Shinya Yao1, Hirokazu Tanaka2 and Zensaku Kozuka1
1Department of Materials Science and Processing, Faculty of Engineering, Osaka University, Osaka
When commercial Pr6O11 powders were repeatedly deoxidized and oxidized by heating up and cooling down under a constant oxygen partial pressure, the starting temperature Tde of the phase transition PrOx(σ)→Pr2O3(A) was dependent on the number of heating runs. There are at least two factors affecting the Tde value. The temperature Tde was obviously highest on the first heating run. In order to clarify the factor increasing Tde, PrnO2n-2 phases quenched into ice-water on various heating runs were subjected to powder X-ray diffraction (XRD) analysis. For all the CaF2-related structures on the first heating run, additional peaks were observed; the main two peaks appeared around 2θ=29°ree. These peaks were not observed for the hexagonal Pr2O3(A) and all the phases re-oxidized from the Pr2O3(A). The following properties of the microstructure exhibiting the additional peaks in XRD analysis were clarified: (i) it does not disappear through phase transitions among the CaF2-related structures, (ii) it disappears once the phases change to hexagonal Pr2O3(A), (iii) after that, it never appears again even for the CaF2-related structure; and (iv) its existence does not have any effect on the phase transitions among CaF2-related phases, but prevents the phase transition PrOx(σ)→Pr2O3(A). For the cubic Pr2O3(C) produced from commercial Pr6O11 powders at 873 K uncler a very low oxygen partial pressure, the corresponding additional peaks around 2θ=29°ree reduced to one. The additional peaks are possibly interpreted as the diffractions due to a superlattice structure involved in the bulk. PrO2 involved in commercial Pr6O11 powders makes it difficult to detect the additional peaks. The formation of the phases exhibiting the additional peaks in XRD analysis may be deeply related to the production processes of the commercial Pr6O11 powders.
praseodymium oxides, thermal stability, phase transition, rare-earth oxides, potentiostatic technique, solid electrolyte, stabilized zirconia, superlattice, microstructure
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