A classical molecular dynamics study of the correlation between the Bredig transition and thermal conductivity of stoichiometric uranium dioxide

Thermophysical properties of uranium dioxide are investigated by classical molecular dynamics for temperatures from 300 K to 3000 K. An increase of specific heat in the temperature range from 1300 K to 2500 K is noted. Comparison with a theoretical model shows that the origin of this behavior is only due to anharmonicity. Such characteristic features of the Bredig transition as the peak in specific heat and high ionic conductivity are investigated. We show that one more important feature was left unnoticed: the rise in the lattice contribution to thermal conductivity at high temperatures. An explanation is provided for this effect which is specific to superionic conductors. Reasonable agreement with experimental data up to 3000 K is obtained for thermal conductivity, even in the absence of electronic excitations.

► Stoichiometric uranium dioxide studied by NPT and NVE classical molecular dynamics.
► Broad peaks in specific heat and thermal expansion are found at 2500–2700 K.
► Ionic conductivity increases above 2500 K, but there is no partial melting.
► Lattice contribution to thermal conductivity increases starting at ∼2500 K.
► This is due to migration of oxygen ions via a high-temperature exchange mechanism.