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We provide experimental and theoretical evidence for a novel type ofpressure-induced insulator-metal transition characterized by site-selectivedelocalization of the electrons. M\"ossbauer spectroscopy, X-ray diffractionand electrical transport measurements on Fe$_2$O$_3$ to 100 GPa, along withdynamical mean-field theory (DFT+DMFT) calculations, reveal this site-selectiveMott transition between 50 and 68 GPa, such that the metallization can bedescribed by ($^\rm{VI}$Fe$^{3+\rm{HS}}$)$_2$O$_3$ [$R\bar{3}c$ structure]$\overrightarrow{\tiny\rm{50~GPa}}$($^\rm{VIII}$Fe$^{3+\rm{HS~VI}}$Fe$^\rm{M}$)O$_3$ [$P2_1/n$ structure]$\overrightarrow{\tiny\rm{68~GPa}}$ ($^\rm{VI}$Fe$^\rm{M}$)$_2$O$_3$ [$Aba2$structure]. Within the $P2_1/n$ crystal structure, characterized by twodistinct coordination sites (VI and VIII), we observe equal abundances offerric ions (Fe$^{3+}$) and ions having delocalized electrons (Fe$^\rm{M}$),and only at higher pressures is a fully metallic $Aba2$ structure obtained, allat room temperature. The transition is characterized bydelocalization/metallization of the $3d$ electrons on half the Fe sites, with asite-dependent collapse of local moments. Above $\sim$50 GPa, Fe$_2$O$_3$ is astrongly correlated metal with reduced electron mobility (large bandrenormalizations) of m*/m$\sim$4 and 6 near the Fermi level. Upondecompression, we observe a site-selective (metallic) to conventional Mottinsulator phase transition ($^\rm{VIII}$Fe$^{3+\rm{HS~VI}}$Fe$^\rm{M}$)O$_3$$\overrightarrow{\tiny\rm{50~GPa}}$ ($^\rm{VIII}$Fe$^{3+\rm{HS~VI}}$Fe$^{3+\rm{HS}}$)O$_3$ within the same $P2_1/n$ structure, indicating a decoupling ofthe electronic and lattice degrees of freedom, characteristic of a true Motttransition. Our results show that the interplay of electronic correlations andlattice may result in rather complex behavior of the electronic structure andmagnetic state.

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