Oxygen nonstoichiometry and high-temperature transport in SrFe1-xWxO3-δ

The complex oxides in the series SrFe1-xWxO3-delta were synthesized and attested to have cubic structure. The oxygen content was measured by coulometric titration technique, and electrical conductivity was measured by four-probe d.c. method in the oxygen partial pressure range 10(-18)-0.5 atm and at temperatures 650-950 degrees C. Thermal expansion tests were performed in air within temperature interval 20-1000 degrees C. The partial molar thermodynamic functions of labile oxygen ions were calculated using coulometric titration data. Conductivity analysis was utilized to separate partial contributions from oxygen ions, n-and p-type electron charge carriers. The concentration and mobility of p-type charge carriers were calculated by combining data from oxygen content and conductivity measurements. It is shown that partial replacement of iron for tungsten results in a strong disordering of the oxygen sub-lattice that renders the perovskite to brownmillerite phase transition suppressed. The observed ion conductivity level in the sample x=0.1 at T<850 degrees C occurs somewhat higher than in the undoped ferrite while electron contribution remains nearly the same. The increase in the concentration of the dopant results in filling of oxygen vacancies and in respective decrease of the oxygen ion conductivity level. The thermal expansion coefficient is observed to monotonously decrease with the increase in tungsten content. (c) 2007 Elsevier B.V. All rights reserved.