Ion-electron transport in strontium ferrites: relationships with structural features and stability

The total electrical conductivity of strontium ferrites, including intergrowth Sr(4)Fe(6)O(13+delta), Sr(3)Fe(2)O(6+delta) with a Ruddlesden-Popper structure, and SrFeO(2.5+delta) where the cubic perovskite lattice transforms into vacancy-ordered brownmillerite at P(O(2)) < 10 Pa and T < 850degreesC, was measured at 650-1000degreesC in the oxygen partial pressure range 10(-15) Pa to 50 kPa. The data were used in order to determine partial ion, p- and n-type electron contributions in the vicinity of electron-hole equilibrium point. The ferrites with brownmillerite and Ruddlesden-Popper structures exhibit substantial ion transport due to thermally-activated disordering of oxygen vacancies and oxygen ions in the perovskite structural slabs, whereas the ion conductivity of Sr(4)Fe(6)O(13+delta) remains below 0.01 S cm(-1) in the studied conditions. The bonding energy of oxygen ions, evaluated from the formation enthalpy of n-type charge carriers, increases in the sequence Sr(4)Fe(6)O(13+delta) < SrFeO(3+delta) < Sr(3)Fe(2)O(6+delta). These values correlate with thermodynamic stability of strontium ferrites at low p(O(2)). The transition of SrFeO(2.5+delta) brownmillerite into disordered cubic phase above 850 degreesC leads to higher stability in reducing atmospheres. The level of p-type conductivity is mainly governed by the concentration of electron holes, which was calculated from the oxygen content determined by coulometric titration technique. The hole mobility, which is quite similar for all strontium ferrites and has a temperature-activated character, varies in the range 0.005-0.05 cm(2) V(-1) s(-1) indicative of small-polaron conduction mechanism. (C) 2004 Elsevier SAS. All rights reserved.