Formation of C-type rare earth structures in the Ce1-xNdxO2-.DELTA. system: a factor in the decrease in oxide-ion conductivity

Abstract

It was found that oxide-ion conductivity of Nd-doped CeO2 (Ce1-xNdxO2-x/2) increased with an increase in Nd content x up to x = 0.2, showing a maximum value at x = 0.2, and then decreased with x. In order to clarify the origin of the decrease in the conductivity of the heavily Nd-doped CeO2, Rietveld analyses of the X-ray diffraction and neutron diffraction data were carried out, assuming two phases with oxygen deficient fluorite-type and rare-earth C-type structures (0.45 ≤ x ≤ 0.7). The oxide-ion conductivity of the heavily Nd-doped CeO2 is discussed in relation to the formation of the rare-earth C-type structure, which derives from the oxygen ordering of the defect fluorite-type structure.

Synthesis and characterization of Th1−xLnxO2−x/2 mixed-oxides

“…4). This kind of structural transition from Fm3m to Ia3 space group was widely reported for Ce 1Àx Ln x O 2Àx/2 solid solutions [35,[80][81][82][83][84]34,85,86], but, to our knowledge, the formation of a Ctype Th 1Àx Ln x O 2Àx/2 single phase sample was never reported up to now. The field of existence of single-phase C-type compounds appeared to be very narrow.…”

“…A + C polyphased oxides were then obtained even for the lowest thorium content (i.e. Th 0.03 Nd 0.97 O 1.515 ), confirming the very low ''solubility'' of thorium [18,22,24] and more generally of tetravalent cations [34,84,87,81] within the A-type structure characteristic of lanthanide sesquioxides from lanthanum to neodymium. Besides, the refined unit cell parameter a of Th 0.51 Nd 0.…”

C-type related order in the defective fluorites La2Ce2O7 and Nd2Ce2O7 studied by neutron scattering and ab initio MD simulations

“…In this study the refined C-type phase in Nd 2 Ce 2 O 7 comes out as more than 50 wt%, whereas a similar study done by Hagiwara et al obtained B32%. 9 The increased intensity of the supercell peaks when comparing PND to PXRD data also suggests that the oxygen lattice is the key to the observation of a superstructure within these compositions. We therefore believe that there are two set of domains, with and without ordering, exhibiting close to the same stoichiometry.…”

“…However, at doping levels above B15-20 mole% the ionic conductivity decreases. 8,9 Changes in the degree of local ordering, or clustering, are often suggested to rationalize the variations in the conducting properties with composition. [9][10][11] Identifying the underlying atomic structure is consequently essential to explain these macroscopic properties.…”

“…Nonetheless, when ceria is doped with La 2 O 3 and Nd 2 O 3 the trends in conductivity and lattice parameter due to doping levels are similar as when ceria is doped with C-type sesquioxides. 9,10,16 La 2 Ce 2 O 7 and Nd 2 Ce 2 O 7 are proposed as systems on the verge of transitioning into more ordered superstructures of the perfect fluorite structure; the pyrochlore and C-type structure respectively (see Fig. 1a-c).…”