SNO2 SIO2 AND SN/SB-OXIDE/SIO2 GEL-DERIVED COMPOSITES .1. STRUCTURAL EVOLUTION FROM A MOSSBAUER STUDY/

SiO2/SnO2 = 80/20 gel samples are prepared from Si(OEt)(4) and SnCl4 o r Sn(OEt)(4). Samples doped with Sb oxide contents up to 4% are also s tudied. The evolution of these samples after treatment from room tempe rature to 1050 degrees C is studied by Sn-119 Mossbauer spectroscopy a nd X-ray analysis. Results indicate that SnO2 separation depends on pr eparation method and Sn- or Sb-oxide precursors. In the case of Sn(OEt )(4), hydrolysis is less complete than for SnCl4 and thermal curing af fords charcoal separation which leads to extensive reduction of Sn(IV) to Sn(II). Products obtained above 800 degrees C are constituted by a well-defined SnO2 phase in the SiO2 matrix. Segregation of Sn(II) and subsequent oxidation to Sn(IV) are invoked to account for this final morphology. The presence of Sb oxide lowers Sn(IV) reduction and favou rs both SnO2 and SiO2 crystallization, via the occurrence of a Sn/Sb-o xide metastable phase observed in the 700-800 degrees C interval.