Charge disproportionation and site-selective local magnetic moments in the post-perovskite-type Fe2O3 under ultra-high pressures

Abstract

The archetypal 3d Mott insulator hematite, Fe 2 O 3 , is one of the basic oxide components playing an important role in mineralogy of Earth's lower mantle. Its high pressuretemperature behavior, such as the electronic properties, equation of state, and phase stability is of fundamental importance for understanding the properties and evolution of the Earth's interior. Here, we study the electronic structure, magnetic state, and lattice stability of Fe 2 O 3 at ultra-high pressures using the density functional plus dynamical mean-field theory (DFT+DMFT) approach. In the vicinity of a Mott transition, Fe 2 O 3 is found to exhibit a series of complex electronic, magnetic, and structural transformations. In particular, it makes a phase transition to a metal with a post-perovskite crystal structure and site-selective local moments upon compression above 75 GPa. We show that the site-selective phase transition is accompanied by a charge disproportionation of Fe ions, with Fe 3±δ and δ ∼ 0.05-0.09, implying a complex interplay between electronic correlations and the lattice. Our results suggest that site-selective local moments in Fe 2 O 3 persist up to ultra-high pressures of ∼200-250 GPa, i.e., sufficiently above the core-mantle boundary. The latter can have important consequences for understanding of the velocity and density anomalies in the Earth's lower mantle.

Pressure-induced high-spin/low-spin disproportionated state in the Mott insulator FeBO3

“…We observed significant increase of the compressibility and hence decrease of bulk modulus from K 0 ~ 200 (LP) to ~ 154 GPa (HP) for the fixed bulk modulus pressure derivative K’ = 4. These results reveal an unusual softening of the lattice that follows the spin transition above ~ 50 GPa, which is in contrast to the anticipated hardening of the lattice at the HS-LS and/or Mott transition 6 , 11 , 12 , 20 , 24 , 25 . This also may be considered as a possible indirect confirmation of the idea of appreciable dynamical lattice effects.…”

“…Most notably, it exhibits apparently resilient non-metallic behavior above 100 GPa 19 . This behavior is different, e.g., to FeGaO 3 and Fe 2 O 3 which exhibit the classical band-width controlled Mott transition at ~ 50 GPa, characterized by a complete collapse of magnetic interactions 23 , and a site-selective Mott IMT at a similar pressure range 24 , 25 , respectively. In this regard, FeBO 3 and its high-pressure behavior is of particular interest as a possible example material documenting different mechanisms for electronic transitions.…”

Verwey-Type Charge Ordering and Site-Selective Mott Transition in Fe4O5 under Pressure

“…It was recently shown that under pressure, iron oxides demonstrate unexpected complexity of their compositions, such as FeO 2 , Fe 4 O 5 , Fe 5 O 6 , Fe 5 O 7 , Fe 7 O 9 , etc., with unusual crystal structures and complex electronic and magnetic properties, − which can be systematized by homologous structural series n FeO· m Fe 2 O 3 (with the exception of FeO 2 ) . Moreover, recent high-pressure (HP) studies of Fe 2 O 3 and Fe 3 O 4 suggest that these materials undergo a site-selective Mott transition, accompanied by a site-dependent collapse of magnetic moments. − In both cases it goes along with site-dependent delocalization (metallization) of the 3d electrons. As a result, the high-spin to low-spin (HS-LS) state crossover and metallization at crystallographically different Fe sites occur at different pressures.…”

Effect of epitaxial strain on the electronic structure and magnetic correlations in infinite-layer (Nd,Sr)NiO$_2$

“…DFT+DMFT has been proven to be among the most advanced theoretical methods for studying the electronic properties of strongly correlated materials, such as correlated transition metal oxides, heavy-fermions, and Fe-based superconductors, e.g., to study the phenomena of a Mott transition, collapse of local moments, large orbital-dependent renormalizations, etc. [48][49][50][51][52][53][54][55][56][57][58][59][60]. We use this advanced computational method to explore the effects of electronic correlations and Sr doping on the electronic structure, magnetic correlations, and exchange couplings of the unstrained (Nd,Sr)NiO 2 .…”