Influence of dipolar interactions on the conduction mechanism of Li+-β-alumina: Molecular Dynamics study

We present Molecular Dynamics simulation results of stoichiometric (s; Li2Al22O34) and nonstoichiometric (ns; Li2+0.43Al22O34+0.21) Li+-β-alumina in the temperature range 300-700 K. Structural properties and ion migration are studied. Particular attention is paid to the role of polarization interactions. All types of ions are considered to be polarizable. Induced dipole moments are calculated in two different ways. For oxygen ion polarizabilities less than 0.3 Å3 an iterative method is used. For higher polarizability, a switching function is introduced to prevent a polarization catastrophe. The induced dipole moments are determined through an equation of motion. The simulations confirm the picture in which Li+ ions move out of the conduction plane attaching themselves to the spinel blocks. A competition between the dipolar interactions of the Li+ with each other and with the highly polarized oxygen ions situated in the spinel blocks near the conduction planes was observed. At oxygen ion polarizabilities less than 3.0 Å3, the Li+ dipole moments show a head to tail arrangement in the x-y plane, which lowers the Li+-Li+ dipolar energy, stimulates the collective motion and therefore the self-diffusion. However, at a polarizability of 3.5 Å3, the dipolar interactions of the oxygen ions become dominant, and the head to tail pattern vanishes. The Li+ are now attached to the spinel blocks, leading to a decrease of the Li+ ion diffusivity.