High-pressure magnetic, electronic, and structural properties ofMFe2O4(M=Mg,

Abstract

Magnetic, electronic, and structural properties of MFe 2 O 4 (M = Mg,Zn,Fe) ferric spinels have been studied by 57 Fe Mössbauer spectroscopy, electrical conductivity, and powder and single-crystal x-ray diffraction (XRD) to a pressure of 120 GPa and in the 2.4-300 K temperature range. These studies reveal for all materials, at the pressure range 25-40 GPa, an irreversible first-order structural transition to the postspinel CaTi 2 O 4 − type structure (Bbmm) in which the HS Fe 3+ occupies two different crystallographic sites characterized by six-and eightfold coordination polyhedra, respectively. Above 40 GPa, an onset of a sluggish second-order high-to-low spin (HS-LS) transition is observed on the octahedral Fe 3+ sites while Fe 3+ occupying bicapped trigonal prism sites remain in the HS state. Despite an appreciable resistance decrease, corroborating with the transition to the LS state, MgFe 2 O 4 and ZnFe 2 O 4 remain semiconductors at this pressure range. However, in the case of Fe 3 O 4 , the second-order HS-LS transition on the Fe 3+ octahedral sites corroborates with a clear trend to a gap closure and formation of a semimetal state above 50 GPa. Above 65 GPa, another structural phase transition is observed in Fe 3 O 4 to a new Pmma structure. This transition coincides with the onset of nonmagnetic Fe 2+ , signifying further propagation of the gradual collapse of magnetism corroborating with a sluggish metallization process. With this, half of Fe 3+ sites remain in the HS state. Thus, this paper demonstrates that, in a material with a complex crystal structure containing transition metal cation(s) in different environments, a HS-LS transition and delocalization/metallization of the 3d electrons does not necessarily occur simultaneously and may propagate through different crystallographic sites at different degrees of compression.

Magnetic and electronic properties of magnetite across the high pressure anomaly

“…In the post-spinel phase, the Néel temperature becomes about twice smaller, and it increases with a noticeably reduced rate dT NP /dP ≈ 8 K/GPa. This reduction may be related to a decrease of the leading antiferromagnetic superexchange interaction strength between the A and B sublattices of iron ions via oxygen ions due to a significant reduction of the average value of the Fe(A)-O-Fe(B) bond angle from 123.5° in the spinel phase ( P = 0 GPa) to about 114.6° in the post-spinel phase ( P = 33 GPa), as evaluated from the present data (Supplementary Table 2) and results 20 .

Figure 8The structural, magnetic and electronic phase diagram of magnetite. The phase boundary of the spinel – post-spinel structural phase transition is constructed using the present results (red solid circles) and data 30 (red open circles).

…”

Pressure-Induced Site-Selective Mott Insulator-Metal Transition inFe2O3

“…For example, in Mn 2 O 3 , corundum-type ε-Mn 2 O 3 (and below T ¼ 1200 K cubic α-Mn 2 O 3 ) transforms upon compression to a distorted perovskite structure [60], and this structural transition coincides with an insulator-metal transition [61]. Similar electronic transformations could be expected in other sesquioxides, e.g., Cr 2 O 3 [62] or Ti 2 O 3 [63], and in materials with a complex crystal structure (or that acquire a complex structure under pressure) containing transition-metal cations in different coordination polyhedra: for example, in magnetite [14] or Fe-bearing bridgmanite [18,19]. Thus, such effect(s) can occur in crystalline oxides comprising Earth and planetary mantles.…”

“…Nevertheless, the abovementioned scenarios of Mott transitions do not explain the experimentally observed details of electronic and structural transformations, e.g., in many iron-bearing materials. In particular, recent experimental studies of ferric and ferrous spinels [14][15][16][17] and Fe-bearing bridgmanite (MgSiO 3 -peroskite) [18,19] reveal a complex coexistence of local Fe 3þ high-and low-spin states under pressure. In spite of the appreciable 3d band broadening from various lattice responses that contribute to the unit-cell reduction, upon compression to a pressure of a megabar, all of these compounds continue to have concurrent resilient magnetic (high-spin) and nonmetallic ground-state features of strong electron correlations.…”

Oxygen Vacancy Substitution Linked to Ferric Iron in Bridgmanite at 27 GPa

“…Scanning electron microscope (SEM Microfocus X-ray diffraction (XRD). XRD analysis was performed on the recovered samples using a micro focused X-ray diffractometer (Brucker AXS D8 Discover) equipped with a two-dimensional solid-state (2001), although a CaTi 2 O 4 -type structure was proposed by a more recent study (Greenberg et al, 2017). Pt peaks from sample capsules also appeared in the sample diffraction patterns due to the limited spatial resolution of the diffractometer (Figure 2).…”