On the origin of positive deviation of the defect structure of complex oxides

Defect structure of a nonstoichiometric compound is normally negatively deviated from the ideal behavior as the nonstoichiometry or defect concentration increases, because of increasing coulombic attraction among charge-compensating defects. It has been sporadically reported that the defect structure deviates in the opposite way or positively from the ideal behavior particularly for hyperstoichiometric complex oxides, e.g., La2 − xSrxNiO4 + δ and the like. As has been the case with the negative deviation, authors have been trying to interpret the positive deviation conventionally by introducing the activity coefficients of the defects involved in the redox equilibrium, but with the coefficients evaluated or interpreted in different ways depending on the authors: typically, in terms of regular solution where the ionic defect activity-coefficient quotient for the redox reaction is assumed to be linearly proportional to the oxygen excess; in terms of the upward shift of electron chemical potential with the nonstoichiometry-induced electron concentration like a rigid band electron gas; in terms of neighboring-sites exclusion and electron-energy splitting due to coulombic repulsion among the same type defects. We have recently found that the positive deviation may be universally attributed to hole degeneracy. Here, we will critically review the earlier interpretations, present a new interpretation based on the hole degeneracy, and demonstrate its universal validity by reinterpreting quantitatively all the positive deviation behaviors of all those oxides reported so far.