Angus Gray-Weale's Research

A superionic crystal or solid electrolyte is one in which some ions move so freely that the material conducts electricity as well as a molten salt does. A superionic crystal has a transition on heating to high ionic conductivity. The crystal remains rigid, but conducts electricity well, as though only one ionic species has ‘melted’ while the others stayed fixed in the crystal lattice. Such materials are used in fuel cells (e.g. yttria-stabilised zirconias), and other clean energy technologies. High conductivity generally only occurs at high temperature and this compromises performance.



The above figure shows an inherent structure from a simulation of lead(II) fluoride, showing the tendency of interstitials and vacancies to cluster. An inherent structure is a mechanically stable configuration of the particles, obtained by minimising the potential energy. Minimisation removes vibrational motions that obscure the structure.

Below are four snapshots showing structures from molecular dynamics trajectories for a model of lead(II) fluoride, and the corresponding inherent structures. The yellow particles are on-site anions, the blue are lead ions, the green are interstitial fluoride ions, and the red are vacancies, i.e. the centers of empty tetrahedra. The upper configurations are taken from molecular dynamics runs, just below and just above the superionic transition. The lower configurations are the corresponding inherent structures. The cluster of four interstitials around a vacancy at the top left of the lead fluoride inherent structure at 950 K is that observed in doped lead fluoride at low temperature by neutron scattering experiments. The inherent structure show the sharp increase in disorder at the superionic transition, and their energy distribution gives us the 'equation of state' for the defects.