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.

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The crystal structure of the technologically relevant, high-temperature proton conductor BaZr0.7Ce0.2Y0.1O3- (BZCY72) has been studied by high-resolution neutron powder diffraction performed on a deuterated sample in the temperature range 10-1173 K, complemented with synchrotron X-ray diffraction in the range RT-1173 K. A volume discontinuity on heating indicates a first-order phase transition from orthorhombic (space group Imma) to rhombohedral symmetry ( 3 Rc ) between 85 and 150 K. A further transition to cubic symmetry ( 3 Pm m ) takes place at ~ 570 K, indicated to be second order from the temperature dependence of the octahedral tilt angle. The stability field of the cubic phase was extended on cooling in the dehydrated state to 85 K. Expansion/contraction of the unit-cell volume on heating in low vacuum and air, respectively observed by neutron diffraction and synchrotron X-ray diffraction, was described with a point-defect model involving the temperature dependence of the water content and thermal expansion. Isotropic strain in the hydrated state is apparent on analysis of the broadening of the neutron-diffraction reflections during heating and cooling cycles. Rietveld refinement of the low-temperature neutron data and Fourier nuclear-density maps were employed to locate the deuterium position at a distance of ~ 0.90 Å from the bonding oxygen at 10 K.

In this work, ZrO2−CeO2 mixed oxide nanotubes with 50, 70, and 90 mol % CeO2 were synthesized following a very simple, high yield procedure, and their properties were characterized by synchrotron radiation XRD and by high resolution electron microscopy. The 50, 70, and 90 mol % CeO2 nanotubes exhibited the tetragonal phase (t′-form and t′′-form, P42/nmc space group) or the cubic phase (Fm3m space group). The nanotube walls were composed of nanoparticles with an average crystallite size ranging from 4.7 to 7.6 nm. Electron microscopy observations confirmed the size of these nanoparticles by direct observation. The SEM and TEM results showed that individual nanotubes were composed of a curved sheet of these nanoparticles. By SEM analysis, the nanotubes were found to have lengths of around 1−8 μm, diameters of around 500 nm, and wall thicknesses of 20 nm. Elemental analysis showed that Ce:Zr ratios appeared to be constant across space, suggesting compositional homogeneity in the samples. The 90 mol % CeO2 nanotubes exhibited the highest value of specific surface area, 101 m2·g−1, which compared with about 28 m2·g−1 for the other two compositions.