Oxygen and cation ordered perovskite, Ba2Y2Mn4O11

A three-step route has been developed for the synthesis of a new oxygen-ordered double perovskite, BaYMn₂O_5.5 or Ba₂Y₂Mn₄O₁₁. (i) The A-site cation ordered perovskite, BaYMn₂O_5+δ, is first synthesized at δ≈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 BaYMn₂O_5.0 phase is then readily oxygenated to the δ≈1 level by means of 1-atm-O₂ annealing at low temperatures. (iii) By annealing this fully oxygenated BaYMn₂O_6.0 in flowing N₂ gas at moderate temperatures the new intermediate oxygen content oxide, BaYMn₂O_5.5 or Ba₂Y₂Mn₄O₁₁, is finally obtained. From thermogravimetric observation it is seen that the final oxygen depletion from δ≈1.0 to 0.5 occurs in a single sharp step about 600°C, implying that the oxygen stoichiometry of BaYMn₂O_5+δ is not continuously tunable within 0.5<δ<1.0. For BaYMn₂O_5.5 synchrotron X-ray diffraction analysis reveals an orthorhombic crystal lattice and a long-range ordering of the excess oxygen atoms in the YO_0.5 layer. The magnetic behavior of BaYMn₂O_5.5 (with a ferromagnetic transition at ∼133 K) is found different from those previously reported for the known phases, BaYMn₂O_5.0 and BaYMn₂O_6.0.