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Abstract: SiO2/SnO2=80/20 gel samples are prepared from Si(OEt)4 and SnCl4 or Sn(OEt)4. Samples doped with Sb oxide contents up to 4% are also studied. The evolution of these samples after treatment from room temperature to 1050°C is studied by 119Sn Mössbauer spectroscopy and X-ray analysis. Results indicate that SnO2 separation depends on preparation method and Sn- or Sb-oxide precursors. In the case of Sn(OEt)4, hydrolysis is less complete than for SnCl4 and thermal curing affords charcoal separation which leads to extensive reduction of Sn(IV) to Sn(II). Products obtained above 800°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 favours both SnO2 and SiO2 crystallization, via the occurrence of a Sn/Sb-oxide metastable phase observed in the 700–800°C interval.

Abstract: The thermal behaviour of samples with nominal composition 80/20=SiO2/SnO2, class B, 76.8/19.2/4.0=SiO2/SnO2/Sb2O5, class C1, and 76.8/19.2/2.0/2.0=SiO2/SnO2/Sb2O5/Sb2O3, class C2, is studied in the interval 25–1050°C by various instrumental methods. Results on these classes of samples, obtained from alkoxide precursors, are compared themselves and with samples of class A obtained from Si(OEt)4 and SnCl4. The segregation and crystallization of SnO2 occurs at 400°C in the presence of microdomains of SnO2·nH2O in the SiO2 gel matrix (class A), whereas it is observed at 700°C for samples B and C composed of Sn and Sb cations homogeneously dispersed in SiO2. This fact implies different mechanisms of SnO2 nucleation and growth. The crystallization of SiO2 is observed at 1200°C for samples A, at 1050°C for B and at 800°C for C. For this latter, the presence of Sb-oxide/ SiO2 reactive glass is invoked to the low-temperature crystallization of SiO2.