Understanding Proton Transfer Mechanisms in Perovskite Oxides

Proton transfer plays a critical role in various chemical and physical processes, particularly in the context of perovskite oxides. Recent studies illustrate that this transfer often occurs over a remaining barrier, as highlighted by observable hydrogen/deuterium (H/D) isotope effects. While the repulsion between hydrogen and boron atoms is reduced in some configurations, the activation enthalpy remains significantly influenced by the elongation of the B-O bond and the barriers associated with proton transfer.

The importance of hydrogen/boron (H/B) repulsion is further emphasized by differences in activation enthalpies between cubic perovskites. Notably, perovskites with pentavalent B-site cations exhibit higher activation enthalpies compared to their tetravalent counterparts. This difference can be attributed to various structural and dynamical factors, which impact proton conductivity, especially when symmetry is reduced within the crystal structure.

Comparative analyses of protonic defects in materials like yttrium-doped BaCeO3 and SrCeO3 reveal how the arrangement of lattice oxygen affects proton mobility. In SrCeO3, the structure's large orthorhombic distortion results in two distinct oxygen sites with differing acid/base properties. This complexity necessitates that long-range proton transport involves transfers between these chemically different sites, leading to increased activation enthalpy and reduced conductivity compared to BaCeO3.

The mobility of protonic defects in perovskites is highly sensitive to changes in crystallographic symmetry, as well as local chemical perturbations caused by acceptor dopants. Although aliovalent dopants are typically chosen based on their ionic radii to maintain structural integrity, this approach can fail for proton conductivity. For instance, BaZrO3 shows significantly different proton mobilities when doped with scandium compared to yttrium, despite similarities in ionic size. This discrepancy highlights how the interaction between the dopant and the surrounding oxygen plays a crucial role in proton affinity.

Moreover, empirical data suggests that the highest proton conductivities are found in oxides with a perovskite structure, largely due to their unique framework of corner-sharing octahedral BO6 units. The high coordination numbers achieved in these structures contribute to the effective transport of protons, indicating that understanding these mechanisms is vital for future advancements in materials science and engineering.

In summary, the intricate relationship between structural characteristics, dopant effects, and proton transfer barriers in perovskite oxides underscores the complexity of ionic transport in these materials and provides essential insights for developing improved solid electrolytes and fuel cell technologies.