Direct Cation-Cation Interactions Induced by Mg Dopants for Electron-Gas Behavior in alpha-Fe2O3

Abstract

alpha-Fe2O3 and its derivatives have been extensively studied for many years, but fascinating structural chemistry and physical behaviors continue to be reported in these systems to date. Here, we show that Mg doping in alpha-Fe2O3 could induce direct cation-cation interactions, which result in electron-gas behaviors. Rather than a simple replacement of Fe in FeO6 octahedra, the Mg dopant in alpha-Fe2O3 is eight-coordinated by six oxygen and two Fe in neighboring FeO6 octahedra via direct Fe-Mg-Fe interactions. The Mg2+ dopant takes both Fe3+ and empty octahedral interstitial sites so that one Mg-doped octahedron is sandwiched by two neighboring FeO6 octahedra, yielding an arrangement of three neighboring Fe-Mg-Fe polyhedra over the medium-range order. The valence electrons confined in the Fe-Mg bonds exhibit two-dimensional electron-gas (2DEG) characteristics, which may provide a new approach to other 2DEG systems, and the couplings with Fe 3d spins and the distortion of local symmetry lead to the spin-glass-like (SGL) magnetism, distinct from the antiferromagnetism of alpha-Fe2O3 and SGL/superparamagnetic behaviors in related materials due to the ultrasmall particle size. This study illustrates the complexity of the doped structure not only in local symmetries but also in the chemical bonding and electron spins for emerging physical properties even in low-cost oxides.