Fast ion diffusion, superionic conductivity and phase transitions of the nuclear materials UO2 and Li2O

Abstract

Uranium and lithium oxides are superionic conductors whose solid-state diffusion coefficients are comparable to that of liquids. They are also of technological importance to the nuclear industry. A detailed study of their physical and thermodynamic properties is of great interest for enhanced understanding of these systems. We review our lattice dynamics and molecular dynamics studies carried out on UO 2 and Li 2 O in their normal as well as superionic phase. Lattice dynamic calculations have been carried out using shell model in the quasiharmonic approximation. The calculated elastic constants, phonon frequencies and specific heat are in good agreement with the reported experimental data. Lattice dynamics calculations of the thermodynamic properties help validate the interatomic potentials required for undertaking molecular dynamics simulations to study the diffusion behavior in UO 2 and Li 2 O. The diffusion constant of Li in Li 2 O and O in UO 2 have been determined and compared with the experiment. The calculated superionic transition temperature of Li 2 O is 1000 K while that of UO 2 is 2300 K. With the help of molecular dynamics simulations the microscopic picture of the local structure of the lattice just beyond the transition temperature has been deduced. The structural behavior of UO 2 and Li 2 O, have also been simulated at high pressure. UO 2 is found to undergo a sluggish transformation to a high-pressure cotunnite phase at about 40 GPa. Similarly Li 2 O reveals an anti-fluorite to anti-cotunnite transformation (as reported in angle dispersive X-ray studies) at about 50 GPa. Our lattice dynamical calculations show up similar transformations for UO 2 and Li 2 O at 70 and 25 GPa respectively.

Generating structural distributions of atomistic models of Li2O nanoparticles using simulated crystallisation

“…Lithium oxides are of interest in technologically important areas such as energy storage 28,29 and nuclear. 30,31 Recently, nanostructured (energy) materials have been exploited for their high surface area and short pathways for Li-ion transport with implications for fast charge and high current output 28 in rechargeable batteries. Moreover, the nanomaterial can sustain a high (elastic) expansion and contraction, associated with intercalation of the Li charge carriers, compared to structural collapse suffered by the parent bulk material.…”