Quimica do estado solido : preparação, caracterização, comportamento termico e reações topoquimicas em tiometalatos do tipo AM52 (A=K, Ag, Ba e M=Fe)

AUTOR(ES)
DATA DE PUBLICAÇÃO

1993

RESUMO

This is a study on the preparation, characterization, ion-exchange and thermal bahavior of iron potassium sulfide, KFeS2. The structure of this solid is basically composed by edge-sharing FeS4 tetrahedra forming negatively charged linear chains [FeS2]n. Potassium ions act as counter-ion. The Fe-Fe distance within the chains is greater than the distance between the Fe ions from neighbouring chains. These facts impart a quasi-unidimensional character to this compound. The caracterization of the products obtained was made by chemical analysis and physical measurements: X-ray diffraction, Mössbauer spectroscopy, infrared spectroscopy, thermogravimetric analysis and scanning eletron microscopy. The complementary characteristics of diffraction and Mössbauer were essential in this characterization. The thermal behavior of KFeS2 and of its ion-exchange products under different atmospheres were studied and theeffect of mechanical stress on the crystals before these reactions were verified. A detailed study of alI steps involving the oxidation of KFeS2 was possible, showing different phase mixtures in which every component could be identified and, in some cases, isolated. Under inert atmospheres (N2, Ar), KFeS2 showed a good heat resistance at least to 600°C. Ion-exchange reactions of KFeS2 yielded AgFeS2 and a-Ba(FeS2)2. in a similar way to the topochemical reactions in which a substance reacts in heterogeneous medium without breakdown of its structure, i.e., some part of the original structure remains intact. The rearrangements on the original structure differ strongly depending on the exchanged ion. In the case of alkaline-earth ions, the linear chain and the quasi-unidimensional character are unchanged and little modifications on the original structure arise. When Ag is exchanged for K, the resulting compound does not presents a chain structure and FeS4 tetrahedra are bound together at alI sides to form an infinite three-dimensional network. Some of the Fe ions move within the struture in this case. The ion-exchange reactions were studied varying parameters like time, temperature, and reaction atmosphere. New compounds were obtained by partial cation exchange with formulae KxAgy[FeS2] and KxBay[FeS2] (x+y=1) and their thermal behavior was determined. X-ray diffraction and infrared spectroscopy were used together with scanning electron microscopy to follow the thermal transformations of the solids to allow a detailed study of alI the reactions listed and to relate the morphological changes with oxidation, ion-exchange topochemical reactions and mechanicaI stress on this system.

ASSUNTO(S)

quimica do estado solido

ACESSO AO ARTIGO

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