MR31A-1820
Stable Spin and Magnetic Arrangements in Fe2O3 Post-Perovskite
Fe2O3 post-perovskite is the high-pressure stable phase of Fe2O3 hematite (P > 60 GPa) and is the Fe end-member of (Mg,Fe)(Si,Fe)O3 post-perovskite. The stable spin arrangement and spin state of Fe2O3 post-perovskite are currently unknown. A better understanding of the high-pressure magnetic signature of Fe2O3 post-perovskite will help explain changes in the magnetic patterns on planetary bodies (e.g., the moon and Mars) due to shock events and could shed light on properties of the D" layer. We use ab initio calculations to calculate the stable magnetic arrangements and spin states at lower-mantle pressures. Antiferromagnetic arrangements are generally found to be more stable than ferromagnetic. At pressures below 70 GPa, we find the most stable magnetic arrangement has anti-parallel spins between the A- and B-sites (all Fe atoms on the A, bipolar prismatic site, are spin up, all Fe atoms on the B, octahedral sites, are spin down). However, at pressures above 70 GPa, the most stable arrangement has anti-parallel spins between the A-site iron atoms and between the B-site iron atoms. Although iron in both sites are high-spin below 70 GPa, the B-site iron atoms transform to low- spin, and the A-site iron atoms remain high-spin above 70 GPa. We also conducted Synchrotron Mössbauer spectroscopy of Fe2O3 post-perovskite at 70 GPa in the laser-heated diamond anvil cell at Sector 3 of Advanced Photon Source. The measured spectrum indicates spin ordering in Fe2O3 post-perovskite consistent with our calculations. Also the measured quadrupole splitting and magnetic hyperfine field suggest that at least half of iron is in the high-spin state, which are largely consistent with the calculations.
MR31A-1821
Predominant Intermediate-Spin Ferrous Iron in Lowermost Mantle Post-Perovskite and Perovskite
Silicate post-perovskite and perovskite are believed to be the dominant minerals of the lowermost mantle and
the lower mantle, respectively, and their properties, which can be strongly influenced by the electronic state
of iron in these phases, affect our understanding of the nature of the deep Earth. To date, in these minerals
the electronic spin state of iron remains unknown under lowermost-mantle pressure-temperature conditions,
although recent studies have showed an electronic spin crossover from high-spin to low-spin in ferropericlase
over an extended pressure-temperature range of the lower mantle (i.e., Lin et al., Science, 2007) and from
high-spin to intermediate-spin in silicate perovskite near the top of the lower mantle (McCammon et al.,
Nature Geoscience, 2008). Here we report the spin and valence states of iron in post-perovskite and
perovskite at pressure-temperature conditions relevant to the lowermost mantle using in situ X-ray emission,
X-ray diffraction, and synchrotron Mossbauer spectroscopies in a laser-heated diamond cell. Perovskite and
post-perovskite display extremely high quadrupole splitting (QS) of approximately 4 mm/s and relatively high
center shift in the synchrotron Mossbauer spectra at 110 GPa and 134 GPa, respectively. Our results show
that Fe2+ exists predominantly in the intermediate-spin state with a total spin number of one in both
phases (Lin et al., Nature Geoscience, 2008). Together with recent results on the effects of the spin
transition in the lower-mantle ferropericlase (see a recent review by Lin and Tsuchiya, PEPI, 2008), here we
will address how the electronic spin states in lower-mantle phases and their associated effects affect our
understanding on the composition, geophysics, and dynamics of the lower mantle..
References:
1. Lin, J. F., H. C. Watson, G. Vanko, E. E. Alp, V. B. Prakapenka, P. Dera, V. V. Struzhkin, A. Kubo, J.
Zhao, C. McCammon, W. J. Evans, Intermediate-spin ferrous iron in lowermost mantle post-perovskite and
perovskite, Nature Geosciences, 2008.
2. Lin, J. F., and T. Tsuchiya, Spin transition of iron in the Earth's lower mantle, Phys. Earth Planet. Inter.,
doi:10.1016/j.pepi.2008.01.005, 2008.
3. Lin, J. F., G. Vanko, S. D. Jacobsen, V. Iota-Herbei, V. V. Struzhkin, V. B. Prakapenka, A. Kuznetsov, and
C.-S.Yoo, Spin transition zone in Earth's lower mantle, Science, 317, 1740-1743, 2007.
MR31A-1822
Spin transition in Fe3+ in Mg-silicate perovskite
In recent years, spin transitions have been reported in both perovskite (Pv) (Badro et al., 2004) and magnesiowüstite (Mw) (Lin et al., 2005). Although no density change has been detected in ferrous iron bearing Pv (Lundin et al., 2008), large changes in density have been found to occur with the spin transition in Mw (Fei et al., 2007). This difference has been attributed to the diverse coordination and oxidation states of Fe in Pv and has made analysis of previous experimental data difficult. We have measured synchrotron Mössbauer spectroscopy (SMS) and X-ray diffraction (XRD) on Mg-silicate Pv with all Fe in Fe3+ (0.9MgSiO3- 0.1Fe2O3) up to 136 GPa in the laser heated diamond anvil cell with an argon pressure medium. Our SMS shows that Fe3+ enters both the dodecahedral and octahedral sites at the entire pressure range we studied (47-136 GPa). We found that both high and low spin Fe3+ exist in the octahedral site between 47 and 63 GPa and then all Fe3+ in the octahedral site becomes low spin at higher pressures, while Fe3+ in the dodecahedral site remains high spin to 136 GPa. Pressure-volume data, measured up to 106 GPa using the gold pressure scale (Tsuchiya, 2003), shows that the spin transition does not effect density but does result in a 10% increase in the bulk modulus near 60 GPa. Below 60 GPa, ferric Pv is 9% more compressible than ferrous Pv, but above 60 GPa, ferric Pv is 17% less compressible than ferrous perovskite. This change in compressibility is likely due to a change in compression mechanism: at low pressure compression is accomplished by a combination of the gradual spin collapse and lattice compression. However, after the spin transition is complete, only lattice compression is at work at higher pressures. If the lower mantle has as much Fe3+ as suggested by McCammon et al. (1997) and Frost et al. (2004), this may result in a global discontinuity at the mid-mantle. If not, local enrichment of Fe3+ can result in a discontinuity, which can be used for the investigation of compositional heterogeneities in the lower mantle. It is notable that some discontinuities have been observed in recent seismologic studies at 900-1800 km depths either locally or globally (Kawakatsu and Niu, 1994; Stunff et al., 1995; Courtier and Revenaugh, 2008).
MR31A-1823
Modeling Mossbauer Spectra to Determine the Spin State of Iron in (Mg,Fe)SiO3 Perovskite
The spin state of iron in (Mg,Fe)SiO3 perovskite at lower mantle conditions is still uncertain. Mössbauer spectroscopy measures the quadrupole splitting, isomer shift, and hyperfine field of iron, which provides constraints on its spin state and valence state. The experimental quadrupole splitting values of iron in perovskite are higher than known high-spin and low-spin quadrupole splitting values, suggesting a possible intermediate-spin state in iron. Currently, no experimental or computational benchmarks exist for the quadrupole splitting of intermediate-spin iron. We use ab initio calculations to calculate the quadrupole splitting of high-, low-, and intermediate-spin iron as a function of pressure and the valence state of iron in (Mg,Fe)SiO3 perovskite. We find at all pressures that high-spin Fe2+ has the largest quadrupole splitting value and the quadrupole splitting of intermediate spin Fe2+ is approximately the same as low-spin iron. In contrast, low-spin Fe3+ has a larger quadrupole splitting than both intermediate- and high-spin Fe3+. Our results suggest the high quadrupole splitting observed experimentally in (Mg,Fe)SiO3 perovskite is due to high-spin Fe2+ rather than intermediate-spin Fe2+. Potential sources of error in the calculations are discussed.
MR31A-1824
Spin transition in ferrous iron in MgSiO3 perovskite under pressure
We present a density functional study of the pressure-induced spin transition in ferrous iron in MgSiO3 perovskite. We address the influence of iron concentration and configuration (structural and magnetic), as well as technical issues such as the nature of the exchange correlation (XC) functional (CA-LDA versus PBE- 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 XC functionals. We also address changes of atomic structure around Fe atoms and electronic structure including the blue shift accompanying the spin transition. Research supported by NSF/EAR 013533, 0230319, and NSF/ITR 0428774 (VLab). Computations were performed at the Minnesota Supercomputing Institute and Indiana Universityfs BigRed system.
MR31A-1825
Structural and spin transitions in Fe2O3
The wide range of intriguing characteristics exhibited by Fe2O3 with pressure and temperature has renewed the attention of the scientific community in the last decade. Experimental and theoretical efforts are on to address and unravel the complexity of the system. The ambient pressure phase, hematite (α- Fe2O3) transforms to a new structural phase (HP1). That the HP1 phase is orthorhombic perovskite (Pbnm) or Rh2O3-II type (Pbcn) is still a debate and yet to be explored theoretically. Experimentally, the succeeding high pressure phase (HP2) was proposed to be Cmcm type post-perovskite (without any structural assignment). From the spin transition point of view, there has been a long-standing issue of an isostructural high spin (HS) to low spin (LS) transition. And experimental data till date are divided into two horizons -- one assigning the spin transition in the hematite phase and the other in the HP1 phase. In this work, motivated by these exotic unresolved controversies of the system, we have tried to gain an insight of the system from first principles density functional calculations. Our results favor the Rh2O3-II type as the HP1 phase, in agreement with recent experiments. However, we do contradict with the experiment in case of HP2 phase. Our results favor a new structural phase with Pmc21 symmetry as HP2. We also show that the phenomenon of HS to LS transition can rather be captured by a simple mechanism and this we believe, might help in removing the boundary between the two horizons as mentioned above.
MR31A-1826
Structural instability of FeOOH at high pressure
Powder x-ray diffraction measurements on goethite (α-FeOOH) using argon pressure medium, were performed at room-temperature and 0 - 60 GPa. A first order phase transition from α-FeOOH (Pbnm) to ε-FeOOH (P21mn) occurs at 42±3 GPa with a discontinuous molar volume decrease of ~3.0%. Mössbauer and resistivity studies to 62 GPa reveal the onset of magnetic moment collapse concurrent with an insulator-metal transition, an unambiguous indication of a Mott-Hubbard correlation breakdown. We suggest that at P>60 GPa, the mechanical-energy density, provided by the high pressure, overcomes the coulombic-energy density resulting in appreciable changes in Fe-(OH) interactions and manifests as a first-order structural phase transition.
MR31A-1827
High-temperature compression of ferropericlase and the effect of temperature on iron spin transition
Iron-bearing lower mantle minerals have been proposed to undergo the spin transition at certain P-T conditions. One of the remarkable effects of the spin transition is on the density since this transition is accompanied by a volume shrinkage. The precise density profile of the mantle based on laboratory measurements is important for understanding the composition of the mantle. In order to collect the density data and understand the nature of the spin transition of an iron-bearing lower mantle mineral, we operated high-temperature compression experiments of ferropericlase (Fp) with a composition of (Mg0.81,Fe 0.19)O with in-situ X-ray diffraction method in an externally-heated diamond anvil cell from 19 to 85 GPa at a constant temperature of 859-869 K. Room-temperature experiments with a laser-annealing technique were also carried out on the same material. Anomalous volume reductions that cannot be explained by normal compression behavior were observed at 60-82 GPa and 58-64 GPa at a high temperature and room temperature, respectively. These volume reductions are likely related to the spin transition of ferrous iron in Fp. The observed density changes across this spin transition at 865 and 300 K are about 1.6% and 0.6%, respectively. The spin transition pressure interval expands with increasing temperature. Very recently, in order to obtain further high-temperature data, we operated a new in-situ X-ray diffraction experiment in a laser-heated diamond cell with a membrane system. In this new system, we were able to regulate the gas pressure in the membrane of the DAC and, therefore, compress the sample at a high temperature during the laser-heating. We collected the volume data at high temperature of 1600-1700 K from 20 to 120 GPa. We will discuss the temperature effect of the spin transition in Fp from our external- and laser-heated and room-temperature compression experiments.
MR31A-1828
Dynamical effects of ionic size fluctuation: Ab initio and numerical simulations study
Recent researches suggest that the spin transition of iron in the mantle minerals can cause several new exotic phenomena, which might give important consequences on the mantle mineralogy. Theoretical free energy modelling combining with density functional computation proposed a broad pressure region where high-spin and low-spin iron coexist along the mantle geotherm. Across this high-spin/low-spin mixed state, any properties are expected to vary continuously. This transient behavior has been successfully confirmed experimentally. More recently, surprising elastic softening has also been reported across the spin crossover. However, the physical backgrounds of dynamical effect of the spin crossover are still little understood. In this study, we have performed density functional and thousands atom numerical simulations to model the mixed spin state in the lower mantle mineral phases. Calculations suggest that the dynamical fluctuation of ionic size cound drive several anomalous behaviors during the spin crossover in particular in the thermoelastic property, even though the structure remains macroscopically isosymmetric. Research supported by Ehime U Project Fund, in part by JSPS
MR31A-1829
Significance of short-range order for high- to low-spin transition in (Mg,Fe)O under pressure
Correct interpretation of the seismic structure of Earth in terms of materials composing our planet requires the detailed knowledge of the physical properties of various metal oxides. (Mg,Fe)O is one of the host materials of the lower mantle. Under pressure this material exhibits high- to low-spin transition that effects its elastic, electrical and optical properties. Here we report the results of our ab initio study of this transition and discuss the significance of short range order and Fe distributioin for the electronic structure and elastic properties of the high- and low-spin phases.
MR31A-1830
Spin state and valence state of iron in Earth's lower mantle from synchrotron Mössbauer spectra of perovskite and post-perovskite up to 1.5 Mbar
The electronic spin state and valence state of iron are fundamental parameters that govern the physical properties and chemical behavior of iron-bearing phases in the Earth's interior, including their densities, sound velocities, thermal conductivities, and chemical potentials. Of particular importance are pressure- induced changes in the spin state and valence state of iron in the predominant lower mantle phase perovskite and its high-pressure polymorph post-perovskite. These issues remain controversial due to limited experimental data on highly compressed samples, and due to the lack of theoretical guidance for interpreting experimental results. Here we present new synchrotron Mössbauer spectra of perovskite and post-perovskite samples under pressures up to 145 GPa. Samples were synthesized and characterized in the laser-heated diamond anvil cell at SPring-8, from gel starting material with the composition (Mg0.9Fe0.1)SiO3. Synchrotron Mössbauer measurements were carried out at beamlines 3-ID and 16-ID at the Advanced Photon Source. Our spectra of perovskite can be fitted with two or three iron sites with quadruple splitting of 2.97 and 0.5 mm/sec, respectively. For post-perovskite, the spectrum can be fitted with a single component that has a small quadruple splitting. We interpret these results on the basis of calculated hyperfine parameters of iron with various crystallographic sites and spin/valence states, and discuss implications for the physics and chemistry of the lowermost mantle. This work is supported by NSF through a collaborative project EAR 07-38973.
MR31A-1831
Thermodynamics properties of ferropericlase
The thermodynamics properties of ferropericlase (Mg(1-x)FexO, xFe ~ 0.19), have been investigated by first principles using a combination of newly developed techniques designed to address materials of such complexity. The strongly correlated nature of ferrous iron has been successfully addressed previously in static calculations by using a first principles LDA+U approach. However, investigation of thermodynamics properties of the solid solution presents further challenges, particularly the inclusion of vibrational effects without which results are not predictive. We have developed a vibrational virtual crystal model (VVCM) to address this issue. The acoustic velocities of the VVCM are, by construction, precisely the same as those of the real solid solution. We present here unusual anomalies on the thermodynamics properties caused by the spin crossover transition. Research supported by NSF/EAR 0635990, and NSF/ITR 0428774 (VLab). Computations were performed at the Minnesota Supercomputing Institute.
MR31A-1832
Viscosity hills in the lower-mantle: influence of iron high-to-low spin transition in ferropericlase
The recently discovered iron spin transition in major mantle minerals at high pressures should have important geophysical implications for the physical properties of the lower mantle. Here, we investigate the consequences of this transition on the viscosity of ferropericlase Mg1-xFexO (x = 0.1875) along lower mantle geotherm. The viscosity was described as a thermally activated mechanism within the elastic strain energy model, in which the activation energies are related to bulk and shear wave velocities. Those velocities at high temperatures and high pressures were computed using the first principles total energy calculations of the materials elasticity. Due to the strong elastic softening, resulting from the spin transition, there is a large reduction in the activation free energies of the materials rheology, leading to a viscosity minimum at a pressure corresponding to the mid-lower mantle. This could provide an explanation for the mantle viscosity undulations obtained from geoid inversion and postglacial rebound studies.
MR31A-1833
Magnetism in Metal Oxides Under Pressure
The nature of the magnetic collapse in transition metal oxides under pressure has been a source of recent
theoretical interest with multiple explanations presented in the literature[1,2]. In order to better
understand this phenomenon we present the results of two sets of
calculations utilizing different electronic structure methods. We perform quantum Monte Carlo (QMC)
calculations of FeO under pressure. As QMC uses no approximate functional and solves the many-body,
correlated Schrödinger equation, these calculations are often much more accurate than standard Density
Functional Theory (DFT) approaches. Therefore we use QMC to calculate total energies of FeO in both the
high spin and low spin states. In addition we use DFT calculations to gain a qualitative understanding of the
mechanism for the transition. As a post processing step to the DFT calculations, a tight binding Hamiltonian
is constructed aiding in the physical interpretation. Results are presented for FeO, MnO and NiO in the B1
structure. As the pressure is increased, the bandwidth increases much faster than the crystal field splitting
suggesting that it plays the dominant role in the magnetic collapse.
1 J. Kunes et al. Nature Materials. 7, 198 (2008)
2 R. E. Cohen et al. Science. 275, 654 (1997)
MR31A-1834
Single-crystal elasticity of Al-bearing ferropericlase at high pressures up to 61 GPa
Electronic transitions in Fe, from a high-spin state low-spin state, are promoted by pressure and has been observed in the Fe-bearing minerals ferropericlase and silicate perovskite of the lower mantle. In essence, high-spin (HS) and low-spin (LS) Fe behave like different elements. As a consequence, any change in elastic properties associated with a HS-LS transition in Fe in a host mineral will likely result in a change in sound velocities and seismic velocity profiles. Ferropericlase (FP), (Mg, Fe)O, is thought to be the second most abundant mineral in the Earth's lower mantle. The electronic spin transition of iron in FP can occur above 40 to 90 GPa at room temperature, depending on the Fe content, and causes visible changes in physical properties. Its effect on physical properties may change our current understanding about the heterogeneity and modeling of lower mantle. Aluminum in ferropericlase is not as abundant as in (Mg, Fe)SiO3 perovskite. However, incorporation of Al into ferropericlase could possibly cause significant changes of its physical properties, as is found for Fe. We have performed Brillouin scattering experiments on Al-bearing ferropericlase at high pressures up to 61GPa. At ambient condition, incorporation of aluminum into ferropericlase results in a slight drop in the shear modulus, and no observed change in bulk modulus. Our results show a "softening" effect of the spin transition on the elasticity of Al-bearing ferropericlase at pressures over 40 GPa. This suggests that the incorporation of aluminum increases the pressure of spin transition. Our high pressure single-crystal velocity data shows that softening is most pronounced in the elastic modulus C12, which decreases significantly within transition region with mixed spin states. C11 is also likely to soften, although the effect appears to be less pronounced than for C12, whereas C44 does not display any softening within the pressure range of our observations.
MR31A-1835
Compressibility and Optical Properties of Single Crystal Magnesite and Siderite up to 75 GPa
The stability and properties of carbonates at mantle conditions are of great importance for the carbon cycle, controlling the fate of subducting carbonate rich crust and mantle fertilization processes. Theoretical and experimental observations indicate that magnesite is the product of reaction of carbonates with silicate minerals, and it has been found to be stable within lower mantle conditions. A siderite component is likely to be present at mantle conditions in equilibrium with the surrounding iron bearing phases. We conducted compression experiments on natural single crystals of magnesite and Mg-siderite. By using neon as pressure transmitting medium we collected diffraction data in nearly hydrostatic conditions up to ~ 75GPa. The compression curve of magnesite is in good agreement with literature data. Siderite shows a steep volume decrease between 40 and 50 GPa which is most likely related to the change of ionic size associated with the spin pairing reported in siderite at ~ 50GPa. Siderites single crystals, about 10μ thick, were initially colorless and assumed a green color, progressively deeper, after ~ 40GPa and turned into red at the highest pressure. The color variations were reversible, quantitative absorption data are going to be presented. Siderite is not a major phase of the earth's mantle, therefore its density and optical properties are not likely to significantly affect the earth's density profile or the thermal equilibrium, however it might help to learn in detail the effect of spin transition on mineral physical properties.