Crystal Field-induced Lattice Expansion upon Reversible Oxygen Uptake/Release in YbMn x Fe 2-x O 4

We successfully form the solid solutions YbMnxFe2-xO4 for x = 0.25, 0.50, 0.75, and 1.0 in order to study the mechanism of oxygen release and uptake as a function of Mn substitution. High-resolution synchrotron x-ray diffraction (SXRD) reveals that YbMnxFe2-xO4 readily uptakes oxygen and undergoes a structural transition from R-3m to P-3 to become the hyper-stoichiometric YbMnxFe2-xO4.5, which demonstrates their potential as oxygen storage materials. X-ray photoelectron spectroscopy (XPS) implies Mn2+ and Fe2+ oxidize to Mn3+ and Fe3+ after the structural transition. Thermogravimetric analysis (TGA) as well as in situ SXRD measurements at elevated temperatures show that O2 uptake commences at 200 ⁰C but the structural transition not until 300 ⁰C. The structural evolution under methane and air, monitored by in situ SXRD, implies promising reversibility and structural stability in this series. By performing structural refinements, we find that Mn substitution causes the lattice parameters, a and c, to evolve in a diametric fashion. Strongly anisotropic expansion of the lattice occurs in all the reduced phases YbMnxFe2-xO4 (R-3m) and oxidized phases YbMnxFe2-xO4.5 (P-3). We propose that this phenomenon can be attributed to d-electron filling and crystal field effects for the Mn and Fe cations.