Polycrystalline samples of hexagonal P63 cm (space group No. 185) Dy1–x Y x MnO3+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 P63 cm phase. Upon slow cooling from the stoichiometric P63 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 (Hex2) and δ ≈ 0.40 (Hex3). The thermal expansion coefficient (TEC) values of the Hex2 and P63 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.

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In situ neutron powder diffraction experiments have been performed on samples of SrMnO y , La 0.1 Sr 0.9 MnO y , and La 0.2 Sr 0.8 MnO y (2.5 e y e 3) to determine the existence of oxygen-vacancy ordering and study order-disorder transformations in the La x Sr 1-x MnO y system. The vacancy ordered monoclinic Sr 7 Mn 7 O 13 , tetragonal (La x Sr 1-x ) 5 Mn 5 O 13 and orthorhombic (La x Sr 1-x ) 2 Mn 2 O 5 (x ) 0, 0.1, and 0.2) phases have been observed, together with already described La 0.1 Sr 0.9 MnO 2.55 . Reversible vacancy order-disorder transitions have been observed for the La-containing (La x Sr 1-x ) 5 Mn 5 O 13 phases. Oxygen-vacancy disordered cubic or pseudocubic phases have also been observed with increasing stability range in y with increasing x. Preliminary phase diagrams of the system have been derived as a function of temperature and oxygen content.

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