Influence of Lattice Vibrations on Ionic Conduction—A New Model for Enhanced Ionic Conductivity

Ionic conductivity in solid-state materials plays a crucial role in various emerging applications, such as all-solid-state batteries and fuel cells. Understanding the fundamentals of ion conduction is essential for the rational design of next-generation devices with enhanced properties.
Although ionic conduction is generally well understood at different levels, some observations remain unincorporated into the theoretical descriptions of material behavior. Protonic conductivity is a particularly interesting case due to the extremely small mass and high charge density of protons. For example, in oxide materials with perovskite structure, proton conduction has been observed to follow a polaron-like mechanism, suggesting a close connection between lattice dynamics and ion conduction.
To investigate the influence of lattice dynamics on proton mobility, we employed nuclear resonance vibrational spectroscopy (NRVS) technique to study lattice vibrations. Since NRVS is sensitive only to Mössbauer-active isotopes, we used Y-doped barium stannates, which have a perovskite structure. By combining this technique with inelastic neutron scattering we built ab initio model, that allowed us to reconstruct the full phonon spectrum of the material and correlate it with proton conductivity. By comparing the activation energies of proton transport with the crystallographic structure and phonon dispersion, we propose a new framework to describe how lattice vibrations influence proton transport. The high mobility of protons allows their movement within the lattice to be decoupled from that of the lattice itself, as the frequency of proton vibrations and their jump rate are higher than those of the lattice vibrations. This allows us to treat the proton as a particle moving through a dynamic potential energy landscape. As a result, the activation energy for proton hopping is time-dependent and significantly reduced by certain lattice vibration modes.
In this work, we demonstrate that this contribution is substantial, even surpassing other structural factors, such as lattice parameters or interatomic distances. Our proposed model not only explains the behavior of other known perovskite-type proton conductors but also accurately describes their conductivity on a quantitative level. By revealing the connection between atomic structure and phonon behavior, we suggest how this knowledge can be applied to design materials with improved ionic conductivity.