EXAFS AND XRD STUDY OF THE STRUCTURAL EVOLUTION DURING ISOTHERMAL SINTERING OF SNO2 XEROGELS

The formation of an ordered (crystalline) phase during isothermal sint ering of SnO2 monolithic xerogels, at 200, 250, 300, 400, 500, 600 and 700 degrees C, has been analyzed by the combined use of EXAFS and XRD techniques. For the desiccated gel (110 degrees C), EXAFS results sho w the formation of small microcrystallites with the incipient cassiter ite structure. Between 110 and 250 degrees C, the dehydratation reacti on leads to an amorphization evidenced by a decrease of the long and s hort range crystallographic order. It is due to fissure formation in t he xerogel network. For higher temperatures, a continuous coagulation of the crystallites occurs, leading to grain growth. Grain and pore gr owth obeys the same kinetic relation, so that the microstructure grows by simple enlargement while its morphology is static.