Stabilization of tetra- and pentavalent Fe ions in Fe-substituted Li2MnO3 with layered rock-salt structure

Origins of various valence states of various Fe ions above the 3+ state for 30% Fe-substituted Li2MnO3 [Li1+x(Fe0.3Mn0.7)1−xO2] with a layered rock-salt structure (R3¯m) have been investigated using magnetization measurement and Fe57 Mössbauer spectroscopy at less than 300K. The temperature dependence of magnetization data revealed that small spontaneous magnetization [1.26(9)Gcm3∕g at 5K] appeared at less than 81K and cusps were observed at 24K under both zero field and field cooling runs. Analyses of Fe57 Mössbauer spectra showed that all samples contained tetravalent (Fe4+) and pentavalent (Fe(5−δ)+) iron ions (20%–40%) along with trivalent ones. High-valence (Fe(5−δ)+) iron might be the result of Fe(3+δ)+∕Fe(5−δ)+ charge disproportionation from tetravalent iron, as in SrMn0.5Fe0.5O3. Small paramagnetic iron doublets (2%–5%) with highly negative isomer shift values (−0.5to−0.6mm∕s) were detected at less than 10K. They might be pentavalent Fe(5+δ)+ ion. The Fe(5+δ)+ ion can be stabilized under high excess lithium content x (>0.28) according to the chemical formula Li1+x(Fe0.3Mn0.7)1−xO2 close to Li2MO3 composition and by suppressing 3d cation disordering to the Li layer. The unique valence state of Fe ions (>4+ state) might be stabilized in the Mn–Li layer of the Li2MnO3 structure. These results mean that Fe ions at 5+ states are stable ions in oxides not only in a perovskite-related structure but also in a rock-salt-related one.