Positron Lifetime Calculations Research Articles

Positron annihilation in structural vacancies in Al‐rich NiAl alloys

AbstractPositron annihilation in structural vacancies in B 2 Al‐rich NiAl alloys is studied theoretically. For this purpose we use the atomic superposition method combined with molecular dynamics employed for relaxations of random vacancy configurations. We found that the defect lifetime decreases with the increasing Ni vacancy concentration. Calculated positron lifetimes are discussed in terms of the ‘vacancy‐superstructure' effect and compared with experimental positron lifetime data available. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Positron lifetime calculations for stacking fault tetrahedra in copper

AbstractWe investigate the positron response of simulated stacking fault tetrahedra in Cu. We find that in both the regular and truncated stacking fault tetrahedra positron lifetimes are about 10‐15 ps larger than the bulk lifetime. This is an indication that the vacancy‐like defects observed in neutron irradiated Cu by positron annihilation lifetime measurements do not arise during the formation of the stacking fault tetrahedra but rather during the interaction of the stacking fault tetrahedra with other defects. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Energetics of positron states trapped at vacancies in solids

We report a computational first-principles study of positron trapping at vacancy defects in metals and semiconductors. The main emphasis is on the energetics of the trapping process including the interplay between the positron state and the defect's ionic structure and on the ensuing annihilation characteristics of the trapped state. For vacancies in covalent semiconductors the ion relaxation is a crucial part of the positron trapping process enabling the localization of the positron state. However, positron trapping does not strongly affect the characteristic features of the electronic structure, e.g., the ionization levels change only moderately. Also, in the case of metal vacancies, the positron-induced ion relaxation has a noticeable effect on the calculated positron lifetime and momentum distribution of annihilating electron-positron pairs.

Positron lifetime calculations of defects in chromium containing hydrogen or helium

The characteristics of defects in chromium containing hydrogen or helium atoms have been investigated by positron lifetime quantum calculations. On the basis of calculated results, the behaviors of empty nano-voids and nano-voids with hydrogen or helium were discussed. It was found that hydrogen and helium in larger three-dimensional vacancy clusters change the annihilation characteristics dramatically. The hydrogen and helium atoms are trapped by lattice vacancies. Helium bound with vacancies can form larger size cluster in chromium. These results provide physical insight for positron interactions with defects in chromium and can be used for prediction of hydrogen or helium generation for the design of fission and fusion reactors. The positron lifetime calculations were performed by the standard DFT density functional theory method. The electron wave functions have been obtained in the local density approximation LDA to the DFT.

Positron lifetime calculations of defects in vanadium containing hydrogen

Positron lifetime quantum-mechanical calculations were carried out for the investigation of defects in vanadium containing hydrogen atoms. The convergence of electronic structure calculations for supercell size is studied for vacancies, vacancy-clusters (nano-voids) and vacancy-clusters containing hydrogen. The electron wave functions have been obtained in the local density approximation (LDA) to the density functional theory (DFT). On the basis of the calculated results, the behaviours of empty nano-voids and nano-voids with hydrogen are discussed. It is found that hydrogen in larger three-dimensional vacancy-clusters change the annihilation characteristics drastically. The hydrogen atoms are trapped by lattice vacancies. These results provide physical insight for positron interactions with defects in vanadium and can be used for the prediction of hydrogen generation useful for the design of a fusion reactor.

Positron study of electron irradiation-induced vacancy defects in SiC

Based on positron annihilation experiments, we have proposed that in 3C-SiC isolated silicon vacancies are responsible for positron trapping after electron irradiation. We have also proposed that in hexagonal SiC one type of vacancy defects survives after annealing at 1000 °C which is attributable to carbon–vacancy–carbon–antisite complexes or silicon–vacancy–nitrogen pairs, while carbon vacancies, silicon vacancies and divacancies are excluded. In this study, from the theoretical calculations of positron lifetime and Doppler broadening of annihilation radiation, the above proposals are confirmed.

Characterization of defects in a martensitic CuAlNi shape-memory alloy

A water-quenched martensitic CuAlNi shape-memory alloy was investigated by a combination of coincidence Doppler broadening and positron-lifetime spectroscopy, supported by positron-lifetime calculations. We find a high defect concentration in the as-quenched samples. The positron-lifetime calculations suggest that the defects are not only single vacancies but also vacancies associated with dislocations and stacking faults. Annealing in the martensitic phase has no significant influence on the vacancy concentration but results in a different chemical environment around the vacancies. After aging in the austenitic phase the vacancy concentration decreases significantly.

Publisher's Note: Two-component density functional theory calculations of positron lifetimes for small vacancy clusters in silicon [Phys. Rev. B 71, 205215 (2005)

Publisher's Note: Two-component density functional theory calculations of positron lifetimes for small vacancy clusters in silicon [Phys. Rev. B 71, 205215 (2005)

Two-component density functional theory calculations of positron lifetimes for small vacancy clusters in silicon

The positron lifetimes for various vacancy clusters in silicon are calculated within the framework of the two-component electron-positron density functional theory. The effect of the trapped positron on the electron density and on the relaxation of the structure is investigated. Our calculations show that, contrary to the usual assumption, the positron-induced forces do not compensate in general for electronic inward forces. Thus, geometry optimization is required in order to determine positron lifetime accurately. For the monovacancy and the divacancy, the results of our calculations are in good agreement with the experimental positron lifetimes, suggesting that this approach gives good estimates of positron lifetimes for larger vacancy clusters, required for their correct identification with positron annihilation spectroscopy. As an application, our calculations show that fourfold trivacancies and symmetric fourfold tetravacancies have positron lifetimes similar to monovacancies and divacancies, respectively, and can thus be confused in the interpretation of positron annihilation experiments.

The observation of the lattice defect formation during hydrogenation and dehydrogenation in La(Ni,Sn)5 by in-situ positron lifetime measurement

AbstractTo clarify the effect of Sn substitution for Ni of LaNi5 on the lattice defect formation during the hydrogenation and dehydrogenation processes, in-situ positron lifetime measurements were per-formed in LaNi4.93Sn0.27. During the hydrogenation, the mean positron lifetime, τm, monotonically increased up to 175 ps which is almost same as calculated positron lifetime for vacancy. This shows vacancy introduction by hydrogenation. The τm mean positron lifetime decreased down to 135 ps with hydrogen content during the dehydrogenation. It shows that the vacancies are removed from the lattice during the dehydrogenation. These results show that vacancies in LaNi4.93Sn0.27 are intro-duced and removed reversibly during the hydrogenation and dehydrogenation. The concentra-tions of vacancy and dislocation were 1 × 10−5 and 6 × 109 cm−2, respectively. These values are two orders lower than those in LaNi5.

A positron study of the defect structures in the D0 3 and B2 phases in the Fe–Al system

The positron annihilation technique was used to identify the nature of the vacancy-type defects in the D0 3 and B2 phases of the Fe–Al system. Seven alloys with Al concentrations in the range 22.7–48 at.% Al and with different thermal treatments were examined. Positron lifetime calculations for the expected defects in the two phases were also performed in order to facilitate the defect identification. In the B2 phase, two types of defects were identified: a thermal complex formed by a Fe-divacancy and an Al antisite, and a Fe-vacancy. No constitutional vacancies were found in the D0 3 phase.

First-principles calculations of positron lifetimes of lattice defects induced by hydrogen absorption

Positron lifetime spectroscopy is a sensitive tool for the study of vacancy-type defects in solids. Our positron lifetime measurements have revealed that huge numbers of excess vacancies are formed in addition to dislocations during the first hydrogen absorption process in some hydrogen storage materials. In this work, we have performed first-principles calculations of positron lifetimes of vacancies as well as the bulk state in LaNi 5–H and Pd–H systems in order to identify lattice defects formed during hydrogen absorption and during isochronal annealing after hydrogen desorption. By comparison between the experimental and theoretical positron lifetimes, one of the defect components formed during hydrogen absorption can be ascribed to the annihilation at the vacancy clusters composed of two or three vacancies in both LaNi 5–H and Pd–H systems. During isochronal annealing after hydrogen desorption in LaNi 5, the positron lifetime of the vacancy-cluster component in LaNi 5 increases from 190 to over 400 ps, which indicates the formation of three-dimensional vacancy clusters. On the other hand, the vacancy-cluster component in Pd decreases from 200 to 160 ps during isochronal annealing. This suggests that not three-dimensional but two-dimensional vacancy clusters are formed in the recovery process of vacancies in Pd.

Positron Lifetime Calculations of Defects in Nickel Containing Hydrogen at Various Temperatures

Positron Lifetime Calculations of Defects in Nickel Containing Hydrogen at Various Temperatures

Defect structure in non-stoichiometric CoTi with CsCl-type ordered structure

abstractPositron lifetimes in CsCl-type intermetallic compounds Co100–XTiX (X ¼ 48 –50:3) water-quenched and furnace-cooled from 1323 K have been measured to study their defect structures. Positron mean lifetimes in water-quenched specimens of both Co-rich and Ti-rich compositions are much longer than the calculated positron lifetime in the perfect lattice, and no matrix component is observed. The positron mean lifetimes in the furnace-cooled specimens of Ti-rich compositions do not have large difference from those for the water-quenched specimens, whereas the furnace-cooled specimens of Co-rich compositions show short positron mean lifetimes, which are almost equal to the calculated lifetime in the perfect lattice. These results indicate that Co-antistructure atoms and Co-vacancies are formed in CoTi as constitutional defects in accord with the model proposed by Bradley and Taylor.

Theoretical Calculations of Positron Lifetimes for Metal Oxides

Our recent positron lifetime measurements for metal oxides suggest that positron lifetimes of bulk state in metal oxides are shorter than previously reported values. We have performed theoretical calculations of positron lifetimes for bulk and vacancy states in MgO and ZnO using first-principles electronic structure calculations and discuss the validity of positron lifetime calculations for insulators. By comparing the calculated positron lifetimes to the experimental values, it was found that the semiconductor model well reproduces the experimental positron lifetime. The longer positron lifetime previously reported can be considered to arise from not only the bulk but also from the vacancy induced by impurities. In the case of cation vacancy, the calculated positron lifetime based on semiconductor model is shorter than the experimental value, which suggests that the inward relaxation occurs around the cation vacancy trapping the positron.

Theoretical calculation of positron lifetimes for LaNi5–H system

Positron lifetime spectroscopy is a powerful tool for the study of vacancy-type defects in solids. Our positron lifetime measurements have revealed that huge numbers of excess vacancies are formed in addition to dislocations during the first hydrogen absorption process of LaNi5 and excess vacancies becomes mobile and form microvoids by a thermal activation process. For further investigation, theoretical approaches using electronic structure calculations are indispensable. In this work, we have performed theoretical calculations of positron lifetime for LaNi5–H system using first principles electronic structure calculations. By comparison between the theoretical and experimental positron lifetimes, one of the defect components during hydrogen absorption can be ascribed to the annihilation at vacancy-clusters composed of two or three Ni vacancies. The positron lifetime of the vacancy-cluster component increases over 400 ps during isochronal annealing after hydrogen desorption. The vacancy-cluster may contain not only Ni vacancies but also La vacancies, since the vacancy-cluster composed of only Ni vacancies cannot yield such a long positron lifetime.

Stability of large vacancy clusters in silicon

Using a density-functional-based tight-binding method we investigate the stability of various vacancy clusters up to a size of 17 vacancies. Additionally, we compute the positron lifetimes for the most stable structures to compare them to experimental data. A simple bond-counting model is extended to take into account the formation of new bonds. This yields a very good agreement with the explicitly calculated formation energies of the relaxed structures for ${V}_{6}$ to ${V}_{14}.$ The structures, where the vacancies form closed rings, such as ${V}_{6}$ and ${V}_{10},$ are especially stable against dissociation. For these structures, the calculated dissociation energies are in agreement with experimentally determined annealing temperatures and the calculated positron lifetimes are consistent with measurements.

Positron lifetime and coincidence Doppler broadening study of vacancy-oxygen complexes in Si: experiments and first-principles calculations

Positron lifetime and coincidence Doppler broadening (CDB) techniques, combined with first-principles calculations of positron annihilation characteristics, are employed to study vacancy-oxygen (VO) complexes in Czochralski-grown silicon (Cz-Si). In the experiments, the positron lifetimes and CDB spectra were measured as functions of post-irradiation annealing temperature for a series of electron-irradiated Cz-Si samples. Though the longer lifetimes for the defects are nearly constant at about 300 ps, the CDB spectra exhibit a distinct stage around 350 °C, indicating a marked change in the defect nature after the post-irradiation annealing. These experimental results are compared with the calculated positron lifetimes and CDB spectra for various vacancy-oxygen complexes in Si. This comparison clarifies that, after annealing at 350 °C, the irradiation-induced divacancies and A centers aggregate into more stable V 3O and V 4O 2.

Positron characteristics of various SiO 2 polymorphs

Theoretical calculations of positron lifetimes, affinities and core electron contributions to Doppler broadening spectra are carried out for several phases (polymorphs) of SiO 2. An interesting feature of SiO 2 is that not all polymorphs have the same coordination of Si and O atoms, which also affects positron properties. The calculated quantities are compared with available experimental data and discussed with reference to SiO 2/Si interfaces.

Theoretical Calculation of Positron Lifetimes in CoAl and CoTi

Positron states for the bulk and vacancy in intermetallic compounds CoAl and CoTi crystallizing in the B2 structure have been calculated using the DV-Xα electronic structure calculation and compared with experimental results. The calculated positron lifetimes in CoAl can explain the compositional dependence of the positron lifetimes in CoAl. The calculated positron lifetime at the Ti vacancy in CoTi is longer than that at the Co vacancy. The charge transfer from Ti to Co leads to the difference of the localization of the positron wave function at the vacancy. At the Co vacancy site surrounded by the positively charged Ti atoms, the positron wave function does not localize as well as at the Ti vacancy site, which decrease the positron lifetime. The calculated results suggest that the positron lifetimes in intermetallic compounds can not be directly correlated to those of the pure component and the charge transfer between the constituent atoms affects the localization of the positron wave function. Intermetallic compounds are of considerable interest due to their physical and mechanical properties. In intermetal- lic compounds having a wide range of concentrations around the stoichiometry, structural defects are introduced in order to compensate deviations from the stoichiometry. Several prop- erties show compositional dependence originating from the structural defects. Main structural defects are vacancies on the sublattice and anti-structure atoms occupying the other sublattice. Positron annihilation spectroscopy is a sensitive tool for vacancy-type defects in solids. In a perfect crystal, the positron wave function is delocalized in the interstitial region because of the repulsion from the ion cores. If vacancy-type defects exist in solids, the positron wave function is localized at the defect where the electron density is lower than the other regions. The localization of the positron at the defect results in a longer lifetime of the positron compared to the bulk life- time because the positron lifetime is inversely proportional to the electron density where the positron annihilated. Positron annihilation spectroscopy is very useful to study the behav- ior of the structural defects in intermetallic compounds. If the vacancy is mainly responsible for the deviation from the stoichiometry, longer positron lifetimes should be measured compared to the bulk lifetime. However, interpretation of the experimental positron lifetime for intermetallic compounds is more complicated than for pure metals, because several types of vacancies may exist in intermetallic compounds. The dif- ferent types of vacancies may yield the different positron life- times. In order to characterize measured positron lifetimes in intermetallic compounds, a theoretical analysis is indispens- able. However, little theoretical work has been done for inter- metallic compounds. In this work, we have performed calculations of positron lifetimes in intermetallic compounds CoAl and CoTi with the B2 structure based on first-principles electronic structure cal- culations. The calculated results are compared with the exper- imental positron lifetimes in order to identify the defect type in intermetallic compounds and estimate the reliability of the calculated positron lifetimes. 2. Computational Method