Using molecular dynamics, the thermophysical
properties of the (Ux,Th1−x)O2 system have been
investigated between 300 and 3600 K. The thermal
dependence of lattice parameter, linear thermal
expansion coefficient, enthalpy and specific heat at
constant pressure is explained in terms of defect
formation and diffusivity on the oxygen sublattice.
Vegard’s law is approximately observed for solid
solution thermal expansion below 2000 K. Different
deviations from Vegard’s law above this temperature
occur owing to the different temperatures at which
the solid solutions undergo the superionic transition
(2500–3300 K). Similarly, a spike in the specific
heat, associated with the superionic transition,
occurs at lower temperatures in solid solutions that
have a high U content. Correspondingly, oxygen
diffusivity is higher in pure UO2 than in pure ThO2.
Furthermore, at temperatures below the superionic
transition, oxygen mobility is notably higher in
solid solutions than in the end members. Enhanced
diffusivity is promoted by lower oxygen-defect
enthalpies in (Ux,Th1−x)O2 solid solutions. Unlike
in UO2 and ThO2, there is considerable variety of
oxygen vacancy and oxygen interstitial sites in solid
solutions generating a wide range of property values.
Trends in the defect enthalpies are discussed in
terms of composition and the lattice parameter of
(Ux,Th1−x)O2.