Hole-trapping effect on thermoelectric power of mixed ionic electronic conductor BaTiO3

The effect of hole-trapping on the thermopower of a mixed ionic electronic conductor, e.g., BaTiO3, is analyzed in terms of irreversible thermodynamics by taking trapped holes as a fourth kind of electronic charge carrier in addition to free electrons, free holes and mobile oxide ions. It is found that the effect manifests itself in two ways: thermostatically in the ionic thermopower via the thermodynamic factor and dynamically in the electronic thermopower via the electrical conductivity contribution of the trapped holes. The thermopowers of both 99.995% pure, undoped and 1.8 m/o Al-doped BaTiO3, that were measured against oxygen activity in the range of -18 < log aO2 < or = 0 at elevated temperatures of 800 degrees to 1100 degrees C [H.-I. Yoo and C. R. Song, J. Electroceram., 2001, 6, 61, ref. 6], are reanalyzed by taking into account the hole-trapping for the doped case. It is found that while the reduced heats-of-transport of free electrons and holes are, respectively, close to their thermal energy k(B)T (k(B) being the Boltzmann constant), that of trapped holes is close to their migration energy that is essentially the same as the trapping energy onto the acceptors doped, 1.04 eV.