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Under high pressures the hydrogen bonds were predicted to transform from a highly asymmetric soft O-H center dot center dot center dot O to a symmetric rigid configuration in which the proton lies midway between the two oxygen atoms. Despite four decades of research on hydroxyl containing compounds, pressure induced hydrogen bond symmetrization remains elusive. Following single crystal x-ray diffraction, Mossbauer and Raman spectroscopy measurements supported by ab initio calculations, we report the H-bonds symmetrization in iron oxyhydroxide, FeOOH, resulting from the Fe3+ high-to-low spin crossover at above 45 GPa.

Relaxation phenomena show up in standard energy domain Mossbauer spectra via line broadening. The evaluation of such spectra is in most cases done by adopting the stochastic theory mainly developed in the 60s and 70s. Due to the time structure and the polarization of the synchrotron radiation nuclear resonance forward scattering in the time domain gives valuable information on relaxation mechanisms. We report here mainly on Nuclear Forward Scattering (NFS) experiments investigating the paramagnetic relaxation of the Fe3+ ion in (NH)(4)(Al0.95Fe0.05)-Fe-57(SO4)(2) . 12H(2)O and briefly on recent investigations on charge fluctuations in Eu3S4.

The Earth’s crust–mantle boundary, the Mohorovicic discontinuity, has been traditionally considered to be the interface between the magnetic crust and the non-magnetic mantle1. However, this assumption has been questioned by geophysical observations2,3 and by the identification of magnetic remanence in mantle xenoliths4, which suggest mantle magnetic sources. Owing to their high critical temperatures, iron oxides are the only potential sources of magnetic anomalies at mantle depths5. Haematite (α-Fe2O3) is the dominant iron oxide in subducted lithologies at depths of 300 to 600 kilometres, delineated by the thermal decomposition of magnetite and the crystallization of a high-pressure magnetite phase deeper than about 600 kilometres6. The lack of data on the magnetic properties of haematite at relevant pressure–temperature conditions, however, hinders the identification of magnetic boundaries within the mantle and their contribution to observed magnetic anomalies. Here we apply synchrotron Mossbauer source spectroscopy in laser-heated diamond anvil cells to investigate the magnetic transitions and critical temperatures in Fe2O3 polymorphs7 at pressures and temperatures of up to 90 gigapascals and 1,300 kelvin, respectively. Our results show that haematite remains magnetic at the depth of the transition zone in the Earth’s mantle in cold or very cold subduction geotherms, forming a frame of deep magnetized rocks in the West Pacific region. The deep magnetic sources spatially correlate with preferred paths of the Earth’s virtual geomagnetic poles during reversals8 that might not reflect the geometry of the transitional field. Rather, the paths might be an artefact caused by magnetized haematite-bearing rocks in cold subducting slabs at mid-transition zone depths. Such deep sources should be taken into account when carrying out inversions of the Earth’s geomagnetic data9, and especially in studies of planetary bodies that no longer have a dynamo10, such as Mars. Synchrotron Mossbauer source spectroscopy is used to reveal that haematite remains magnetic in cold subducting slabs at the depth of the transition zone in the Earth’s mantle, with implications for the locations of magnetic poles during inversions of the Earth’s magnetic field.

We have investigated the electronic configuration of iron in Fe-, Al-containing magnesium silicate perovskite, i.e., bridgmanite, the main component of the lower mantle, at conditions of the deep Earth's interior using the energy domain Synchrotron Mossbauer Source technique. We show that the high ferric iron content observed previously in quenched samples is preserved at high temperatures and high pressures. Our data are consistent with high-spin to intermediate-spin (HS-IS) crossover in Fe2+ at high pressures and ambient temperature. We see no evidence of spin crossover in Fe3+ occupying the A-position of bridgmanite. On laser heating at pressures above ∼40 GPa we observe a new doublet with relative area below 5% which is assigned to Fe3+ in the octahedral (B-site) position in bridgmanite. We conclude that at lower mantle conditions Fe3+ remains predominantly in the HS state, while Fe2+ occurs solely in the IS state.

Nature Communications 9(1), 4142 (2018). doi:10.1038/s41467-018-06457-x, A Verwey-type charge-ordering transition in magnetite at 120 K leads to the formation of linear units of three iron ions with one shared electron, called trimerons. The recently-discovered iron pentoxide (Fe$_4$O$_5$) comprising mixed-valent iron cations at octahedral chains, demonstrates another unusual charge-ordering transition at 150 K involving competing formation of iron trimerons and dimerons. Here, we experimentally show that applied pressure can tune the charge-ordering pattern in Fe$_4$O$_5$ and strongly affect the ordering temperature. We report two charge-ordered phases, the first of which may comprise both dimeron and trimeron units, whereas, the second exhibits an overall dimerization involving both the octahedral and trigonal-prismatic chains of iron in the crystal structure. We link the dramatic change in the charge-ordering pattern in the second phase to redistribution of electrons between the octahedral and prismatic iron chains, and propose that the average oxidation state of the iron cations can pre-determine a charge-ordering pattern., Published by Nature Publishing Group UK, [London]

Before the Archean to Proterozoic Transition (APT) the tectonic regime was dominated by microplates floating on a low viscosity mantle. Such a regime restricted chemical exchange between the shallow and deeper mantle reservoirs. After the APT, a more global convection regime led to deep subduction of slabs. We propose that the improved vertical mixing of the mantle favoured the release to the Earth's surface of an oxygen excess initially trapped in the deep mantle. This excess built up when the primordial lower mantle was left with a high Fe3+/(Fe2++Fe3+) ratio (#Fe3+), after metallic iron segregated down into the core. Our synchrotron-based in situ experiments suggest a primordial Fe3+excess of ∼20 % for the mantle iron. By comparison with the #Fe3+ of the present mantle, this Fe3+excess would correspond to 500-1000 times the O2 content in the Earth's atmosphere. The tectonic transition greatly facilitated the ascent of oxidised lower mantle material towards the Earth's surface, inducing a continuous arrival of O2 at the Earth's surface and into the atmosphere.

Geochemical, cosmochemical, geophysical, and mineral physics data suggest that iron (or iron-nickel alloy) is the main component of the Earth's core. The inconsistency between the density of pure iron at pressure and temperature conditions of the Earth's core and seismological observations can be explained by the presence of light elements. However, the low shear wave velocity and high Poisson's ratio of the Earth's core remain enigmatic. Here we experimentally investigate the effect of carbon on the elastic properties of iron at high pressures and temperatures and report a high-pressure orthorhombic phase of iron carbide, Fe7C3. We determined the crystal structure of the material at ambient conditions and investigated its stability and behaviour at pressures up to 205 GPa and temperatures above 3,700 K using single-crystal and powder X-ray diffraction, Mössbauer spectroscopy, and nuclear inelastic scattering. Estimated shear wave and compressional wave velocities show that Fe7C3 exhibits a lower shear wave velocity than pure iron and a Poisson's ratio similar to that of the Earth's inner core. We suggest that carbon alloying significantly modifies the properties of iron at extreme conditions to approach the elastic behaviour of rubber. Thus, the presence of carbon may explain the anomalous elastic properties of the Earth's core.

We describe a new methodology to collect energy domain Mossbauer spectra of inclusions in natural diamonds using a Synchrotron Mossbauer Source (SMS). Measurements were carried out at the Nuclear Resonance beamline ID18 at the European Synchrotron Radiation Facility (Grenoble, France). We applied this non-destructive approach to collect SMS spectra of a ferropericlase inclusion still contained within its diamond host from Juina (Brazil). The high spatial resolution of the measurement (~ 15 μm) enabled multiple regions of the 190 × 105 μm 2 inclusion to be sampled and showed that while Fe 3 + /Fe tot values in ferropericlase were below the detection limit (0.02) overall, there was a magnetic component whose abundance varied systematically across the inclusion. Hyperfine parameters of the magnetic component are consistent with magnesioferrite, and the absence of superparamagnetism allows the minimum particle size to be estimated as ~ 30 nm. Bulk Fe 3 + /Fe tot values are similar to those reported for other ferropericlase inclusions from Juina, and their variation across the inclusion can provide constraints on its history.

The presence of carbonates in inclusions in diamonds coming from depths exceeding 670 km are obvious evidence that carbonates exist in the Earth’s lower mantle. However, their range of stability, crystal structures and the thermodynamic conditions of the decarbonation processes remain poorly constrained. Here we investigate the behaviour of pure iron carbonate at pressures over 100 GPa and temperatures over 2,500 K using single-crystal X-ray diffraction and Mössbauer spectroscopy in laser-heated diamond anvil cells. On heating to temperatures of the Earth’s geotherm at pressures to ∼50 GPa FeCO3 partially dissociates to form various iron oxides. At higher pressures FeCO3 forms two new structures—tetrairon(III) orthocarbonate Fe43+C3O12, and diiron(II) diiron(III) tetracarbonate Fe22+Fe23+C4O13, both phases containing CO4 tetrahedra. Fe4C4O13 is stable at conditions along the entire geotherm to depths of at least 2,500 km, thus demonstrating that self-oxidation-reduction reactions can preserve carbonates in the Earth’s lower mantle., Carbonates are shown to exist in the lower mantle as seen in diamond inclusions, but thermodynamic constraints are poorly understood. Here, the authors synthesise two new iron carbonate compounds and find that self-oxidation-reduction reactions can preserve carbonates in the mantle.

Review of scientific instruments 88(8), 084501 - (2017). doi:10.1063/1.4998985, A portable double-sided pulsed laser heating system for diamond anvil cells has been developed that is able to stably produce laser pulses as short as a few microseconds with repetition frequencies up to 100 kHz. In situ temperature determination is possible by collecting and fitting the thermal radiation spectrum for a specific wavelength range (particularly, between 650 nm and 850 nm) to the Planck radiation function. Surface temperature information can also be time-resolved by using a gated detector that is synchronized with the laser pulse modulation and space-resolved with the implementation of a multi-point thermal radiation collection technique. The system can be easily coupled with equipment at synchrotron facilities, particularly for nuclear resonance spectroscopy experiments. Examples of applications include investigations of high-pressure high-temperature behavior of iron oxides, both in house and at the European Synchrotron Radiation Facility using the synchrotron Mössbauer source and nuclear inelastic scattering., Published by American Institute of Physics, [S.l.]

Physical review / B covering condensed matter and materials physics 93(20), 201110(2016). doi:10.1103/PhysRevB.93.201110, Magnetic properties of NiO have been studied in the multimegabar pressure range by nuclear forward scattering of synchrotron radiation using the 67.4 keV Mössbauer transition of $^{61}$Ni. The observed magnetic hyperfine splitting confirms the antiferromagnetic state of NiO up to 280 GPa, the highest pressure where magnetism has been observed so far, in any material. Remarkably, the hyperfine field increases from 8.47 T at ambient pressure to ∼ 24 T at the highest pressure, ruling out the possibility of a magnetic collapse. A joint x-ray diffraction and extended x-ray-absorption fine structure investigation reveals that NiO remains in a distorted sodium chloride structure in the entire studied pressure range. Ab initio calculations support the experimental observations, and further indicate a complete absence of Mott transition in NiO up to at least 280 GPa., Published by Inst., Woodbury, NY

Review of scientific instruments 86(11), 114501 - (2015). doi:10.1063/1.4935304, Developments in pulsed laser heating applied to nuclear resonance techniques are presented together with their applications to studies of geophysically relevant materials. Continuous laser heating in diamond anvil cells is a widely used method to generate extreme temperatures at static high pressure conditions in order to study the structure and properties of materials found in deep planetary interiors. The pulsed laser heating technique has advantages over continuous heating, including prevention of the spreading of heated sample and/or the pressure medium and, thus, a better stability of the heating process. Time differentiated data acquisition coupled with pulsed laser heating in diamond anvil cells was successfully tested at the Nuclear Resonance beamline (ID18) of the European Synchrotron Radiation Facility. We show examples applying the method to investigation of an assemblage containing ε-Fe, FeO, and Fe$_3$C using synchrotron Mössbauer source spectroscopy, FeCO$_3$ using nuclear inelastic scattering, and Fe$_2$O$_3$ using nuclear forward scattering. These examples demonstrate the applicability of pulsed laser heating in diamond anvil cells to spectroscopic techniques with long data acquisition times, because it enables stable pulsed heating with data collection at specific time intervals that are synchronized with laser pulses., Published by American Institute of Physics, [S.l.]