Phonon Catalysis

Over the last decade the Atomistic Simulation & Energy (ASE) research group has developed two methods for modeling phonons, and their interactions via molecular dynamics (MD) simulations, namely Green-Kubo Modal Analysis (GKMA) and Interface Conductance Modal Analysis (ICMA). These methods use the normal modes of a system, which are computed in the harmonic limit, as a basis set for decomposing the heat flow in a system, in a very general way. They can be applied to any form of solid or rigid molecule. These techniques allow one to compute how much each individual normal mode/phonon in a given system contributes to thermal transport. However, the same approach can also be extended to other properties to determine the extent to which specific phonons contribute to other phenomena. For example, the same modal analysis techniques used in GKMA and ICMA can be used to determine which modes/phonons are responsible for a chemical reaction, a chemical transformation, ion/mass diffusion, or a phase change etc. Furthermore, once the modes that primarily contribute to a particular phenomenon are identified, they can in concept be externally excited to accelerate the transformation, which is a phenomenon the ASE group has termed “phonon catalysis”. This talk will show a first example of this phenomenon observed in MD simulations, whereby a Li-ion conductor has several specific phonons excited and its diffusivity increases by 4-6 orders of magnitude. Importantly, the modes responsible for the diffusion are excited to the same temperature that corresponds with the observed diffusivity, but the bulk temperature of the material remains effectively unchanged. This first example shows what may be possible with targeted phonon excitation, as phonon catalysis may be a new approach to stimulating or even suppressing chemical transformations in real world applications (e.g., a solid oxide fuel cell that operates at room temperature, with the transport and reaction kinetics of 1000°C).