Molecular dynamics simulation of UO2 nanocrystals surface

In this article we investigated surface of nanocrystals (NCs) of uranium dioxide (UO2) using molecular dynamics (MD) under isolated (non-periodic) boundary conditions in the approximation of rigid ions and pair potentials (RIPI). It is shown that a cubic shape of NCs is metastable and equilibrium is reached in the process of structural relaxation to the octahedral shape during long simulations of 1000 ns (200 million MD steps), which increase with the size of NC. We measured the size dependences of the lattice parameter and the surface energy density of cubic and octahedral NCs with volumes up to 1000 nm3 (50 000 particles) at temperatures of 2200 K and 2300 K. For the surfaces {1 0 0} and {1 1 1} we obtained the energy densities σ100 = 1.60 ± 0.02 J/m2, σ111 = 1.14 ± 0.03 J/m2 and surface tension constant γ111 = 0.841 ± 0.008 J/m2. The resulting ratio of σ100/σ111 = 1.41 ± 0.04 within the error coincides with the experimental value of 1.42 ± 0.05 measured for microscopic cavities in UO2 monocrystals.

► We perform MD simulation of isolated UO2 nanocrystals with surface at 2200 K and 2300 K. ► Cubic crystals transform to octahedral shape during long simulations of 100–1000 ns. ► Lattice parameter and specific energy are measured across crystals of 768–49152 ions. ► We get linear extrapolations to macroscopic values via reciprocal size dependences. ► Surface energy density and specific heat capacity show size independence.