Topotactic phase transformation of the brownmillerite SrCoO2.5 to the perovskite SrCoO3-δ

Oxygen stoichiometry is one of the most important elements in determining the physical properties of transition metal oxides (TMOs). A small fractional change in the oxygen content, resulting in the variation of valence state of the transition metal, can drastically modify the materials functionalities. In particular, TMOs with mixed valences have attracted attention for many energy applications. Previous studies also showed that the ability to control the number of d-band electron populations and detailed spin configurations is critical for improved catalytic performance of TMOs. In this context, SrCoO$_{x}$ (2.5 < x < 3.0) is an ideal class of materials due to the existence of two structurally distinct topotatic phases, i.e. the brownmillerite SrCoO$_{2.5}$(BM-SCO) and the perovskite SrCoO$_{3}$. Especially, BM-SCO has atomically-ordered one-dimensional vacancy channels, which can accommodate additional oxygen. Moreover, SrCoO$_{x}$ exhibits a wide spectrum of physical properties depending on the oxygen stoichiometry. Since SrCoOx has only a single knob to control the Co valence state without cation doping, it is an attractive material for studying the valence state dependent physical properties. However, so far, the growth of high quality single crystalline materials has not been much studied due to difficulty in controlling the right oxidation state. In this work, we report on the epitaxial growth of BM-SCO single crystalline films on SrTiO3 by pulsed laser epitaxy. In order to examine the topotactic phase transformation to the perovskite SrCoO$_{3-δ}$, some of samples were subsequently in-situ annealed at various oxygen pressure P(O$_{2}$). We found that post-annealing in high P(O$_{2}$) (> several hundreds of Torr) could fill some of oxygen vacancies accompanying systematic evolution in electronic, magnetic, and thermoelectric properties.