Synthesis and Oxygen Storage Capacities of Hexagonal Dy_1–xY_xMnO_3+δ

Polycrystalline samples of hexagonal P 6 3 cm (space group No. 185) Dy 1– x Y x MnO 3+δ were synthesized over complete solubility ranges under reducing conditions with oxygen partial pressures of 5 ppm to air at high temperatures (1200–1400 °C). All samples displayed unusually large, reversible oxygen content excess (0 ≤ δ ≤ 0.35) at low temperatures of ∼200–400 °C under air and oxygen atmospheres in thermogravimetric measurements. During heating, stoichiometric samples showed rapid uptakes of oxygen at 200–300 °C and equally fast releases of oxygen at 275–375 °C when materials transformed back to the stoichiometric P 6 3 cm phase. Upon slow cooling from the stoichiometric P 6 3 cm phase, all samples displayed rapid uptakes of oxygen at 200–350 °C. Similar changes were observed as a function of oxygen partial pressure at constant temperature. Increased, reversible changes in oxygen content were also achieved by high-pressure annealings in oxygen and hydrogen reduction at 400 °C. Thermogravimetric and X-ray diffraction measurements indicate the presence of two new structural phases at δ ≈ 0.25 (Hex 2 ) and δ ≈ 0.40 (Hex 3 ). The thermal expansion coefficient (TEC) values of the Hex 2 and P 6 3 cm phases were determined to be (8.4–11.6) × 10 –6 K –1 and (2.1–5.6) × 10 –6 K –1, respectively, and the chemical expansion (CE) associated with the transition between these phases was found to be (0.82–3.48) × 10 –2 mol –1 .