Theoretical and experimental evidence of a site-selective Mott transition in Fe2O3 under pressure

Abstract

We provide experimental and theoretical evidence for a novel type of pressure-induced insulator-metal transition characterized by site-selective delocalization of the electrons. Mössbauer spectroscopy, X-ray diffraction and electrical transport measurements on Fe${}_2$O${}_3$ to 100 GPa, along with dynamical mean-field theory (DFT+DMFT) calculations, reveal this site-selective Mott transition between 50 and 68 GPa, such that the metallization can be described by ($^\rm{VI}$Fe${}^{3+\mathrm{HS}}$)${}_2$O${}_3$ [$R\bar{3}c$ structure] $\overrightarrow{50\mathrm{GPa}}$ ($^\rm{VIII}$Fe${}^{3+\mathrm{HS}\mathrm{VI}}$Fe$^\rm{M}$)O${}_3$ [$P{2}_1/n$ structure] $\overrightarrow{68\mathrm{GPa}}$ ($^\rm{VI}$Fe$^\rm{M}$)${}_2$O${}_3$ [$Aba2$ structure]. Within the $P{2}_1/n$ crystal structure, characterized by two distinct coordination sites (VI and VIII), we observe equal abundances of ferric ions (Fe${}^{3+}$) and ions having delocalized electrons (Fe$^\rm{M}$), and only at higher pressures is a fully metallic $Aba2$ structure obtained, all at room temperature. The transition is characterized by delocalization/metallization of the $3d$ electrons on half the Fe sites, with a site-dependent collapse of local moments. Above $\sim$50 GPa, Fe${}_2$O${}_3$ is a strongly correlated metal with reduced electron mobility (large band renormalizations) of m*/m$\sim$4 and 6 near the Fermi level. Upon decompression, we observe a site-selective (metallic) to conventional Mott insulator phase transition ($^\rm{VIII}$Fe${}^{3+\mathrm{HS}\mathrm{VI}}$Fe$^\rm{M}$)O${}_3$ $\overrightarrow{50\mathrm{GPa}}$ ($^\rm{VIII}$Fe${}^{3+\mathrm{HS}\mathrm{VI}}$Fe${}^{3+\mathrm{HS}}$)O${}_3$ within the same $P{2}_1/n$ structure, indicating a decoupling of the electronic and lattice degrees of freedom, characteristic of a true Mott transition. Our results show that the interplay of electronic correlations and lattice may result in rather complex behavior of the electronic structure and magnetic state.

18 pages, 5 figures

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