Investigation of the recovery and annealing kinetics of deformedα-iron by magnetic measurements

Isochronal annealing experiments in the temperature range 25–700° C revealed the existence of three annealing stages (stages IV, V and VI) in the annealing spectrum of cold-worked Fe-0.006 wt.% C by observing the associated changes in maximum magnetic susceptibility (_max) and magnetic coercivity (H _cr). Stage IV centered around 220° C appears only in the recovery of heavily cold-worked samples, activated by 1.1 eV, is attributed to the free-migration of vacancies. Stage IV disappears in the recovery spectrum of low-deformed samples and is inferred to the complete capture of vacancies originated during plastic deformation by carbon-atoms in the matrix. The recovery stage V, attributed to the dissocation of carbon-vacancy pairs, is found to be activated by an energy 1.8 eV. The binding energy between the carbon atom and vacancy is found to be 0.7 eV. The mechanism responsible for this recovery stage is enhanced by increasing the degree of plastic deformation in the matrix. The recovery stage VI appears above 450° C, activated by 2.8 eV, and the process is related to the climb motion of dislocations during the recrystallization process.     

Ferroelectricity associated with the metastable phase “III” of AgNO3

The dielectric constant, dc resistance, D-E ferroelectric hysteresis loop and dilatometric analysis of the three phases I, II, and III of AgNO₃ single crystals has been studied over the temperature range 100–200° C. A ferroelectric behaviour of the metastable phase III was detected here for the first time similar to what happened in KNO₃. The ferroelectric is attributed here to Ag⁺-ion vacancy formation in the unit cell of AgNO₃. The energy activating the process of vacancy formation was found to beE _v=2.6 eV. It was found that an ionic shift from one lattice point to another requires an amount of energy to overcome a potential barrierE _m=0.1 eV. A model is suggested to explain such behaviour. Dilatometric analysis indicated that this metastable phase transition III is accompanied by an expansion of the unit cell.     

Elucidating hydrogen assisting vacancy formation in metals: Mo and Nb as examples

Impurity H is inclined to be trapped by some defects with more space such as vacancy and grain boundary when it dissolves in a metal. Inversely, H can also enhance formation of large amount of vacancies even if there are few vacancies in intrinsic metals initially. Using first-principles simulation combined with statistical model, Mo and Nb as examples, we quantitatively calculate the concentration of vacancies including pure vacancy and H _n -vacancy (H _n V) complexes. The concentrations of vacancies in the form of H _n V notably increase due to the decrease of effective formation energy of vacancy from H stimulation. Our finding solves a long-standing puzzle on the atomistic mechanism underlying H assisting vacancy formation in some metals.     

μ+SR study of vacancies in thermal equilibrium in ferromagnets

Muon spin precession frequencies and transverse relaxation rates have been measured on demagnetized iron, cobalt, and FeCo alloys (3 at%–50 at% Co) between room temperature and the Curie temperatureT _c. The increase of the relaxation rate in iron between 930 K and 1010 K could be quantitatively attributed to the trapping of positive muons by vacancies in thermal equilibrium, resulting in an enthalpy of monovacancy formation ofH _1V ^F =(1.7±0.1) eV. the smallest vacancy concentrations detected are = 10⁻⁸.     

Influence of annealing on the conductivity and photoconductivity of p-type CdTe

On annealing p-type CdTe, considerable change in conductivity takes place. Samples of high resistivity were used for the measurements. Each of a set of samples was annealed at different temperature. After annealing, the temperature dependence of the conductivity and the relaxation curves of the photoconductivity were measured. Analysis of the first set of curves yielded value of energyE _a corresponding to the level occurring in given samples. It was established that this acceptor level is due to V^cd, or V^cd complexes, and is situated at 0.3 eV above the valence band edge. Concentration of these levels is increased by annealing. Furthermore, an energy value ofH=0.79 eV was found, corresponding very probably to the formation energy of a vacancy V^cd.Analysis of the relaxation curves yielded the temperature dependence of _S , _T and the energy distanceE _M of the impurity level that is responsible for photoconductivity. A value of (E _g -E _M )=0.09–0.12 eV was found for all samples studied. This level therefore lies below the conduction band edge and its concentration amounts to 10¹⁴–10¹⁵ cm^–3. The level is probably due to foreign impurities.Two sets of samples were used: both as-grown and Sb-doped. The results for both sets were not much different from each other.Ke Karlovu 3, Praha 2, Czechoslovakia.     

The low-field galvanomagnetic transport coefficients of cubic metals: General formulae and experiments on the hall coefficient in Al(Ge)

The scattering of conduction electrons by defects is described by a low-field transport relaxation time _k ^x . It depends on the electron wavevectork and on the direction of the electric fieldE/E, but not on the magnetic fieldH. From a discussion of Kohler's rule written in terms of the exact relaxation time _k ^* it follows that this approximation is very good for low values ofH/, with being the resistivity forH=0. Assuming _k ^x to be known, the linearized Boltzmann equation is solved by a Jones-Zener expansion up to termsH ³. For metals with cubic symmetry we derive simple formulae for the coefficients of transverse and longitudinal magnetoresistance and for the two leading terms of the Hall coefficientR(H)=R ₀+R ₂(H/)². Simplifications occur for a _k with cubic symmetry and for metals with special Fermi surfaces.These formulae are used to interpret experimental results of the magnetic field dependence ofR(H) in Al(Ge) at 4 K. In this dilute aluminium alloyR ₀ is highly positive andR ₂ strongly negative. By irradiating a 3,000 ppm Al(Ge) sample with reactor neutrons at 4 K, an increasing concentration of self-interstitials and vacancies is added to the germanium impurities resulting in a decrease of bothR ₀ and |R ₂|. This is discussed in a three-group model of the Fermi surface of aluminium.This work was supported by the German Bundesministerium für Forschung und Technologie     

Some physical properties of KCl+Pb2+ crystals

This paper presents the results of measurements of the mechanical and electrical properties of as-received pure and lead-doped crystals of potassium chloride. The critical resolved shear stress of the crystals obeys the Franks relationship in the whole concentration range. In addition, the correlation between ₀ and the Vickers hardness numberH was found and the equation is of the form ₀=k (H–H ₀). The solubility of Pb²⁺ in as-received KCl crystals was observed, from measurements of the electrical conductivity, to be low—the successive saturation of the solid solution started already in an environment of 5 mole ppm in agreement with data from the mechanical measurements. The density of cation vacancies and their mobility were represented by the following euqations ₁ T=6·25×10⁴ exp(–·75/kT) cm² K/volt. sec,n ₁=6·95×10²³ exp(–2·12/kT) vacancies/cm³ The value of 0sd46±0·02 eV was found as a rough estimate of the association energy.The authors wish to express their thanks to Professor Dr. J. Z.Damm and Ing. E.Mariani for their stimulating interest in the present work.     

The loss of single vacancies in metals during quenching

The Lomer equation, describing the rate of change of single vacancy concentration in a metal specimen during exponential quenching, was numerically solved for different quenching temperatures and different quenching rates. A relation has been found between the constantb, which characterizes the initial quenching rate, and the quenching temperature leading to the same percentual loss of vacancies. This relation enables us to determine the conditions of quenching, yielding the predetermined percentage of retained vacancies.The possibility is discussed of applying the above results in measurements of the formation energyEf^F, to estimate the migration energyE ^M and to determine the change of the mean number of jumps during quenching.The author would like to express his gratitude to Dr. K. Míek, head of the Metal Physics Department, for his interest and useful comments.     

Vacancy formation energies from positron trapping measurements

The threshold temperatureT _t at which thermally generated vacancies produce measurable positron trapping is a linear function of the energy of self-diffusionQ. SinceQ is also linearly related to the vacancy formation energyE _1v ^f , a measurement ofT _t leads directly to a determination of the latter. It is possible to make a precise determination ofE _1v ^f without approaching the melting point—a major advantage in dealing with refractories or with metals having a high vapour pressure in the solid state.     

Mass-spectrometric analysis of diffusion and solubility of helium in cerium-gadolinium ceramics with a submicrocrystalline structure

Diffusion and solubility of helium in Ce_0.8Gd_0.2O_{1.9 − δ} ceramics (δ = 0, 0.015) with a submicrocrystal structure are studied by thermodesorption of helium from preliminarily saturated (in the gas phase) crystals at temperatures of 613 and 673 K in the saturated pressure range 0–21 MPa. It is shown that, in this ceramics (δ = 0), the defect-trap diffusion mechanism operates. The main positions for dissolution are neutral anion vacancies formed as a result of thermal dissociation of impurity-vacancy complexes and saturated up to ∼1 × 10¹⁹ cm⁻³ at P = 6 MPa and T = 673 K. The dissociation energy of the complex and the energy of helium dissolution in the neutral anion vacancy are estimated at ∼2 eV and below −0.3 eV, respectively.     

Defects in electron irradiated GaP studied by positron lifetime spectroscopy

Defects induced by electron irradiation were investigated in GaP. The irradiation was performed at 15 K with an incident electron energy of 2 MeV and a fluence of 10¹⁸cm^–2. Annealing experiments were carried out in the temperature range between 100 and 1000 K.Ga and P vacancies were detected after electron irradiation and the different annealing behaviour of the two types of vacancies was observed. The recovery stage between 100 and 400 K was attributed to the annealing of Ga vacancies and the recovery at temperatures above 900 K to the annealing of P vacancies.We also performed Hall measurements to determine the location of the Fermi level in the bandgap during the annealing. Two different ionization levels of the P vacancy were found which can be attributed to the transitions V _P ⁺ /V _P ^o and V _P ^o /V _P ^– .Temperature-dependent measurements were performed to study the effect of shallow positron traps.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Vacancy induced changes in the electronic structure of titanium carbide—I. Band structure and density of states

Results of a self-consistent “Augmented Plane Wave” (APW) band-structure calculation are presented for substoichiometric titanium carbide with 25% vacancies on the carbon sublattice sites (TiC_{0 75}) assuming a model structure with ordered vacancies Comparison with an earlier APW study on stoichiometric TiC reveals that the carbon vacancies induce two pronounced peaks in the density of states (DOS), 0.4 eV below and 0.8 eV above the Fermi energy E_γ, thus forming electronic states in a region where the DOS for stoichiometric TiC exhibits a minimum So-called “vacancy states” with an important amount of charge on the vacancy site are found to be derived from Ti 3d states extending into the vacancy muffin tin sphere An angular momentum decomposition with respect to the center of the vacancy muffin tin sphere shows that the s character predominates for the occupied and the p character for the unoccupied “vacancy states” The theoretical findings explain features near E_γ, observed in recently published X-ray emission spectra Furthermore, we find a slight increase of electronic charge in the carbon muffin tin spheres as compared with stoichiometnc TiC.     

Monovacancy formation energy in copper,silver, and gold by positron annihilation

Monovacancy formation energies in copper, silver, and gold have been deduced from the temperature variation of the peak counting rate in the angular correlation curve of positron annihilation radiation from these metals. The counting rate was temperature dependent over the entire temperature range, including temperatures so low that no trapping of positrons at vacancies is effective. At these temperatures the increase in counting rate results from thermal expansion of the lattice. By separating this thermal expansion effect from the vacancy trapping effect at higher temperatures, we obtained values for the monovacancy formation energyE _1v for copper, silver, and gold to 1.29±0.02 eV, 1.16±0.02 eV, and 0.97±0.01 eV, respectively.     

Charge compensation and the luminescence of Cr3+ in KMgF3

Studies of the photoluminescence spectra of Cr³⁺ ions in KMgF₃ crystals co-doped with Cr³⁺ and Ni²⁺ ions are reported. Several crystal field sites are identified by the different R-line spectra due to the ² E⁴ A ₂ transition and broadband luminescences associated with the ⁴ T ₂⁴ A ₂ transitions. Cr³⁺ ions substituting without local charge compensation on the octahedral Mg²⁺ site give rise to a low temperature R line in photoluminescence at =702.3 nm with a radiative decaytime of 3 ms at T=14 K. At T=300 K this isotropic centre gives rise to an unpolarized broadband ⁴ T ₂⁴ A ₂ emission, which results from the thermal occupancy of an excited ⁴ T ₂ state just above the ² E level which, at lower temperature, gives rise to emission in the R-line. Other crystal field sites are due to some Cr³⁺ ions having Mg²⁺ or K⁺ vacancies in nearest-neighbour positions, these vacancies being required to maintain charge neutrality in doped fluoride perovskites. The Cr³⁺–K⁺ vacancy complex results in the centre having trigonal symmetry, and low temperature, photoluminescence via R ₁ and R ₂ lines at 716.8 nm and 716.0 nm, respectively. Finally, Cr³⁺ ions having a nearest neighbour Mg²⁺ vacancy have tetragonal symmetry, experiencing weak crystal fields. In consequence, the ⁴ T ₂ level lies below ² E and the photoluminescence spectrum at low temperature takes the form of a polarized broad ⁴ T ₂⁴ A ₂ band with peak at 760 nm and radiative decaytime of 54 s.     

The density of quenched gold

A precision hydrostatic differential method for measuring small density differences of solid bodies was used to determine the activation energy of vacancy formation and migration in quenched gold. The values were found to beE _f =0·76 eV, andE _m =0·51 eV.E _f is substantially lower than the well established value 0·96 eV, whereasE _m is close to the value 0·56 eV determined by Jeannette and Machlin for oxygen-free gold.The concentration of vacancies determined from our measurement is of the same order as the absolute calorimetric determination by DeSorbo.A short communication was read at the Conference on Point Defects in Quenched Metals, ANL, June 1964.     

Point defects related to 260 K thermostimulated luminescence in α-Al2O3

Abstract

Thermo- and photostimulated processes are studied in reduced hydrogen containing α-Al₂O₃ excited by UV light. It is found that UV excitation in F absorption band at 90 K results in a ionization of the F-centers and capture of released electrons at defects thus producing an anisotropy absorption band at 4.2 eV and the dominant thermoluminescence (TL) peak at 260 K. The 260 K TSL peak is accompanied by complete bleaching of the 4.2 eV absorption band and vice versa—by light stimulation in the region of the 4.2 eV band the 260 K TSL peak disappear and released electrons recombine with F⁺-centers. Both the effect of the preliminary high-temperature thermal treatment of samples on formation of 4.2 eV-centers and the observed dichroism characteristics allows to conclude that corresponding complex defect contains hydrogen and can involve vacancy pair.     

Formation energies and electronic structures of native point defects in potassium dihydrogen phosphate

The formation energies and the equilibrium concentration of vacancies,interstitial H,K,P,O and antisite structural defects with P and K in KH 2 PO 4 (KDP) crystals are investigated by ab initio total-energy calculations.The formation energy of interstitial H is calculated to be about 2.06 eV and we suggest that it may be the dominant defect in KDP crystal.The formation energy of an O vacancy (5.25 eV) is much higher than that of interstitial O (0.60 eV).Optical absorption centres can be induced by defects of O vacancies,interstitial O and interstitial H.We suggest that these defects may be responsible for the lowering of the damage threshold of the KDP.A K vacancy defect may increase the ionic conductivity and therefore the laser-induced damage threshold decreases.     

Dielectric relaxation and computer simulation studies of rutile-structured MnF2 crystals

The fluorides of the rutile structure are relatively simple ionic materials with tetragonal symmetry for which the dominant intrinsic defect has not been established. The present experiments involve low-temperature dielectric relaxation measurements on Er³⁺-and Y³⁺-doped MnF₂ single crystals. Unexpectedly, dielectric loss peaks were observed at cryogenic temperatures, involving very low activation energies,E. For both dopants a prominent peak is observed for samples oriented parallel to thec-axis withE 6 meV and in perpendicular orientations withE=37 meV for Er³⁺ and 46 meV for Y³⁺ doping. Such lowE-values are probably too small to be controlled by lattice migration of a defect. Rather, we expect that they are due to a very low symmetry configuration created when the ions near the defect move off symmetry to a more stable configuration. Computer simulation calculations have been carried out which are much improved over early studies of this system in terms of the code used and the F-F interatomic potentials. The results show that the energy per defect for the anion Frenkel (1.53 eV) is lower than that of the Schottky (1.99 eV). It was also shown that the fluorine interstitial, F_i, adopts a split-interstitial form. This defect associates strongly with trivalent dopants Er and Y to produce a low symmetry dipolar structure with the necessary off-symmetry configuration to explain the experimental findings. Since there is no alternative way to explain these low temperature relaxations in terms of impurities associated with Mn vacancies, as would be required by the Schottky model, we conclude that these experiments serve to establish the nature of the intrinsic defect in MnF₂ as anion Frenkel.