Crystal field-induced lattice expansion upon reversible oxygen uptake/release in YbMnxFe2−xO4

Abstract

We successfully form the solid solutions YbMn_xFe_2−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 YbMn_xFe_2−xO₄ readily take up oxygen and undergo a structural transition from Rm to P to become hyper-stoichiometric YbMn_xFe_2−xO₄.₅, which demonstrates their potential as oxygen storage materials. X-ray photoelectron spectroscopy (XPS) implies that Mn²⁺ and Fe²⁺ oxidize to Mn³⁺ and Fe³⁺ after the structural transition. Thermogravimetric analysis (TGA) and in situ SXRD measurements at elevated temperatures show that O₂ uptake commences at 200 °C but the structural transition does not until 300 °C. 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. Strong anisotropic expansion of the lattice occurs in all the reduced phases YbMn_xFe_2−xO₄ (Rm) and oxidized phases YbMn_xFe_2−xO_4.5 (P). We propose that this phenomenon can be attributed to d-electron filling and crystal field effects for the Mn and Fe cations.