The crystal structures of a number of nanocrystalline ZrO 2 -CeO 2 solid solutions, synthesized by a pH-controlled nitrate-glycine gel-combustion process, were studied. By using a synchrotron X-ray diffractometer, small peaks of the tetragonal phase, which correspond to forbidden reflections in the case of a perfect cubic fluorite structure, were clearly detected. By monitoring the most intense of these reflections, 112, as a function of the CeO 2 content, the tetragonal-cubic phase boundary was found to be at 85 (5) mol% CeO 2 . For a CeO 2 content up to 68 mol%, a tetragonal phase with c/a > 1 (known as the t 0 form) was detected, whereas, between 68 and 85 mol% CeO 2 , the existence of a tetragonal phase with c/a = 1 and oxygen anions displaced from their ideal positions in the cubic phase (the t 00 form) was verified. Finally, solid solutions with higher CeO 2 contents exhibit the cubic fluorite-type phase.

In order to evaluate promising directions in the development of mixed-conducting membrane materials for oxygen separation and partial oxidation of natural gas, a series of ferritebased ceramics were studied, including La 1-x Sr x Fe 1-y Ga y O 3-δ (x = 0.5 -0.8; y = 0 -0.4), La 1-x Sr x Fe 1-y Al y O 3-δ (x = 0.7 -1.0; y = 0 -0.5), La 0.3 Sr 0.7 Fe 0.7-x Al 0.3 Cr x O 3-δ (x = 0.1 -0.2), (Sr 2 Fe 3 ) 1-x (SrCo) x O z (x = 0 -0.8), CaFe 0.5 Al 0.5 O 2.5+δ and Ln 3-x Ca x Fe 5 O 12-δ (Ln = Gd, Y; x = 0 -0.5). The maximum oxygen permeation is observed for perovskite-type solid solutions with high oxygen deficiency, which exhibit, however, excessive thermal and chemically induced expansion. As for cobaltite-and gallate-based mixed conductors, the increase in ionic transport is accompanied with increasing limiting role of the surface exchange processes. The stability of perovskite-related ferrites in reducing atmospheres, which is comparable to that of LaFeO 3-δ and iron oxide, may be moderately increased or decreased by donor-or acceptor-type doping, respectively. In addition, the substitution of iron with cations having a more stable oxidation state, such as Ga 3+ , Al 3+ or Cr 3+/4+ , partly prevents the lattice expansion induced by oxygen nonstoichiometry variations, although the solubility of these dopants in the ferrite lattice is limited.

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