Characterization of Oxygen Storage and Structural Properties of Oxygen-Loaded Hexagonal RMnO3+δ (R = Ho, Er, and Y)

• Published in: Chemistry of materials Vol. 27; no. 18; pp. 6259 - 6267
• Main Authors: Abughayada, C, Dabrowski, B, Kolesnik, S, Brown, D. E, Chmaissem, O
• Format: Journal Article
• Language: English
• Published: American Chemical Society 22-09-2015
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/acs.chemmater.5b01817

Single-phase polycrystalline samples of stoichiometric RMnO3+δ (R = Er, Y, and Ho) were achieved in the hexagonal P63 cm structure through solid state reaction at ∼1300 °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 > δ > 0) at an unusually low temperature range of ∼190–325 °C, indicating the industrial usefulness of RMnO3+δ materials for lower cost thermal swing adsorption processes for oxygen separation from air. Further increase of the excess oxygen intake to δ ∼ 0.38 was achieved for all the investigated materials when annealed under high pressures of oxygen. The formation of three oxygen stable phases with δ = 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 δ = 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 Pca21.