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Powder samples of pure anatase were produced using laser-induced pyrolysis of titanium alkoxides, and the catalysts were prepared using conventional wet impregnation methods. The diffraction patterns were interpreted in microstructural terms by Fourier analysis of their peak profiles. The transition temperature for the anatase-rutile transition in these catalysts was found between 500° and 550 °C. For the reflections of the anatase phase, a decrease of their Bragg (2θ) positions was observed up to 550 °C when the presence of the rutile phase becomes important. The response of the anatase structure to the thermal treatment is anisotropic with the c-axis showing the highest sensitivity to the observed expansion of the lattice. The rutile Bragg reflections are sharper than those of the anatase phase. The corresponding microstructural parameters indicate that, in all cases, the transformation is accompanied by an increase of the crystallites and/or of the lattice perfection. The evolution of these parameters is influenced by the presence of vanadium. The V-treated surface layer must be particularly distorted and apparently act as a restraint to perfecting by thermal treatments. Only the transition to rutile is capable of overcoming that restraint by allowing crystallite growth at the expense of the smaller and distorted anatase crystallites.

Rheotaxial growth and thermal oxidation (RGTO) for depositing thin films is a recognized technique in preparing gas sensitive semiconducting oxides. This paper presents a study performed by x-ray diffraction and scanning Auger microscopy of the mechanisms of growth and formation of the thin films of the new ternary compound Sn1−xFexOy with an iron content in the range O < x < 25 at. %. A structural model of this compound, which is found to be stable over a very large range of Sn/Fe ratios, can be derived by partially substituting Fe3+ ions in Sn4+ sites. This is an easy substitution in view of the similar values shown by the ionic radii (Fe3+ = 0.64 Å, Sn4+ = 0.71 Å) and the Pauling electronegativity (Fe3+ = 1.8, Sn4+ = 1.8) of these two ions. Experimental data, showing that this material is an excellent CO sensor, are reported.