Structural modulation and spin glassiness upon oxidation in oxygen storage material LnFeMnO4+x for Ln = Y, Lu, and Yb

The mixed valence multiferroic LnFe(2+)Fe(3+)O(4) (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 LnFe(3+)Mn(3+)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, Ln(3+), does not seem to influence such modulations since we observe identical satellite reflections for all three samples of Ln = Y, Lu, and Yb. Both LnFeMnO(4) and LnFeMnO(4.5) display spin glassy behavior with 2D short-range magnetic ordering being observed in LnFeMnO(4). Analysis of the neutron diffraction data reveals a correlation length of similar to 10 nm. Upon oxidation to LnFeMnO(4.5), the short-range magnetic order is significantly suppressed.

Automated summary

By using neutron, electron, and synchrotron X-ray diffraction studies, we have observed a structural modulation in the mixed valence multiferroic LnFeMnO(4.5). Bond valence analysis suggests that oxygen insertion likely occurs at the middle of the Fe/Mn-O bipyramid layers. The nature of the lanthanide, Ln(3+), does not seem to influence such modulations.