Estudo de soluções sólidas binárias de sais iônicos duplos de metais bivalentes através da espectroscopia de vida média do pósitron (EVMP) e da espectroscopia de alargamento da raia de aniquilação por efeito Doppler (EARAD)

Abstract

Studies applying Positron Anihilation Lifetime Spectroscopy (PALS) in solids provide evidence of the importance of physical and chemical properties of the central ion and the importance of the ligand in positronium formation. PALS has been used as a potentially important technique for differentiation and study of systems constituted by solids or by mixtures of isomorphic compounds, specially in the low concentration range, lower than 5% of the guest compound, in which X-Raydiffraction used in the characterization of solids is limited. In previous work[1] the first ionic solid solutions and mixtures constituted from alumens KAl(SO4)2.12H2O and KCr(SO4)2.12H2O were studied with the objective of investigating the physical-chemical properties of cations Al 3+ and Cr3+, related to the formation and inhibition of positronium formation in ionic solid solutions. The results demonstrated in these solutions the phenomenon of total inhibition of positronium formation, that appliesto the Stern-Volmer equation, according to the Spur Model. In this work, PALS has been used combined with EARAD, as an important analytictechnique in the characterization of binary ionic solid solutions, however, involving divalent ions of transition metal never studied before instead of trivalent ions, as studied in previous systems [1]. The compounds involved in this work were the double salts of general formula:MK2(SO4)2.6H2O, (M= Zn2+, Cu2+, Ni2+ e Co2+) and (NH4)2(SO4)2.6H2O, where M= Zn2+, Cu2+, that were synthesized and later characterized by atomic absorption, thermogravimetry and x-ray powder diffraction. PALS and EARAD measurements were made in isolated compounds and in systems prepared from them. Traditional measurements of PALS in mechanical prepared mixtures were used as blanks in comparison with the results of the PALS of the solid solutions. The total inhibition of positronium formation phenomenon has been observed in solid solutions, providing evidence of the structural difference between these systems (monophasic) and the mixtures (biphasic). The experimental results in solid solutions were traditionally described on the basis of the SPUR model of positronium. Comparing the values of the constants of total inhibition, k, the following tendency has been observed:k (Zn(1x)Cu(x)(NH4)2(SO4)2.6H2O) > k (Zn(1-x)Ni(x)K2(SO4)2.6H2O >k (Zn(1-x)Cu(x)K2(SO4)2.6H2O k (Zn(1-x)Co(x)K2(SO4)2.6H2O.