PROTONS IN LAERO3

The ac electrical conductivity of Ca-, Sr-, and Ba-doped LaErO3 has been measured with the van der Pauw method as a function of P(H2O) (10(-5)-10(-2) atm) P(O2) (10(-20)-1 atm), and temperature (500-1200-degrees-C). The total conductivity is modelled assuming a defect structure which comprises protons and oxygen vacancies compensating the acceptor dopants. Furthermore, it is assumed, and supported by EMF measurements, that the conductivity has contributions from protons, oxygen vacancies, and electron holes. Data fitting allows the pre-exponential terms and enthalpies of mobilities and equilibrium constants to be determined. The alkaline-earth-doped LaErO3 is mainly a protonic conductor at high P(H2O) and low P(O2) and temperature. The equilibrium constant for the reaction H2O(g) + V(o)'' = 2H(i)' + O(o)x is found to decrease with increasing temperature; oxygen vacancies are the dominant carrier at high temperatures and protons at lower temperatures. The type of dopant examined plays only a modest role on the type of defects that are preferred in this system.