Characterization of Oxygen Storage and Structural Properties of Oxygen-Loaded Hexagonal R MnO 3+δ ( R = Ho, Er, and Y)
Single-phase polycrystalline samples of stoichiometric RMnO3+delta (R = Er, Y, and Ho) were achieved in the hexagonal P6(3)cm structure through solid state reaction at, similar to 1300 degrees C. Thermogravimetric measurements in oxygen atmospheres demonstrated that samples with the larger Ho and Y show rapid and reversible incorporation of large amounts of excess oxygen (0.3 > delta> 0) at an unusually low temperature range of similar to 190-325 degrees C, indicating the industrial usefulness of RMnO3+delta materials for lower cost thermal swing adsorption processes for oxygen separation from air. Further increase of the excess oxygen intake to delta similar to 0.38 was achieved for all the investigated materials when annealed under high pressures of oxygen. The formation of three oxygen stable phases with 6 = 0, 0.28, and 0.38 was confirmed by thermogravimetric measurements, synchrotron X-rays, and neutron diffraction. In situ synchrotron diffraction proved the thermal stability of these single phases and the regions of their creation and coexistence, and demonstrated that the stability of the delta = 0.28 phase increases with the ionic size of the R ion. Structural modeling using neutron powder diffraction for oxygen excess phases describes the formation and details of a large R3c superstructure observed for HoMnO3.28 by tripling the c-axis of the original parent unit cell. Modeling of the RMnO3.38 (R = Y and Er) oxygen-loaded phase converged on a structural model consistent with the symmetry of Pca2(1).