  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$_{3-\delta}$, 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.
