Magnetic Collapse in Fe3Se4 under High Pressure

Electronic structure and magnetic properties of Fe3Se4 are calculated using the density functional approach. Due to the metallic properties, magnetic moments of the iron atoms in two nonequivalent positions in the unit cell are different from ionic values for Fe3+ and Fe2+ and are equal to M1=2.071μB and M2=−2.042μB, making the system ferrimagnetic. The total magnetic moment for the unit cell is 2.135μB. Under isotropic compression, the total magnetic moment decreases non-monotonically and correlates with the non-monotonic dependence of the density of states at the Fermi level N(EF). For 7% compression, the magnetic order changes from the ferrimagnetic to the ferromagnetic. At 14% compression, the magnetic order disappears and the total magnetic moment becomes zero, leaving the system in a paramagnetic state. This compression corresponds to the pressure of 114 GPa. The magnetic ordering changes faster upon application of an isotropic external pressure due to the sizeable anisotropy of the chemical bondings in Fe3Se4. The ferrimagnetic and paramagnetic states occur under pressures of 5.0 and 8.0 GPa, respectively. The system remains in the metallic state for all values of compression.

Structural Diversity of Magnetite and Products of Its Decomposition at Extreme Conditions

Magnetite, Fe₃O₄, is the oldest known magnetic mineral and archetypal mixed-valence oxide. Despite its recognized role in deep Earth processes, the behavior of magnetite at extreme high-pressure high-temperature (HPHT) conditions remains insufficiently studied. Here, we report on single-crystal synchrotron X-ray diffraction experiments up to ∼80 GPa and 5000 K in diamond anvil cells, which reveal two previously unknown Fe₃O₄ polymorphs, γ-Fe₃O₄ with the orthorhombic Yb₃S₄-type structure and δ-Fe₃O₄ with the modified Th₃P₄-type structure. The latter has never been predicted for iron compounds. The decomposition of Fe₃O₄ at HPHT conditions was found to result in the formation of exotic phases, Fe₅O₇ and Fe₂₅O₃₂, with complex structures. Crystal-chemical analysis of iron oxides suggests the high-spin to low-spin crossover in octahedrally coordinated Fe³⁺ in the pressure interval between 43 and 51 GPa. Our experiments demonstrate that HPHT conditions promote the formation of ferric-rich Fe-O compounds, thus arguing for the possible involvement of magnetite in the deep oxygen cycle.

Evolution of electronic and magnetic properties of nominal magnetite nanoparticles at high pressure probed by x-ray absorption and emission techniques

We present a study of electronic and magnetic properties of nominal magnetite nanoparticles (NPs) (~6 nm) at high pressure in the presence of silicon pressure medium using x-ray absorption near edge structure (XANES), x-ray magnetic circular dichroism (XMCD) and non-resonant x-ray emission spectroscopy (XES). XANES data show a reduction of Fe charge state, a change in local environment around Fe at tetrahedral sites, and a reduced occupation of Fe 4p  orbitals, not seen in previous pressure studies of bulk magnetite. XMCD data show a continuous magnetic moment reduction of ~50% between ambient pressure and 20 GPa, similar to what was observed in previous bulk magnetite studies. XES spectra of NPs indicate a gradual change in spin configuration away from the high-spin state consistent with a postulated charge transfer from Fe 4p  to 3d states and the observed reduction in XMCD signal. Taken together, the results point to substantial differences in the response of electronic and magnetic properties of the nano-counterparts of bulk magnetite at high pressure.

Magnetic and electronic properties of magnetite across the high pressure anomaly

The magnetite Fe₃O₄, being anciently known magnetic material to human kind and remaining in leading positions for development of advanced technologies presently, demonstrates a number of puzzling physical phenomena, being at focus of extensive research for more than century. Recently the pressure-induced anomalous behavior of physical properties of magnetite in vicinity of the structural phase transition, occurring at P ~ 25–30 GPa, has attracted particular attention, and its nature remains unclear. Here we study the magnetic and electronic properties of magnetite across high pressure anomaly and in the pressure-induced phase by means of ⁵⁷Fe synchrotron Moessbauer spectroscopy and neutron diffraction. The hyperfine interaction parameters behavior was systematically analysed over pressure 0–40 GPa and temperature 10–290 K ranges. In the high pressure phase the ferrimagnetic order formation below T_NP ~ 420 K was observed and spin arrangement symmetry was deduced. The structural, magnetic and electronic phase diagram of magnetite in the discussed pressure range is established.

Pressure-induced structural and spin transitions of Fe3S4

Greigite (Fe₃S₄), isostructural with Fe₃O₄ has recently attracted great scientific interests from material science to geology due to its complicated structure and electronic and magnetic configurations. Here, an investigation into the structural, magnetic and electronic properties of Fe₃S₄ under high pressure has been conducted by first-principle calculations based on density functional theory. The results show that a first-order phase transition of Fe₃S₄ would occur from the inverse spinel (SP) structure to the Cr₃S₄-type (CS) structure at 3.4 GPa, accompanied by a collapse of 9.7% in the volume, a redistribution of iron cations, and a half-metal to metal transition. In the CS-Fe₃S₄, Fe²⁺ located at octahedral environment firstly undergoes a transition from high-spin (HS) state to low-spin (LS) state at 8.5 GPa and Fe³⁺ subsequently does at 17 GPa. The Equation of State for different phases of Fe₃S₄ are also determined. Our results not only give some clues to explore novel materials by utilizing Fe₃S₄ but also shed light on the fundamental information of Fe₃O₄, as well as those of other SP-AB₂X₄ compounds.

Unraveling the complexity of iron oxides at high pressure and temperature: Synthesis of Fe5O6

The iron-oxygen system is the most important reference of rocks' redox state. Even as minor components, iron oxides can play a critical role in redox equilibria, which affect the speciation of the fluid phases chemical differentiation, melting, and physical properties. Until our recent finding of Fe4O5, iron oxides were assumed to comprise only the polymorphs of FeO, Fe3O4, and Fe2O3. Combining synthesis at high pressure and temperature with microdiffraction mapping, we have identified yet another distinct iron oxide, Fe5O6. The new compound, which has an orthorhombic structure, was obtained in the pressure range from 10 to 20 GPa upon laser heating mixtures of iron and hematite at ~2000 K, and is recoverable to ambient conditions. The high-pressure orthorhombic iron oxides Fe5O6, Fe4O5, and h-Fe3O4 display similar iron coordination geometries and structural arrangements, and indeed exhibit coherent systematic behavior of crystallographic parameters and compressibility. Fe5O6, along with FeO and Fe4O5, is a candidate key minor phase of planetary interiors; as such, it is of major petrological and geochemical importance. We are revealing an unforeseen complexity in the Fe-O system with four different compounds-FeO, Fe5O6, Fe4O5, and h-Fe3O4-in a narrow compositional range (0.75 < Fe/O < 1.0). New, finely spaced oxygen buffers at conditions of the Earth's mantle can be defined.

Abnormal elastic and vibrational behaviors of magnetite at high pressures

Magnetite exhibits unique electronic, magnetic, and structural properties in extreme conditions that are of great research interest. Previous studies have suggested a number of transitional models, although the nature of magnetite at high pressure remains elusive. We have studied a highly stoichiometric magnetite using inelastic X-ray scattering, X-ray diffraction and emission, and Raman spectroscopies in diamond anvil cells up to ~20 GPa, while complementary electrical conductivity measurements were conducted in a cubic anvil cell up to 8.5 GPa. We have observed an elastic softening in the diagonal elastic constants (C₁₁ and C₄₄) and a hardening in the off-diagonal constant (C₁₂) at ~8 GPa where significant elastic anisotropies in longitudinal and transverse acoustic waves occur, especially along the [110] direction. An additional vibrational Raman band between the A_1g and T_2g modes was also detected at the transition pressure. These abnormal elastic and vibrational behaviors of magnetite are attributed to the occurrence of the octahedrally-coordinated Fe²⁺-Fe³⁺-Fe²⁺ ions charge-ordering along the [110] direction in the inverse spinel structure. We propose a new phase diagram of magnetite in which the temperature for the metal-insulator and distorted structural transitions decreases with increasing pressure while the charge-ordering transition occurs at ~8 GPa and room temperature.

Synthesis of superparamagnetic nanotubes as MRI contrast agents and for cell labeling

AIMS

Magnetic nanoparticles have been studied widely as MRI contrast agents to increase the sensitivity of this technique. This work describes the synthesis and characterization of magnetic nanotubes (MNTs) as a novel MRI contrast agent.

METHODS

MNTs with high saturation magnetization were fabricated by the synthesis of superparamagnetic iron oxide nanoparticles (SPIONs) directly in the pores of silica nanotubes (SNTs). The MNTs were characterized by electron microscopy, superconducting quantum interference device and MRI. Preliminary studies on in vitro cytotoxicity and cell labeling were carried out.

RESULTS

The MNTs retained the superparamagnetic characteristics in bulk solutions with a considerably high saturation magnetization of 95 emu/gFe. The nuclear magnetic resonance (NMR) relaxivities for MNTs of 500 nm in length and of 60 nm in diameter were r(1) = 1.6 +/- 0.3 mM(-1)s(-1) and r(2) = 264 +/- 56 mM(-1)s(-1) and, for the MNTs of 2 microm in length and 70 nm in diameter, the r(1) and r(2) were 3.0 +/- 1.3 and 358 +/- 65 mM(-1)s(-1), respectively. In vitro cell labeling showed promising results with excellent labeling efficiency. No cellular toxicity was observed in vitro.

CONCLUSIONS

The integration of SPIONs with SNTs imparts the superparamagnetic characteristics of SPIONs onto the SNTs, creating unique magnetic nanoparticles with multifunctionality. The MNTs showed promising results as a MRI contrast agent with high NMR relaxivities, little cytotoxicity and high cell-labeling efficiency.

Novel pressure-induced magnetic transition in magnetite (Fe3O4)

Fe K-edge x-ray magnetic circular dichroism of magnetite (Fe3O4) powders was measured with synchrotron radiation under variable pressure and temperature conditions in diamond anvil cell. The magnetic dichroism was observed to decrease discontinuously by approximately 50% between 12 and 16 GPa, independent of temperature. The magnetic transition is attributed to a high-spin to intermediate-spin transition of Fe2+ ions in the octahedral sites and could account for previously observed structural and electrical anomalies in magnetite at this pressure range. The interpretation of x-ray magnetic circular dichroism data is supported by x-ray emission spectroscopy and theoretical cluster calculations.

Deviatoric stress: a nuisance or a gold mine?

Both synchrotron radiation and deviatoric stress were once considered to be nuisances. Now synchrotron radiation is one of the most important tools available to scientists of all disciplines and deviatoric stress is one of the most useful aspects of x-ray diffraction at extreme conditions. Samples in high-pressure devices are under true hydrostatic pressure only when surrounded by a fluid, thus limiting true hydrostatic pressure studies at ambient temperatures to pressures below about 11 GPa. Elevated temperature is able to extend this limit but has rarely been used for this purpose. Instead, noble gases have been used as pressure media as their solids are especially soft. Deviatoric stress and resultant anisotropic elastic strain in solid samples and solid media have led to many subtle errors in determinations of elastic properties and crystal structures, especially in the days before it was realized that they could be measured and were potentially a valuable source of information. In recent years, measuring anisotropic elastic strain by x-ray diffraction has provided new insights into materials strength, elastic properties, crystal structures, mechanisms of phase transitions, slip systems, lattice preferred orientation, and, of course, ways to make corrections when deviatoric stress is indeed a nuisance.

Subducted banded iron formations as a source of ultralow-velocity zones at the core–mantle boundary

Ultralow-velocity zones (ULVZs) are regions of the Earth's core-mantle boundary about 1-10 kilometres thick exhibiting seismic velocities that are lower than radial-Earth reference models by about 10-20 per cent for compressional waves and 10-30 per cent for shear waves. It is also thought that such regions have an increased density of about 0-20 per cent (ref. 1). A number of origins for ULVZs have been proposed, such as ponding of dense silicate melt, core-mantle reaction zones or underside sedimentation from the core. Here we suggest that ULVZs might instead be relics of banded iron formations subducted to the core-mantle boundary between 2.8 and 1.8 billion years ago. Consisting mainly of interbedded iron oxides and silica, such banded iron formations were deposited in the world's oceans during the late Archaean and early Proterozoic eras. We argue that these layers, as part of the ocean floor, would be recycled into the Earth's interior by subduction, sink to the bottom of the mantle and may explain all of the observed features of ULVZs.

The Bakerian Lecture, 1983 - The Earth’s core: its composition, formation and bearing upon the origin of the Earth

The density of the outer core is about 3 % smaller than pure iron, which implies that the core contains a substantial amount of one or more low atomic mass elements. Candidates which have been suggested on various grounds include S, H, C, O, Si, and Mg. Plausible models of accretion of the Earth encounter difficulties in trapping sufficient S, H and C to explain the density deficit. On the other hand, entry of Si and Mg is not favoured by thermodynamic arguments. Oxygen is the most abundant element in the Earth and would be a prime candidate if it could be shown to be extensively soluble in molten iron at core temperatures and pressures. New experimental data on the solubility of FeO in molten iron are reviewed. They demonstrate that at atmospheric pressure, FeO is extensively soluble in iron at 2500 °C and that complete miscibility probably occurs above 2800 °C. Moreover, liquid iron in equilibrium with magnesiowüstite (Mg 0.8 Fe 0.2 )O also dissolves large quantities of FeO above 2800 °C. The solubility of FeO in molten iron is considerably increased by high pressures, because of the small partial molar volume of FeO in the Fe─FeO melt. If the core formed by segregation of metal originally dispersed throughout the Earth, it seems inevitable that it would have dissolved large amounts of FeO. The density of the outer core can be matched if it contains about 35 mol % FeO, a quantity that is readily explained by the new experimental data. Solution of FeO in iron causes the melting point of the metal phase to be depressed below the solidus temperature of the silicate phase assemblages in the mantle. A model for the formation of the core is described, based upon Fe-FeO phase relations at high temperatures and pressures. The model implies the presence of a high content of FeO in the Bulk Earth. This can be explained if the Earth accreted from a mixture of two components: A, a highly reduced, metal-rich devolatilized assemblage and B, a highly oxidized, volatile-rich assemblage similar to C1 chondrites. The formation of these components in the solar nebula is discussed. The large amount of FeO now inferred to be present in the Earth was mainly produced during accretion by oxidation of metallic iron from component A by water from component B. This two-component mixing model also provides an attractive explanation of some aspects of the chemistry of the Earth’s mantle including the abundances of siderophile and volatile elements.