  We have synthesized A-site ordered (AO)- and A-site disordered
(AD)-Y1/4Sr3/4CoO3-delta (YSCO) with various oxygen deficiency delta, and have
made a comparative study of the structural and physical properties. We have
found that A-site (Y/Sr) ordering produces the unconventional oxygen-vacancy
ordered (OO) structure, and that the magnetic and transport properties of both
AO- and AD-YSCO strongly depend on the oxygen-vacancy (or excess oxygen)
ordering pattern. AO-YSCO with a stoichiometric delta of 0.5 has the
unconventional OO structure reflecting Y/Sr ordering pattern. With decreasing
delta from 0.5, the overall averaged OO structure is essentially unchanged
except for an increase of occupancy ratio for the oxygen-vacant sites. At delta
= 0.34, excess oxygen atoms are ordered to form a novel superstructure, which
is significant for the room-temperature ferromagnetism of AO-YSCO. In AD-YSCO,
on the other hand, the quite different OO structure, which is of a
brownmillerite-type, is found only in the vicinity of delta = 0.5.
