HR: 1340h
AN: MR23A-0040    [Abstracts]
TI: Absence of Charge Localization in Magnetite Under Pressure
AU: * Wenzel, M J
EM: mjwenzel@eps.berkeley.edu
AF: University of California, Berkeley, 307 McCone Hall, Berkeley, CA 94720 United States
AU: Steinle-Neumann, G
EM: g.steinle-neumann@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, D-95440 Germany
AB: The magnetic and electronic properties of Fe₃O₄ magnetite have been the subject of intensive study. In particular, there is great interest in charge-localization at the Verwey transition, a two-order-of-magnitude drop in electrical conductivity below a critical temperature (T_{V}~120 K). The effect of pressure has been less-extensively studied, but it has also been implicated in charge localization [1]. We examine the proposed localization with first-principles calculations and find no evidence for it. Above TV, magnetite has the inverse spinel structure with cubic symmetry. The three Fe in the formula unit are nominally distributed with 1/3 each as Fe³⁺ in a tetrahedral (t) site, and Fe³⁺ and Fe²⁺ in two octahedral (o) sites. It has been demonstrated that the two o sites are equivalent, i.e., are effectively both occupied by Fe^2.5+. The low-T phase has reduced symmetry, and calculations [2,3] show that this structure does exhibit charge localization. Using in-situ Mössbauer spectroscopy to probe the electronic environment of Fe, Pasternak et al. [1] argue for charge localization with increasing pressure. They see evidence for a transition through an intermediate state of Fe³⁺ (t) and Fe²⁺, Fe³⁺ (o), the classic inverse spinel, to the normal spinel with Fe²⁺ (t) and 2 Fe³⁺ (o). Here we test the hypothesis of charge ordering as a function of compression by means of density functional based computations. We have applied all electron computations (LAPW) with the generalized gradient approximation (GGA) as well as LDA+U schemes to the exchange and correlation potentials to investigate the electronic and magnetic structure of the Fe sites, breaking the symmetry-derived equivalency of the o sites. In addition to monitoring the magnetic moments, and the electronic density of states associated with them, we examine the hyperfine fields and electronic field gradients, and evaluate the charge density with formal Bader charges. We are, however, not able to find any discernable charge localization, electronic or magnetic differences for the two octahedral sites for compression range explored (V/V0 > 0.80), leaving the changes in the Mössbauer spectra unexplained. References: [1] Pasternak et al., J. Magnetism Magnetic Mat. 265: L107-L112, 2003. [2] Leonov et al., Phys. Rev. Lett. 93: 1-4, 2004. [3] Jeng et al., Phys. Rev. Lett. 93: 1-4, 2004.}
DE: 3914 Electrical properties
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005