Abstract
Neptunium dioxide has been treated in reducing and oxidizing atmospheres between 570 and 2880 K. The results of ceramographic, X-ray diffraction, thermal and electron microprobe analysis were used to construct a partial neptunium-oxygen phase diagram. The highest valency of neptunium in oxides is four at temperatures above 900 K. The dioxide exhibits a hypostoichiometric homogeneity range (NpO 2−x) at temperatures above 1300 K; on cooling to room temperature hypostoichiometric compositions decompose into NpO 2 and metallic Np. The neptunium-oxygen system shows characteristics which are intermediate between those of the isotype uranium-oxygen and plutonium-oxygen systems.
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