A new look on the nature of high-spin to low-spin transition in Fe2O3

TL;DR

This study uses density functional theory to analyze pressure-induced spin and structural transitions in Fe2O3, revealing that spin state changes are primarily governed by octahedral volume rather than structural phase transitions.

Contribution

The paper introduces a simple octahedral volume parameter, Voct, as a key indicator for spin state transitions in Fe2O3, bridging experimental discrepancies and enhancing understanding of the transition mechanism.

Findings

GGA captures both spin and structural transitions in Fe2O3.

Spin transition correlates with octahedral volume Voct, independent of structural phase.

Analysis reconciles experimental controversies regarding transition phases.

Abstract

Iron sesquioxide (Fe2O3) displays pressure and temperature induced spin and structural transitions. Our calculations show that, density functional theory (DFT), in the generalized gradient approximation (GGA) scheme, is capable of capturing both the transitions. The ambient pressure corundum type phase (hematite or alpha-Fe2O3), having R_3c symmetry, gets distorted by the application of pressure and transforms to a distorted corundum type or Rh2O3(II) phase with Pbcn symmetry, in agreement with recent experiments. GGA + U calculations show the same trend but shift the transition pressures to higher values. Experimentally, the onset of the structural transition begins in the vicinity of the spin transition pressure and whether the system undergoes spin transition in the corundum type (HP1) or in the Rh2O3(II) type (HP2) phase, is still a controversial issue. With a relatively simple, but general, octahedral structural parameter, Voct (the octahedral volume around iron ions), we show that in order to acquire a low spin (LS) state from a high spin (HS) one, the system does not necessarily need to change the crystal structure. Rather, the spin transition is a phenomenon that concerns the cation octahedra and the spin state of the system depends mainly on the value of Voct, which is governed by two distinct equations of state, separated by a well defined volume gap, for the HS and LS states respectively. Analysis of the results on the basis of octahedral volume allows to sum up and bridge the gap between two experimental results and thus provides a better description of the system in the region of interest.