A Bound State of Positrons with Sulphate Ions

Abstract

Positron lifetime spectra are obtained in four different single crystals of sulphates: sodium, strontium, barium, and lead sulphate. The spectra can be resolved into four components, although only the intermediate ones are significant since the other components, with a very low intensity percentage, are associated to positronium formed at the interface of the source with the specimen. The intermediate short-lived component is associated with the annihilation of the outer electrons of the oxygen ions. The annihilation of positrons in cation vacancies or in interstitial impurities may also contribute to this component. Annihilation in other defects, like clusters of vacancies, is associated with the intermediate long-lived component, whose main contribution seems to be a bound state of positrons with electrons belonging to the covalent bonds between sulphur and oxygen.Des spectres de duree de vie du positon sont obtenus dans quatre differentes sortes de monocristaux de sulfates, plus concretement, de sodium, strontium, baryum et plomb. L'analyse des spectres en quatre composants montre que le temps de vie le plus long et le plus court, avec des intensites trts basses, sont d h a I'annihilation du positonium forme l'interface entre la source et I'echantillon. Le composant intermediaire de la duree de vie courte est associe a l'annihilation des positons avec les electrons exterieurs des ions oxygene. L'annihilation dans les lacunes des cations ou avec des impuretes interstitielles peut &tre aussi une contribution a ce composant. L'annihilation dans d'autres defauts, comme des amas de lacunes, est associee au composant intermediaire de longue duree de vie, dont la principale contribution, comme il en a etC deduit, est I'annihilation des positons pieges par les electrons des liaisons covalentes entre le soufre et I'oxygene.

Alternation Mechanism of Lead Sulfate toward Lead Sulfide under Sulfuric Acid Conditions

“…Figure 4a shows reaction time variations of the positron lifetimes τ 1 and τ 2 for the powder samples which reacted in sulfuric acid solution at 348, 383, 413, and 443 K. The positron lifetime τ 1 , initially similar to that of the PbS crystal (see the solid line), gradually increases with the reaction time and approaches ∼225 ps at 20 days corresponding to the positron lifetime in the bulk of PbSO 4 (see the dashed line). 23 This clearly captures the picture that the bulk state of PbS is gradually shifted toward PbSO 4 resulting from the reaction in sulfuric acid solution. It is of interest that the PbSO 4 formation is visible by locally probing the matrix of PbS and PbSO 4 , in agreement with the results of coherent XRD (see Figure 2).…”

Positron lifetime studies of CaSO4: Dy thermoluminescence phosphors

“…The first life time component may be attributed to the annihilation of positrons at Ca vacancies. The second longer component may be due to the bound state of positrons with sulphate ions [7]. It was proposed that 2 4 …”