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.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films