Charge order and three-site distortions in the Verwey structure of magnetite

Abstract

The mineral magnetite (Fe(3)O(4)) undergoes a complex structural distortion and becomes electrically insulating at temperatures less than 125 kelvin. Verwey proposed in 1939 that this transition is driven by a charge ordering of Fe(2+) and Fe(3+) ions, but the ground state of the low-temperature phase has remained contentious because twinning of crystal domains hampers diffraction studies of the structure. Recent powder diffraction refinements and resonant X-ray studies have led to proposals of a variety of charge-ordered and bond-dimerized ground-state models. Here we report the full low-temperature superstructure of magnetite, determined by high-energy X-ray diffraction from an almost single-domain, 40-micrometre grain, and identify the emergent order. The acentric structure is described by a superposition of 168 atomic displacement waves (frozen phonon modes), all with amplitudes of less than 0.24 ångströms. Distortions of the FeO(6) octahedra show that Verwey's hypothesis is correct to a first approximation and that the charge and Fe(2+) orbital order are consistent with a recent prediction. However, anomalous shortening of some Fe-Fe distances suggests that the localized electrons are distributed over linear three-Fe-site units, which we call 'trimerons'. The charge order and three-site distortions induce substantial off-centre atomic displacements and couple the resulting large electrical polarization to the magnetization. Trimerons may be important quasiparticles in magnetite above the Verwey transition and in other transition metal oxides.

Speed limit of the insulator–metal transition in magnetite

“…Nevertheless, the exact mechanism of the insulator-metal, or Verwey, transition has long remained inaccessible [2][3][4][5][6][7][8]. Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase [9]. Here we investigate the Verwey transition with pump-probe x-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator-metal transition.…”

“…This transition is accompanied by a change in the crystal symmetry from a high-temperature cubic inverse spinel to a monoclinic phase, the fine details of which have eluded crystallographers for decades. An important step was taken recently, demonstrating that the low-temperature structure consists of a network of three-Fe-site distortions dubbed trimerons, encompassing two outer 3+ and a central 2+ octahedral Fe B site ions [9]. In addition, the t 2g orbitals within a trimeron are ordered, meaning that the trimeron lattice (see Fig.…”

Order-disorder phase transition on the (100) surface of magnetite

“…15 The charge order in the monoclinic phase is complex, 39 with many inequivalent states of the octahedral iron. Therefore, it is likely that models such as in Fig.…”

Orbital occupancy evolution across spin- and charge-ordering transitions in YBaFe 2 O 5

“…The crystal structure of the charge-ordered magnetite at low temperatures has only recently been solved [6,7], and its complexity extends to the Verwey transition at 120 K [8][9][10] between the two magnetite forms.…”