Defect segregation to grain boundaries in BaZrO3 from first-principles free energy calculations
Journal article, 2017
Acceptor-doped BaZrO3 is currently the most promising ceramic proton conductor for application in electrolyzers and reactor membranes. Its overall proton conductivity is, however, limited by space charge formation due to defect segregation to grain boundaries (GBs). In this contribution, we determine the vibrational contribution to the free energy of GB formation and defect segregation in the symmetric Sigma 3 (112) tilt GB in BaZrO3 in order to elaborate on the high temperature GB defect chemistry. We consider segregation of several different defect species: oxygen vacancies (v(O)(center dot center dot) ), protons (OHO center dot) and the four trivalent acceptor dopants Sc, In, Y and Gd (M '(Zr). The calculations reveal that the segregation free energy of v(O)(center dot center dot) decreases with temperature while it remains fairly constant for (OHO center dot ), which increase the relative stability of similar to with respect to (OHO center dot ) with increasing temperature. As a consequence the onset for hydration of the GB core is shifted towards lower temperatures. For the dopants we find that both the segregation energy and entropy correlate with the ionic radius, where they are positive for the smaller dopants (Sc and In) while negative for the larger (Y, Gd). In turn, this leads to similar segregation free energies for all species, which are close to zero, at high temperatures where dopants are mobile, implying that dopant segregation is almost entirely driven by segregated positively charged defects. Neglecting the phonon contribution erroneously leads to the conclusion that the thermodynamic driving force for dopant segregation increases with increasing dopant radii, and it is therefore important to consider free energies when predicting the high temperature defect chemistry of BaZrO3 GBs.