The structure and proton transport mechanisms in the superprotonic phase of C_SH₂PO₄: An ab initio molecular dynamics study

Anhydrous proton conducting materials have attracted a burst of interest in recent years because of their potential application in fuel cells. However, a detailed microscopic picture of proton transport mechanisms has not been fully established. To gain a deeper understanding of such materials, the superprotonic phase of cesium dihydrogen phosphate (CDP), CsH₂PO₄, was studied using Car-Parrinello ab initio molecular dynamics (CPAIMD) simulations. Unlike previous classical molecular dynamics studies of related systems, such as CsHSO₄, proton transfer is explicitly included in the present CPAIMD simulations. A detailed analysis of structural, dynamical and spectroscopic properties of CDP is reported. The cubic structure of the system is dynamically disordered as seen from the broad Cs - Cs and Cs - P radial distribution functions (RDF), but the fluctuations in the hydrogen bond distances (from the O - O RDF) are much smaller than those of the Cs - Cs and Cs - P RDFs. The orientations of PO₄ groups are disordered as well, but a complete tumbling motion of the PO₄ group was not observed within the time scale of the present CPAIMD simulations (∼20 ps). In fact, the reorientation rate of the PO₄ group was found to be smaller than the proton hopping rate, contrary to previously held notions. Therefore, proton diffusion was observed to occur via the slow reorientation of PO₄ followed by a fast proton hopping to a neighboring PO₄ unit. A Grotthuss type structural diffusion mechanism, which does not require PO₄ reorientation, was also observed with an associated order of magnitude faster time scale.