ELECTRON-MICROSCOPY STUDY OF THE FLUORITE[--]RHOMBOHEDRAL (RIII) TRANSFORMATION IN THE U-LA-O SYSTEM

Unit cell parameters of a rhombohedral fluorite-related superstructure formed in the U-La-O system, for which several sets of cell parameters have been proposed, have been unequivocally determined by HRTEM. The calculated hexagonal parameters are a(H) = 0.393(1) nm, c(H) = 1.890(4) nm. Microstructural aspects such as microdomains formation and twinning accompanying the reversible fluorite (disordered) <--> rhombohedral (ordered) transformation are revealed. The presence of twins, a characteristic feature of the rhombohedral phase, is evidently derived from the pre-existent disorder in the fluorite phase.