Surface Segregation Entropy of Protons and Oxygen Vacancies in BaZrO3.

The perovskite BaZrO3 has attracted considerable attention in the recent decade due to its high temperature proton conducting properties, and possible application as electrolyte in intermediate temperature fuel cells and electrolyzers. In this contribution, we performed, for the first time, first-principles calculations of the phonon contribution to the defect thermodynamics of the ZrO2 terminated (001) surface of BaZrO3. The approach allows us to determine both the segregation enthalpy and entropy of defects, which we apply to two fundamental defects in BaZrO3; fully charged oxygen vacancies (vO••) and protonic defects (OHO•). The calculations show that both defects exhibit favorable segregation enthalpies of -65 and -125 kJ/mol, respectively. Further, the vibrational formation entropy of the surface vO•• is significantly higher than that of the bulk vO••, due to smaller local structural relaxations of the surface defect, leading to a finite surface segregation entropy of 53 J/mol K. OHO•, on the other hand, displays nearly identical vibrational spectra at the surface and in the bulk, and the segregation entropy is therefore negligible. Hence, phonons not only stabilize the surface vO•• compared to the bulk defect thermodynamically at high temperatures, but also affect the relative stability of vO•• and OHO• at the surface. Finally, we apply a simplified space charge model to the (001) surface, and show that neglect of phonons results in strongly underestimated surface concentrations of vO••. [ABSTRACT FROM AUTHOR]