Defect chemistry modelling of oxygen-stoichiometry, vacancy concentrations, and conductivity of (La 1− xSr x) yMnO 3± δ

Abstract

Two precise algorithms are devised for the calculation of defect concentrations in A-site acceptor doped ABO 3 perovskites. The two models contain nine species including cation vacancies on the A- and B-site. The small polaron model is based on three redox levels of the B-ion. A large polaron model, based on delocalised electrons, electron holes and all B-ions being trivalent is given in Appendix A. The sequential mathematical method allows us to calculate the high temperature oxygen partial pressure dependent properties of (La 1− x Sr x ) y MnO 3± δ in a unified manner irrespective of the type of defect regime. Simulations are shown for a pO 2 span from 10 −30 to 10 5 atm. The three required equilibrium constants for (La 1− x Sr x ) y MnO 3± δ had to be changed significantly from values given in literature in order to match the observed stoichiometry span. The main results shown are calculated by the small polaron model containing only ionic species – the B-ion may be Mn B′ (Mn 2+), Mn B x (Mn 3+), and Mn B ⋅(Mn 4+). The A/B-ratio= y greatly influences the oxygen stoichiometry, oxygen ion vacancy- and cation vacancy concentrations and the total conductivity. Calculations are given for the range 0.87≤ y≤1.13 for a Sr doping of 10% at 1000°C. The defect model can simultaneously describe the observed stoichiometry and conductivity dependence on pO 2, if the electronic mobility is decreased by up to 50% at pO 2<10 −10 and pO 2>10 −2 atm.