⁵⁷Fe Mössbauer studies of Sr<inf>2-x</inf>Ca <inf>x</inf>FeReO<inf>6</inf> double perovskite compounds

We present a 57Fe Mössbauer study of the Sr 2-xCaxFeReO6 double perovskite series, with x = 0, 0.1, 0.2, 0.5, 1.0, 1.5 and 2.0. The analysis at 300 and 16 K shows iron in three different metal neighbour environments, due to the misplacement of Fe and Re ions (antisite defects). Judging from the isomer shift and hyperfine magnetic field values, iron ions on the perfect ordered structure show intermediate valence state (between Fe3+ and Fe2+), with an effective electronic configuration 3d(5+y), where y decreases from 0.3 to 0.2 as x increases from 0 to 2. Changes in quadrupole shift values for x ≥ 1.0 confirm the cubic to monoclinic structural transition observed in these compounds.