Superionic hcp-Fe alloys and their seismic velocities in Earth’s inner core

Earth’s inner core (IC) is less dense than pure iron, indicating the existence of light elements within it1. Si, S, C, O, and H have been suggested to be the candidates2,3, and the properties of Fe-light-element alloys were studied to constrain the IC composition4-19. Light elements have a significant influence on seismic velocities4-13, melting temperatures15-17, and thermal conductivities of Fe-alloys18,19. However, the state of the light elements in the IC is rarely considered. Using ab initio molecular dynamics (AIMD) simulations, we found that H, O, and C in hexagonal close-packed (hcp) Fe transform to a superionic state under IC conditions, showing high diffusion coefficients like liquid. It suggests the IC can be in superionic state rather than normal solid state. The liquid-like light elements lead to a significant reduction in the seismic velocities approaching the seismological observation of the IC20,21. The significant decrease in shear wave velocity (VS) gives an explanation on the soft IC21. In adddtion, the light-element convection in the IC has potential influence on the IC seismological structure and magnetic field.

Thermal Jamming of Ions in the Superionic State of UO2

ABSTRACTThe oxygen ions in the high temperature superionic state of uranium dioxide (UO2) are known to be in an arrested or jammed state, exhibiting characteristic features of jammed kinetics such as low dimensional string-like ion hopping and dynamical heterogeneity (DH). This thermally-jammed state entails a configurational entropic cost. Using atomistic simulations and the 2PT method, we compute the solid-like (vibrational) and hard sphere-like (configurational) contributions to the total entropy across a temperature range of 1500 K to 2800 K that envelop both the onset of superionic conduction (2000 K) and the second order λ-transition (2610 K). To properly account for the thermally jammed state of the ions, we use an equation of state that is appropriate for the metastable fluid branch. Our simulation results are in excellent agreement with the entropy data extracted from specific heat experiments with a mean error of less than 2%.

A computational study on the superionic behaviour of ThO2

This study reports DFT and MD study of lattice dynamical, mechanical and anionic transport behaviour of ThO2 in the superionic state. Softening of B1u phonon mode is a precursor to the dynamical disorder of the oxygen sublattice and 〈001〉 is the easy direction for anion migration in the superionic state.