Positron and positronium annihilation patterns in zeolites and bulk ceramics

Abstract

.43. Gt, 61.72.Qq, 78.70.Bj Positron lifetime (LT) measurements have revealed that in porous media the annihilation pattern is probably dominated by a near-saturation level trapping both for the positron and the positronium as well, which masks the contribution from the "free annihilation", i.e., annihilation from delocalised positrons. In this work we compare positron lifetime spectra of crystalline zeolites with bulk ceramic materials fabricated by sintering from the same constituents as the zeolites and analyse common features and differences between them. For the dense ceramic samples the contribution from long-living components to the annihilation pattern is substantially lower than that for the zeolites. The results are analysed with the aim to extract new knowledge for the zeolites and for the sensitivity limits for free volume studies in ceramics. 1 Introduction By now, positron annihilation techniques (PAT) have become extensively used for structural investigation of solids. In the several decades of the evolution of PAT, the interest has shifted somewhat from electronic structural studies to the investigation of structural imperfections where positron trapping has played an important role. In contrast to bulk solids, porous media present a more complex picture: positron lifetime (LT) data revealed the coexistence of annihilation modi with long lifetime components, ascribed to the annihilation of orthopositronium (o-Ps) in various free or confined states [1][2][3]. If trapped, Ps remains and decays in the trap. The simple quantum-well model [4,5] -correlating the o-Ps lifetime to the size of the trapping sites where it annihilates -was applied with success. and was developed further recently (e.g., [6]), taking into account non-spherical geometry, too. The long lifetime values indicate a low interaction of o-Ps with its surroundings and call attention to the necessity to appropriately record also the 3γ-decaying modes in LT spectroscopy, customarily tuned to observe 2γ-annihilation events only. The o-Ps states are sensitive to additives (e.g., surface-bound ions, gases) which may transform the long-living states into other, shorter-living ones, annihilating via 2γ-annihilation. It has also been shown [7,8] that changes in the 3γ/2γ-ratio may also strongly influence the PAT parameters due to the changes induced in the detection efficiency and sophisticated corrections are necessary.

Determination of Free-Volume Properties in Polymers Without Orthopositronium Components in Positron Annihilation Lifetime Spectroscopy

“…The four-lifetime analyzed lifetime and intensity results in those selected zeolites and molecular sieves are listed in Table . The results of Table are comparable to those existing data in zeolite systems. − It is noted that in the PATFIT analysis of PALS data, we have fixed the shortest lifetime τ 1 = 0.125 ns, which corresponds to the p -Ps lifetime so that the results for the other three lifetimes are more stable and consistent. We also attempted to constrain I 1 = 1/3 of o -Ps intensity in the PATFIT analysis.…”

“…This is likely because of the hydrates absorbed inside the cavities and some intergrain defect structures in those materials. Although some people have attempted to apply high temperature and pumping to remove hydrates and impurities from the inner surfaces, the PALS results are known not to be able to mirror the α,β cages’ sizes precisely. − In this study for the purpose of calibration τ 2 - R equation, instead of using ideal α,β cage sizes, we use the experimental o -Ps lifetimes (τ 3 and τ 4 ) according to eq to calculate the mean hole radii. For a real substrate with multicomponents of o -Ps lifetime, i , we calculate the mean radius from o -Ps lifetime R̅ by the following equation R̅ = ∑ i R i false( normalτ normalo − normalP normals false) · I i / ∑ i I i where R i (τ o -Ps ) and I i are the corresponding i th o -Ps lifetime according to eq and intensity, respectively.…”

“…Finally, we attempted to extend eq for radius >5 Å similar to the extension of eq for the hole R > 5 Å based on the infinite potential spherical model . Because the positron is different from o -Ps in the annihilation mechanism, the positron can undergo only 2-γ annihilation in condensed matter, whereas o -Ps can annihilate via 2-γ pick-off or 3-γ process, and the observed value of τ 2 in large holes of condensed matter remains in the region of 0.5 to 0.6 ns. − Therefore, it is also seen in Figure that the deviation from eq from the data in zeolite materials become more noticeable for radius >4 Å. The fitting of eq to converge to τ 2 ≈ 0.6 ns is mathematically difficult.…”

“…Thus, the semiempirical fitting of the quantum mechanical model of infinitive potential spherical for τ 2 - R relationship breaks down for the hole radius R > 5 Å. It will be impractical to use τ 2 - R relationship to determine free-volume dimension for R > 5 Å. Fortunately, in most of molecular substrates with R > 5 Å, some fractions of o -Ps component remain observable because quenching and inhibition effects of o -Ps are reduced in large holes, − except for those with strong chemical-reacting functional groups, which are localized on the inner surface of holes.…”

“…The dashed line is approximated linear fit eq 10. Legends: epoxy-pressure (Figure 3, ref 24); PS-press: polystyrene pressure (Figure5); PS-temp: polystyrenetemperature (Figure7); MSA: molecular sieves A-type (this work); zeolite MSA/zeolite type A (this work); MS13X (this work); Sodalite (this work); NaAzeolite; 42 Zeolite4A;44 Zeolite Y;47 NaA: Na-zeolite;46 and 13X: zeolite-13X.45…”