Defect structure studies using positron annihilation spectroscopy

The positron annihilation method is a new addition to the range of sensitive complementary nuclear techniques available for materials’ research. The preferential sensitivity of positrons towards micro-defect domains which are not assessable by other techniques makes it an attractive tool for many materials science problems. The present paper is intended as a brief introduction on the principle of measurements and its potential is exemplified with the help of results on some metallic and ceramic systems.     

Investigation of the electronic structure of ferro- and paramagnetic nickel by positron annihilation

Positron angular correlation curves have been measured for nickel in the [100], [110], and [111] directions at room temperature, 330°C and 380°C. Mijnarend's approach is used for their interpretation. For ferromagnetic nickel th 6th zone Fermi Surface (FS) obtained in this way is similar to that obtained from other experimental measurements as well as from band structure calculations. At 380°C the regrouping of the 5th zone electrons at the Curie point is well visible, and information is obtained about the paramagnetic nickel FS in the 6th and 5th zones. Besides, at this temperature the FS in the 6th zone is larger and rounded off. This effect, still needing further experimental confirmation, is attributed to thermal vibrations of the lattice.     

Temperature dependence of positron annihilation in iron and vanadium

The Doppler broadened lineshape is measured as a function of temperature for iron and vanadium; vacancies produce a strong trapping effect in γ-Fe, but a weak effect in V. Threshold temperaturesT _t for trapping are 1280±25 and 1370±30 K. Empirical linear relationships betweenT _t , self-diffusion energyQ ^SD and monovacancy formation energyH _1v ^F are discussed and used to determineH _1v ^F values for γ-Fe and V.     

Mössbauer spectroscopic and positron annihilation studies of iron and tin containing aluminium alloys

The application of Mössbauer spectroscopy and positron annihilation to study phase transitions, precipitation, dissolution and vacancy-tin interaction in iron and tin containing aluminium alloys was shown on three examples.     

Experimental and theoretical positron annihilation studies on bulk nickel silicides

Bulk nickel silicides (NiSi and NiSi₂) have been studied using the experimental positron lifetime and depth-resolved positron beam measurements. Ab-initio calculations of positron lifetime for the silicides have also been carried out using the atomic superposition method. For NiSi phase, it is found that the theoretically computed positron lifetime compares favourably with the experimentally deduced value indicating that NiSi is defect free. However, for NiSi₂, the experimental positron lifetime does not compare well with the theoretical value suggesting that NiSi₂ contains vacancy defects. This is further supported by the positron diffusion lengths deduced from the VEPFIT analysis of the positron beam results.     

Fascinating electronic games in iron complexes

Summary Coordination compounds of transition metal ions with open-shell electron configurations may exhibit dynamic electronic-structure phenomena, depending on the nature of the coordinating ligand sphere. The change of spin state with temperature (?thermal spin-crossover?), light-induced electron transfer processes leading to long-lived metastable charge and spin states (e.g., ?LIESST? effect), are some of the fascinating electronic games encountered in transition metal compounds, which are presently under extensive study by chemists and physicists. M?ssbauer spectroscopy plays a dominant role in the investigation of such phenomena in iron compounds, as will be demonstrated in this paper. This work will focus on selected examples of ?thermal spin-crossover? in iron(II) complexes and switching between different spin states by irradiation with light of different wavelength (LIESST effect), demonstrating that M?ssbauer spectroscopy besides other physical techniques proves to be a highly elegant tool for following the spin state conversion and the concomitant changes of molecular and crystal structure properties. Finally, M?ssbauer emission spectroscopy, both time integral and time differential, has been employed to generate and identify long-lived excited spin states by making use of the nuclear disintegration of⁵⁷Co as an intrinsic molecular light source (NIESST=nuclear decay-induced excited-spin-state trapping). Lifetime measurements by optical techniques on LIESST states and by time-differential M?ssbauer coincidence spectroscopy on NIESST states prove that the relaxation pathways in the mechanisms for LIESST and NIESST are identical. Dedicated to Prof. Harry B. Gray on the occasion of his 60th birthday. Paper presented at ICAME-95, Rimini, 10–16 September 1995.     

Investigations of neutron irradiated iron by positron annihilation and correlated internal friction measurements

Positron annihilation experiments have been performed on iron after low temperature neutron irradiation and during isochronal annealing. A good correlation exists between internal friction and positron annihilation data for iron with interstitial impurities, proving that these impurities react with vacancies during annealing (complex-formation).     

On positron annihilation in zinc

The purpose of this work is to understand Mogensen's and Petersen's positron annihilation curves for zinc. Mijnarends' approach is used as an auxiliary method of localizing inhomogeneities of the electronic density in momentum space, as defined in the paper. Evidence is found for a new effect consisting of a strong enhancement of the annihilation probability in the lenses obtained by the intersection of the Fermi surface with HMC surfaces. This effect, not the anisotropy of the Fermi surface, is the main reason for the anisotropy of the annihilation curves. Paper presented at 3rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973).     

Direct evidence of Cd vacancies in CdSe nanoparticles: positron annihilation studies

Poly vinyl alcohol (PVA) capped CdSe nanoparticles having size in the range of 7–17 nm have been synthesized through chemical route and characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Positron coincidence Doppler broadening (CDB) measurements have been carried out in these nanoparticles. It is observed that the electron momentum distributions show a variation in the core electron momentum region with the particle size. In order to examine the influence of defects, first principle calculations of electron momentum distributions in bulk CdSe and in the presence of Cd as well as Se vacancy defects have been performed. Comparison of experimental data with the calculated momentum distribution reveals the presence of Cd vacancy defects, the concentration of which decreases with the increase in the particle size. The present study also indicates possible Se enrichment on the surface of the nanoparticles with the decrease in the particle size.     

Relevance of slow positron beam research to astrophysical studies of positron interactions and annihilation in the interstellar medium

The processes undergone by positrons in the interstellar medium (ISM) from the moments of their birth to their annihilation are examined. Both the physics of the positron interactions with gases and solids (dust grains), and the physical conditions and characteristics of the environments where the processes of energy loss, positronium formation, and annihilation taking place, are reviewed. An explanation is given as to how all the relevant physical information are taken into account in order to calculate annihilation rates and spectra of the 511 keV emission for the various phases of the ISM; special attention is paid to positron interactions with dust and with polycyclic aromatic hydrocarbons. An attempt is made to show to what extent the interactions between positrons and interstellar dust grains are similar to laboratory experiments in which beams of slow positrons impinge upon solids and surfaces. Sample results are shown for the effect of dust grains on positron annihilation spectra in some phases of the ISM which, together with high resolution spectra measured by satellites, can be used to infer useful knowledge about the environment where the annihilation is predominantly taking place and ultimately about the birth place and history of positrons in the Galaxy. The important complementarity between work done by the astrophysical and the solid-state positron communities is strongly emphasized and specific experimental work is suggested which could assist the modeling of the interaction and annihilation of positrons in the ISM.     

Isochronal annealing studies of deformed FeSi alloys using positron annihilation spectroscopy

A series of FeSi samples were deformed to a thickness reduction of 16%. They were isochronally annealed for one hour at different temperatures and characterized by the Doppler broadening of the annihilation radiation (DBAR) measured at room temperature. Optical microscopy (OM) is used to investigate the microstructure of the deformed samples before and after annealing. The S parameter data show a decrease with the increase of the annealing temperature. At 973 K a significant decrease sets in. The microstructures of the alloys, investigated by OM, show that recrystallization is completed at 1173 K.     

Evolution,migration and clustering of helium-vacancy complexes in RAFM steel- depth resolved positron annihilation Doppler broadening study

Indian Reduced Activation Ferritic Martensitic steel is implanted with 130 keV helium ions to a fluence of 5 × 10¹⁴ and 1 × 10¹⁶ ions/cm² and investigated using positron annihilation spectroscopy. The samples were characterised by defect sensitive S and W-parameters using depth resolved slow positron beam. A dose dependency is observed in the nucleation and growth of helium bubbles with annealing temperature. An experimental evidence for the migration of smaller helium-vacancy complexes is observed via the variation in thickness/width of irradiated layer with temperature. The S–W plot clearly shows the regions corresponding to defect annealing, bubble nucleation and growth.     

Defect structure and dielectric properties of Bi-based pyrochlores probed by positron annihilation

Positron annihilation lifetime (PAL) and Doppler broadening (DB) techniques have been performed to identify structural defects of the bismuth based pyrochlore systems with generic formula (Bi_1.5Zn_0.5)(Zn_0.5−x/3Ti_xNb_1.5−2x/3)O₇ (x = 0, 0.25, 0.5,1.0, 1.5). We found that all studied compounds contain substantial amount of the lattice vacancy defects, the variation of the annihilation lifetime suggests that the defects structure undergoes significant changes. The complex defects could be produced with increasing content of Ti, resulting in a drop in the intensity I₂ in the Ti-rich sample. At 1 MHz their dielectric constant (?′) varies from 150 for Ti-poor system to 210 for Ti-rich system and loss tangent (tan δ) remains rather low level. The high dielectric constant response of the BZTN ceramics is attributed to loosening state of cations located in the center of octahedral, so favor off-center displacement. The occurrence of complex defects help to enhance the dielectric constant.     

SHI induced modification in CR-39 polycarbonate by positron annihilation lifetime studies

CR-39 is a polycarbonate widely used as SSNTD for recording nuclear charged particles and in other applications. Latent ion tracks produced in the polymers due to the damage produced by the passage of Swift Heavy Ions contain amorphous material with highest degree of disorder, changing the free volume properties which have strong correlation with the macroscopic properties of the material. Positron annihilation lifetime spectroscopy (PALS) provides direct information about the dimension, content and hole size distribution of free volume in polymers. The effect of irradiation of ⁴⁰Ar (14.9 MeV/n) ions on CR-39 polycarbonate by Positron Annihilation Lifetime Spectroscopy (PALS) is reported here. PALS provides a non-destructive and non-interfering probe, having high detection efficiency for free volume hole properties. From o-Ps lifetime mean free volume hole radius and average free volume of the micro-voids have been calculated. PAL measurement shows an increase in free volume on irradiation.     

Energetics and electronic structure of refractory elements in the dislocation of NiAl

Using DMol and the discrete variational method within the framework of the density functional theory,we study the alloying effects of Nb,Ti,and V in the [100](010) edge dislocation core of NiAl.We find that when Nb(Ti,V) is substituted for Al in the center-Al,the binding energy of the system reduces 3.00 eV(2.98 eV,2.66 eV).When Nb(Ti,V) is substituted for Ni in the center-Ni,the binding energy of the system reduces only 0.47 eV(0.16 eV,0.09 eV).This shows that Nb(Ti,V) exhibits a strong Al site preference,which agrees with the experimental and other theoretical results.The analyses of the charge distribution,the interatomic energy and the partial density of states show that some charge accumulations appear between the impurity atom and Ni atoms,and the strong bonding states are formed between impurity atom and neighbouring host atoms due mainly to the hybridization of 4d5s(3d4s) orbitals of impurity atoms and 3d4s4p orbitals of host Ni atoms.The impurity induces a strong pinning effect on the [100](010) edge dislocation motion in NiAl,which is related to the mechanical properties of the NiAl alloy.     

Properties of positron annihilation in metals

Positron annihilation in bulk metals has been studied by examining the angular distribution of the annihilation photons in polycrystalline samples of magnesium, aluminum, copper, zinc, indium, tin, lead, and bismuth. It has been shown that conduction electrons as well as core electrons take part in this process. The conduction electron densities and Fermi energies have been determined. It is found that the electron density in the vicinity of a positron is significantly higher than the density of the free electron gas. We believe that this is due to the formation of Wheeler complexes and we estimate its charge. We have analyzed various means of measuring the conduction electron density and conclude that the positron method gives the most reliable information. Fiz. Tverd. Tela (St. Petersburg) 41, 929–934 (June 1999)     

Preliminary studies on a variable energy positron annihilation lifetime spectroscopy system

There are many advantages in being able to perform positron annihilation lifetime spectroscopy (PALS) using a variable energy positron beam, the most obvious being the easy identification of different defect types at different depths. The difficulty in conducting variable energy (VE) PALS studies lies in the fact that a “start” signal is required to signal the entry of the positron into the target. Two methods have been used to overcome this problem, namely the bunching technique, which employs radio frequency (RF) cavities and choppers, and secondly the use of secondary electrons emitted from the target. The latter technique is in terms of experimental complexity much simpler, but has in the past suffered from poor time resolution (typically ∼500 ps). In this work, we present a series of computer simulations of a design based on the secondary electron emission from thin C-foils in transmission mode which shows that significant improvements in time resolution can be made with resolutions ∼200 ps being in principle possible.     

Isothermal annealing of room temperature deformed iron studied with the positron annihilation technique

Positron-annihilation lineshape parameter measurements were performed during isothermal annealing of room temperature deformed iron. An isothermal annealing effect is seen in impure iron while in pure iron no effect is measured.