Proton Conductivity in Hydrogen Phosphate/Sulfates from a Coupled Molecular Dynamics/Lattice Monte Carlo (cMD/LMC) Approach

The ionic conductivity of solid acids of CsHnXO4 type (X = P, S; n = 2, 1) varies upon chemical substitution P ↔ S and between different crystal structures (Tc = 503 K for CsH2PO4). We apply a recently developed coupled molecular dynamics/lattice Monte Carlo simulation approach (cMD/LMC1,2) to explain both the phosphate/sulfate and temperature/phase-related variations of the proton conductivity on a molecular level. Our simulation method elucidates the relative importance of the two key components of the Grotthuss-type proton conduction mechanism, proton hopping and structural reorientation, as a function of the chemical/thermodynamical conditions. We find that the chemical substitution leads to a substantial change in the proton hopping rate, which however results only in a modest variation of the proton diffusivity. The variation of the temperature of CsH2PO4 results in a significant response of the anion rotation frequency, which turns out to be the rate-limiting process for proton conduction. In parti...