Synthesis and Oxygen Storage Capacities of Hexagonal Dy1-xYxMnO3+δ

Polycrystalline samples of hexagonal P6(3)cm (space group No. 185) Dy1-xYxMnO3+delta were synthesized over complete solubility ranges under reducing conditions with oxygen partial pressures of 5 ppm to air at high temperatures (1200-1400 degrees C). All samples displayed unusually large, reversible oxygen content excess (0 <= delta <= 0.35) at low temperatures of similar to 200-400 degrees C under air and oxygen atmospheres in thermogravimetric measurements. During heating, stoichiometric samples showed rapid uptakes of oxygen at 200-300 degrees C and equally fast releases of oxygen at 275-375 degrees C when materials transformed back to the stoichiometric P63cm phase. Upon slow cooling from the stoichiometric P63cm phase, all samples displayed rapid uptakes of oxygen at 200-350 degrees 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 degrees C. Thermogravimetric and X-ray diffraction measurements indicate the presence of two new structural phases at delta approximate to 0.25 (Hex(2)) and delta approximate to 0.40 (Hex(3)). The thermal expansion coefficient (TEC) values of the Hex(2) and P6(3)cm phases were determined to be (8.4-11.6) x 10(-6) K-1 and (2.1-5.6) x 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) x 10(-2) mol(-1).