Novel methods of synthesis and wet-chemical redox analysis for magnetoresistive double-perovskite Sr₂FeMoO(6-w)

An efficient encapsulation synthesis technique and an accurate wet-chemical analysis method for the redox state have been developed for the Sr₂FeMoO(6-w) double-perovskite compound known as a magnetoresistance (MR) material. Single-phase Sr₂FeMoO(6-w) samples are reproducibly obtained from Sr-Fe-Mo-O mixtures under very low oxygen partial pressures utilizing an Fe/FeO redox couple as the oxygen trap which is encapsulated together with the starting material mixture in an evacuated synthesis ampoule. Coulometric oxidation titration of the lower-valent iron and/or molybdenum species, formed upon dissolving the sample material in acidic solution, was found to reveal the oxygen content of a Sr₂FeMoO(6-w) sample with high reproducibility. Furthermore, the initial potential level in the titration is believed to indicate the nature of the reduced species. In the samples synthesized with the present encapsulation technique at 1150°C the oxygen non-stoichiometry was found to be w ≃ 0.03 ± 0.02. No indication of lower-valent molybdenum was observed.