Synchrotron Mossbauer Spectroscopy Research Articles

Influence of ligand substitution on magnetic hyperfine interaction in Dy6-based single-molecule magnets/toroics

161Dy time-domain synchrotron Mossbauer spectroscopy (SMS), just recently presented as a novel tool to investigate Dy(III)-based single-molecule magnets (SMMs), has now been applied on two polynuclear Dy6-based wheel compounds possessing special features of single-molecule toroics (SMTs). The magnitude of the magnetic hyperfine field revealed for the Dy6 ring system [Dy6(teaH)6(NO3)6]·8MeOH is with B0 = 574.3(5) T characteristic for Dy(III) with a 6H15/2 ground state. The ligand substituted compound [Dy6(Me-teaH)6(NO3)6]·6MeCN possesses within the experimental errors the same magnetic hyperfine characteristics (B0 = 574.9(5) T). Consequently, the herein studied organic ligand variation has no perceptible influence on the magnetic hyperfine field sensed by the 161Dy(III) nuclei in these particular homometallic Dy(III)6-wheels.

Pressure-Induced Structural Transition to the Polar Phase in GdFe3(BO3)4

The GdFe3(BO3)4 crystal has attracted great interest as a magnetic-field-induced multiferroic. In this paper, we show that multiferroic properties in this crystal can be induced by high pressure. At high pressures up to 50 GPa, created in diamond anvil cells, the structural and vibrational (phonon) properties of the GdFe3(BO3)4 crystal were studied. The structural phase transition was detected at about 23–25 GPa by Raman and synchrotron Mossbauer (NFS) spectroscopy. First-principle calculations of the crystal lattice dynamics at pressures below and above the structural transition were carried out. It was established that at pressures above the structural transition, the space group R32 of GdFe3(BO3)4 is changed to the polar space group R3 and the crystal becomes a ferroelectric. At the R32 → R3 transition, the displacement of the boron ion B(2) and oxygen O results in the formation of boron-oxygen tetrahedrons B(2)O4 instead of the plane BO3 triangles. Meanwhile, the triangle oxygen environment of boron i...

Redox and structural controls on tin isotopic fractionations among magmas

Recent analytical developments have made possible the determination of the isotopic composition of tin in igneous rocks. In order to establish a framework to interpret the mass-dependent tin isotopic signatures of planetary materials, seven geologically-relevant silicate glasses (basalt, rhyolite, enstatite and anorthite glasses) were synthesized with moderate amounts of 119Sn (on the order of a weight percent). Redox conditions were controlled during sample synthesis to set the redox ratio (Sn2+/Sntot) from stannous (Sn2+) to stannic (Sn4+) glasses. The mean force constants of tin bonds in these glasses were determined by synchrotron nuclear resonant inelastic X-ray scattering (NRIXS) in order to determine the reduced isotopic partition function ratios (β-factors) of these glasses. Clues on the coordination chemistry and the valence state of tin in these glasses were also derived from synchrotron Mossbauer spectroscopy (SMS). The force constants of tin drastically increases from Sn2+-bearing to Sn4+-bearing glasses and varies significantly with the glass composition at a given redox state. The average coordination number of tin likely controls these variations with glass composition as suggested by SMS results. It is concluded that large isotope fractionation is expected between materials containing Sn2+ and Sn4+ respectively even at magmatic temperatures and that the coordination chemistry of tin in silicates strongly affect its isotope partitioning behavior. Our experimental data are finally used to interpret available Sn isotope data collected in terrestrial rocks. The incompatible behavior of Sn4+ in mantle minerals leads to a enrichment in heavy isotopes in mantle melts and to the depletion in heavy isotopes in solid residues of melting with a magnitude consistent with the isotope fractionation between Sn2+ and Sn4+ predicted by NRIXS data. Finally, we show that during fractional crystallization of basalt and considering the effect of tin coordination number in minerals and melts, the partitioning of Sn4+ into ilmenite leads to an enrichment in light isotopes in the residual melt.

Mechanosynthesized particles of Galfenol magnetostrictive composition and their polyurethane-bonded magnetoactive composites probed by synchrotron Mossbauer spectroscopy

Galfenol magnetostrictive particles have been mechanochemically synthesized and used as functional component of polyurethane-based composite to achieve magnetoactive elastic response. Synchrotron Mossbauer study was performed on energetic and time scale to reveal local and magnetic structure behavior responsible for the promising composite functionality. Mechanosynthesized particles local heterogeneity as well as particles magnetic domains behavior in external magnetic field were investigated.

Spin Crossover and the Magnetic P–T Phase Diagram of Hematite at High Hydrostatic Pressures and Cryogenic Temperatures

The magnetic properties of the α-Fe2O3 hematite at a high hydrostatic pressure have been studied by synchrotron Mossbauer spectroscopy (nuclear forward scattering (NFS)) on iron nuclei. Time-domain NFS spectra of hematite have been measured in a diamond anvil cell in the pressure range of 0–72 GPa and the temperature range of 36–300 K in order to study the magnetic properties at a phase transition near a critical pressure of ~50 GPa. In addition, Raman spectra at room temperature have been studied in the pressure range of 0–77 GPa. Neon has been used as a pressure-transmitting medium. The appearance of an intermediate electronic state has been revealed at a pressure of ~48 GPa. This state is probably related to the spin crossover in Fe3+ ions at their transition from the high-spin state (HS, S = 5/2) to a low-spin one (LS, S = 1/2). It has been found that the transient pressure range of the HS–LS crossover is extended from 48 to 55 GPa and is almost independent of the temperature. This surprising result differs fundamentally from other cases of the spin crossover in Fe3+ ions observed in other crystals based on iron oxides. The transition region of spin crossover appears because of thermal fluctuations between HS and LS states in the critical pressure range and is significantly narrowed at cooling because of the suppression of thermal excitations. The magnetic P–T phase diagram of α-Fe2O3 at high pressures and low temperatures in the spin crossover region has been constructed according to the results of measurements.

Single-crystal equations of state of magnesiowüstite at high pressures

Solid solutions of (Mg,Fe)O with high iron enrichment may be an important component of ultralow-velocity zones at Earth’s core-mantle boundary. However, to date there have been few high-precision studies on the elastic properties of these materials. In this study we present results on the compression of (Mg_(0.22)Fe_(0.78))O magnesiowustite in both neon and helium pressure media using single-crystal diffraction to ~55 GPa. In addition, our sample was characterized by time-domain synchrotron Mossbauer spectroscopy at ambient pressure using an extended time range that resulted in vastly improved energy resolution. The combination of these high-resolution techniques tightly constrains the presence of a defect-structure component at room pressure due to 4.7 mol% tetrahedrally coordinated ferric iron, resulting in a renormalized composition of (Mg_(0.215)Fe_(0.762)□_(0.023))O. Both high-pressure diffraction data sets are well described by a third-order Birch-Murnaghan equation of state. The best fit-parameters for a crystal with cubic structure in helium are K_(0T) = 148(3) GPa, K′_(0T) = 4.09(12), and V0 = 78.87(6) A^3. Increasing differential stress in the neon-containing sample chamber was correlated with increasing apparent distortion of the initially cubic unit cell, requiring a lower-symmetry hexagonal cell to fit the data above ~20 GPa. For fit equations of state, we determine the pressure-dependent correlation ellipses for the equation of state parameters and compare with previously published single-crystal diffraction data from (Mg,Fe)O crystals in a helium medium. We make two main observations from the data sets using a helium pressure medium: K_(0T) decreases as a function of increasing iron content from periclase to wustite and K′_(0T) is consistent with an approximately constant value of 4.0 that is independent of iron content, at least up to the iron concentration measured here. In combination with previously reported thermal parameters, we compute the density of magnesiowustite with this composition at core-mantle boundary conditions and discuss the implications.

Magnetic phase separation and strong enhancement of the Néel temperature at high pressures in a new multiferroic Ba3TaFe3Si2O14

The high-pressure properties of a new multiferroic of the langasite family Ba3TaFe3Si2O14 were investigated in diamond-anvil cells (DAC) in the temperature range of 4.2–295 K by a new method of synchrotron Mossbauer spectroscopy. Strong enhancement of the Neel temperature T N was observed at pressures above 20 GPa associated with the structural transformation. The highest value of T N is about 130 K which is almost five times larger than the value at ambient pressure (about 27 K). It was suggested that the high value of T N appears due to redistribution of Fe ions over 3f and 2d tetrahedral sites of the langasite structure. In this case, the short Fe-O distances and favorable Fe-O-Fe bond angles create conditions for strong superexchange interactions between iron ions, and effective two-dimensional (2D) magnetic ordering appears in the (ab) plane. The separation of the sample into two magnetic phases with different T N values of about 50 and 130 K was revealed, which can be explained by the strong 2D magnetic ordering in the ab plane and 3D ordering involving inter-plane interaction.

Electronic transitions of iron in almandine-composition glass to 91 GPa

Valence and spin states of Fe were investigated in a glass of almandine (Fe3Al2Si3O12) composition to 91 GPa by X-ray emission spectroscopy and energy- and time-domain synchrotron Mossbauer spectroscopy in the diamond-anvil cell. Changes in optical properties, total spin moment and Mossbauer parameters all occur predominantly between 1 bar and similar to 30 GPa. Over this pressure range, the glass changes from translucent brown to opaque and black. The total spin moment of the glass derived from X-ray emission spectroscopy decreases by similar to 20%. The complementary Mossbauer spectroscopy approaches reveal consistent changes in sites corresponding to 80-90% Fe2+ and 10-20% Fe3+. The high-spin Fe2+ doublet exhibits a continuous decrease in isomer shift and increase in line width and asymmetry. A high-spin Fe3+ doublet with quadrupole splitting of similar to 1.2 minis is replaced by a doublet with quadrupole splitting of similar to 1.9 minis, a value higher than all previous measurements of high-spin Fe3+ and consistent with low-spin Fe3+. These observations suggest that Fe3+ in the glass undergoes a continual transition from a high-spin to a low-spin state between 1 bar and similar to 30 GPa. Almandine glass is not expected to undergo any abrupt transitions in electronic state at deep mantle pressures.

Temperature of Earth's core constrained from melting of Fe and Fe0.9Ni0.1 at high pressures

The melting points of fcc- and hcp-structured Fe_(0.9)Ni_(0.1) and Fe are measured up to 125 GPa using laser heated diamond anvil cells, synchrotron Mossbauer spectroscopy, and a recently developed fast temperature readout spectrometer. The onset of melting is detected by a characteristic drop in the time-integrated synchrotron Mossbauer signal which is sensitive to atomic motion. The thermal pressure experienced by the samples is constrained by X-ray diffraction measurements under high pressures and temperatures. The obtained best-fit melting curves of fcc-structured Fe and Fe_(0.9)Ni_(0.1) fall within the wide region bounded by previous studies. We are able to derive the γ–ϵ–l triple point of Fe and the quasi triple point of Fe_(0.9)Ni_(0.1) to be 110 ± 5GPa, 3345 ± 120K and 116 ± 5GPa, 3260 ± 120K, respectively. The measured melting temperatures of Fe at similar pressure are slightly higher than those of Fe_(0.9)Ni_(0.1) while their one sigma uncertainties overlap. Using previously measured phonon density of states of hcp-Fe, we calculate melting curves of hcp-structured Fe and Fe_(0.9)Ni_(0.1) using our (quasi) triple points as anchors. The extrapolated Fe_(0.9)Ni_(0.1) melting curve provides an estimate for the upper bound of Earth's inner core–outer core boundary temperature of 5500 ± 200K. The temperature within the liquid outer core is then approximated with an adiabatic model, which constrains the upper bound of the temperature at the core side of the core–mantle boundary to be 4000 ± 200K. We discuss a potential melting point depression caused by light elements and the implications of the presented core–mantle boundary temperature bounds on phase relations in the lowermost part of the mantle.

Equation of state and spin crossover of (Mg,Fe)O at high pressure, with implications for explaining topographic relief at the core-mantle boundary

Iron-bearing periclase is thought to represent a significant fraction of Earth’s lower mantle. However, the concentration of iron in (Mg,Fe)O is not well constrained at all mantle depths. Therefore, understanding the effect of iron on the density and elastic properties of this phase plays a major role in interpreting seismically observed complexity in the deep Earth. Here we examine the high-pressure behavior of polycrystalline (Mg,Fe)O containing 48 mol% FeO, loaded hydrostatically with neon as a pressure medium. Using X-ray diffraction and synchrotron Mossbauer spectroscopy, we measure the equation of state to about 83 GPa and hyperfine parameters to 107 GPa at 300 K. A gradual volume drop corresponding to a high-spin (HS) to low-spin (LS) crossover is observed between ~45 and 83 GPa with a volume drop of 1.85% at 68.8(2.7) GPa, the calculated spin transition pressure. Using a newly formulated spin crossover equation of state, the resulting zero-pressure isothermal bulk modulus K 0T,HS for the HS state is 160(2) GPa with a K ′0T,HS of 4.12(14) and a V 0,HS of 77.29(0) A3. For the LS state, the K 0T,LS is 173(13) GPa with a K ′0T,LS fixed to 4 and a V 0,LS of 73.64(94) A3. To confirm that the observed volume drop is due to a spin crossover, the quadrupole splitting (QS) and isomer shift (IS) are determined as a function of pressure. At low pressures, the Mossbauer spectra are well explained with two Fe2+-like sites. At pressure between 44 and 84, two additional Fe2+-like sites with a QS of 0 are required, indicative of low-spin iron. Above 84 GPa, two low-spin Fe2+-like sites with increasing weight fraction explain the data well, signifying the completion of the spin crossover. To systematically compare the effect of iron on the equation of state parameters for (Mg,Fe)O, a spin crossover equation of state was fitted to the pressure-volume data of previous measurements. Our results show that K 0,HS is insensitive to iron concentration between 10 to 60 mol% FeO, while the spin transition pressure and width generally increases from about 50–80 and 2–25 GPa, respectively. A key implication is that iron-rich (Mg,Fe)O at the core-mantle boundary would likely contain a significant fraction of high-spin (less dense) iron, contributing a positive buoyancy to promote observable topographic relief in tomographic images of the lowermost mantle.

Spin and valence states of iron in Al-bearing silicate glass at high pressures studied by synchrotron Mossbauer and X-ray emission spectroscopy

High-pressure synchrotron Mossbauer (SMS) and X-ray emission (XES) spectroscopic measurements were conducted to investigate the spin and valence states of iron in (A1,Fe)-bearing magnesium silicate glass (Mg0.79Fe0.10Al0.10Si0.96O3) up to 126 GPa and 300 K. By analyzing the Fe K beta emission spectra using the integrated relative difference (IRD) method, which accounts for the spectral broadening effects, the derived total spin momentum (S) of the iron in the glass shows no observable changes with pressure within the experimental uncertainties. A two-doublet fitting model representing two diverse local iron atomic environments was used to satisfactorily simulate the high-pressure SMS spectra of iron in the glass. The doublet with an averaged quadrupole splitting (QS) value of 1.94(+/- 0.25) mm/s and chemical shift (CS) of 1.02(+/- 0.25) minis at ambient conditions was assigned to be high-spin Fe2+, whereas the second doublet with QS = 0.83(+/- 0.25) mm/s and CS = 0.49(+/- 0.25) mm/s was assigned to be high-spin Fe3+. Increasing pressure continuously elevates the QS of Fe2+ from similar to 2 mm/s at ambient pressure to 3.5 mm/s at 126 GPa, while Fe3+ only exhibits a slight increase in the QS to 1.34(+/- 0.25) mm/s. Comparing with previous experimental and theoretical studies on the local geometries and hyperfine parameters of silicate glasses and minerals, we conclude that the occurrence of the extremely high QS of Fe2+ in our glass above similar to 40-50 GPa can be associated with the enhanced density and diverse distortions and geometries of the local Fe2+ environments. Our combined XES and SMS results show that both Fe2+ and Fe3+ ions in Al-bearing silicate remain in the high-spin state, rather than undergoing a spin-pairing transition as proposed previously. Assuming that the silicate glass results can be used as an analog for understanding silicate melts, our results here indicate that iron ions likely experience significant changes in the local environments yet remain overall in the high-spin state in silicate melts at the extreme pressure and temperature conditions of the deep mantle.

Spin transition of Fe2+ in ringwoodite (Mg,Fe)2SiO4 at high pressures

Electronic spin transitions of iron in the Earth’s mantle minerals are of great interest to deep-Earth researchers because their effects on the physical and chemical properties of mantle minerals can significantly affect our understanding of the properties of the deep planet. Here we have studied the electronic spin states of iron in ringwoodite (Mg 0.75 Fe 0.25 ) 2 SiO 4 using synchrotron Mossbauer spectroscopy in a diamond-anvil cell up to 82 GPa. The starting samples were analyzed extensively using transmission and scanning electron microscopes to investigate nanoscale crystal chemistry and local iron distributions. Analyses of the synchrotron Mossbauer spectra at ambient conditions reveal two non-equivalent iron species, (Fe 2+ ) 1 and (Fe 2+ ) 2 , which can be attributed to octahedral and tetrahedral sites in the cubic spinel structure, respectively. High-pressure Mossbauer measurements show the disappearance of the hyperfine quadrupole splitting ( QS ) of the Fe 2+ ions in both sites at approximately 45–70 GPa, indicating an electronic high-spin (HS) to low-spin (LS) transition. The spin transition exhibits a continuous crossover nature over a pressure interval of ~25 GPa, and is reversible under decompression. Our results here provide the first experimental evidence for the occurrence of the spin transition in the spinel-structured ringwoodite, a mantle olivine polymorph, at high pressures.

Local structure variations observed in orthoenstatite at high pressures

The site-specific behavior of iron in an orthoenstatite-structured ^(57)Fe-enriched (M1)(M2)Si_(2)O_(6) powdered sample was explored using synchrotron Mossbauer spectroscopy and diamond-anvil cells in two independent experiments at ambient temperature. In one experiment, NaCl was used as the pressure-transmitting medium (ambient pressure to 36 GPa), and in the other experiment, Ne surrounded the sample (4.1 to 26.8 GPa). The hyperfine parameters of the M1 and M2 sites at room pressure are in excellent agreement with previous literature values obtained using conventional Mossbauer spectroscopy and yield (Mg_(0.980)Fe_(0.020(5))(Mg_(0.760)Fe_(0.240))Si_(2)O_6 as the chemical formula. Analyses of both data sets reveal a change in the trend or discontinuity in the hyperfine parameters around 10 GPa, indicative of a transformation in orthopyroxene. However, the detailed behaviors of the iron sites with pressure appear to depend on the local stress conditions provided by the different pressure media. Our observations may help explain the reported variations in structural transition behavior for orthopyroxenes at high pressures.

High-spin-low-spin transition in magnesiowüstite (Mg0.75,Fe0.25)O at high pressures under hydrostatic conditions

The spin states of Fe2+ ions in (Mg0.75,Fe0.25)O magnesiowustite crystals at hydrostatic pressures up to 90 GPa created in a diamond-anvil cell with helium as a pressure-transmitting medium have been investi-gated by transmission and synchrotron Mossbauer spectroscopy at room temperature. An electron transition from the high-spin (HS) state to the low-spin (LS) state (HS-LS crossover) has been observed in the pressure range of 55–70 GPa. The true HS-LS transition occurs in a narrow pressure range and the extension of the electron transition to ∼15 GPa is attributed to the effect of the nearest environment and to thermal fluctuations between the high-spin and low-spin states at finite temperatures. It has been found that the lowest pressure at which the electron HS-LS transition can occur in the Mg1 − x Fe x system is 50–55 GPa.

Spin state of ferric iron in MgSiO3 perovskite and its effect on elastic properties

Abstract Recent studies have indicated that a significant amount of iron in MgSiO 3 perovskite (Pv) is Fe 3+ (Fe 3+ /ΣFe = 10–60%) due to crystal chemistry effects at high pressure ( P ) and that Fe 3+ is more likely than Fe 2+ to undergo a high-spin (HS) to low-spin (LS) transition in Pv in the mantle. We have measured synchrotron Mossbauer spectroscopy (SMS), X-ray emission spectroscopy (XES), and X-ray diffraction (XRD) of Pv with all iron in Fe 3+ in the laser-heated diamond-anvil cell to over 100 GPa. Fe 3+ increases the anisotropy of the Pv unit cell, whereas Fe 2+ decreases it. In Pv synthesized above 50 GPa, Fe 3+ enters into both the dodecahedral (A) and octahedral (B) sites approximately equally, suggesting charge coupled substitution. Combining SMS and XES, we found that the LS population in the B site gradually increases with pressure up to 50–60 GPa where all Fe 3+ in the B site becomes LS, while Fe 3+ in the A site remains HS to at least 136 GPa. Fe 3+ makes Pv more compressible than Mg-endmember below 50 GPa because of the gradual spin transition in the B site together with lattice compression. The completion of the spin transition at 50–60 GPa increases bulk modulus with no associated change in density. This elasticity change can be a useful seismic probe for investigating compositional heterogeneities associated with Fe 3+ .

Nuclear resonant X-ray spectroscopy of (Mg,Fe)SiO3 orthoenstatites

We present nuclear resonant inelastic X-ray scattering (NRIXS) and synchrotron Mossbauer spectroscopy (SMS) measurements, both nuclear resonant X-ray spectroscopic methods, on synthetic samples of orthoenstatite-structured (Mg,^(57)Fe)SiO_3, a representative component in Earth's upper mantle. All measurements were performed at ambient conditions. NRIXS spectra were measured for three samples of orthoenstatite containing 20, 13, and 7 mol% FeSiO_3. The Debye sound velocities were determined from the low-energy region of the partial phonon density of states (PDOS). With known density and bulk modulus, the shear modulus, compressional and shear wave velocities have been computed. The sound velocities obtained from NRIXS are in good agreement with sound velocities obtained using Brillouin spectroscopy and ultrasonic methods for similar compositions. An important advantage of NRIXS is access to additional thermodynamic information, such as the average force constant, mean-square displacement, obtained from the PDOS. We discuss the contribution of the vibrational spectra to these quantities. In addition to the PDOS, the electronic environment of the iron sites in (Mg_(0.87)^(57)Fe_(0.13))SiO_3 orthoenstatite was determined using ^(57)Fe SMS and conventional Mossbauer spectroscopy. Evaluation of the Mossbauer spectra reveals two distinct iron sites, which are well distinguished by their hyperfine fields. The minority and majority sites are consistent with high-spin Fe^(2+) in the M1 and M2 sites, respectively.

Behavior of iron in (Mg,Fe)SiO3 post‐perovskite assemblages at Mbar pressures

The electronic environment of the iron sites in post‐perovskite (PPv) structured (57Fe,Mg)SiO3 has been measured in‐situ at 1.12 and 1.19 Mbar at room temperature using 57Fe synchrotron Mössbauer spectroscopy. Evaluation of the time spectra reveals two distinct iron sites, which are well distinguished by their hyperfine fields. The dominant site is consistent with an Fe3+‐like site in a high spin state. The second site is characterized by a small negative isomer shift with respect to α‐iron and no quadrupole splitting, consistent with a metallic iron phase. Combined with SEM/EDS analyses of the quenched assemblage, our results are consistent with the presence of a metallic iron phase co‐existing with a ferric‐rich PPv. Such a reaction pathway may aid in our understanding of the chemical evolution of Earth's core‐mantle‐boundary region.

High-spin-low-spin transition and the sequence of the phase transformations in the BiFeO3 crystal at high pressures

The transition of Fe3+ ions from the high-spin (HS) state (S = 5/2) to the low-spin (LS) state (S = 1/2) has been observed in the BiFeO3 multiferroic crystal at high pressures in the range 45–55 GPa. This effect has been studied in high-pressure diamond-anvil cells by means of two experimental methods using synchrotron radiation: nuclear resonant forward scattering (NFS or synchrotron Mossbauer spectroscopy) and FeKβ high-resolution X-ray emission spectroscopy (XES). The HS-LS transition correlates with anomalies in the magnetic, optical, transport, and structural properties of the crystal. At room temperature, the transition is not stepwise, but is extended in a pressure range of about 10 GPa due to thermal fluctuations between the high-spin and low-spin states. It has been found that the transition of the BiFeO3 insulator to the metal occurs only in the low-spin phase and the cause of all phase transitions is the HS-LS crossover.

Nuclear resonant spectroscopy

Nuclear resonant scattering techniques with synchrotron radiation (SR) are introduced on a basic level. We focus on the theoretical background and on experimental aspects of two popular methods with a widening range of applications, nuclear resonant inelastic x-ray scattering and synchrotron Mossbauer spectroscopy. The inelastic method provides specific vibrational information, e.g., the phonon density of states. The Mossbauer method permits determination of hyperfine interactions. All nuclear resonance techniques take full advantage of the unique properties of SR: intensity, collimation, time structure, and polarization. As a result both methods discussed here have led to novel applications for materials under extreme conditions, proteins with biological functionality, and magnetic nanostructures.

A synchrotron Mössbauer spectroscopy study of (Mg,Fe)SiO3perovskite up to 120 GPa

The electronic environment of the Fe nuclei in two silicate perovskite samples, Fe_(0.05)Mg_(0.95)SiO_3 (Pv05) and Fe_(0.1)Mg_(0.9)SiO_3 (Pv10), have been measured to 120 GPa and 75 GPa, respectively, at room temperature using diamond anvil cells and synchrotron Mossbauer spectroscopy (SMS). Such investigations of extremely small and dilute ^(57)Fe-bearing samples have become possible through the development of SMS. Our results are explained in the framework of the “three-doublet” model, which assumes two Fe2+-like sites and one Fe^(3+)-like site that are well distinguishable by the hyperfine fields at the location of the Fe nuclei. At low pressures, Fe^(3+)/∑Fe is about 0.40 for both samples. Our results show that at pressures extending into the lowermost mantle the fraction of Fe^(3+) remains essentially unchanged, indicating that pressure alone does not alter the valence states of iron in (Mg,Fe)SiO_3 perovskite. The quadrupole splittings of all Fe sites first increase with increasing pressure, which suggests an increasingly distorted (noncubic) local iron environment. Above pressures of 40 GPa for Pv10 and 80 GPa for Pv05, the quadrupole splittings are relatively constant, suggesting an increasing resistance of the lattice against further distortion. Around 70 GPa, a change in the volume dependence of the isomer shift could be indicative of the endpoint of a continuous transition of Fe3+ from a high-spin to a low-spin state.