57Fe Moessbauer spectroscopy investigation of the FeII/III mixed-valence state and the B-site order in double perovskite A2FeMoO6

B-site ordered double-perovskite samples A2FeMoO6, with A=Ca, Sr or Ba were synthesized in a reducing atmosphere using an encapsulation technique. The samples were characterized by 57Fe Moessbauer spectroscopy. The valence of Fe was found to be in an intermediate II/III state, being close to the midpoint for A=Sr and slightly shifted towards II for Ba. The degree of B-site ordering was found to be more than 95%. Using various synthesis temperatures the ordering could be tuned. Fe atoms replacing Mo at the antisite and adjacent to it were identified in the Moessbauer data. Presence of antiphase boundaries was also seen