Structural modulation and spin glassiness upon oxidation in oxygen storage material LnFeMnO_4+x for Ln = Y, Lu, and Yb

The mixed valence multiferroic LnFe²⁺Fe³⁺O₄ (where Ln = Y, Lu, and Yb) can reversibly uptake oxygen into its lattice, which is evidenced by a crystallographic phase transition along with the appearance of structural modulations. In this study, we show that the Mn-substituted version of this multiferroic can also be readily oxidized to LnFe³⁺Mn³⁺O_4.5 revealing similar oxygen storage behavior. Through neutron, electron, and synchrotron x-ray diffraction studies, we observe a structural modulation that we attribute to a displacement wave in the fully oxidized compound. This wave exhibits commensurability with a wavevector q = (−2/7, 1/7, 0). Bond valence summation analysis of plausible interstitial oxygen positions suggests that oxygen insertion likely occurs at the middle of the Fe/Mn-O bipyramid layers. The structural modulation of LnFeMnO_4.5 is two-dimensional, propagates along the ab-plane, and is highly symmetric as 12 identical modulation vectors are observed in the diffraction patterns. The nature of the lanthanide, Ln³⁺, does not seem to influence such modulations since we observe identical satellite reflections for all three samples of Ln = Y, Lu, and Yb. Both LnFeMnO₄ and LnFeMnO_4.5 display spin glassy behavior with 2D short-range magnetic ordering being observed in LnFeMnO₄. Analysis of the neutron diffraction data reveals a correlation length of ∼10 nm. Upon oxidation to LnFeMnO_4.5, the short-range magnetic order is significantly suppressed.