Structural Transition and Pair Formation inFe3O2BO3

Abstract

We observe for the first time a structural phase transition in the oxyborate Fe3O2BO3 which occurs along three leg ladders present in this material. X-ray diffraction shows that this transition at 283 K is associated with a new phase where atomic displacements occur in alternate directions perpendicular to the axis and within the plane of the ladders. Magnetic data show that these displacements lead to the formation of singlet pairs which dissociate close to the structural transition. Anomalies in the transport properties also occur close to 283 K showing that the structural transition is related to a charge ordering phenomenon in a low dimensional structure.

Uniaxial magnetic anisotropy in Co2.25Fe0.75OBartolomé1, Arauzo2, Kazak3 et al. 2011Phys. Rev. B

“…The temperature dependence of this coercive field enters via the dependence of the wall energy on the anisotropy constant K 1 , γ ∝ √ AK 1 , where A is proportional to the exchange coupling. 25 For comparison, at the same temperature T = 10 K, and θ H = 0 one obtains H b C M r = γ /l = 8.73×10 6 BO 3 , but not enough to explain the observed increase, and K 1 may be similar in both compounds, it can be concluded that the main reason for the increase in coercivity field is the increase of the number of defects in the crystal upon the introduction of Fe, which in turn reduces strongly the average distance between defects, l.…”

“…6,7 Short and long Fe-Fe bonds are formed in the 4-2-4 triads in the direction perpendicular to the axis of the ladder, with a doubling of the cell parameter on the c axis that affects the magnetic properties of the material. In this work the magnetic properties of the substitutional compound Co 2.25 Fe 0.75 O 2 BO 3 are studied and compared to those of the parent compounds Co 3 O 2 BO 3 and Fe 3 O 2 BO 3 -the two homometallic oxyborates with ludwigite structure available now.…”

“…At present there exist quite some experimental data for Fe 3 O 2 BO 3 : x-ray diffraction, 5,6,8 neutron powder diffraction (NPD), 8,9 magnetic measurements, 6,10 Mössbauer spectroscopy (MS), 5,[10][11][12][13][14] electron paramagnetic resonance, 15 and theoretical works. 7,16 The properties of this compound are summarized as follows:…”

“…Early MS experiments 5 have shown that the sites 1 and 3 are occupied by Fe 2+ ions, while sites 2 and 4 were apparently occupied by Fe 3+ ions. (1) It undergoes a structural transition at T CD = 283 K, when being cooled, in the form of a subtle dimerization of iron ion pairs along 3LLs type I and charge localization, 6,11 while the Fe 2+ (sites 1 and 3) play no role in this transition. 11 Above T CD the ions in the sites 2 and 4 are occupied by Fe 3+ cations with one additional electron per rung, while below T CD mixed-valence ion pairs are formed.…”

Transport properties of the transverse charge-density-wave systemFe3O2BO3

“…3 The structural transition at T S Ϸ 283 K is accompanied by a change in the activation energy of the electrical resistivity. 1 An important distinction between the Fe 3 O 2 BO 3 ludwigite and the common Peierls systems is the existence of local moments belonging to the Fe ions which interact with the CDW; besides, the mechanism responsible for the CDW formation is different from that of the Peierls distortion in onedimensional metals. 2 Here we are dealing with an excitonic instability due to the coupling between an empty band and a fully occupied one.…”