Search

The melting curve of H2O has been measured by in situ Raman spectroscopy in an externally heated diamond anvil cell up to 22 GPa and 900 K. The Raman-active OH-stretching bands and the translational modes of H2O as well as optical observations are used to directly and reliably detect melting in ice VII. The observed melting temperatures are higher than previously reported x-ray measurements and significantly lower than recent laser-heating determinations. However, our results are in accord with earlier optical determinations. The frequencies and intensities of the OH-stretching peaks change significantly across the melting line while the translational mode disappears altogether in the liquid phase. The observed OH-stretching bands of liquid water at high pressure are very similar to those obtained in shock-wave Raman measurements. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In situ high P-T Raman spectra of solid CO2 up to 67 GPa and 1660 K have been measured, using a micro-optical spectroscopy system coupled with a Nd:YLF laser heating system in diamond anvil cells. A metallic foil was employed to efficiently absorb the incoming Nd:YLF laser and heat the sample. The average sample temperature was accurately determined by detailed balance from the anti-Stokes/Stokes ratio, and was compared to the temperature of the absorber determined by fitting the thermal radiation spectrum to the Planck radiation law. The transformation temperature threshold and the transformation dynamics from the molecular phases III and II to the polymeric phase V, previously investigated only by means of temperature quench experiments, was determined at different pressures. The P-T range of the transformation, between 640 and 1100 K in the 33–65 GPa pressure interval, was assessed to be a kinetic barrier rather than a phase boundary. These findings lead to a new interpretation of the high P-T phase diagram of carbon dioxide. Furthermore, our approach opens a new way to perform quantitative in situ Raman measurements under extremely high pressures and temperatures, providing unique information about phase relations and structural and thermodynamic properties of materials under these conditions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The concentrations of Fe, Al, and Ni and their distributions were determined for all known natural assemblages of ferropericlase (fPer) and bridgmanite (Bridg), coexisting as inclusions in deep-mantle diamonds from Brazil, Canada, Guinea, and South Australia. Based upon these data, it is likely that some areas within the deep lower mantle are iron-rich and differ markedly from a pyrolitic composition. In the lowermost lower mantle, Bridg is Al-rich and fPer is Ni-poor, witnessing the presence of a free metallic phase in the mineral-forming environment. The iron partitioning in the Bridg + fPer association … in juvenile diamond inclusions is as low as 0.1-0.2. During ascent of the diamonds with their inclusions to the surface, the … eventually increases to values of 0.4-0.5 and even as high as 0.7. The details of the element partitioning between natural Bridg and fPer in the lower mantle are as follows: iron in Bridg is ferrous Fe2+ in the A site, substituting for Mg2+; almost all iron in fPer is ferrous Fe2+; the share of ferric Fe3+ iron in fPer is Fe3+/ΣFe = 8-12 at %; iron concentrations in both Bridg and fPer increase with depth (pressure), reflecting the increasing Fe content in the lower part of the lower mantle, different from that of a pyrolitic model. Al in Bridg is mainly in the cation site B and partly in the cation site A, in both cases substituting for Si, Mg, and Fe with vacancy formation; and in the case of Al positioning into both B and A sites, a charge-balanced reaction occurs. The natural samples show very diverse … values and elemental distribution that cannot be simply explained by our current understanding on alumina dissolution in Bridg and the spin transition of Fe2+ in fPer. These major differences between experimental results and observations in natural samples demonstrate the complex, inhomogeneous iron speciation and chemistry in the lower mantle. [ABSTRACT FROM AUTHOR]

The article comments on a study on the use of a laser to generate conditions similar to those inside the cores of Earth-like exoplanets, which references a research paper by R. G. Kraus et al., published within the issue of "Science." Topics discussed include the crystallization of molten iron under extreme conditions, factors for habitability of exoplanets, and theory on the formation of Earth-like exoplanets.

Magnetite exhibits unique electronic, magnetic, and structural properties in extreme conditions that are of great research interest. Previous studies have suggested a number of transitional models, although the nature of magnetite at high pressure remains elusive. We have studied a highly stoichiometric magnetite using inelastic X-ray scattering, X-ray diffraction and emission, and Raman spectroscopies in diamond anvil cells up to ~20 GPa, while complementary electrical conductivity measurements were conducted in a cubic anvil cell up to 8.5 GPa. We have observed an elastic softening in the diagonal elastic constants (C11 and C44) and a hardening in the off-diagonal constant (C12) at ~8 GPa where significant elastic anisotropies in longitudinal and transverse acoustic waves occur, especially along the [110] direction. An additional vibrational Raman band between the A1g and T2g modes was also detected at the transition pressure. These abnormal elastic and vibrational behaviors of magnetite are attributed to the occurrence of the octahedrally-coordinated Fe2+-Fe3+-Fe2+ ions charge-ordering along the [110] direction in the inverse spinel structure. We propose a new phase diagram of magnetite in which the temperature for the metal-insulator and distorted structural transitions decreases with increasing pressure while the charge-ordering transition occurs at ~8 GPa and room temperature. [ABSTRACT FROM AUTHOR]

Studying the velocity-density profiles of iron and iron-silicon alloy at high pressures and temperatures is critical for understanding the Earth's core as well as the interiors of other planetary bodies. Here we have investigated the compressional wave velocity (VP) and density (p) profiles of polycrystalline bcc-Fe and Fe0.85Si0.15 alloy (8 wt.% Si) using in situ high-energy resolution inelastic X-ray scattering (HERIX) and synchrotron X-ray diffraction spectroscopies in an externally-heated diamond anvil cell (EHDAC) up to 15 GPa and 700 K. Based on the measured velocity-density (VP-p) and velocity-pressure (VP-P) relations of bcc-Fe at simultaneous high pressure and temperature (P-T) conditions, our results show a strong VP reduction at elevated temperatures at a constant density. Comparison of the VP-p profiles between the bcc-Fe and bcc-Fe0.85Si0.15 alloy indicates that the alloying effect of additional 8 wt.% Si on the VP-p relationship of bcc-Fe is predominant via a constant density decrease of approximately 0.6 g/cm³ (7%). Compared with the literature velocity results for bcc and hcp Fe-Si alloys, the bcc-Fe and Fe-Si alloys exhibit higher VP than their hcp phase counterparts at the given bcc-hcp transition pressures. Our results here strongly support the notion that high temperature has a strong effect on the VP of Fe and that the VP-p profile of Fe can be affected by structural and magnetic transitions. Analyses on literature elastic constants of the bcc Fe-Si alloys, as a function of P-T and Si content, show that the bcc phase displays extremely high VP anisotropy of 16-30% and VS splitting anisotropy of 40-90% at high temperatures, while the addition of Si further enhances the anisotropy. Due to the extremely high elastic anisotropy of the bcc Fe-Si alloy, a certain portion of the bcc Fe-Si alloy with the lattice-preferred orientation may produce VP and VS anisotropies to potentially account for the observed seismic anisotropy in the inner core. [ABSTRACT FROM AUTHOR]

High-pressure synchrotron Mössbauer (SMS) and X-ray emission (XES) spectroscopic measurements were conducted to investigate the spin and valence states of iron in (Al,Fe)-bearing magnesium silicate glass (Mg0.79Fe0.10Al0.10Si0.96O3) up to 126 GPa and 300 K. By analyzing the Fe Kβ 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) mm/s 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 ~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 ~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. [ABSTRACT FROM AUTHOR]

In situ synchrotron X-ray diffraction experiments on natural magnesiosiderite [(Mg0.35Fe0.65)CO3] were conducted using resistive and laser-heated diamond-anvil cells (DACs) up to 78 GPa and 1200 K. Based on thermal elastic modeling of the measured pressure-volume curves at given temperatures, we have derived thermal equation of state (EoS) parameters and the spin-crossover diagram of magnesiosiderite across the spin transition. These results show the spin crossover broadened and shifted toward higher pressures at elevated temperatures. Low-spin magnesiosiderite is 6% denser and 8% more incompressible than the high-spin phase at 1200 K and high pressures. Within the spin crossover from 53 to 63 GPa at 1200 K, magnesiosiderite exhibits anomalous thermal elastic behaviors, including a dramatic increase in the thermal expansion coefficient by a factor of 20 and a drop in the isothermal bulk modulus and the bulk sound velocity by approximately 75 and 50%, respectively. Compared with the end-member magnesite [MgCO3] at relevant pressure-temperature conditions of the subducted slabs, the high-spin magnesiosiderite with 65 mol% FeCO3 is approximately 21-23% denser and its unit-cell volume is 2^1% larger, whereas the low-spin state is 28-29% denser and 2% smaller than the end-member magnesite. Since ferromagnesite with 20 mol% of iron has been proposed to be a potential deep-carbon carrier, our results here indicate that the dense low-spin ferromagnesite can become more stable than high-spin ferromagnesite at pressures above approximately 50 GPa, providing a mechanism for (MgFe)-bearing carbonate to be a major carbon host in the deeper lower mantle. [ABSTRACT FROM AUTHOR]

The recent discovery of iron ferropnictide superconductors has received intensive concern in connection with magnetically involved superconductors. Prominent features of ferropnictide superconductors are becoming apparent: the parent compounds exhibit an antiferromagnetic ordered spin density wave (SDW) state, the magnetic-phase transition is always accompanied by a crystal structural transition, and superconductivity can be induced by suppressing the SDW phase via either chemical doping or applied external pressure to the parent state. These features generated considerable interest in the interplay between magnetism and structure in chemically doped samples, showing crystal structure transitions always precede or coincide with magnetic transition. Pressure-tuned transition, on the other hand, would be more straightforward to superconducting mechanism studies because there are no disorder effects caused by chemical doping; however, remarkably little is known about the interplay in the parent compounds under controlled pressure due to the experimental challenge of in situ measuring both of magnetic and crystal structure evolution at high pressure and low temperatures. Here we show from combined synchrotron Mössbauer and X-ray diffraction at high pressures that the magnetic ordering surprisingly precedes the structural transition at high pressures in the parent compound BaFe2As2, in sharp contrast to the chemical-doping case. The results can be well understood in terms of the spin fluctuations in the emerging nematic phase before the long-range magnetic order that sheds light on understanding how the parent compound evolves from a SDW state to a superconducting phase, a key scientific inquiry of iron-based superconductors. [ABSTRACT FROM AUTHOR]

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 (Mg0.75Fe0.25)2SiO4 using synchrotron Mössbauer spec-troscopy 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 Mössbauer spectra at ambient conditions reveal two non-equivalent iron species, (Fe2+)1 and (Fe2+)2, which can be attributed to octahedral and tetrahedral sites in the cubic spinel structure, respectively. High-pressure Mössbauer measurements show the disappearance of the hyperfine quadrupole splitting (QS) of the Fe2+ 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. [ABSTRACT FROM AUTHOR]

Compressional wave velocity-density (VP - p) relations of candidate Fe alloys at relevant pressure-temperature conditions of the Earth's core are critically needed to evaluate the composition, seismic signatures, and geodynamics of the planet's remotest region. Specifically, comparison between seismic VP - &rgr; profiles of the core and candidate Fe alloys provides first-order information on the amount and type of potential light elements--including H, C, O, Si, and/or S--needed to compensate the density deficit of the core. To address this issue, here we have surveyed and analyzed the literature results in conjunction with newly measured VP-p results of hexagonal closest- packed (hep) Fe and hcp-Fe0.85Si0.15 alloy using in situ highenergy resolution inelastic X-ray scattering and X-ray diffraction. The nature of the Fe-Si alloy where Si is readily soluble in Fe represents an ideal solid-solution case to better understand the lightelement alloying effects. Our results show that high temperature significantly decreases the VP of hcp-Fe at high pressures, and the Fe-Si alloy exhibits similar high-pressure VP - &rgr; behavior to hcp-Fe via a constant density offset. These VP-p data at a given temperature can be better described by an empirical power-law function with a concave behavior at higher densities than with a linear approximation. Our new datasets, together with literature results, allow us to build new VP - &rgr; models of Fe alloys in order to determine the chemical composition of the core. Our models show that the VP -p profile of Fe with 8 wt % Si at 6,000 Κ matches well with the Preliminary Reference Earth Model of the inner core. [ABSTRACT FROM AUTHOR]

The electronic spin and valence states of iron in lower-mantle silicate perovskite have been previously investigated at high pressures using various experimental and theoretical techniques. However, experimental results and their interpretation remain highly debated. Here we have studied a well-characterized silicate perovskite starting sample [(Mg0.9,Fe0.1)SiO3] in a chemically inert Ne pressure medium at pressures up to 120 GPa using synchrotron Mössbauer spectra. Analyses of the Mössbauer spectra explicitly show a high-spin to low-spin transition of the octahedral-site Fe3+ occurring at ~13--24 GPa, as evidenced from a significant increase in the hyperfine quadrupole splitting. Two quadrupole doublets of the A-site Fe2+, with extremely high-QS values of 4.1 and 3.1 mm/s, occur simultaneously with the spin transition of the octahedral-site Fe3+ and continue to develop to 120 GPa. It is conceivable that the spin-pairing transition of the octahedral-site Fe3+ causes a volume reduction and a change in the local atomic-site configurations that result in a significant increase of the quadrupole splitting in the dodecahedral-site Fe2+ at 13--24 GPa. Our results here provide a coherent explanation for recent experimental and theoretical results on the spin and valence states of iron in perovskite, and assist in comprehending the effects of the spin and valence states of iron on the properties of the lower-mantle minerals. [ABSTRACT FROM AUTHOR]

Ferromagnesite [(Mg,Fe)CO3] has been proposed as a candidate host mineral for carbon in the Earth's mantle. Studying its physical and chemical properties at relevant pressures and temperatures helps our understanding of deep-carbon storage in the planet's interior and on its surface. Here we have studied high-pressure vibrational and elastic properties of magnesian siderite [(Mg0.35Fe0.65)CO3] across the electronic spin transition by Raman and X-ray diffraction spectroscopies in a diamond-anvil cell. Our results show an increase in Raman shift of the observed lattice modes of magnesian siderite across the spin transition at 45 GPa as a result of an ~8% unit-cell volume collapse and a 10% stiffer lattice (higher bulk modulus). C-O bond lengthening in the strong, rigid (CO3)2- unit across the spin transition contributes to a competitive decrease in Raman shift, most evident in the Raman shift decrease of the symmetric stretching mode. Combined vibrational and elastic results are used to derive the mode Grüneisen parameter of each mode, which drops significantly across the transition. These results suggest that the low-spin state has distinctive vibrational and elastic properties compared to the high-spin state. Analyses of all recent experimental results on the (Mg,Fe)CO3 system show no appreciable compositional effect on the transition pressure, indicating weak iron-iron exchange interactions. Our results provide new insight into understanding the effects of the spin transition on the vibrational, elastic, and thermodynamic properties of (Mg,Fe)CO3 as a candidate carbon-host in the deep mantle. [ABSTRACT FROM AUTHOR]

High-pressure phonon density of states (PDOS) of Fe2O3 across structural and electronic transitions has been investigated by nuclear resonant inelastic x-ray scattering (NRIXS) and first-principles calculations together with synchrotron Mössbauer, x-ray diffraction, and x-ray emission spectroscopies. Drastic changes in elastic, thermodynamic, and vibrational properties of Fe2O3 occur across the Rh2O3(II)-type structural transition at 40-50 GPa, whereas the Mott insulator-metal transition occurring after the structural transition only causes nominal changes in the properties of the Fe2O3. The observed anomalous mode-softening behavior of the elastic constants is associated with the structural transition at 40-50 GPa, leading to substantial changes in the Debye-like part of the PDOS in the terahertz acoustic phonons. Our experimental and theoretical studies provide new insights into the effects of the structural and electronic transitions in the transition-metal oxide (TMO) compounds. [ABSTRACT FROM AUTHOR]

Abstract  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, incompressibility, radiative thermal conductivity, electrical conductivity, 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 ferropericlase, (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} 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 ψ angle between the compression direction 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 transition 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. [ABSTRACT FROM AUTHOR]

Silicate melts at the top of the transition zone and the core-mantle boundary have significant influences on the dynamics and properties of Earth's interior. MgSiO3-rich silicate melts were among the primary components of the magma ocean and thus played essential roles in the chemical differentiation of the early Earth. Diverse macroscopic properties of silicate melts in Earth's interior, such as density, viscosity, and crystal-melt partitioning, depend on their electronic and short-range local structures at high pressures and temperatures. Despite essential roles of silicate melts in many geophysical and geodynamic problems, little is known about their nature under the conditions of Earth's interior, including the densification mechanisms and the atomistic origins of the macroscopic properties at high pressures. Here, we have probed local electronic structures of MgSiO3 glass (as a precursor to Mg-silicate melts), using high-pressure x-ray Raman spectroscopy up to 39 GPa, in which high-pressure oxygen K-edge features suggest the formation of tricluster oxygens (oxygen coordinated with three Si frameworks; [3]O) between 12 and 20 GPa. Our results indicate that the densification in MgSiO3 melt is thus likely to be accompanied with the formation of triculster, in addition to a reduction in nonbridging oxygens. The pressure-induced increase in the fraction of oxygen triclusters >20 GPa would result in enhanced density, viscosity, and crystal-melt partitioning, and reduced element diffusivity in the MgSiO3 melt toward deeper part of the Earth's lower mantle. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Synchrotron-based X-ray emission spectroscopy (XES) is well suited to probing the local electronic structure of 3 d transition metals such as Fe and Mn in their host phases. The laser-heated diamond anvil cell technique is uniquely capable of generating ultra-high static pressures and temperatures in excess of 100 GPa and 3000 K. Here X-ray emission spectroscopy and X-ray diffraction have been interfaced with the laser-heated diamond cell for studying the electronic spin states of iron in magnesiowüstite-(Mg0.75,Fe0.25)O and its crystal structure under lower-mantle conditions. X-ray emission spectra of the ferrous iron in a single crystal of magnesiowüstite-(Mg0.75,Fe0.25)O indicate that a high-spin to low-spin transition of ferrous iron occurs at 54 to 67 GPa and 300 K and the ferrous iron remains in the high-spin state up to 47 GPa and 1300 K. This pilot study points to the unique capability of the synchrotron-based XES and X-ray diffraction techniques for addressing the issue of electronic spin transition or crossover in 3 d transition metals and compounds under extreme high- P–T conditions. [ABSTRACT FROM AUTHOR]

This study demonstrates the use of monochromatic synchrotron X-ray radiation of 40 keV for high-precision equation-of-state studies on sets of single crystals analysed individually in the same diamond-anvil pressure cell. Angle-dispersive zone-axis diffraction patterns were obtained from crystals of wustite-Fe0.93O and magnesiowüstite-(Mg0.73Fe0.27)O to 51 GPa in a hydrostatic helium pressure medium. The rhombohedral phase of Fe0.93O was observed above 23 GPa, and its isothermal bulk modulus ( K0) was determined to be 134 (±4) GPa, assuming K′ = 4. The rhombohedral phase of Fe0.93O is more compressible than B1-structured Fe0.93O, with K0 = 146 (±2) GPa. Magnesiowüstite-(Mg0.73Fe0.27)O remains cubic over the experimental pressure range, and has a bulk modulus of 154 (±3) GPa with K′ = 4.0 (±0.1). [ABSTRACT FROM AUTHOR]

Iron is the most abundant transition-metal element in the mantle and therefore plays an important role in the geochemistry and geodynamics of the Earth's interior. Pressure-induced electronic spin transitions of iron occur in magnesiowüstite, silicate perovskite and post-perovskite. Here we have studied the spin states of iron in magnesiowüstite and the isolated effects of the electronic transitions on the elasticity of magnesiowüstite with in situ X-ray emission spectroscopy and X-ray diffraction to pressures of the lowermost mantle. An observed high-spin to low-spin transition of iron in magnesiowüstite results in an abnormal compressional behaviour between the high-spin and the low-spin states. The high-pressure, low-spin state exhibits a much higher bulk modulus and bulk sound velocity than the low-pressure, high-spin state; the bulk modulus jumps by ∼35 per cent and bulk sound velocity increases by ∼15 per cent across the transition in (Mg0.83,Fe0.17)O. Although no significant density change is observed across the electronic transition, the jump in the sound velocities and the bulk modulus across the transition provides an additional explanation for the seismic wave heterogeneity in the lowermost mantle. The transition also affects current interpretations of the geophysical and geochemical models using extrapolated or calculated thermal equation-of-state data without considering the effects of the electronic transition. [ABSTRACT FROM AUTHOR]

We describe an in situ high pressure-temperature Raman technique for studying materials in laser-heated diamond anvil cells using a Nd:YLF laser (1053 nm) as the heating source and an ion laser as the Raman exciting source. Here we introduce the method of laser heating transparent samples using a metallic foil (Pt,Re, or W) as the laser absorber (internal heating furnace) in a diamond cell. The YLF laser is used to effectively laser-heat one side of a metal foil 5–15 μm thick with a small hole of 10–20 μm in diameter at the center. The foil, in turn, heats a transparent sample while the Raman signals excited by an Ar+ or Kr+ laser are measured. Temperature of the laser-heated foil is measured by means of spectroradiometry whereas the average temperature of the heated sample is independently determined from the intensity ratios of the anti-Stokes/Stokes excitation pairs. The intrinsic temperature-dependent asymmetry of the Raman spectra arises from the principle of the detailed balance and is independent of sample properties other than the temperatures. The average determined by the signal-to-noise ratio of anti-Stokes/Stokes excitation pairs gives the sample temperature with the statistical accuracy of the Raman spectra. Transparent samples such as CO2 have been heated up to 1600 K and 65 GPa and Raman spectra have been measured with temperature uncertainty of 50–100 K. In situ Raman spectroscopy by laser heating represents a powerful technique to characterize high pressure-temperature properties of materials including molecular systems present in planetary interiors. [ABSTRACT FROM AUTHOR]

Magnesiowüstite [(Mg,Fe)O] is the second most abundant mineral of Earth's lower mantle. Understanding its stability under lower mantle conditions is crucial for interpreting the physical and chemical properties of the whole Earth. Previous studies in an externally heated diamond anvil cell suggested that magnesiowüstites decompose into two components, Fe-rich and Mg-rich magnesiowüstites at 86 GPa and 1,000 K. Here we report an in situ study of two magnesiowüstites [(Mg[sub 0.39], Fe[sub 0.61])O and (Mg[sub 0.25],Fe[sub 0.75])O] at pressures and temperatures that overlap with mantle conditions, using a laser-heated diamond anvil cell combined with synchrotron x-ray diffraction. Our results show that addition of Mg in wüstite (FeO) can stabilize the rock-salt structure to much higher pressures and temperatures. In contrast to the previous studies, our results indicate that Mg-rich magnesiowüstite is stable in the rock-salt structure in the lower mantle. The physical and chemical properties of magnesiowüstite should change gradually and continuously in the lower mantle, suggesting that it does not make a significant contribution to seismic-wave heterogeneity of the lower mantle. Stable Mg-rich magnesiowüstite in lower-most mantle can destabilize FeO in the core-mantle boundary region and remove FeO from the outer core. [ABSTRACT FROM AUTHOR]

Mars experienced a dynamo process that generated a global magnetic field ~4.3 (or earlier) to 3.6 billion years ago (Ga). The cessation of this dynamo strongly affected Mars' history and is expected to be linked to thermochemical evolution of Mars' iron-rich liquid core, which is strongly influenced by its thermal conductivity. Here, we directly measured thermal conductivities of solid iron-sulfur alloys to pressures relevant to the Martian core and temperatures to 1023 Kelvin. Our results show that a Martian core with 16 weight % sulfur has a thermal conductivity of ~19 to 32 Watt meter-1 Kelvin-1 from its top to the center, much higher than previously inferred from electrical resistivity measurements. Our modeled thermal conductivity profile throughout the Martian deep-mantle and core indicates a ~4-to 6-fold discontinuity across the core-mantle boundary. The core's efficient cooling resulting from the depth-dependent, high conductivity diminishes thermal convection and forms thermal stratification, substantially contributing to cessation of Martian dynamo. [ABSTRACT FROM AUTHOR]

Earth’s inner core is predominantly composed of solid iron (Fe) and displays intriguing properties such as strong shear softening and an ultrahigh Poisson’s ratio. Insofar, physical mechanisms to explain these features coherently remain highly debated. Here, we have studied longitudinal and shear wave velocities of hcp-Fe (hexagonal close-packed iron) at relevant pressure–temperature conditions of the inner core using in situ shock experiments and machine learning molecular dynamics (MLMD) simulations. Our results demonstrate that the shear wave velocity of hcp-Fe along the Hugoniot in the premelting condition, defined as T/Tm (Tm: melting temperature of iron) above 0.96, is significantly reduced by ~30%, while Poisson’s ratio jumps to approximately 0.44. MLMD simulations at 230 to 330 GPa indicate that collective motion with fast diffusive atomic migration occurs in premelting hcp-Fe primarily along [100] or [010] crystallographic direction, contributing to its elastic softening and enhanced Poisson’s ratio. Our study reveals that hcp-Fe atoms can diffusively migrate to neighboring positions, forming open-loop and close-loop clusters in the inner core conditions. Hcp-Fe with collective motion at the inner core conditions is thus not an ideal solid previously believed. The premelting hcp-Fe with collective motion behaves like an extremely soft solid with an ultralow shear modulus and an ultrahigh Poisson’s ratio that are consistent with seismic observations of the region. Our findings indicate that premelting hcp-Fe with fast diffusive motion represents the underlying physical mechanism to help explain the unique seismic and geodynamic features of the inner core. [ABSTRACT FROM AUTHOR]

Electronic states of iron in Earth's mantle minerals including ferropericlase, silicate perovskite, and post-perovskite have been previously investigated at high pressures and/or temperatures using various experimental techniques, including X-ray emission and Mössbauer spectroscopies. Although such methods have been used to infer changes in the electronic spin and valence states of iron in lower mantle minerals, they do not directly probe the 3d electronic states quantitatively. Here we use 1s2p resonant X-ray emission spectroscopy (RXES) at the Fe K pre-edge to directly probe and assess the 3d electronic states and the crystal-field splittings of Fe2+ in the lower-mantle ferropericlase [(Mg0.75,Fe0.25) O] at pressures up to 90 GPa. The pre-edge features from X-ray absorption spectroscopy in the partial fluorescence yield (PFY-XAS) and RXES results explicitly show three excited states for high-spin Fe2+ (a lower-energy 4T1g state, a 4T2g state, and a higher-energy 4T1g state) and a single ²Eg state for low-spin Fe2+, attributed to the (t2g)0(eg)³ excited configuration. This latter feature begins to appear at 48 GPa and grows with pressure, while the peaks related to high-spin Fe2+ vanish above 80 GPa. The observed pre-edge features are consistent with purely quadrupolar transitions resulting from the centrosymmetric character of the Fe2+ site. The K pre-edge RXES spectra at the incident energy of 7112 eV, which are similar to the Fe L-edge spectra, are also used successfully to quantitatively obtain consistent results on the spin transition of Fe2+ in ferropericlase under high pressures. Owing to the superior sensitivity of the RXES technique, the observed electronic states and their energy separations provide direct information on the local electronic structures and crystal-field splitting energies of the 3d electronic shells of Fe2+ in ferropericlase at relevant pressures of the Earth's lower mantle. [ABSTRACT FROM AUTHOR]

Reaching the goal of economical photoelectrochemical (PEC) water splitting will likely require the combination of efficient solar absorbers with high activity electrocatalysts for the hydrogen and oxygen evolution reactions (HER and OER). Toward this goal, we synthesized an amorphous FeOOH (a-FeOOH) phase that has not previously been studied as an OER catalyst. The a-FeOOH films show activity comparable to that of another OER cocatalyst, Co-borate (Co-Bi), in 1 M Na2CO3, reaching 10 mA/cm2 at an overpotential of 550 mV for 10 nm thick films. Additionally, the a-FeOOH thin films absorb less than 3% of the solar photons (AM1.5G) with energy greater than 1.9 eV, are homogeneous over large areas, and act as a protective layer separating the solution from the solar absorber. The utility of a-FeOOH in a realistic system is tested by depositing on amorphous Si triple junction solar cells with a photovoltaic efficiency of 6.8%. The resulting a-FeOOH/a-Si devices achieve a total water splitting efficiency of 4.3% at 0 V vs RHE in a three-electrode configuration and show no decrease in efficiency over the course of 4 h. [ABSTRACT FROM AUTHOR]

Light elements in Earth's core play a key role in driving convection and influencing geodynamics, both of which are crucial to the geodynamo. However, the thermal transport properties of iron alloys at high-pressure and -temperature conditions remain uncertain. Here we investigate the transport properties of solid hexagonal close-packed and liquid Fe-Si alloys with 4.3 and 9.0 wt % Si at high pressure and temperature using laser-heated diamond anvil cell experiments and first-principles molecular dynamics and dynamical mean field theory calculations. In contrast to the case of Fe, Si impurity scattering gradually dominates the total scattering in Fe-Si alloys with increasing Si concentration, leading to temperature independence of the resistivity and less electron-electron contribution to the conductivity in Fe-9Si. Our results show a thermal conductivity of ~100 to 110 Wm21K21 for liquid Fe-9Si near the topmost outer core. If Earth's core consists of a large amount of silicon (e.g., > 4.3 wt %) with such a high thermal conductivity, a subadiabatic heat flow across the core-mantle boundary is likely, leaving a 400- to 500-km-deep thermally stratified layer below the core-mantle boundary, and challenges proposed thermal convection in Fe-Si liquid outer core. [ABSTRACT FROM AUTHOR]

Determination of the full elastic constants (cij) of methane hydrates (MHs) at extreme pressuretemperature environments is essential to our understanding of the elastic, thermodynamic, and mechanical properties of methane in MH reservoirs on Earth and icy satellites in the solar system. Here, we have investigated the elastic properties of singe-crystal cubic MH-sI, hexagonal MH-III, and orthorhombic MH-III phases at high pressures in a diamond anvil cell. Brillouin light scattering measurements, together with complimentary equation of state (pressure-density) results from X-ray diffraction and methane site occupancies in MH from Raman spectroscopy, were used to derive elastic constants of MH-sI, MH-II, and MH-III phases at high pressures. Analysis of the elastic constants for MH-sI and MH-II showed intriguing similarities and differences between the phases' compressional wave velocity anisotropy and shear wave velocity anisotropy. Our results show that these high-pressure MH phases can exhibit distinct elastic, thermodynamic, and mechanical properties at relevant environments of their respective natural reservoirs. These results provide new insight into the determination of how much methane exists in MH reservoirs on Earth and on icy satellites elsewhere in the solar system and put constraints on the pressure and temperature conditions of their environment. [ABSTRACT FROM AUTHOR]

Lattice strength and structural phase transitions of gadolinium (Gd) were determined under nonhydrostatic compression up to 55 GPa using an angle-dispersive radial x-ray diffraction technique in a diamond-anvil cell at room temperature. Three new phases of fcc structure, dfcc structure, and new monoclinic structure were observed at 25 GPa, 34 GPa, and 53 GPa, respectively. The radial x-ray diffraction data yield a bulk modulus K0=36(1) GPa with its pressure derivate K0'=3.8(1) at the azimuthal angle between the diamond cell loading axis and the diffraction plane normal and diffraction plane ψ=54.7°. With K0' fixed at 4, the derived K0 is 34(1) GPa. In addition, analysis of diffraction data with lattice strain theory indicates that the ratio of differential stress to shear modulus (t/G) ranges from 0.011 to 0.014 at pressures of 12-55 GPa. Together with estimated high-pressure shear moduli, our results show that Gd can support a maximum differential stress of 0.41 GPa, while it starts to yield to plastic deformation at 16 GPa under uniaxial compression. The yield strength of Gd remains approximately a constant with increasing pressure, and reaches 0.46 GPa at 55 GPa. [ABSTRACT FROM AUTHOR]

Acoustic compressional and shear wave velocities (VP, VS) of anhydrous (AHRG) and hydrous rhyolitic glasses (HRG) containing 3.28 wt% (HRG-3) and 5.90 wt% (HRG-6) total water concentration (H[sub 2]O[sub t]) have been measured using Brillouin light scattering (BLS) spectroscopy up to 3 GPa in a diamond-anvil cell at ambient temperature. In addition, Fourier-transform infrared (FTIR) spectroscopy was used to measure the speciation of H2O in the glasses up to 3 GPa. At ambient pressure, HRG-3 contains 1.58 (6) wt% hydroxyl groups (OH-) and 1.70 (7) wt% molecular water (H2Om) while HRG-6 contains 1.67 (10) wt% OH- and 4.23 (17) wt% H2Om where the numbers in parentheses are ±1s. With increasing pressure, very little H2Om, if any, converts to OH- within uncertainties in hydrous rhyolitic glasses such that HRG-6 contains much more H2Om than HRG-3 at all experimental pressures. We observe a nonlinear relationship between high-pressure sound velocities and H2Ot, which is attributed to the distinct effects of each water species on acoustic velocities and elastic moduli of hydrous glasses. Near ambient pressure, depolymerization due to OH[sup -] reduces VS and G more than VP and KS. VP and KS in both anhydrous and hydrous glasses decrease with increasing pressure up to ~1-2 GPa before increasing with pressure. Above ~1-2 GPa, VP and KS in both hydrous glasses converge with those in AHRG. In particular, VP in HRG-6 crosses over and becomes higher than VP in AHRG. HRG-6 displays lower VS and G than HRG-3 near ambient pressure, but VS and G in these glasses converge above ~2 GPa. Our results show that hydrous rhyolitic glasses with ~2-4 wt% H2Om can be as incompressible as their anhydrous counterpart above ~1.5 GPa. The nonlinear effects of hydration on high-pressure acoustic velocities and elastic moduli of rhyolitic glasses observed here may provide some insight into the behavior of hydrous silicate melts in felsic magma chambers at depth. [ABSTRACT FROM AUTHOR]

Diopside is one of the most important end-members of clinopyroxene, which is an abundant mineral in upper-mantle petrologic models. The amount of clinopyroxene in upper-mantle pyrolite can be ~15 vol%, while pyroxenite can contain as high as ~60 vol% clinopyroxene. Knowing the elastic properties of the upper-mantle diopside at high pressure-temperature conditions is essential for constraining the chemical composition and interpreting seismic observations of region. Here we have measured the single-crystal elasticity of Fe-enriched diopside (Di80Hd20, Di-diopside, and Hd-hedenbergite; also called Fe-enriched clinopyroxene) at high-pressure conditions up to 18.5 GPa by using in situ Brillouin light-scattering spectroscopy (BLS) and synchrotron X-ray diffraction in a diamond-anvil cell. Our experimental results were used in evaluating the effects of pressure and Fe substitution on the full single-crystal elastic moduli across the Di-Hd solid-solution series to better understand the seismic velocity profiles of the upper mantle. Using the third- or fourth-order Eulerian finite-strain equations of state to model the elasticity data, the derived aggregate adiabatic bulk and shear moduli (KS0, G0) at ambient conditions were determined to be 117(2) and 70(1) GPa, respectively. The first- and second-pressure derivatives of bulk and shear moduli at 300 K were (∂KS/∂P)T = 5.0(2), (∂²KS/∂P²)[sub T] = -0.12(4) GPa-1 and (∂G/∂P)T = 1.72(9), (∂²G/∂P²)T = -0.05(2) GPa-1, respectively. A comparison of our results with previous studies on end-member diopside and hedenbergite in the literatures shows systematic linear correlations between the Fe composition and single-crystal elastic moduli. An addition of 20 mol% Fe in diopside increases KS0 by ~1.7% (~2 GPa) and reduces G0 by ~4.1% (~3 GPa), but has a negligible effect on the pressure derivatives of the bulk and shear moduli within experimental uncertainties. In addition, our modeling results show that substitution of 20 mol% Fe in diopside can reduce V[sub P] and V[sub S] by ~1.8% and ~3.5%, respectively, along both an expected normal mantle geotherm and a representative cold subducted slab geotherm. Furthermore, the modeling results show that the VP and VS profiles of Fe-enriched pyroxenite along the cold subducted slab geotherm are ~3.2% and ~2.5% lower than AK135 model at 400 km depth, respectively. Finally, we propose that the presence of Fe-enriched pyroxenite (including Fe-enriched clinopyroxene, Fe-enriched orthopyroxene, and Feenriched olivine), can be an effective mechanism to cause low-velocity anomalies in the upper mantle regions atop the 410 km discontinuity at cold subudcted slab conditions. [ABSTRACT FROM AUTHOR]

In this study, we measured the sound velocities of single-crystal periclase by Brillouin light scattering (BLS) combined with in situ synchrotron X-ray diffraction (XRD) up to ~30 GPa and 900 K in an externally heated diamond-anvil cell (EHDAC). Our experimental results were used to evaluate the combined effects of pressure and temperature on the elastic moduli of single-crystal periclase using third-order Eulerian finite-strain equations. All of the elastic moduli increased with increasing pressure but decreased with increasing temperature, except the off-diagonal modulus C12, which remained almost constant up to ~30 GPa and 900 K. The derived aggregate adiabatic bulk and shear moduli (KS0, G0) at ambient conditions were 162.8(±0.2) and 130.3(±0.2) GPa, respectively, consistent with literature results. The pressure derivatives of the bulk [(∂KS/∂P)300 K] and shear moduli [(∂G/∂P)300 K] at ambient conditions were 3.94(±0.05) and 2.17(±0.02), respectively, whereas the temperature derivatives of these moduli [(∂KS/∂T)P and (∂G/∂T) P]] at ambient conditions were -0.025(±0.001) and -0.020(±0.001) GPa/K, respectively. A comparison of our experimental results with the high-pressure (P) and high-temperature (T) elastic moduli of ferropericlase (Fp) in the literature showed that all the elastic moduli of Fp were linearly correlated with the FeO content up to approximately 20 mol%. These results allowed us to build a comprehensive thermoelastic model for Fp to evaluate the effect of Fe-Mg substitution on the elasticity and seismic parameters of Fp at the relevant P-T conditions of the lower mantle. Our modeling results showed that both the increase of the Fe content in Fp and the increasing depth could change the compressional wave anisotropy (AVP) and shear wave splitting anisotropy (AVS) of Fp in the upper parts of the lower mantle. Furthermore, using our modeling results here, we also evaluated the contribution of Fp to seismic lateral heterogeneities of thermal or chemical origin in the lower mantle. Both the thermally induced and Fe-induced heterogeneities ratios (RS/P = ∂lnVS/∂lnVP) of Fp from 670 to 1250 km along a representative lower mantle geotherm increased by ~2-5% and ~15%, respectively. The thermally induced RS/P value of Fp20 is ~30% higher than Fp10, indicating that the Fe content has a significant effect on the thermally induced RS/P of Fp. Compared to the seismic observation results (RS/P = 1.7-2.0) in the upper regions of the lower mantle, the Fe-induced RS/P value of Fp is more compatible than the thermally induced RS/P value of Fp20 (the expected composition of Fp in the lower mantle) within their uncertainties. Thus, we propose that Fe-induced lateral heterogeneities can significantly contribute to the observed seismic lateral heterogeneities in the Earth's lower mantle (670-1250 km). [ABSTRACT FROM AUTHOR]

Objectives: Staphylococcus lugdunensis, a species of CoNS, has become an important hospital pathogen because of increasing resistance to β-lactam antibiotics such as methicillin and oxacillin. Methicillin resistance is mainly due to the acquisition of the staphylococcal cassette chromosome (SCC) mec (SCCmec). Little is known about the structure of SCCmec in methicillin- or oxacillin-resistant CoNS.Methods: WGS was performed to determine the structure of SCCmec elements of two clinical S. lugdunensis isolates: CMUH-22 and CMUH-25.Results: These elements were found to be flanked by DRs and IRs with unique mosaic structures and a common integration site in the 3' end of the rlmH gene. The sequences of the regions located between rlmH and the ISSau4-like transposase genes of both elements were similar to those of SCCmec Vt of Staphylococcus aureus PM1. The SCCmec (type V, 5C2&4) of CMUH-25 harboured a novel ccrC complex and a C2-like mec complex in opposite orientations, similar to the type V SCCmec of S. aureus WIS. The sequences of the ccrA4B4 genes and J1 and J2 regions of CMUH-25 were similar to those of the SCC element of Staphylococcus haemolyticus NCTC 11042. In contrast, portions of the sequence of the J1 region of type Vt (5C2) SCCmec in strain CMUH-22 were highly similar to portions of those of Staphylococcus epidermidis RP62A and the composite SCCmec type V of S. aureus WAMRSA40.Conclusions: These observations suggest that the SCCmec elements of CMUH-25 and CMUH-22 evolved separately and assembled through different recombination events. [ABSTRACT FROM AUTHOR]

Electronic structures of electron- and hole-doped BaFe2As2 and nondoped SrFe2As2 have been studied systematically by x-ray emission spectroscopy at Fe and As K-absorption edges. The electron and hole doping causes slight increase of the integrated absolute difference (IAD) values of the Fe Kβ x-ray emission spectra which correlate to the local magnetic moment. Pressure decreases the IAD values and local magnetic moment, and induces the lower-spin states in these compounds. The pre-edge peak intensity of the XAS spectra at the Fe K-absorption edge increases with pressure in both compounds. This indicates an increase of the Fe 3d-As 4p hybridization. It was found that pressure induced a discontinuous increase of the prepeak intensity of the PFY-XAS spectra at the As K-absorption edge at low pressures in the BaFe2As2 systems. Our results may suggest that the Fe 3d-As 4p hybridization plays a key role in suppressing the AFM order by the doping or pressure and fluctuation of the local magnetic moment and the electron-electron correlation may also play a role on the physical properties of the iron superconductors. [ABSTRACT FROM AUTHOR]

Brillouin light scattering and impulsive stimulated light scattering have been used to determine the full elastic constants of magnesiosiderite [(Mg0.35Fe0.65)CO3] up to 70 GPa at room temperature in a diamond-anvil cell. Drastic softening in C11, C33, C12, and C13 elastic moduli associated with the compressive stress component and stiffening in C44 and C14 moduli associated with the shear stress component are observed to occur within the spin transition between ~42.4 and ~46.5 GPa. Negative values of C12 and C13 are also observed within the spin transition region. The Born criteria constants for the crystal remain positive within the spin transition, indicating that the mixed-spin state remains mechanically stable. Significant auxeticity can be related to the electronic spin transition-induced elastic anomalies based on the analysis of Poisson's ratio. These elastic anomalies are explained using a thermoelastic model for the rhombohedral system. Finally, we conclude that mixed-spin state ferromagnesite, which is potentially a major deep-carbon carrier, is expected to exhibit abnormal elasticity, including a negative Poisson's ratio of-0.6 and drastically reduced VP by 10%, in Earth's midlower mantle. [ABSTRACT FROM AUTHOR]

Single-crystal elasticity of candidate minerals in the Earth's mantle, such as that of ferropericlase and bridgmanite, etc., is very important for understanding the seismic observations, geodynamic flow patterns, and testing geochemical and mineralogical models of the planet's deep interior. Determination of the full elastic tensor typically requires measuring both compressional and shear wave velocities (vP and vS) of the candidate single crystal as a function of crystallographic orientations at high pressures, but it has been a huge technical challenge obtaining vP at pressures above 25 GPa using Brillouin light scattering coupled with in a diamond-anvil cell due to the spectral overlap of the sample vP with the vS of the diamond window. In this study, we present a new method to derive the full elastic tensor (Cij) of single crystals using only measured vS of a given crystal platelet as a function of the azimuthal angle. Experimentally determined vP and vS results from Brillouin measurements for cubic periclase (MgO) and spinel (MgAl2O4), tetragonal stishovite (SiO2), and orthorhombic zoisite [Ca2Al3Si3O12(OH)] at ambient conditions are used as examples to demonstrate the application of our approach from theoretical analyses and experimental prospective. For high-symmetry cubic minerals, such as cubic MgO and spinel, a suitable crystallographic plane with small tradeoffs between any two Cij in vS is required for the method to work well such that the obtained Cij using measured vS velocities alone can be within 3% of the values derived from using both vS and vP. Our analyses show that the (-1,0.5,0.2) platelet for periclase and the (1,1,0) platelet for spinel are respective optimal orientations for applying our method. For lower symmetry minerals, such as tetragonal stishovite and orthorhombic zoisite, three crystallographic planes, that are orthogonal to each other and are tilted at least 20° from the principal crystallographic planes, can be used to provide reliable constraints on Cij using measured vS alone. We have extended this method to derive Cij of the (-1,0.5,0.2) platelet for periclase at pressures of 5.8 and 11.3 GPa, in a high-pressure diamond-anvil cell to demonstrate the usefulness of the approach in studying the elasticity of Earth's mantle minerals at relevant pressure-temperature conditions. Our proposed approach can be extended to all other crystal systems at high pressures to overcome the constant lack of experimental vP velocities at above 25 GPa, potentially providing new experimental and theoretical approaches in constraining the elastic tensor of the materials in the Earth's deep interior, which will be an effective strategy to solve one of the most relevant difficulties involved in the experimental study of the elastic properties (especially elastic anisotropy) of minerals of the lower mantle. [ABSTRACT FROM AUTHOR]

The presence of water in the Earth's deep mantle is an issue of increasing interest in the field of highpressure mineralogy. An important task for further advancing research in the field is to create homogeneous single crystals of candidate deep-mantle water-bearing minerals of 1 mm or larger in size, which is required for applying them for the time-of-flight (TOF) single-crystal Laue diffraction method with a third-generation neutron instrument. In this study, we perform several experiments to demonstrate an improved methodology for growing hydrous crystals of such large sizes at relevant transition zone and lower-mantle conditions via very slow cooling over a maximum period of 1 day. Successfully synthesized crystals using this methodology include dense hydrous magnesium silicate (DHMS) phase E, hydrous wadsleyite, hydrous ringwoodite, and bridgmanite (silicate perovskite). It is also demonstrated that these hydrous crystals can be grown from deuterium enriched starting materials in addition to those having a natural hydrogen isotope ratio. Magnitudes of chemical and crystallographic heterogeneities of the product crystals were characterized by comprehensive analysis of X-ray precession photography, single-crystal X-ray diffraction (SCXRD), field-emission scanning electron microscope (FE-SEM), electron probe microanalyzer (EPMA), secondary ion mass spectroscopy (SIMS), powder X-ray diffraction (PXRD), and TOF neutron powder diffraction (TOF-NPD). The product crystals were confirmed to be inclusion free and crystallographically homogeneous. Compositional and isotopic differences of major elements and hydrogen isotope abundances were lower than 1 and 3%, respectively, among intracrystals and intercrystals within each recovered sample capsule. Phase E crystals up to 600 µm in the largest dimension were grown at a constant temperature of 1100 °C kept for 3 h. Using a lattice parameter-to-temperature relation of phase E, the thermal gradient in the sample capsules for the phase E synthesis has been evaluated to be 20 °C/mm. Hydrous wadsleyite crystals up to 1100 µm in the largest dimension were grown at 1390 °C with a temperature reduction of 70 °C during heating for 10 h. Hydrous ringwoodite crystals up to 1000 µm in the largest dimension were grown at around 1400 °C with a temperature reduction of 110 °C during heating for 12 h. Bridgmanite crystals up to 600 µm in the largest dimension were grown at 1700 °C with a temperature reduction of 30 °C during heating for 12 h. A TOF single-crystal diffraction instrument has been successfully used for analyzing one of the hydrous wadsleyite crystals, which demonstrated that single crystals appropriate for their expected usage are created using the method proposed in the present study. [ABSTRACT FROM AUTHOR]

Pressure dependence of the Yb valence in YbNiGe3 has been investigated up to 15.6 GPa at 300 K and up to 7.7 GPa at 17 K by means of x-ray absorption spectroscopy in Lα1 partial fluorescence yield mode and resonant x-ray emission spectroscopy around the Yb L3 absorption edge. The Yb valence in YbNiGe3 at ambient pressure strongly fluctuates with the mean valence of v = 2.52 ± 0.01 at 300 K. The Yb valence rapidly changes toward a trivalent state with pressure up to ∼5 GPa, slowly increases up to ∼ 10 GPa, and then reaches a saturated value of v ∼ 2.87 at 15.6 GPa. The Yb valence at 17 K slightly decreases compared to that at 300 K; v ∼ 2.45 at ambient pressure and ∼2.72 at 7.7 GPa. We found that the pressure-induced change in the intensity of a quadrupole component in the x-ray absorption spectra shows the same trend as the Yb valence in YbNiGe3. In contrast to YbNiGe3, the Yb valence in YbNiSi3 is nearly 3 at ambient pressure with almost no temperature dependence. [ABSTRACT FROM AUTHOR]

Perovskite oxides ABO3 are important materials used as components in electronic devices. The highly compact crystal structure consists of a framework of corner-shared BO6 octahedra enclosing the A-site cations. Because of these structural features, forming a strong bond between A and B cations is highly unlikely and has not been reported in the literature. Here we report a pressure- induced first-order transition in PbRuO3 from a common orthorhombic phase (Pbnm) to an orthorhombic phase (Pbn21) at 32 GPa by using synchrotron X-ray diffraction. This transition has been further verified with resistivity measurements and Raman spectra under high pressure. In contrast to most well-studied perovskites under high pressure, the Pbn21 phase of PbRuO3 stabilized at high pressure is a polar perovskite. More interestingly, the Pbn21 phase has the most distorted octahedra and a shortest Pb–Ru bond length relative to the average Pb–Ru bond length that has ever been reported in a perovskite structure. We have also simulated the behavior of the PbRuO3 perovskite under high pressure by first principles calculations. The calculated critical pressure for the phase transition and evolution of lattice parameters under pressure match the experimental results quantitatively. Our calculations also reveal that the hybridization between a Ru:t2g orbital and an sp hybrid on Pb increases dramatically in the Pbnm phase under pressure. This pressure-induced change destabilizes the Pbnm phase to give a phase transition to the Pbn21 phase where electrons in the overlapping orbitals form bonding and antibonding states along the shortest Ru–Pb direction at P > Pc. 4. [ABSTRACT FROM AUTHOR]

Bismuth vanadate (BiVO4) is a promising photoelectrode material for the oxidation of water, but fundamental studies of this material are lacking. To address this, we report electrical and photoelectrochemical (PEC) properties of BiVO4 single crystals (undoped, 0.6% Mo, and 0.3% W:BiVO4) grown using the floating zone technique. We demonstrate that a small polaron hopping conduction mechanism dominates from 250 to 400 K, undergoing a transition to a variable-range hopping mechanism at lower temperatures. An anisotropy ratio of ∼3 was observed along the c axis, attributed to the layered structure of BiVO4. Measurements of the ac field Hall effect yielded an electron mobility of ∼0.2 cm² V-1 s-1 for Mo and W:BiVO4 at 300 K. By application of the Gārtner model, a hole diffusion length of ∼100 nm was estimated. As a result of low carrier mobility, attempts to measure the dc Hall effect were unsuccessful. Analyses of the Raman spectra showed that Mo and W substituted for V and acted as donor impurities. Mott–Schottky analysis of electrodes with the (001) face exposed yielded a flat band potential of 0.03–0.08 V versus the reversible H2 electrode, while incident photon conversion efficiency tests showed that the dark coloration of the doped single crystals did not result in additional photocurrent. Comparison of these intrinsic properties to those of other metal oxides for PEC applications gives valuable insight into this material as a photoanode. [ABSTRACT FROM AUTHOR]

We report on direct measurements of the Yb valence in the heavy Fermion compound YbCuAl as a function of temperature and pressure using resonant x-ray emission spectroscopy. The increase of the Yb2+ component at T < 100 K and ambient pressure, well described by the single impurity Anderson model, is found to compensate for the thermal contraction of the unit cell volume. Under pressure, the Yb valence increases continuously up to 25 GPa, albeit a marked leveling off close to the critical pressure, at P ≥ 13 GPa. This finding is reminiscent of a recent report on YbCu2Si2 and further confirms the interplay between electronic and magnetic fluctuations near the magnetic instability point of rare-earth intermediate-valence materials. [ABSTRACT FROM AUTHOR]

Ferropericlase [(Mg,Fe)O] is one of the most abundant minerals of the earth's lower mantle. The high-spin (HS) to low-spin (LS) transition in the Fe2+ ions may dramatically alter the physical and chemical properties of (Mg,Fe)O in the deep mantle. To understand the effects of compression on the ground electronic state of iron, electronic and magnetic states of Fe2+ in (Mg0.75Fe0.25)O have been investigated using transmission and synchrotron Mössbauer spectroscopy at high pressures and low temperatures (down to 5 K). Our results show that the ground electronic state of Fe2+ at the critical pressure Pc of the spin transition close to T = 0 is governed by a quantum critical point (T = 0, P = Pc) at which the energy required for the fluctuation between HS and LS states is zero. Analysis of the data gives Pc = 55 GPa. Thermal excitation within the HS or LS states (T > 0 K) is expected to strongly influence the magnetic as well as physical properties of ferropericlase. Multielectron theoretical calculations show that the existence of the quantum critical point at temperatures approaching zero affects not only physical properties of ferropericlase at low temperatures but also its properties at P-T of the earth's lower mantle. [ABSTRACT FROM AUTHOR]

Many particle-physics models that extend the standard model predict the existence of long-range spin-spin interactions. We propose an approach that uses the Earth as a polarized spin source to investigate these interactions. Using recent deep-Earth geophysics and geochemistry results, we create a comprehensive map of electron polarization within the Earth induced by the geomagnetic field. We examine possible long-range interactions between these spin-polarized geoelectrons and the spin-polarized electrons and nucleons in three laboratory experiments. By combining our model and the results from these experiments, we establish bounds on torsion gravity and possible long-range spin-spin forces associated with the virtual exchange of either spin-one axial bosons or unparticles. [ABSTRACT FROM AUTHOR]

Temperature and pressure dependences of the electronic structure of the heavy-fermion system CePd2Si2 have been investigated using partial fluorescence yield x-ray absorption spectroscopy and resonant x-ray emission spectroscopy at the Ce L3 edge. The temperature dependence has also been measured for CeRh2Si2 for comparison. In both compounds Ce is in a weakly mixed valence state at ambient pressure, mostly ∫1 with a small contribution from the ∫0 component. No temperature dependence of the Ce valence is observed at temperatures as low as 8 K. In CePd2Si2 at 19 K, however, the Ce valence shows a continuous increase with pressure, indicating pressure-induced derealization of the 4∫ states. Theoretical calculations based on the single impurity Anderson model reproduce the experimental results well. Pressure dependence of the difference between the ground state valence and the measured valence including the final state effect is also discussed. [ABSTRACT FROM AUTHOR]

We have investigated the temperature and pressure dependency of the electronic structure of Yb-filled skutterudites, YbFe4Sb12 and Yb0.88Fe4Sb12, using x-ray absorption and emission spectroscopies. An anomalous increase of the Yb valence, which is beyond the conventional Anderson model picture, is found to coincide with the onset of the ferromagnetic order in the x = 0.88 sample below 20 K. In contrast, the nearly stoichiometric YbFe4Sb12 is paramagnetic down to 2 K and the Yb valence is independent of temperature. This evidences a close interplay between the magnetic instability of the Fe 3d electrons and valence instability of the Yb 4f electrons. Under pressure, a sudden increase in the valence is found to occur around 13 GPa for YbFe4Sb12 and 17 GPa for Yb0.88Fe4Sb12. [ABSTRACT FROM AUTHOR]

We performed an x-ray spectroscopic study combining resonant x-ray emission spectroscopy (RXES) and photoelectron spectroscopy on the superconducting ternary silicide YbGa1.15Si0.85 and nonsuperconducting ternary germanide YbGaxGe2-x (x = 1.0 and 1.1). The Yb valence for all three compounds is found to be about 2.3. In YbGa1.15Si0.85 no temperature dependence of the Yb valence is observed in the RXES spectra in the temperature range of 7–300 K, while the valence shows a drastic increase under pressure from the Yb2+ state partially including itinerant electrons to the localized Yb3+ state. Differences are observed in the valence-band spectra of the photoelectron spectroscopy between YbGa1.15Si0.85 and YbGaxGe2-x, which may be attributed to the difference of crystal structure. We conclude that both the crystal structure of the planar GaSi layer in YbGa1.15Si0.85 and the resultant electronic structure may have a crucial role in the occurrence of superconductivity. [ABSTRACT FROM AUTHOR]

The pressure-induced phase transformations in pure LiAlH4have been studied using in situ Raman spectroscopy up to 7 GPa. The analyses of Raman spectra reveal a phase transition at approximately 3 GPa from the ambient pressure monoclinic -LiAlH4phase (P21/c) to a high pressure phase (-LiAlH4, reported recently to be monoclinic with space groupI41/b) having a distorted AlH4-tetrahedron. The Al−H stretching mode softens and shifts dramatically to lower frequencies beyond the phase transformation pressure. The high pressure -LiAlH4phase was pressure quenchable and can be recovered at lower pressures (∼1.2 GPa). The Al−H stretching mode in the quenched state further shifts to lower frequencies, suggesting a weakening of the Al−H bond. [ABSTRACT FROM AUTHOR]

WN6 phase discovered at 126-165 GPa after heating of W in nitrogen. XRD refinements reveal a unit cell in space group R̅3m which is consistent with the WN6 structure with armchairlike hexazine (N6) rings, while strong A1g Raman mode confirms its N─N single bonds. Density functional theory (DFT) calculations reveal balanced contributions of attractive interactions between W and covalent N6 rings, and repulsions between N6 rings that make WN6 ultrastiff and tough. The WN6 phase displays long bond lengths in the nearest N-N and pressure-enhanced electronic band gap, which pave the way for finding novel nitrides. [ABSTRACT FROM AUTHOR]