The nature of defects in reduced pure and Nb‐doped TiO2 rutile is investigated at high temperature by Seebeck coefficient measurements. The classical point defect model approach allows a good fit of the theoretical curves to experimental data. In undoped rutile, the main defects should be titanium interstitials in a large oxygen partial pressure range. For small departures from stoichiometry, the influence of impurities and band to band thermal excitation is discussed. In the case of Nb‐doped rutile, two kinds of compensation may occur. Some conclusions related to transport phenomena in such materials are drawn.

The electrical conductivity of rutile doped with 0.04–3 at.% niobium is reported as a function of oxygen pressure in the temperature range 1273–1623 K. The charge compensation is discussed in terms of a point defect model, under the assumption of a substitutional incorporation of niobium into the titanium rutile sublattice. Two kinds of charge compensation occur according to the temperature and the oxygen pressure, via an electronic or a lattice defect, in a Ti1−yNbyO2 or a Ti1−yNbyO2+y/2 solid solution, respectively. In the overstoichiometric range of the oxide, the data, when applied to undoped rutile, allow some conclusions about the atomic or electronic transport properties, both in the high and low temperature regimes.

Ultrafine tetragonal ZrOz powder was prepared by hydrothermal treatment at 100 MPa of amorphous hydrous zirconia with distilled water and LiCl and KBr solutions. The resulting powder consisted of well-crystallized particles; at 2OO0C, the particle size was 16 nm and at 500°C, 30 nm. Under hydrothermal conditions tetragonal ZrOt appears to crystallize topotactically on nuclei in the amorphous hydrous zirconia.

The phase diagram of the system ZrOz-CeOz was reinvestigated using hydrothermal techniques. Cubic, tetragonal, and monoclinic solid solutions are present in this system. The tetragonal solid solution decomposes to monoclinic and cubic solid solutions by a eutectoid reaction at 105Oo+5O0C. The solubility limits of the tetragonal and cubic solid solutions are about 18 and 70 mol% CeOZ, respectively, at 14OO0C, and about 16 and 80 mol% Ce02, respectively, at 12OO0C. Solubility limits of the monoclinic and cubic solid solutions are about 1.5 and 88 mol% CeOz at 1000°C, and 1.5 and 98 mol% CeOz at 800°C, respectively. The compound Ce2Zr,010 is not found in this system.

Ultrafine tetragonal ZrO2 powder was prepared by hydrothermal treatment at 100 MPa of amorphous hydrous zirconia with distilled water and LiCl and KBr solutions. The resulting powder consisted of well‐crystallized particles; at 200°C, the particle size was 16 nm and at 500°C, 30 nm. Under hydrothermal conditions tetragonal ZrO2 appears to crystallize topotactically on nuclei in the amorphous hydrous zirconia.