This study investigates the combined effects of Ca non-stoichiometry and sintering atmosphere on the defect chemistry, microstructural evolution, and dielectric response of CaCu3Ti4O12 (CCTO). Ca-excess compositions (Ca1+αCu3Ti4O12, α = 0.0 or 0.1) were synthesized via coprecipitation, calcined at 850 °C, and sintered at 1050 or 1100 °C under air or pure oxygen. XRD, SEM, and XPS characterized structure, microstructure, and surface oxidation state, while positron annihilation lifetime spectroscopy (PALS) directly probed open-volume defects and subgrain structures, evidencing subgrain boundaries within CCTO grains. Impedance spectroscopy (20 Hz–5.0 MHz, 20–200 °C) revealed three dielectric relaxation processes attributed to bulk grains, subgrain boundaries, and grain boundaries with deep trap states, with activation energies of 0.32–0.75 eV. The Ca-excess sample sintered in oxygen exhibited enhanced permittivity (εr´ = 3.95 ×10⁴, tanδ = 0.230 at 1.0 kHz), highlighting sintering atmosphere engineering as a strategy to tailor defect-mediated dielectric behavior in CCTO ceramics.

Kinetic Monte Carlo (KMC) simulations are widely used to investigate diffusion-mediated processes that unfold on time scales inaccessible to molecular dynamics. However, conventional atomistic KMC requires extensive activation-barrier catalogues-especially in multicomponent systems where local environments induce strong many-body effects. Here, we introduce the minimum Direct Two-Body Barrier (mDTBB) approach, a compact strategy in which the activation energy barrier for a given event is defined as the minimum energy among its relevant two-body interactions. This substantially reduces the number of required barriers while retaining the essential physics associated with saddle-point energetics. We validate mDTBB approach using two wellestablished test cases: (i) vacancy mediated solute clustering in supersaturated Al-Mg-Cu alloys, and (ii) surface diffusion during Au deposition on TiO2 (110). In both systems, the simulation trends closely reproduce experimental observations, and the resulting energetics are consistent with those obtained from significantly more computationally demanding methods. We further apply mDTBB in a predictive setting to Ga monolayer formation on GaN, demonstrating its ability to deliver physically grounded insights in regimes where experimental data are scarce. Overall, mDTBB emerges as a versatile and computationally efficient framework for modeling bulk and surface atomistic processes without the need for extensive barrier parametrisation.

Supramolecular crystalline structures were developed in films prepared from blends of a bio-based waterborne polyurethane (WBPU) based on castor oil/tartaric acid (CO-TA) with two different WBPUs synthesized from polycaprolactone diol (PCL) and dimethylol-propionic acid (PCL-DMPA) or TA (PCL-TA). Partial crystallization of the PCL segments generated topologies that strongly affected the final properties of the films. Depending on the internal emulsifier used in the synthesis of the PCL-based WBPUs and its proportion in the blend, the crystalline structures differed, resulting in lamellar, dendritic, or fiber-like networks. Several techniques were employed in the study: differential scanning calorimetry (DSC) to investigate the crystallization kinetics of the blends; optical and confocal microscopies to identify the topologies; positron annihilation lifetime spectroscopy (PALS) to correlate crystallinity with the mean nanohole volume, and dynamic mechanical analysis (DMA) to characterize the mechanical properties of the films. The crosslinked network from the CO-TA based WBPU and the microphase separation in the blends led to constrained crystal growth, resulting in distinct topologies. The work presents a novel approach to generate supramolecular structures, which had a positive effect on the films properties. The relationship between crystallinity and free volume, as measured by PALS, is also analyzed in terms of chain mobility.

The natural aging of polymers obtained by polymerization of chemically modified oleic or lauric acids with a precursor based on soybean oil was evaluated by comparing infrared spectra, color parameters, contact angles, thermal degradation temperatures, dynamic mechanical properties, and the characteristic nanoscale parameters of positron annihilation lifetime spectra, obtained for samples recently prepared and aged at room temperature for 24 months. The results denoted that physicochemical changes occurred in the materials due to aging, which were evidenced as variations in some structural and physical properties in all the studied polymers. These changes were explained in terms of the network characteristics (i.e., crosslinking density, pendant dangling chains, steric impediment, presence of labile double bonds) and the competitive degradative processes (i.e., crosslinking, chain scission, formation of small volatile molecules) that take place during aging. Results from positron annihilation lifetime spectroscopy indicated that although the average free volume of the tested samples remained almost constant after aging, the number of nanoholes decreased because smaller voids disappeared, thus changing free volume hole size distribution. This study is the result of a careful and systematic experimental design, lasting more than 2 years, therefore representing a considerable investment of time, effort, and resources to produce findings that are not only scarce but also crucial for advancing the development of new materials, especially those based on biomass.Practical Applications: The physicochemical and mechanical characteristics of these polymers, with high green carbon content and environmental benefits, can be tailored for use in different applications by selecting the type and proportion of the precursor in the formulation. In turn, the effects of natural aging are of fundamental importance when defining the type and duration of use. Their characteristics, both before and after natural aging, make them good candidates for structural, decorative, and surface protection applications, especially in indoor environments.

The sensitivity of positron annihilation characteristics to changes in the molecular packing in network-forming polymers has been demonstrated since the early 1980s. Positron annihilation lifetime spectroscopy (PALS) is a unique technique that can provide direct information on the free volume in polymers through the experimental parameters of the free volume hole distribution, their mean value, and volume fraction. This knowledge is currently applied for PALS investigations on the main processes that govern the molecular organization in some green polymers when subjected to different synthesis procedures or environmental conditions (humidity, physical aging, temperature). In this article, which includes a wide repertoire of works published in the last two decades, results of PALS studies on eco-sustainable polymer systems based on starch, chitosan, or vegetable oils, are analyzed and discussed. Many examples are taken from the direct experience of the authors.

Mn-doped CeO2 nanoparticles were synthesized using a microwave-assisted hydrothermal method and analyzed through photoluminescence spectroscopy, positron annihilation lifetime spectroscopy (PALS), and electrical resistance measurements, with the aim of determining the effect of Mn incorporation on the electrical properties of ceria. The present study reveals that the initial introduction of Mn results in the proliferation of small defective structures, attributed to the substitution of Ce4+ by Mn2+ and Mn3+. This transformation leads to the increased formation of Ce3+ species, which are linked to the presence of faulty structures associated with oxygen vacancies. The photoluminescence analysis showcases a notable rise in the number of neutral, singly, and doubly ionized oxygen vacancies following the addition of Mn. Concurrently, in pure sample, electrical measurements indicate that Ce3+ species govern the local charge transfer mechanism, facilitated by the formation of Ce 4f1 orbitals. However, the electrons trapped in various oxygen vacancy-related defect states are responsible for the elevation in activation energy observed in Mn-doped CeO2 samples. Furthermore, electrical measurements exhibit consistent activation energies across all samples when exposed to varying atmospheric conditions. Additionally, the enhanced response in Mn-doped samples is primarily attributed to a significant increase in carrier density.

CeO2 samples were synthesized via the polymeric precursor method at different calcination temperatures. Electric properties were investigated using positron annihilation lifetime spectroscopy, electron paramagnetic resonance spectroscopy, and complex impedance spectroscopy. Reduction in the specific surface area of the particles with increasing calcination temperature along with morphological changes were observed. PALS depicted doubly ionized oxygen vacancies surrounded by two Ce3+ atoms, while the EPR spectroscopy showed a singly ionized oxygen vacancy surrounded by Ce3+ and Ce4+ ions. Impedance measurements unraveled the presence of polarons, while thermal treatments led to a lower electrical conductance, as the calcination temperature increased.

In this study, several methods were employed to investigate the electrical characteristics of β-Ag2MoO4 systems, both Eu-doped and undoped, synthesized using the microwave-assisted hydrothermal method. The focus extended to understanding how synthesis time influences material defects, with doping fixed at 1%. A systematic shift in the silver vacancy (VAg) concentration was observed within the doped β-Ag2MoO4 system. Specifically, this study demonstrated that the incorporation of Eu3+ into polycrystalline β-Ag2MoO4 initially increases the VAg concentration. However, as the synthesis time progresses, the VAg concentration decreases, resulting in alterations in the resulting electrical properties, arising from the intricate interplay between the number of grain boundaries and carrier density. By combining information obtained from photoluminescence, positron annihilation lifetime spectroscopy, and impedance spectroscopy, a comprehensive conduction mechanism was formulated, shedding light on both doped and undoped β-Ag2MoO4 systems.

In this work, a consistent and accurate set of energy barrier values for the migration process, assisted by a vacancy, of atoms of different chemical elements belonging to the two most relevant families of age-hardenable Al-Cu and Al-Zn-based alloys is given. These values were calculated using the Nudged Elastic Band method within the Density Functional Theory framework for a vacancy diffusing in simple configurations of solute atoms, specifically the main alloying elements Cu and Zn involving a single solute atom or more complex configurations, such as pairs of elements of the same or different chemical species. In both cases, in our calculations, the presence of Mg and/or Ag solute atoms is also considered. The results obtained are analyzed in terms of the energetic interaction among vacancies and the different solute atoms. The advantage of having a set of vacancy diffusion energy barrier values calculated under the same criteria is that it can be used as reliable input parameters for future kinetic Monte Carlo simulations to study the precipitation kinetics in a wide type of age-hardenable alloys.

Defects at semiconductors with electron acceptor and donor sites govern the electronic and optoelectronic ap-plications due to their unique electronic properties. This work provides deep insight into the nature of defects and the conduction mechanism in alpha-Ag2WO4. To this aim, a detailed analysis of the results of XRD with Rietveld refinements, FE-SEM images, and measurements of different spectroscopies (impedance, positron annihilation lifetime, and photoluminescence) are carried out on alpha-Ag2WO4 samples synthesized by a simple co-precipitation method. Two types of vacancy defects: cationic O-vacancies, and anionic Ag or Ag-O vacancy complexes are elucidated with a Schottky p-type potential barrier. The results indicate that the Ag vacancies remain constant during thermal treatment, while an opposite effect is found for the oxygen vacancies. This behavior governs the multifunctional properties of alpha-Ag2WO4 semiconductors via a tunneling plus thermionic conduction mechanism.

The effect of citric acid (CA) concentration and water content on the free hole volume of thermoplastic cassava starch films (TPS) was studied. To this aim, continuous in situ positron annihilation lifetime spectroscopy measurements were performed at fixed moisture content and during water desorption. The results show that the increase in CA concentration leads to wider free hole volume distributions with lower mean values. During water desorption, the mean values and width of such distributions systematically decrease with the exposure time, and the evolution of the hole volumes was well-described using the Kohlrausch-Williams-Watts function. The water vapour permeability was significantly higher in films incorporating 5 % (w/w) of CA, in line with the more open network of this material that was revealed in the hole volumes distribution. The Young's modulus of all the developed films increased significantly after partial water desorption, which was attributed to the plasticizer loss reflected in a decrease in the mean hole volume value (between 4 % and 13 %). This work evidences that the control and report of the relative humidity are essential when testing TPS-based films, as their nanostructures are strongly dependent on external conditions.

This study reports the electrical properties of Nd-doped cerium oxide (CeO2) films synthesized by microwave assisted hydrothermal using a two-point probe technique. Positron annihilation lifetime spectroscopy studies evidenced that, as the Nd content rises, a structural disorder occurs. This is caused by an increase in oxygen vacancies surrounded with Nd (defective clusters), with the mean lifetime components ranging between 290 and 300 ps. Particle size estimation showed values from 8.6 to 28.9 nm. Along with the increase of neodymium impurities, also the conductivity increases, due to the hopping conduction mechanism between defective species. This gives rise to a response time of only 6 s, turning these materials candidates to realize gas sensor devices. Ab initio investigations showed that the improved electric conduction is boosted mostly by the reduced Nd2+ than the Ce3+, where the oxygen vacancies play a fundamental role.

An approach using combined results, obtained by different experimental variants of positron annihilation spectroscopy and from kinetic Monte Carlo simulations, made it possible to determine the fundamental mechanisms by which vacancies assist solute atoms to form clusters during the earliest stages of precipitation kinetics in age-hardenable Al-Cu and Al-Cu-Mg alloys. The investigations were performed on the conventional precipitation-hardening system Al-1.74Cu (at. %) alloy aged at 293 and 342 K; these temperatures were chosen since they are low enough to avoid the formation of more stable nanostructures. To understand the influence of a minor alloying element on the solute clustering, the ternary Al-1.74Cu-0.35Mg (at. %) alloy was also studied. Interpreted in terms of the so-called vacancy pump model of solute aggregation, the results obtained made it possible to give a detailed and precise description concerning the role of the solute-vacancy exchanges in the solute clustering dynamics and the energetic stabilization of the formed clusters. It deserves to be pointed out that the results of our simulations for the ternary alloy indicate that, when aging proceeds, the solute atoms transported by vacancies progressively form Cu-Mg coclusters containing different amounts of nonmixed Cu or Mg solute atoms.

ABSTRACT A systematic study on the influence of fillers on the structural properties at micro and nanoscale of vulcanized compounds based on a natural rubber matrix reinforced with silica and/or carbon black is presented. Several compounds with different SiO2/CB ratios (1.5/1, 2.25/1, and 3/1) and total filler contents (55, 60, and 65 phr) were prepared and vulcanized at 150 °C. The experimental techniques used were rheometric, swelling and dynamic mechanical tests, and positron lifetime spectroscopy. From these techniques, cure reaction parameters, the fraction at the maximum degree of swelling, storage modulus, loss tangent, and fractional free volume were measured. Using a recent model based on a hydrodynamic description and the percolation of aggregates in a rubber matrix, it was found that regardless of the filler combinations, the dynamic storage modulus is well represented as a function of the filler volume fraction. Besides, beyond a critical SiO2/CB ratio (2.25/1) in the formulations of the compounds, the loss tangent does not depend on the SiO2/CB ratio. The results obtained show a direct correlation among mechanical properties, swelling and fractional free volume, and the type and amount of fillers in the reinforced compounds.

An experimental study on the evolution of the physicochemical, thermal and nanostructural properties of chitosan samples obtained from squid pens as the deacetylation treatment proceeds is presented. To this aim, potentiometric titration, capillary viscosimetry, infrared spectroscopy, differential scanning calorimetry and positron annihilation lifetime spectroscopy were used. The results obtained are discussed in terms of the influence of the deacetylation time on the deacetylation degree, average molecular weight, thermal parameters and average free nanohole size of the different samples. A way of preparing chitosan matrices with tailored nanostructural characteristics for specific applications through the deacetylation process is explored.

This study reports the synthesis of hybrid nanostructures composed of cerium dioxide and microcrystalline cellulose prepared by the microwave-assisted hydrothermal route under distinct temperature and pH values. Their structural, morphological and spectroscopic behaviors were investigated by X-Rays Diffraction, Field Emission Gun Scanning Electron Microscopy, High-Resolution Transmission Electron Microscopy, and Fourier-Transform Infrared, Ultraviolet–Visible, Raman and Positron Annihilation Lifetime spectroscopies to evaluate the presence of structural defects and their correlation with the underlying mechanism regarding the biocide activity of the studied material. The samples showed mean crystallite sizes around 10 nm, characterizing the formation of quantum dots unevenly distributed along the cellulose surface with a certain agglomeration degree. The samples presented the characteristic Ce–O vibration close to 450 cm−1 and a second-order mode around 1050 cm−1, which is indicative of distribution of localized energetic levels originated from defective species, essential in the scavenging of reactive oxygen species. Positron spectroscopic studies showed first and second lifetime components ranging between 202–223 ps and 360–373 ps, respectively, revealing the presence of two distinct defective oxygen species, in addition to an increment in the concentration of Ce3+-oxygen vacancy associates as a function of temperature. Therefore, we have successfully synthesized hybrid nanoceria structures with potential multifunctional therapeutic properties to be further evaluated against the COVID-19.

Owing to the global upward trend of accidental carbon monoxide (CO) poisoning in the past 30 years, this work aimed to develop Cobalt-doped CeO2 particles by the microwave-assisted hydrothermal route under distinct conditions. Their structural, morphological, spectroscopic and electrical behaviors were investigated to correlate the influence of Co on their properties with the introduction of oxygen vacancies and their sensing capability to assist in the mitigation of CO poisoning cases. The samples were crystalline and had no secondary phases. Two distinct activation energies for the electrical conduction processes were observed due to dopant influence, corroborating the local cluster-to-cluster charge transfer (CCCT) mechanism, resulting in a response time of only 3s for the 4% Co-doped sample. On the other hand, through positron annihilation studies we showed that the oxygen vacancies are preferentially formed near Co ions, reducing the Co ion charge and leading to the formation of neutral VO-Co+2 complex clusters.

Electron-matter coupling is a fascinating way to tune and modify the properties of materials. In this work, we present a study on the formation and nature of vacancy-like defects in alpha-Ag2WO4 samples synthesized in a water or ethanol medium and subsequently submitted to electron beam irradiation at different exposure times. To understand the effects on the geometrical and electronic nature of the generated defects, the data obtained by positron annihilation lifetime spectroscopy were interpreted with the aid of first-principles calculations at the density functional theory level. To complement these results, X-ray diffraction, Raman spectroscopy, photoluminescence emissions, and field emission gun scanning electron microscopy techniques were also used. Based on the positron binding energy and the calculated and experimental positron lifetimes, the defect structure of the nonirradiated and irradiated samples was revealed. As a general feature, it was found that the defect structure is more complex for samples synthesized in ethanol than in water. In particular, the results show that all samples contain defects involving Ag vacancies and that the concentration of this type of defect increases with the irradiation time.

A study on the induced changes in the vacancy-like defect structure and the dielectric properties of K0.5Na0.5NbO3 ceramics by the addition of different amounts, between 0% and 7%, of BaTiO3 is presented. The samples were prepared by a mechanochemically activated solid-state reaction method. The structural evolution due to the orthorhombic to the tetragonal phase transition of the KNN doped samples was followed using X-ray diffraction and Raman spectroscopy. The use of positron annihilation lifetime spectroscopy allowed to identify the defect structure at sub-nanometric scale and the nature of the vacancy-like defects generated by the phase transition. The obtained results are discussed considering the lattice symmetry change and the defects structure formed due to the replacement of alkaline and niobium ions by barium and titanium ones. Additionally, changes in the dielectric properties are discussed in terms of the structural modifications of the different KNN-based ceramics involving different kinds of vacancy-like defects.

We experimentally studied the formation of vacancy clusters and oxygen related defects in uranium oxide (UOx) thin films (<70 nm) changing the stoichiometry in the x = 2.2–3.5 range. Films were deposited on Si(0 0 1) by DC magnetron sputtering varying the substrate temperature (room temperature, 400 °C and 600 °C) and different relative O2 partial pressures in the argon-oxygen mixture. The different species of vacancy-like defects are identified by the combination of depth dependent positron annihilation techniques and by comparison of the experimental data with ab-initio calculations. In samples growth up to 400 °C substrate temperature, di- and tri- vacancies were formed whereas at higher temperature, hexa-vacancies and larger vacancy clusters appear. Film growth at increasing oxygen partial pressure was found not to be correlated with an increase of oxygen defects, but with the formation of more complex vacancy clusters. The presence of oxygen related defects is revealed by identifying preferential positron annihilations with oxygen electrons. Moreover, uranium vacancies inside vacancy clusters are identified by localization of positrons, in agreement with ab-initio calculations.