Charge localization in the Verwey structure of magnetite

Abstract

The thermal evolution of electronic order in the complex Verwey ground state of magnetite (Fe 3 O 4 ) has been determined through 22 high-accuracy synchrotron x-ray structure refinements using three 10-40 μm grains of stoichiometric magnetite. A robust fitting function is introduced to extract values of order parameterlike quantities at zero temperature and at the upper limit of the Verwey phase T u = 123.4 K. The low-temperature structural distortion is found to be almost frozen below the Verwey transition but small changes in lattice and local mode amplitudes and Fe-Fe distances reveal an increase in electron localization on cooling. These distortions confirm that electron localization within trimerons is the driving force behind the Verwey transition. Electron localization is also revealed by anomalous decreases in the largest principal thermal displacement factors of Fe cations as electron-phonon decoupling occurs on cooling.

The Mechanism for the Half‐Metal to Insulator Transition at the Fe3O4/ZnAl2O4 Interface

“…The main reason may be from the distorted lattice and charge transfer. The distortion at the Fe 3 O 4 lattice mainly come from the interface effect rather than orbital ordering, which is different from the bulk and surface phase . Different Fe B sites are created by the effect and different symmetry point group ( Pmm 2), which make the orbital splitting different from the bulk phase, and influence the transition.…”

“…Interestingly, trimeron units composed of two Fe 3+ and one Fe 2+ were reported by Wright et al, who characterized the low‐temperature structure of Fe 3 O 4 using a high‐energy X‐ray method and found that many frozen lattice modes contribute to the Cc superstructure. In their studies, they analyzed the main reason for the phenomenon and deduced that the t 2g electron of the Fe 2+ is delocalized, which causes a decrease in the length of Fe–Fe due to the lowering of Coulomb repulsion, and found the electron localization within trimerons is the driving force behind the phase transition . Above T = 221 K, the short‐range correlations still remain …”

Light scattering from the critical modes of the Verwey transition in magnetite

“…In the structural transition of magnetite, complex atomic displacements, with small amplitudes down to 1 pm, give rise to a change in crystal symmetry, which is reflected in the modifications of a number of physical properties [2][3][4][5]. A monoclinic cell four times larger than the cubic cell accounts for the complete set of structural distortions that take place upon decreasing the temperature from above to below T V .…”

“…On cooling across the transition temperature, also referred to as the Verwey temperature, T V , the former is lowered from cubic to monoclinic, whereas the latter changes from conductive to insulating. In the past two decades, accurate diffraction experiments disclosed the most elusive features of the crystal structure and the charge and orbital order in the monoclinic phase [2][3][4][5]. In particular, the constitutive units of the sublattice of octahedral Fe ions, or B-type Fe ions, are small polarons (SPs) named trimerons, comprised of an extra electron delocalized over three ions and the atomic displacements of the end ions toward the central ion [4,5].…”

“…In the past two decades, accurate diffraction experiments disclosed the most elusive features of the crystal structure and the charge and orbital order in the monoclinic phase [2][3][4][5]. In particular, the constitutive units of the sublattice of octahedral Fe ions, or B-type Fe ions, are small polarons (SPs) named trimerons, comprised of an extra electron delocalized over three ions and the atomic displacements of the end ions toward the central ion [4,5]. It was suggested that, in the cubic phase, dynamic fluctuations of trimerons would aggregate into complexes, responsible for the persistence of local electronic and structural correlations for finite times.…”