Spin Transition Of Iron Research Articles

Single-crystal elasticity of (Mg 0.9Fe 0.1)O to 81 GPa

Ferropericlase is the second most abundant mineral in Earth's lower mantle and knowledge of its elastic properties at relevant conditions is needed to adequately interpret seismic observations in terms of the mineralogy and thermal state of the deep Earth. Here, we report the complete elastic tensor of (Mg 0.9Fe 0.1)O ferropericlase to 81.2 GPa at room temperature measured by Brillouin spectroscopy and X-ray diffraction in the diamond-anvil cell. Our data indicate that the spin transition of iron between 45 and 63 GPa dramatically affects the longitudinal and off-diagonal elements of the elastic stiffness tensor c 11 and c 12, whereas it leaves the shear constant c 44 almost unaffected. Based on our results, the spin transition markedly changes the pressure (and temperature) dependence of the compressional and bulk sound velocities and must be taken into account in any attempt to match average radial seismic velocity profiles with mineral physics observations. The different pressure dependence of compressional (and bulk) and shear sound velocities across the high-spin to low-spin (HS–LS) transition implies that the spin transition might be best observed in the v p / v s ratio and its pressure (and temperature) derivative. We also point out the possibility that the spin transition leads to an anti-correlated temperature dependence of shear and compressional wave speeds in certain pressure–temperature regimes in Earth's lower mantle.

Spin transition in heptanuclear star-shaped iron(III)–antimony(V) NCS- and CN-bridged compounds

The precursor [Fe III(L)Cl] (LH 2 = N, N′-bis(2′-hydroxy-benzyliden)-1,6-diamino-3-azahexane) has been prepared and Mössbauer spectroscopy assigned a high-spin ( S = 5/2) state at room temperature. The precursor is combined with the bridging units [Sb V(X) 6] − (X = CN −, NCS −) to yield star-shaped heptanuclear clusters [(LFe III–X) 6Sb V]Cl 5. The star-shaped compounds are in general high-spin systems at room temperature. On cooling to 20 K some of the iron(III) centers switch to the low-spin state as indicated by Mössbauer spectroscopy, i.e. multiple electronic transitions. While the cyano-bridged complex performs a multiple spin transition the thiocyanate-compound shows no significant population at both temperatures.

Deformation of lower-mantle ferropericlase (Mg,Fe)O across the electronic spin transition

Recent high-pressure studies have shown that an electronic spin transition of iron in ferropericlase, an expected major phase of Earth's lower mantle, results in changes in its properties, including density, incompress- ibility, radiative thermal conductivity, electrical conduc- tivity, and sound velocities. To understand the rheology of ferropericlase across the spin transition, we have used in situ radial X-ray diffraction techniques to examine ferro- periclase, (Mg0.83,Fe0.17)O, deformed non-hydrostatically in a diamond cell up to 81 GPa at room temperature. Compared with recent quasi-hydrostatic studies, the range of the spin transition is shifted by approximately 20 GPa as a result of the presence of large differential stress in the sample. We also observed a reduction in incompressibility and in the unit cell volume of 3% across the spin transition. Our radial X-ray diffraction results show that the {0 0 1} texture is the dominant lattice preferred orientation in ferropericlase across the spin transition and in the low-spin state. Viscoplastic self-consistent polycrystal plasticity simulations suggest that this preferred orientation pattern is produced by {1 1 0}\1-10( slip. Analyzing our radial X-ray diffraction patterns using lattice strain theory, we evaluated the lattice d-spacings of ferropericlase and Mo as a function of the w angle between the compression direc- tion and the diffracting plane normal. These analyses give the ratio between the uniaxial stress component (t) and the shear modulus (G) under constant stress condition, which represents a proxy for the supported differential stress and elastic strength. This ratio in the mixed-spin and low-spin states is lower than what is expected from previous studies of high-spin ferropericlase, indicating that the spin transi- tion results in a reduced differential stress and elastic strength along with the volume reduction. The influence of the spin transition on the differential stress and strength of ferropericlase is expected to be less dominant across the wide spin transition zone at high pressure-temperature conditions relevant to the lower mantle.

Elastic Shear Anisotropy of Ferropericlase in Earth's Lower Mantle

Seismic shear anisotropy in the lowermost mantle most likely results from elastic shear anisotropy and lattice preferred orientation of its constituent minerals, including perovskite, post-perovskite, and ferropericlase. Measurements of the elastic shear anisotropy of single-crystal (Mg0.9Fe0.1)O up to 69 gigapascals (GPa) show that it increased considerably across the pressure-induced spin transition of iron between 40 and 60 GPa. Increasing iron content further enhances the anisotropy. This leads to at least 50% stronger elastic shear anisotropy of (Mg,Fe)O in the lowermost mantle compared to MgO, which is typically used in geodynamic modeling. Our results imply that ferropericlase is the dominant cause of seismic shear anisotropy in the lower mantle.

Cover Picture: Photo‐Induced Spin Transition of Iron(III) Compounds with π–π Intermolecular Interactions (Chem. Eur. J. 14/2009)

AbstractIntroduction of strong intermolecular interactions leads to the observation of the LIESST effect even for iron(III) spin‐crossover (SCO) compounds. For LIESST iron(III) compounds, both stretching and bending modes are considered in the reaction coordinate diagram. The picture depicts the LIESST mechanism in the reaction coordinate diagram considering both stretching and bending modes for SCO iron(III) compounds. For more information see the Full Paper by S. Hayami, O. Sato et al. on page 3497 ff.magnified image

Photo‐Induced Spin Transition of Iron(III) Compounds with π–π Intermolecular Interactions

Iron(III) spin-crossover compounds [Fe(pap)(2)]ClO(4) (1), [Fe(pap)(2)]BF(4) (2), [Fe(pap)(2)]PF(6) (3), [Fe(qsal)(2)]NCS (4), and [Fe(qsal)(2)]NCSe (5) (Hpap=2-(2-pyridylmethyleneamino)phenol and Hqsal=2-[(8-quinolinylimino)methyl]phenol) were prepared and their spin-transition properties investigated by magnetic susceptibility and Mössbauer spectroscopy measurements. The iron(III) compounds exhibited spin transition with thermal hysteresis. Single crystals of the iron(III) compounds were obtained as suitable solvent adducts for X-ray analysis, and structures in high-spin (HS) and low-spin (LS) states were revealed. Light-induced excited-spin-state trapping (LIESST) effects of the iron(III) compounds were induced by light irradiation at 532 nm for 1-3 and at 800 nm for 4 and 5. The activation energy E(a) and the low-temperature tunneling rate k(HL)(T-->0) of iron(III) LIESST compound 1 were estimated to be 1079 cm(-1) and 2.4x10(-8) s(-1), respectively, by HS-->LS relaxation experiments. The Huang-Rhys factor S of 1 was also estimated to be 50, which was similar to that expected for iron(II) complexes. It is thought that the slow relaxation in iron(III) systems is achieved by the large structural distortion between HS and LS states. Introduction of strong intermolecular interactions, such as pi-pi stacking, can also play an important role in the relaxation behavior, because it can enhance the structural distortion of the LIESST complex.

Spin transition in (Mg,Fe)SiO 3 perovskite under pressure

We present a density functional study of the pressure-induced spin transition in ferrous iron (Fe 2+) at the A site in MgSiO 3 perovskite. We address the influence of iron concentration and configuration (structural and magnetic), as well as technical issues such as the influence of the exchange-correlation functional (LDA versus GGA) on the spin transition pressure. Supercells containing up to 160 atoms were adopted to tackle these issues. We show that there are preferred configurations for high-spin and low-spin iron and that the spin transition pressure depends strongly on iron concentration and all the issues above. Across the spin transition, irons move into the middle of distorted octahedra causing drastic changes in the d states configuration and a blueshift in the band gap. Such blueshift should decrease the contribution of ferrous iron to the electrical conductivity and increase its contribution to the radiative conductivity in the lower mantle. Both LDA and GGA results suggest that the spin transition can occur in the pressure range of the lower mantle and of previous experiments. The transition range can encompass the entire lower mantle passing through a mixed-spin state caused by cation disorder and magnetic entropy.

An Iron(II) Spin‐Transition Compound with Thiol Anchoring Groups (Eur. J. Inorg. Chem. 17/2008)

The cover picture shows the solid-state structure and magnetic susceptibility data of an iron(II) spin-transition complex, which was designed, synthesized and characterized in the group of Mario Ruben at the Institute of Nanotechnology in Karlsruhe. The complex, which undergoes a near-room-temperature transition from a closed-shell spin singlet to an open-shell high-spin state, holds promise for the controlled manipulation and readout of the magnetic states of single molecules. In order to integrate and to implement such switchable spin-transition units into electronic devices, the molecule bears protected thiol linkers, which allows its attachment to gold electrodes (bottom). Details are discussed in the Short Communication on p. 2649 ff.

Spin transition in iron(III) and iron(II) complexes

The main stages of the studies on the spin transitions in iron(III) and iron(II) complexes are considered. The types of the spin transitions and the factors responsible for the latter are reported. The problems arising during experiments in this field are discussed.

Iron(II) Spin Transition Complexes with Dendritic Ligands, Part I

AbstractThe ligands G1‐ and G2‐oligo (benzyl ether) (PBE) dendrons and their iron(II) complexes [Fe(Gn‐PBE)3]A2·xH2O (with n = 1, 2 and A = triflate, tosylate) were prepared. The magnetic properties of the complexes were investigated by a SQUID magnetometer. All complexes exhibit gradual spin transition below room temperature. At very low temperatures the magnetic behaviour reflects zero‐field splitting (ZFS) effects. 57Fe‐Mössbauer spectroscopy was performed to distinguish between ZFS of high spin species and spin state conversion into the low spin state. Further characterisation was carried out by thermogravimetric analysis (TGA) and FT‐IR spectroscopy. Structural features have been determined by powder XRD measurements.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Element partitioning between magnesium silicate perovskite and ferropericlase: New insights into bulk lower-mantle geochemistry

In this study, we investigated iron–magnesium exchange and transition-metal trace-element partitioning between magnesium silicate perovskite (Mg,Fe)SiO 3 and ferropericlase (Mg,Fe)O synthetised under lower-mantle conditions (up to 115 GPa and 2200 K) in a laser-heated diamond anvil cell. Recovered samples were thinned to electron transparency by focused ion beam and characterized by analytical transmission electron microscopy (ATEM) and nanometer-scale secondary ion mass spectroscopy (nanoSIMS). Iron concentrations in both phases were obtained from X-ray energy dispersive spectroscopy measurements and nanoSIMS. Our results are the first to show that recently reported spin-state and phase transitions in the lower mantle directly affect the evolution of Fe–Mg exchange between both phases. Mg-perovskite becomes increasingly iron-depleted above 70–80 GPa possibly due to the high spin–low spin transition of iron in ferropericlase. Conversely, the perovskite to post-perovskite transition is accompanied by a strong iron enrichment of the silicate phase, ferropericlase remaining in the Fe-rich phase though. Nanoparticles of metallic iron were observed in the perovskite-bearing runs, suggesting the disproportionation of ferrous iron oxide, but were not observed when the post-perovskite phase was present. Implications on the oxidation state of the Earth and core segregation will be discussed. Transition trace-element (Ni, Mn) concentrations (determined with the nanoSIMS) show similar trends and could thus be used to trace the origin of diamonds generated at depth. This study provides new results likely to improve the geochemical and geophysical models of the Earth's deep interiors.

Spin transition of iron in the Earth's lower mantle

Electronic spin-pairing transitions of iron and associated effects on the physical properties of host phases have been reported in lower-mantle minerals including ferropericlase, silicate perovskite, and possibly in post-perovskite at lower-mantle pressures. Here we evaluate current understanding of the spin and valence states of iron in the lower-mantle phases, emphasizing the effects of the spin transitions on the density, sound velocities, chemical behavior, and transport properties of the lower-mantle phases. The spin transition of iron in ferropericlase occurs at approximately 50 GPa and room temperature but turns into a wide spin crossover under lower-mantle temperatures. Current experimental results indicate a continuous nature of the spin crossover in silicate perovskite at high pressures, but which valence state of iron undergoes the spin crossover and what is its associated crystallographic site remain uncertain. The spin transition of iron results in enhanced density, incompressibility, and sound velocities, and reduced radiative thermal conductivity and electrical conductivity in the low-spin ferropericlase, which should be considered in future geophysical and geodynamic modeling of the Earth's lower mantle. In addition, a reduction in sound velocities within the spin transition is recently reported. Our evaluation of the experimental and theoretical pressure–volume results shows that the spin crossover of iron results in a density increase of 2–4% in ferropericlase containing 17–20% FeO. Here we have modeled the density and bulk modulus profiles of ferropericlase across the spin crossover under lower-mantle pressure–temperature conditions and shown how the ratio of the spin states of iron affects our understanding of the state of the Earth's lower mantle.

Elasticity of (Mg,Fe)O Through the Spin Transition of Iron in the Lower Mantle

Changes in the electronic configuration of iron at high pressures toward a spin-paired state within host minerals ferropericlase and silicate perovskite may directly influence the seismic velocity structure of Earth's lower mantle. We measured the complete elastic tensor of ferropericlase, (Mg(1-x),Fe(x))O (x = 0.06), through the spin transition of iron, whereupon the elastic moduli exhibited up to 25% softening over an extended pressure range from 40 to 60 gigapascals. These results are fully consistent with a simple thermodynamic description of the transition. Examination of previous compression data shows that the magnitude of softening increases with iron content up to at least x = 0.20. Although the spin transition in (Mg,Fe)O is too broad to produce an abrupt seismic discontinuity in the lower mantle, the transition will produce a correlated negative anomaly for both compressional and shear velocities that extends throughout most, if not all, of the lower mantle.

Effects of Fe spin transition on the elasticity of (Mg, Fe)O magnesiowüstites and implications for the seismological properties of the Earth's lower mantle

High‐pressure x‐ray diffraction of (Mg0.80Fe0.20)O at room temperature reveals a discontinuity in the bulk modulus at 40 (±5) GPa, similar to the pressure at which an electronic spin‐pairing transition of Fe2+ is observed by Mössbauer spectroscopy. We determine the zero‐pressure bulk modulus of low‐spin magnesiowüstite to be between KT0 = 136 and 246 GPa, with a pressure derivative (∂KT/∂P)T0 between 5.2 and 3.9. The best fit unit‐cell volume at zero pressure, V0 = 71 (±5) Å3, is consistent with past estimates of the ionic radius of octahedrally‐coordinated low‐spin Fe2+ in oxides. A spin transition at lower‐mantle depths between 1100 and 1900 km (40–80 GPa) would cause a unit‐cell volume decrease (ΔV) of 3.7 (±0.5) to 2.0 (±0.1) percent and bulk sound velocity increase (Δvϕ) of 7.6 (±4) percent at 40 GPa and 7.6 (±1.2) percent at 80 GPa. Even in the absence of a visible seismic discontinuity, we expect the spin transition of iron to imply a correction to current compositional models of the lower mantle, with up to 10 mol percent increase of magnesiowüstite being required to match the seismological data.

Spin Transition Zone in Earth's Lower Mantle

Mineral properties in Earth's lower mantle are affected by iron electronic states, but representative pressures and temperatures have not yet been probed. Spin states of iron in lower-mantle ferropericlase have been measured up to 95 gigapascals and 2000 kelvin with x-ray emission in a laser-heated diamond cell. A gradual spin transition of iron occurs over a pressure-temperature range extending from about 1000 kilometers in depth and 1900 kelvin to 2200 kilometers and 2300 kelvin in the lower mantle. Because low-spin ferropericlase exhibits higher density and faster sound velocities relative to the high-spin ferropericlase, the observed increase in low-spin (Mg,Fe)O at mid-lower mantle conditions would manifest seismically as a lower-mantle spin transition zone characterized by a steeper-than-normal density gradient.

Electrical conductivity of the lower‐mantle ferropericlase across the electronic spin transition

Electrical conductivity of the lower‐mantle ferropericlase‐(Mg0.75,Fe0.25)O has been studied using designer diamond anvils to pressures over one megabar and temperatures up to 500 K. The electrical conductivity of (Mg0.75,Fe0.25)O gradually rises by an order of magnitude up to 50 GPa but decreases by a factor of approximately three between 50 to 70 GPa. This decrease in the electrical conductivity is attributed to the isosymmetric high‐spin to low‐spin transition of iron in ferropericlase. That is, the electronic spin transition of iron results in a decrease in the mobility and/or density of the charge transfer carriers in the low‐spin ferropericlase. The activation energy of the low‐spin ferropericlase is 0.27 eV at 101 GPa, consistent with the small polaron conduction (electronic hopping, charge transfer). Our results indicate that low‐spin ferropericlase exhibits lower electrical conductivity than high‐spin ferropericlase, which needs to be considered in future geomagnetic models for the lower mantle.

Synthesis, structure, spectroscopic and magnetic characterization of a novel spin-crossover iron(II) complex with 1-cyclopropyltetrazole ligands

For the first time a 1-cyclopropyl substituted tetrazole ( C 3 tz) has been used as a potential ligand for iron(II) spin-transition complexes. The complexation of 1-cyclopropyltetrazol with iron(II) tetrafluoroborate yielded a fine powdered product of [Fe( C 3 tz) 6](BF 4) 2 being poorly soluble in most common solvents. Single crystals of complex were grown in situ from a solution of ligand and iron(II) hexafluorophosphate, which yielded a hexagonal prismatic crystalline product of [Fe( C 3 tz) 6](PF 6) 2. A comparison of XRPD data of the homologues [Fe( C 3 tz) 6](BF 4) 2 and [Fe( C 3 tz) 6](PF 6) 2 proves them to be homeotypic. The thermally induced spin-crossover phenomenon of [Fe( C 3 tz) 6](BF 4) 2 complex shows very abrupt spin transitions, with a spin-crossover temperature T 1/2 ≈ 180 K which is found to be ≈50 K above the T 1/2 of all known iron(II) complexes with n-alkyltetrazoles as ligands. The T 1/2 was determined by temperature-dependent 57Fe-Mössbauer, far FT-IR and UV–Vis–NIR spectroscopy as well as temperature dependent magnetic susceptibility measurements (SQUID).

The spin-states and spin-transitions of mononuclear iron(II) complexes of nitrogen-donor ligands

A survey of mononuclear iron(II) complexes with heterocyclic N-donor ligation is presented. A brief introduction to spin-crossover chemistry and low-temperature spin-trapping is provided, since many of these compounds undergo thermal spin-transitions upon cooling or heating. These are highlighted, and the structural changes underlying spin-crossover are discussed where this is known. Materials showing spin-trapping behaviour following thermal quenching or irradiation at very low temperatures are also described.

Electronic spin transition of iron in the Earth's deep mantle

Electronic spin is a quantum property of every electron, associated with its intrinsic angular momentum. Though there are no suitable physical analogies to describe the spin quantum number, there are two possibilities, called spin ‘up’ and spin ‘down.’ The electronic structure of iron in minerals is generally such that valence electrons will more abundantly occupy different spatial orbitals and maintain the same spin than occupy the same spatial orbital and assume opposite spin, called ‘spin‐paired.’To the astonishment of mineral physicists, pressure‐induced electronic spin‐pairing transitions of iron that were predicted nearly 50 years ago recently have been detected in major mantle‐forming oxides and silicates in ultrahigh‐pressure experiments at lower‐mantle pressures [e.g., Badro et al., 2003, 2004; Lin et al., 2005]. If such a spin transition is occurring in the Earth's lower mantle, there may be profound geophysical implications.

Iron(II) spin transition 1,2,4-triazole chain compounds with novel inorganic fluorinated counteranions

New one-dimensional spin transition coordination polymers of formula [Fe(NH 2trz) 3](AF 6) · nH 2O (A = Ti, Zr, Sn; NH 2trz = 4-amino-1,2,4-triazole) have been synthesized in MeOH and EtOH media. These materials display an abrupt and hysteretic spin transition around 200 K as well as a reversible thermochromic effect on cooling. A preliminary evaluation of the lattice dynamics in the high-spin and low-spin states is presented.