When commercial Pr
6O
11 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 PrO
x(σ)→Pr
2O
3(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, Pr
nO
2n−2 phases quenched into ice-water on various heating runs were subjected to powder X-ray diffraction (XRD) analysis. For all the CaF
2-related structures on the first heating run, additional peaks were observed; the main two peaks appeared around 2θ=29°. These peaks were not observed for the hexagonal Pr
2O
3(A) and all the phases re-oxidized from the Pr
2O
3(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 CaF
2-related structures, (ii) it disappears once the phases change to hexagonal Pr
2O
3(A), (iii) after that, it never appears again even for the CaF
2-related structure; and (iv) its existence does not have any effect on the phase transitions among CaF
2-related phases, but prevents the phase transition PrO
x(σ)→Pr
2O
3(A). For the cubic Pr
2O
3(C) produced from commercial Pr
6O
11 powders at 873 K uncler a very low oxygen partial pressure, the corresponding additional peaks around 2θ=29° reduced to one. The additional peaks are possibly interpreted as the diffractions due to a superlattice structure involved in the bulk. PrO
2 involved in commercial Pr
6O
11 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 Pr
6O
11 powders.
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