAl in experiments. All the mechanical properties agree well with experimental data, indicating the reasonable vacancy model of FeAl intermetallic.