The majority of periodic table elements are created within the conventional uranium dioxide fuel matrix during its time in pile. These elements are accommodated in quite different ways within the nuclear ceramic. Ba, Zr, Y and the lanthanide atoms are typically substituted within the uranium dioxide matrix. At high burnup, the number of phases that form and the differences between them can result in the development of highly complex structures and microstructures. Similar effects might be encountered in manufactured advanced actinide fuels for the transmutation of minor actinides or in fuels containing significant amounts of burnable poisons (typically Gd). Those systems constitute then a large class of compounds, generally with fluorite or fluorite-derived structures, exhibiting high radiation tolerance, possibly related to their peculiar ability to accommodate a variety of defects and to exist as nonstoichiometric compounds within a large homogeneity range. Nevertheless, a variety of behaviours and different phases is observed after quenching or annealing in several of the peudo-binary systems of type (Ln,U)O2-x, suggesting that the thermal history of those systems is of paramount importance. Some of these systems, like (U,Ce)O2-x are particularly complex, display an intriguing structural variety, and therefore present a fundamental interest. In this talk we would try to address some of those interesting structural features induced by aliovalent substitutions in UO2.