  A three-step route has been developed for the synthesis of a new
oxygen-ordered double perovskite, BaYMn2O5.5 or Ba2Y2Mn4O11. (i) The A-site
cation ordered perovskite, BaYMn2O5+d, is first synthesized at d ~ 0 by an
oxygen-getter-controlled low-O2-pressure encapsulation technique utilizing FeO
as the getter for excess oxygen. (ii) The as-synthesized, oxygen-deficient
BaYMn2O5.0 phase is then readily oxygenated to the d ~ 1 level by means of
1-atm-O2 annealing at low temperatures. (iii) By annealing this
fully-oxygenated BaYMn2O6.0 in flowing N2 gas at moderate temperatures the new
intermediate-oxygen-content oxide, BaYMn2O5.5 or Ba2Y2Mn4O11, is finally
obtained. From thermogravimetric observation it is seen that the final oxygen
depletion from d ~ 1.0 to 0.5 occurs in a single sharp step about 600 C,
implying that the oxygen stoichiometry of BaYMn2O5+d is not continuously
tunable within 0.5 < d < 1.0. For BaYMn2O5.5 synchrotron x-ray diffraction
analysis reveals an orthorhombic crystal lattice and a long-range ordering of
the excess oxygen atoms in the YO0.5 layer. The magnetic behavior of BaYMn2O5.5
(with a ferromagnetic transition at ~ 133 K) is found different from those
previously reported for the known phases, BaYMn2O5.0 and BaYMn2O6.0.
