Appearance of pentavalent Fe ion as a result of a charge disproportionation in Fe-substituted Li2MnO3

Abstract

20% Fe substituted Li2MnO3 (Li1+x(Fe0.2Mn0.8)1-xO2, 0 < x < 1/3) was prepared using coprecipitation–calcination method. X-ray Rietveld analyses showed that most Fe and Mn ions were on both 4g and 2b sites and small amount of them occupy on 2c and 4h sites in monoclinic Li2MnO3-type structure. Neutron Rietveld analysis revealed that Mn ion was mainly on the 4g site, whereas Fe ion was on both 4g and 2b sites. Temperature dependence of magnetization detected a cusp at 30 K for the sample, originating from its antiferromagnetic nature. The 57Fe Mössbauer spectrum at 300 K consisted of two symmetric doublets with different isomer shifts. From the isomer shifts, Fe ion existed as 3+δ and 4+δ states (0 < δ < 1). Below 200 K, the Fe4+δ component split to two doublets having two isomer shift values assigned as 4+ and 5+ states. At 3 K, the spectrum consisted of magnetically ordered 3+, 4+, and 5+ irons and paramagnetic 4+ state iron. The magnetically ordered pentavalent and tetravalent Fe ions can be stabilized at a highly distorted octahedral 4g site, whereas magnetically ordered trivalent and paramagnetic tetravalent Fe ions exist mainly on the less distorted octahedral 2b site.