The effect of the mixing states of CaF
2-like phases appearing in the Zr-Ce-Pr-O system on their thermochemical behavior has been discussed on the basis of the data accumulated over the past few decades. In the Pr-O system, a series of CaF
2-like Pr
nO
2n-2: Pr
3+4Pr
4+mO
2(4+m)-2 oxides are well-known; the metastable phase (
m = 4) may appear in a mixing state with the
m = 5 or 3 phase for the Pr
nO
2n-2 thin film sputtered on a CaF
2-related Y
2O
3 substrate. In the Zr-Ce-O system, there exist many CaF
2-like and -related phases: tetragonal t, t', t'', cubic c, κ-Ce
2Zr
2O
8, and so on. In the Zr-Pr-O system, the novel CaF
2-type λ-(Zr
1/4Pr
3/4)O
2-y phase with large oxygen nonstoichiometry has been discovered. For the quasi-binary solid solutions, Zr
2/8Ce
(6-x/8)Pr
x/8O
2-y (0≤x≤6), the (c, t'') and λ phases appear in the Ce-rich and Pr-rich regions, respectively. The extremely resembling c, t'' and λ phases can form junctions with coherent interfaces, accompanied by introduction of oxygen vacancy around them, when the samples pre-fired at a sufficiently high temperature. The most excellent results for CH
4 steam reforming test have been obtained with Ni catalyst supported by the oxides possessing the junctions; the catalytic activity of NiO has been reported.
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