Molecular dynamics simulation of UO2 nanocrystals surface

TL;DR

This study used molecular dynamics to analyze the surface properties and shape stability of UO2 nanocrystals, revealing metastable cubic shapes transforming into stable octahedral forms and providing surface energy data consistent with experimental results.

Contribution

It presents the first detailed MD simulation of UO2 nanocrystal surface energies and shape evolution under isolated boundary conditions.

Findings

Cubic nanocrystals are metastable and relax into octahedral shapes.

Surface energy densities for {100} and {111} surfaces were measured.

The ratio of surface energies matches experimental data.

Abstract

In this article we investigated surface of nanocrystals (NC) of uranium dioxide (UO2) using molecular dynamics (MD) under isolated (non-periodic) boundary conditions with the approximation of pair potentials and rigid ions. It is shown that a cubic shape of the model NCs is metastable and the stable equilibrium is reached in the process of structural relaxation to the octahedral shape over a time of 1000 ns (200 million MD steps), which increases with the size of NC. We measured the size dependences of the lattice parameter and the surface energy density for NC of cubic and octahedral shape with volume up to 1000 nm3 (50000 particles) at temperatures of 2200K and 2300K. For the surfaces {100} and {111} we obtained the energy density {\sigma}100=1.602\pm0.016 J/m^2, {\sigma}111=1.137\pm0.032 J/m^2 and surface tension constant {\gamma}111=0.875\pm0.008 J/m^2. The resulting ratio of {\sigma}100/{\sigma}111=1.408\pm0.042 within the error coincides with the experimental value of 1.42\pm0.05 measured for microscopic cavities in monocrystals of UO2.