Fig. 4: The U-O phase diagram.

a The U(IV), U(V) and U(VI) abundance in oxides UO2, U4O9, U3O7, U3O8 at ambient T and b the U(V) abundance in U-O phase diagram under high T. U(V) is present in all UO2+x phases with x above 2.0 and below 3.0 and has the biggest predominance field from x = 2.25 to x = 2.75. The phase diagram in (b) is a simplified version adapted from reviews11,12,19. The experimental data from this study are reported in (b) as black and blue points for the heating and the cooling, respectively.