MICROSTRUCTURAL CHARACTERIZATION OF THE STABILIZED FLUORITE PHASES FORMED IN THE LESS-THAN-OR-EQUAL-TO-Y-LESS-THAN-OR-EQUAL-TO-0.8) SYSTEM(X(0)

The thermal behaviour and stability of the fluorite structure in the C a1-yLayU2O6+x(0 less than or equal to y less than or equal to 0.8) sys tem during ageing have been studied by thermal analysis and X-ray powd er diffraction. When the amount of La3+ incorporated into the Ca1-yLay U2O6+x structure is greater than or equal to 0.6 the complete stabiliz ation of the fluorite phase is achieved. This phase remains stable ove r the temperature range from room temperature-1400 degrees C even afte r being submitted to long term give time. For lower amounts of La3+ a mixture of cubic fluorite and calcium lanthanum diuranate solid soluti on is obtained on annealing. The microstructural characterization carr ied out by electron diffraction and high resolution transmission elect ron microscopy reveals short range order for the high temperature quen ched materials. Annealing of the oxides leads to the formation of orde red nanodomains embedded within the fluorite matrix, but no ordered si ngle phase is isolated. A hexagonal cell of a(H) = 3.8 and c(H) = 18.8 Angstrom has been deduced for the domains.