We have critically reviewed and discussed currently available information regarding the spin and valence states of iron in lower mantle minerals and the associated effects of the spin transitions on ...physical, chemical, and transport properties of the deep Earth. A high‐spin to low‐spin crossover of Fe2+ in ferropericlase has been observed to occur at pressure‐temperature conditions corresponding to the middle part of the lower mantle. In contrast, recent studies consistently show that Fe2+ predominantly exhibits extremely high quadrupole splitting values in the pseudo‐dodecahedral site (A site) of perovskite and post‐perovskite, indicative of a strong lattice distortion. Fe3+ in the A site of these structures likely remains in the high‐spin state, while a high‐spin to low‐spin transition of Fe3+ in the octahedral site of perovskite occurs at pressures of 15–50 GPa. In post‐perovskite, the octahedral‐site Fe3+ remains in the low‐spin state at the pressure conditions of the lowermost mantle. These changes in the spin and valence states of iron as a function of pressure and temperature have been reported to affect physical, chemical, rheological, and transport properties of the lower mantle minerals. The spin crossover of Fe2+ in ferropericlase has been documented to affect these properties and is discussed in depth here, whereas the effects of the spin transition of iron in perovskite and post‐perovskite are much more complex and remain debated. The consequences of the transitions are evaluated in terms of their implications to deep Earth geophysics, geochemistry, and geodynamics including elasticity, element partitioning, fractionation and diffusion, and rheological and transport properties.
Key Points
Spin states of iron in lower‐mantle minerals are reviewed.
Effects of the transitions are addressed.
Implications of the transitions are discussed.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Abstract
Hydrogen-rich superhydrides are believed to be very promising high-
T
c
superconductors. Recent experiments discovered superhydrides at very high pressures, e.g. FeH
5
at 130 GPa and LaH
10
...at 170 GPa. With the motivation of discovering new hydrogen-rich high-
T
c
superconductors at lowest possible pressure, here we report the prediction and experimental synthesis of cerium superhydride CeH
9
at 80–100 GPa in the laser-heated diamond anvil cell coupled with synchrotron X-ray diffraction. Ab initio calculations were carried out to evaluate the detailed chemistry of the Ce-H system and to understand the structure, stability and superconductivity of CeH
9
. CeH
9
crystallizes in a
P6
3
/mmc
clathrate structure with a very dense 3-dimensional atomic hydrogen sublattice at 100 GPa. These findings shed a significant light on the search for superhydrides in close similarity with atomic hydrogen within a feasible pressure range. Discovery of superhydride CeH
9
provides a practical platform to further investigate and understand conventional superconductivity in hydrogen rich superhydrides.
Objective
The neuromodulatory effects of focused ultrasound (FUS) have been demonstrated in animal epilepsy models; however, the safety and efficacy of FUS in humans with epilepsy have not been well ...established. Patients with drug‐resistant epilepsy (DRE) undergoing stereo‐electroencephalography (SEEG) provide an opportunity to investigate the neuromodulatory effects of FUS in humans.
Methods
Patients with DRE undergoing SEEG for localization of the seizure onset zone (SOZ) were prospectively enrolled. FUS was delivered to the SOZ using a neuronavigation‐guided FUS system (ceiling spatial‐peak temporal‐average intensity level = 2.8 W/cm2, duty cycle = 30%, modulating duration = 10 min). Simultaneous SEEG recordings were obtained during sonication and for 3 days after treatment. Seizures, interictal epileptiform discharges, and adverse events after FUS were monitored.
Results
Six patients met the eligibility criteria and completed FUS treatment. A decrease in seizure frequency was observed in two patients within the 3‐day follow‐up; however, one patient presented an increase in the frequency of subclinical seizures. Posttreatment magnetic resonance imaging revealed neither lesion nor brain edema. Significant changes in spectral power of SEEG were noted at the targeted electrodes during FUS treatment. One patient reported subjective scalp heating during FUS, and one patient developed transient naming and memory impairment that resolved within 3 weeks after FUS.
Significance
FUS can be safely delivered to the SOZ of patients with DRE, resulting in significant changes in spectral power of SEEG. A larger sample cohort and pursuing optimal sonication parameters will be required to elucidate the neuromodulatory effects of FUS when used for seizure control.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Two distinct stacking orders in ReS2 are identified without ambiguity and their influence on vibrational, optical properties and carrier dynamics are investigated. With atomic resolution scanning ...transmission electron microscopy (STEM), two stacking orders are determined as AA stacking with negligible displacement across layers, and AB stacking with about a one‐unit cell displacement along the a axis. First‐principles calculations confirm that these two stacking orders correspond to two local energy minima. Raman spectra inform a consistent difference of modes I & III, about 13 cm−1 for AA stacking, and 20 cm−1 for AB stacking, making a simple tool for determining the stacking orders in ReS2. Polarized photoluminescence (PL) reveals that AB stacking possesses blueshifted PL peak positions, and broader peak widths, compared with AA stacking, indicating stronger interlayer interaction. Transient transmission measured with femtosecond pump–probe spectroscopy suggests exciton dynamics being more anisotropic in AB stacking, where excited state absorption related to Exc. III mode disappears when probe polarization aligns perpendicular to b axis. The findings underscore the stacking‐order driven optical properties and carrier dynamics of ReS2, mediate many seemingly contradictory results in the literature, and open up an opportunity to engineer electronic devices with new functionalities by manipulating the stacking order.
Two stacking orders of ReS2 are identified. Stacking AA has negligible displacement across layers and stacking AB has a one‐unit cell displacement along the a‐axis. AB stacking has stronger interlayer coupling than AA. The cross‐layer displacement in AB stacking disrupts excited‐state excitons. Vibrational, optical properties and carrier dynamics in two stacking orders are drastically different.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
In this study, we have investigated the stability of CaCO3 at high pressures and temperatures using synchrotron X‐ray diffraction in laser‐heated diamond anvil cells. Our experimental results have ...shown that CaCO3 in the aragonite structure transforms into CaCO3‐VII (P21/c) at 27 GPa and 1,500 K with a negative Clapeyron slope of −4.3(9) MPa/K. CaCO3‐VII is stable between 23 and 38 GPa at 2,300 K and transforms into post‐aragonite at 42 GPa and 1,400 K. Furthermore, it reacts with stishovite, an abundant form of SiO2 in subducted oceanic crust, forming CaSiO3‐perovskite. The occurrence of CaSiO3‐perovskite via the reaction of CaCO3‐VII and stishovite provides an explanation for the observation of the high concentrations of CaSiO3‐perovskite and some amount of CaCO3 in deep‐mantle inclusions. CaCO3‐VII is thus an important carbon‐bearing phase at the topmost lower mantle and may provide necessary carbon to produce deep‐mantle diamonds.
Key Points
Here we have reported the experimental evidence for a thermally stable phase of CaCO3, CaCO3‐VII, at the top lower mantle
CaCO3‐VII reacts with stishovite at depths of 700–1,000 km to form CaSiO3‐pervoskite and release CO2 to the surrounding mantle
CaCO3‐VII is an important carbon carrier at the Earth's topmost lower mantle
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Knowledge of the physical and chemical properties of candidate deep-carbon carriers such as ferromagnesite (Mg,Fe)CO₃ at high pressure and temperature of the deep mantle is necessary for our ...understanding of deep-carbon storage as well as the global carbon cycle of the planet. Previous studies have reported very different scenarios for the (Mg,Fe)CO₃ system at deep-mantle conditions including the chemical dissociation to (Mg,Fe)O+CO₂, the occurrence of the tetrahedrally-coordinated carbonates based on CO₄ structural units, and various high-pressure phase transitions. Here we have studied the phase stability and compressional behavior of (Mg,Fe)CO₃ carbonates up to relevant lower-mantle conditions of approximately 120 GPa and 2400 K. Our experimental results show that the rhombohedral siderite (Phase I) transforms to an orthorhombic phase (Phase II with Pmm2 space group) at approximately 50 GPa and 1400 K. The structural transition is likely driven by the spin transition of iron accompanied by a volume collapse in the Fe-rich (Mg,Fe)CO₃ phases; the spin transition stabilizes the high-pressure phase II at much lower pressure conditions than its Mg-rich counterpart. It is conceivable that the low-spin ferromagnesite phase II becomes a major deep-carbon carrier at the deeper parts of the lower mantle below 1900 km in depth.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
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.
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IJS, KILJ, NUK, PNG, UL, UM
Molybdenum disulphide is a layered transition metal dichalcogenide that has recently raised considerable interest due to its unique semiconducting and opto-electronic properties. Although several ...theoretical studies have suggested an electronic phase transition in molybdenum disulphide, there has been a lack of experimental evidence. Here we report comprehensive studies on the pressure-dependent electronic, vibrational, optical and structural properties of multilayered molybdenum disulphide up to 35 GPa. Our experimental results reveal a structural lattice distortion followed by an electronic transition from a semiconducting to metallic state at ~19 GPa, which is confirmed by ab initio calculations. The metallization arises from the overlap of the valance and conduction bands owing to sulphur-sulphur interactions as the interlayer spacing reduces. The electronic transition affords modulation of the opto-electronic gain in molybdenum disulphide. This pressure-tuned behaviour can enable the development of novel devices with multiple phenomena involving the strong coupling of the mechanical, electrical and optical properties of layered nanomaterials.
High‐quality single‐crystals of (Al,Fe)‐bearing bridgmanite, Mg0.88 Fe3+0.065Fe2+0.035Al0.14Si0.90O3, of hundreds of micrometer size were synthesized at 24 GPa and 1800 °C in a Kawai‐type apparatus ...from the starting hydrous melt containing ~6.7 wt% water. Analyses of synthesized bridgmanite using petrographic microscopy, scanning electron microscopy, and transmission electron microscopy show that the crystals are chemically homogeneous and inclusion free in micrometer‐ to nanometer‐spatial resolutions. Nanosecondary ion mass spectrometry (NanoSIMS) analyses on selected platelets show ~1,020(±70) ppm wt water (hydrogen). The high water concentration in the structure of bridgmanite was further confirmed using polarized and unpolarized Fourier‐transform infrared spectroscopy (FTIR) analyses with two pronounced OH‐stretching bands at ~3,230 and ~3,460 cm−1. Our results indicate that lower‐mantle bridgmanite can accommodate relatively high amount of water. Therefore, dehydration melting at the topmost lower mantle by downward flow of transition zone materials would require water content exceeding ~0.1 wt%.
Plain Language Summary
Water cycle between surface oceans and Earth's deep interior is a key to understanding the evolution and physical/chemical states of the planet. Early studies show that major transition zone minerals, wadsleyite, and ringwoodite, could accommodate abundant water (1–3 wt%), in the form of lattice‐bonded hydrogen atoms, in their crystal structures. However, water solubility in lower‐mantle bridgmanite, the most abundant mineral in the most volumetric layer of the planet, has remained poorly understood. The scientific challenge here was largely due to difficulties in making large‐sized high‐quality single‐crystals of bridgmanite for reliable characterizations of its water concentration. Here we synthesized single‐crystal bridgmanite of a few hundred micrometers in diameter, which are examined to be inclusion and precipitate free and thus can be used for reliable water concentration measurements using NanoSIMS analyses. Unpolarized and polarized FTIR analyses are used to identify characteristic OH‐stretching bands. Our results show that (Al,Fe)‐bearing bridgmanite could contain as high as 1,020(±70) ppm wt water. This high water concentration in bridgmanite has implications for our understanding of how melting can occur deep in the mantle below the transition zone.
Key Points
High‐quality, inclusion‐free bridgmanite single crystals (Mg0.88Fe3+0.065Fe2+0.035Al0.14Si0.90O3) were synthesized and characterized
The crystals contain ~1,020(±70) ppm wt water using NanoSIMS and show pronounced OH‐stretching bands at ~3230 and ~3460 cm‐1 in FTIR spectra
Dehydration melting at the topmost lower mantle can occur when water content exceeds ~0.1 wt% solubility limit
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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