Pressure-Induced Site-Selective Mott Insulator-Metal Transition in Fe_{2}O_{3}

Abstract

We provide experimental and theoretical evidence for a pressure-induced Mott insulator-metal transition in Fe_{2}O_{3} characterized by site-selective delocalization of the electrons. Density functional plus dynamical mean-field theory (DFT+DMFT) calculations, along with Mössbauer spectroscopy, x-ray diffraction, and electrical transport measurements on Fe_{2}O_{3} up to 100 GPa, reveal this site-selective Mott transition between 50 and 68 GPa, such that the metallization can be described by (^{VI}Fe^{3+HS})_{2}O_{3} [R3[over ¯]c structure][under →]50 GPa(^{VIII}Fe^{3+HS} ^{VI}Fe^{M})O_{3} [P2_{1}/n structure][under →]68 GPa(^{VI}Fe^{M})_{2}O_{3}[Aba2/PPv structure]. Within the P2_{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^{M}), and only at higher pressures is a fully metallic high-pressure structure obtained, all at room temperature. Thereby, 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 approximately 50 GPa, Fe_{2}O_{3} is a strongly correlated metal with reduced electron mobility (large band renormalizations) of m^{*}/m∼4 and 6 near the Fermi level. Importantly, upon decompression, we observe a site-selective (metallic) to conventional Mott insulator phase transition (^{VIII}Fe^{3+HS} ^{VI}Fe^{M})O_{3}[under →]50 GPa(^{VIII}Fe^{3+HS} ^{VI}Fe^{3+HS})O_{3} within the same P2_{1}/n structure, indicating a decoupling of the electronic and lattice degrees of freedom. Our results offer a model for understanding insulator-metal transitions in correlated electron materials, showing that the interplay of electronic correlations and crystal structure may result in rather complex behavior of the electronic and magnetic states of such compounds.