O. Narygina

Body-centered cubic iron-nickel alloy in Earth's core.

Cosmochemical, geochemical, and geophysical studies provide evidence that Earths core contains iron with substantial (5 to 15%) amounts of nickel. The iron-nickel alloy Fe0.9Ni0.1 has been studied in situ by means of angle-dispersive x-ray diffraction in internally heated diamond anvil cells (DACs), and its resistance has been measured as a function of pressure and temperature. At pressures above 225 gigapascals and temperatures over 3400 kelvin, Fe0.9Ni0.1 adopts a body-centered cubic structure. Our experimental and theoretical results not only support the interpretation of shockwave data on pure iron as showing a solid-solid phase transition above about 200 gigapascals, but also suggest that iron alloys with geochemically reasonable compositions (that is, with substantial nickel, sulfur, or silicon content) adopt the bcc structure in Earths inner core.

Optical absorption and radiative thermal conductivity of silicate perovskite to 125 gigapascals.

Mantle convection and plate tectonics are driven by the heat flow from Earths core to the surface. The radiative contribution to heat transport is usually assumed to be negligible. Here, we report the near-infrared and optical absorption spectra of silicate perovskite, the main constituent of the lower mantle, to 125 gigapascals. Silicate perovskite remains quite transparent up to the pressures at the core-mantle boundary. Estimates of radiative thermal conductivity derived from these spectra approach 10 watts meter–1 kelvin–1 at lowermost mantle conditions, implying that heat conduction is dominated by radiation. However, the increase in radiative conductivity with temperature (T) is less pronounced than expected from a T3 dependency.

Portable laser‐heating system for diamond anvil cells

The diamond anvil cell (DAC) technique coupled with laser heating has become the most successful method for studying materials in the multimegabar pressure range at high temperatures. However, so far all DAC laser-heating systems have been stationary: they are linked either to certain equipment or to a beamline. Here, a portable laser-heating system for DACs has been developed which can be moved between various analytical facilities, including transfer from in-house to a synchrotron or between synchrotron beamlines. Application of the system is demonstrated in an example of nuclear inelastic scattering measurements of ferropericlase (Mg(0.88)Fe(0.12))O and h.c.p.-Fe(0.9)Ni(0.1) alloy, and X-ray absorption near-edge spectroscopy of (Mg(0.85)Fe(0.15))SiO(3) majorite at high pressures and temperatures. Our results indicate that sound velocities of h.c.p.-Fe(0.9)Ni(0.1) at pressures up to 50 GPa and high temperatures do not follow a linear relation with density.

Importance of correlation effects in hcp iron revealed by a pressure-induced electronic topological transition

We discover that hcp phases of Fe and Fe(0.9)Ni(0.1) undergo an electronic topological transition at pressures of about 40 GPa. This topological change of the Fermi surface manifests itself through anomalous behavior of the Debye sound velocity, c/a lattice parameter ratio, and Mössbauer center shift observed in our experiments. First-principles simulations within the dynamic mean field approach demonstrate that the transition is induced by many-electron effects. It is absent in one-electron calculations and represents a clear signature of correlation effects in hcp Fe.

Iron spin state in silicate perovskite at conditions of the Earth's deep interior

We present a review of our recent work concerning the spin state of Fe2+ and Fe3+ in iron magnesium aluminium silicate perovskite, the most abundant phase in the Earths interior. Experimental results obtained using Mössbauer spectroscopy (with a radioactive source and a Synchrotron Mössbauer Source) and nuclear forward scattering for a range of different sample compositions in both externally heated and laser-heated diamond anvil cells show clear trends in the variation of hyperfine parameters with pressure and temperature. These trends combined with reported total spin state measurements using X-ray emission spectroscopy on samples of similar composition support the conclusion that Fe2+ undergoes a high-spin to intermediate-spin transition near the top of the lower mantle and an intermediate-spin to low-spin transition near the bottom of the lower mantle. No spin transition is observed to occur in Fe3+ for samples with compositions relevant for the lower mantle.

Hyperspectral μ-XANES mapping in the diamond-anvil cell: analytical procedure applied to the decomposition of (Mg,Fe)-ringwoodite at the upper/lower mantle boundary

We propose a new analytical approach to extract information from μ-X-ray Absorption Spectroscopy (XAS) based mapping techniques, where each pixel of a map contains full XANES (X-ray Absorption Near Edge Structure) information. Data reduction is performed thanks to specifically developed software, XASMAP, which allows automatic normalization, linear combination fits, map reconstruction and spectrum extraction. We illustrate an example of application on data acquired in a laser heated diamond-anvil cell, devoted to the investigation of the behavior of iron during the decomposition of (Mg,Fe)-ringwoodite into perovskite and ferropericlase at conditions relevant to Earth upper/lower mantle boundary. The analysis of 1600 Fe K-edge XANES spectra allows the reconstruction of maps based on iron-speciation, but it also drives to iron-concentration maps in a complex mixture of three crystalline phases. This analytical procedure opens the way to in situ studies at extreme conditions of pressure and temperature for the geosciences, physics and chemistry communities.

Development of micro-XANES mapping in the diamond anvil cell.

Energy-dispersive X-ray absorption spectroscopy is now a well established method that has been applied to a broad range of applications. At the energy-dispersive EXAFS beamline of the ESRF, ID24, the recently achieved 5 x 5 microm focal spot combined with fast acquisition has allowed complex and non-uniform samples to be mapped and images to be obtained where each pixel contains full XAS information. This method has been applied to a study under extreme conditions of pressure and temperature in a diamond anvil cell in transmission mode. The case study was the investigation of the Fe K-edge XANES of (Mg,Fe)SiO(3)-perovskite and (Mg,Fe)O-ferropericlase on decomposition of the spinel-structured olivine [gamma-(Mg,Fe)(2)SiO(4)] at 78 (3) GPa after laser heating at 2200 (100) K.

In situ high-pressure study of LiNbO3-type FeTiO3: X-ray diffraction and Mössbauer spectroscopy

The structure of LiNbO3-type FeTiO3 and the oxidation state of Fe have been investigated using X-ray diffraction and Mössbauer spectroscopy in the diamond anvil cell up to 18 GPa at room temperature. A structural phase transition is observed at 15.7 GPa from LiNbO3-type to perovskite-type, accompanied by a volume collapse of 1.5%. LiNbO3-type FeTiO3, which is shown to contain only ferrous iron up to this pressure, and no charge transfer is observed. In addition to the c/a axial ratio that has been used to distinguish between ilmenite and LiNbO3-type FeTiO3, the hyperfine parameters (isomer shift and quadrupole splitting) provide an efficient way to discriminate between these two phases.

Effect of Spin Transitions in Iron on Structure and Properties of Mantle Minerals

Abstract Iron is an important element in Earth lower mantle minerals. At conditions of the deep Earth’s interior iron ions could undergo high-to-low spin crossover. We discuss evolution of the spin state of iron in ferropericlase (Mg,Fe)O, silicate perovskite and garnet (Mg,Fe)SiO3 at high pressures and temperatures.