New water-containing phase derived from the Sr3Fe2O7-δ phase of the Ruddlesden-Popper structure

Here a novel water-containing derivative phase (with the elongated c axis from 20.2 to 28.0 angstrom) is reported for the n = 2 member of the Srn+1FenO3n+1-delta Ruddlesden-Popper series, i.e., Sr3Fe2O7-delta with the layer sequence of SrO-SrO-FeO2-SrO1-delta-FeO2. The intercalated water molecules in the derivative phase are believed to reside within the (SrO)(2) double-layer block, probably accompanied by additional protons. The latter belief is supported by the fact that upon water incorporation iron was found to be reduced (toward the trivalent state). High-quality samples of the parent Sr3Fe2O7-delta phase were synthesized from a solution-derived precursor powder. For samples synthesized in air cerimetric titration yielded an oxygen content at 7 - delta = 6.44. More oxygen-deficient samples with 7 - delta = 6.32 and 6.18 were obtained by postannealing the as-synthesized material in flowing Ar gas at 400 and 500 degrees C, respectively. Independent of the value of delta, all the Sr3Fe2O7-delta samples transformed to the water derivative when subjected to an ambient (humid) atmosphere, whereas the speed of the topotactic derivative-phase formation increased with increasing delta. On the other hand, reversible dehydration of the derivative phase occurred upon heating in two distinguishable steps below similar to 230 degrees C.