Preparation and electron microscopy of intermediate phases in the interval Ce7O12Ce11O20

Crystals of CeO₂, grown from a flux of sodium tetraborate, were reduced in dry hydrogen to compositions in the interval CeO_1.714CeO_1.818. Selected compositions were studied using a high-resolution electron microscope. The phase CeO_1.714(Ce₇O₁₂) was confirmed to have the same unit cell as does Pr₇O₁₂ and Tb₇O₁₂. CeO_1.818(Ce₁₁O₂₀) was found to be isostructural with Tb₁₁O₂₀. Two new phases in the interval were established unequivocally. One of them, probably Ce₁₉O₃₄, is triclinic with a = 6.627, b = 11.478, c = 10.123, α = 100.9, β = 90.0, and γ = 95.5, has not previously been observed in any rare earth oxide system. Another, Ce₆₂O₁₁₂, appears to be isomorphous with Tb₆₂O₁₁₂. The two intermediate phases previously reported in this interval, Ce₉O₁₆ and Ce₁₀O₁₈, were not observed, nor was Ce₁₂O₂₂. Diffraction evidence also points to three additional phases in the interval not previously seen.