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Valence electron and positron charge densities in SiC are obtained from wave functions derived in a model pseudopotential bandstructure calculation. It is observed that the positron density is maximum in the open interstices and is excluded not only from the ion cores but also, to a considerable degree, from the valence bonds. Electron–positron momentum densities are calculated for the (001–110) plane. The results are used to analyze the positron effect in large gap semiconductors.

The Pr-substituted YBa₂Cu₃O_7-δ(Y123) superconducting systems with the content of 0.0–1.0 have been systematically studied by positron lifetime and infrared absorption experiments. The results show that the short lifetime τ₁ decreases as a function of Pr-substitution x below x=0.6, but when above x=0.6, it increases inversely. The long lifetime τ₂ decreases as a function of Pr-substituted x. Based on the present results, we discuss that Pr⁴⁺ substitutes on Ba²⁺ ion and forms Pr_Ba defects. The variation of τ₁ is caused by the oxygen vacancies and τ₂ is probably affected by the absence of Cu due to Pr_Ba defects. The infrared absorption of Pr-substituted Y123 systems gives three variational modes, located at 560 cm^-1(A₁), 1435 cm^-1(A₂) and 1631 cm^-1(A₃) respectively. Here the A₃ mode shows that with Pr content increasing from x=0.1 to 1.0, the infrared absorption decreases and the peak tends to broaden. So, combining with the results of the positron lifetime spectra, we can argue that there always exists portion of Pr_Ba defects in the Pr-substituted systems, and the Pr⁴⁺ substituted into Ba²⁺ is also increasing with the Pr content increasing. When the Pr content is 0.6, the systems have a metal-insulator transition and Pr_Ba defects begin to domain the samples' microstructure.

To recognize the distributional characteristics of Fe and Ni doping and the fundamental effects of magnetic ions on superconductivity, the X-ray diffraction, positron annihilation technology and simulated calculations are utilized to investigate systemically YBa₂Cu_3-x(Fe, Ni)_xO_7-δ(x=0.0~0.5). The results show that Fe and Ni doping form different kinds of ion clusters and enter the crystal lattice. When occupying Cu(2) sites in the CuO₂ planes, ions gather into Double Square and/or other clusters, which results in a strong electronic localization and would directly influence the pairing and transportation of carriers, so the superconductivity is suppressed dramatically. While doping ions enter the Cu(1) sites through gathering Hexamer and/or other clusters, this induces the localization of holes and weakens the function of carrier reservoir, so carriers cannot be easily transferred to the CuO₂ planes. However, in this case, the pairing and transportation of carriers are not affected directly, thus the superconductivity is suppressed weakly. Besides, the present results indicate that the superconductivity suppressed has no direct correlation with the magnetism of Fe and Ni ion itself.

Results of molecular motion studies carried out in two liquid crystal forming compounds n-p-cyano-p-hexyloxybiphenyl (M18) and n-p-ethoxybenzylidene-p-butylaniline (EBBA) using positron lifetime spectroscopy (PLS) are presented. Temperature dependent positron lifetime measurements have been performed in each compound during the heating cycle of samples prepared by either quenching or slow cooling from the respective liquid crystalline phase of the compounds. In both the compounds, behaviors of the quenched and slow cooled samples are found to be different. The material in the quenched sample, unlike the slow-cooled sample, exhibits strong temperature dependence before it undergoes a glass transition. In each case, the temperature dependence of o-Ps pick-off lifetime in the quenched sample shows broad peaks at various characteristic temperatures. These peaks have been attributed to various intra- and inter-molecular motions associated with these compounds. The characteristic frequencies of some of the modes observed in the present work agree well with the literature reported values obtained from FIR and Raman studies. The present study demonstrates the usefulness of PLS in the study of molecular motions.

The independent particle model (IPM) coupled with empirical pseudopotential method (EPM) was used to compute the thermalized positron charge densities in specific family of binary tetrahedrally coordinated crystals of formula A^NB^8-N. Initial results show a clear asymmetrical positron charge distribution relative to the bond center. It is observed that the positron density is maximum in the open interstices and is excluded not only from the ion cores but also to a considerable degree from the valence bonds. Electron-positron momentum densities are calculated for the (001, 110) planes. The results are used to analyze the positron effects in GeC and SnC. Our computational technique provides the theoretical means of interpreting the k-space densities obtained experimentally using the two-dimensional angular correlation of annihilation radiation (2D-ACAR).

The influence of Nb₂O₅-doped concentration on the positive temperature coefficient of resistance (PTCR) effect, electrical properties and microdefects of (Ba${}_{0.95}$Sr${}_{0.05}$)(TiNb${}_x$)O₃ (BSTN) ceramics were investigated. Firing was conducted at 1350${}^{\circ }$C for 2 h in air. The donor-doped content affected the electrical properties, PTCR effect and formation of the microdefect type of the BSTN samples. The room temperature resistivity of the BSTN specimens first decreased and then increased with increasing donor-doped content in the range of 0.2 mol.% Nb${}^{5+}$ to 0.5 mol.% Nb${}^{5+}$. Moreover, the information on microdefects in BSTN ceramics was demonstrated by coincidence Doppler broadening spectrum. The influence of the defects on the PTCR characteristics of the ceramics was also revealed.

Detailed Mössbauer studies carried out in liquid sodium (Na)-exposed austenitic stainless steel (SS-316) show that there is a partial formation of ferromagnetically (FM)-ordered ferritic zones in the paramagnetic austenitic matrix. Results of low energy positron beam-based Doppler broadening studies imply the occurrence of vacancy kind of defects in the liquid Na-exposed SS-316. Correlating these results, the partial occurrence of FM-ordered zones in the liquid Na-exposed SS-316 is understood to be due to open volume defects, predominantly that of Ni vacancies occurring at the surface and upto a certain depth of liquid Na-exposed stainless steel. These results are elucidated in terms of hyperfine parameters associated with ferritic zones.

Proton-irradiation induced defects in Te-doped GaSb have been studied by photoluminescence (PL) and positron annihilation spectroscopy (PAS). A 2.6 MeV proton irradiation with fluences of 1×10¹⁴ cm^-2, and 3×10¹⁵ cm^-2 was used to produce defects in the Te-doped GaSb samples with free electron concentration of 1×10¹⁷ cm^-3 and 1×10¹⁸ cm^-3 respectively. The change of S parameters in Te-doped samples irradiated with different proton fluences, indicates that the defects induced by proton irradiation are most likely the V_Ga-related defects. The PL spectra of Te-doped GaSb with different proton irradiation doses were measured at 77 K. The results show that the V_Ga-related defects induced by proton irradiation are acceptors in Te-doped GaSb. We have also found that the dopant-induced vacancies which are related to Te have existed in unirradiated samples.

Microstructure of Fe-based amorphous and nanocrystalline soft magnetic alloy has been investigated by X-ray diffraction (XRD), transmission electronic microscopy (TEM) and Doppler broadening positron annihilation technique (PAT). Doppler broadening measurement reveals that amorphous alloys (Finemet, Type I) which can form a nanocrystalline phase have more defects (free volume) than alloys (Metglas, Type II) which cannot form this microstructure. XRD and TEM characterization indicates that the nanocrystallization of amorphous Finemet alloy occurs at 460°C, where nanocrystallites of α-Fe with an average grain size of a few nanometers are formed in an amorphous matrix. With increasing annealing temperature up to 500°C, the average grain size increases up to around 12 nm. During the annealing of Finemet alloy, it has been demonstrated that positron annihilates in quenched-in defect, crystalline nanophase and amorphous-nanocrystalline interfaces. The change of line shape parameter S with annealing temperature in Finemet alloy is mainly due to the structural relaxation, the pre-nucleation of Cu nucleus and the nanocrystallization of α-Fe(Si) phase during annealing. This study throws new insights into positron behavior in the nanocrystallization of metallic glasses, especially in the presence of single or multiple nanophases embedded in the amorphous matrix.

The mechanism of slow positron annihilation at polymeric surfaces has been discussed in terms of positron diffusion at the surface and trapping of positrons and positronium in free volume holes. The one-dimensional diffusion equation has been solved and the rate equations have been set up to describe the various processes supposed to occur when a thermalized positron encounters the polymeric surface. The model has been used to calculate the Doppler broadening of the line shape parameter (S parameter) in polyurethane and polystyrene as a function of incident positron energy and temperature. The results have been compared with the available experimental data. The S parameter vs temperature curves show a remarkable discontinuity at the glass transition temperature (T_g). Large variation in the S parameter has been observed at low energies, suggesting a significant structure of free volume holes near the surface.

The recovery behavior of 20% plastically deformed AlSi_11.35Mg_0.23 in various stages of isochronal annealing is investigated by positron lifetime (LT). Experimental results show that the positron mean lifetime is a function of annealing temperature. The lifetime of the positron annihilating in a perfect lattice is 187.3 ps. It is 229.8 ps in a 20% deformed one. There are two regions in the isochronal annealing, one of them is related to the point defect and the other to the dislocation. The activation enthalpy for the dislocation is calculated from the isothermal study in the dislocation region from 575–675 K by slow and fast cooling and it is 0.16 ± 0.02 and 0.53 ± 0.06 eV, respectively.

This paper reports the results of lifetime measurements on deformed AlSiMgSr alloy. Using the trapping model, we obtained the value of 196.7 ps for the lifetime of the free state, and 210.5 ps for the trapped state. The specific trapping rate per unit defect concentration was calculated to be 0.2280 cm² · s^-1. The concentration of defects was 5.78×10¹⁷cm^-3 when thickness reduction was 18.2%. The dislocation density for the same thickness reduction is 1.63×10¹⁰cm^-2. We have compared these values with a previously used method¹ and have obtained perfect agreement for both methods, demonstrating that this a very powerful tool for detecting and evaluating defect concentration.

Nanocrystalline ZnO particles substituted with different concentrations (0–30%) of Mn were synthesized by using a modified ceramic route and characterized by X-ray diffraction, transmission electron microscopy, selected area electron diffraction and energy dispersive X-ray analysis methods. Positron lifetime and coincidence Doppler broadening measurements were used as probes to identify the vacancy-type defects present in them and monitor the changes while doping. The predominant positron trapping center in the undoped ZnO is identified as the trivacancy-type cluster V_Zn+O+Zn, which is negatively charged, and it transformed to the neutral divacancy V_Zn+O on doping with Mn²⁺ ions. The intensity of the defect-specific positron lifetime component got reduced initially indicating partial occupancy of the vacancies by the doped cations but then recovered on further doping due to the additional Zn vacancies created as a result of the increasing strain introduced by the Mn ions of larger radius. The creation of a new phase ZnMn₂O₄ thereafter changed the course of variation of the annihilation parameters, as the positrons got increasingly trapped in the vacancies at the tetrahedral and octahedral sites of the spinel nanomanganite.

In order to investigate the effect of the incorporated nanoparticles on the photocatalytic property of the hybrid membranes, the uncovered and covered polysulfone/TiO₂ hybrid membranes were prepared. Positron annihilation $\gamma$-ray spectroscopy coupled with a positron beam was utilized to examine the depth profiles of the two membranes. The photocatalytic activities of the membranes were evaluated by the degradation of Rhodamine B (RhB) aqueous solution under the irradiation of Xe lamp. UV-Vis spectroscopy was applied to study the UV transmission through the polysulfone layer. Electrochemical impedance spectroscopy was used to detect the photo-generated charges by the covered membrane during the irradiation. It can be found that UV light can penetrate through the covered layer (about 230nm), and the incorporated nanoparticles can still generate charges under irradiation, which endows the photocatalytic ability of the covered membrane.