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The decomposition of synthetic siderite (FeCO3) in air atmosphere at room temperature conditions was studied. Siderite was formed by mechanochemical reaction of Fe3O4 and graphite at high CO2 pressure in the presence of water. Kinetics of decomposition reaction was studied over period up to 9 days and it is shown that decomposition reaction obeys geometrical contraction solid-state reaction mechanism model. It was found that the water influences not only the kinetics of siderite formation but also its stability. Siderite completely decomposes at ambient conditions yielding magnetite (Fe3O4) and hematite (Fe2O3) which can reversibly re-absorb carbon dioxide.

Water ice becomes a superionic phase under the high pressure and temperature conditions of deep planetary interiors of ice planets such as Neptune and Uranus, which affects interior structures and generates magnetic fields. The solid Earth, however, contains only hydrous minerals with a negligible amount of ice. Here we combine high pressure and temperature electrical conductivity experiments, Raman spectroscopy and first-principles simulations to investigate the state of hydrogen in the pyrite-type FeO2Hx (x ≤ 1), which is a potential H-bearing phase near the core–mantle boundary. We find that when the pressure increases beyond 73 GPa at room temperature, symmetric hydroxyl bonds are softened and the H+ (or proton) becomes diffusive within the vicinity of its crystallographic site. Increasing temperature under pressure, the diffusivity of hydrogen is extended beyond the individual unit cell to cover the entire solid, and the electrical conductivity soars, indicating a transition to the superionic state, which is characterized by freely moving protons and a solid FeO2 lattice. The highly diffusive hydrogen provides fresh transport mechanisms for charge and mass, which dictate the geophysical behaviours of electrical conductivity and magnetism, as well as geochemical processes of redox, hydrogen circulation and hydrogen isotopic mixing in Earth’s deep mantle. Under conditions of Earth’s deep lower mantle, hydrogen ions diffuse freely through the FeOOH lattice framework and electrical conductivity increases rapidly, according to electrical conductivity experiments and first-principles simulations.

This study was conducted to understand the relationship between various critical temperatures and the stability of the secondary phases inside the heat-affected-zone (HAZ) of welded Grade 91 (Gr.91) steel parts. Type IV cracking has been observed in the HAZ, and it is widely accepted that the stabilities of the secondary phases in Gr.91 steel are critical to the creep resistance, which is related to the crack failure of this steel. In this work, the stabilities of the secondary phases, including those of the M23C6, MX, and Z phases, were simulated by computational thermodynamics. Equilibrium cooling and Scheil simulations were carried out in order to understand the phase stability in welded Gr.91 steel. The effect of four critical temperatures—that is, Ac1 (the threshold temperature at which austenite begins to form), Ac3 (the threshold temperature at which ferrite is fully transformed into austenite), and the M23C6 and Z phase threshold temperatures—on the thickness of the HAZ and phase stability in the HAZ is discussed. Overall, the simulations presented in this paper explain the mechanisms that can affect the creep resistance of Gr.91 steel, and can offer a possible solution to the problem of how to increase creep resistance at elevated temperatures by optimizing the steel composition, welding, and heat treatment process parameters. The simulation results from this work provide guidance for future alloy development to improve creep resistance in order to prevent type IV cracking.

If subduction carries hydrous minerals deep into Earth’s mantle, they may “rust” the iron outer core, forming vast sinks of oxygen that can later be returned to the atmosphere.

The structural stability of Ti3AlC2 under high pressure is important for understanding its mechanical properties. Here, we conducted a high hydrostatic pressure synchrotron X-ray diffraction experiment and no structural phase transition was observed. Like most other MAX phases, Ti3AlC2 showed an anisotropic compression behavior. Most importantly, an anomaly in c/a ratio was observed at 20.3 GPa, indicating that a pressure-induced isostructural phase transition occurred here. Analysis of the electronic band structure and Fermi surface revealed that three bands crossed the Fermi surface under compression, which suggested that this isostructural phase transition can be considered to be motivated by an electronic topological transition. The subsequent Hall-effect measurements reconfirmed this variation of the electronic band at the Fermi surface, which can be regarded as the electronic origin for the observed isostructural phase transition. These results enrich the basic property data of Ti3AlC2 and would benefit the further understanding of this promising material.

Carbon dioxide capture/release reactions using magnetite, Fe3O4, and hematite, Fe2O3, as sorbents were studied. Kinetics of mechanically activated chemical reactions between iron oxides and CO2 was investigated as a function of CO2 pressure and planetary ball mill process parameters. It was found that complete carbonation of iron oxides can be accomplished at room temperature and elevated CO2 pressure (10–30 bar). Siderite calcination was studied in vacuum and argon atmospheres. FeCO3 can be decomposed at 367 °C yielding magnetite, carbon and/or iron. This mixture can reversibly re-absorb carbon dioxide in multiple carbonation–calcination cycles. These results suggest that siderite or iron oxides are prospective and efficient reversible sorbents for CO2 capture. Keywords: Iron oxides, CO2 capture capacity, Carbonation, Mechanically activated chemical reactions, Calcination, Reversible sorbent

We report the catalyst-free synthesis of monolithic mesoporous nanopolycrystalline diamond from periodic mesoporous carbon at pressures between 15 and 21 GPa and a temperature of 1300 °C. We investigated the pressure-dependence of the porosity with 3-dimensional electron tomography. We have observed that surface areas increase from 56 to 90, 138 m2 g−1, and porosities increase from 10, 24, to 33% for materials produced at 15, 18, and 21 GPa, respectively. The increased porosity at higher pressure may be due to the earlier onset of the nucleation of diamond at higher pressure.

Origins of the ultralow velocity zones may be classified by their velocity-reduction-ratio, R = δ lnVS/δ lnVP, which ranges from 1.2–1.5 for iron oxides or iron-enriched magnesium oxides, 1.6–2.0 for pyrite-type FeO2Hx, 2.3–2.8 for the eutectic melt of Fe + C, 2.7–3.1 for partial melt of (Mg, Fe)SiO3 + Fe to 3.5–4.5 for iron-rich post-perovskite.

Electrical heterogeneity at the depth of 900-1400 km in Earth’s interior is a key factor to constrain the minor phase composition of the lower mantle. However, prevailing mineralogical models including Fe- or Al-enriched silicates or ferropericlase are insufficient to explain the ultra-high electrical conductivity in local areas of subduction slabs. Here, we measure the electrical conductivity of ε-FeOOH up to 61 GPa. A 3-order abrupt jump of electrical conductivity is observed in 45-50 GPa, reaching 1.24±0.19 × 103 S/m at 61 GPa. Density mean field theory simulations suggest that ε-FeOOH undergoes a Mott-type electronic transition, which leads the conduction mechanism to switch from small polaron conduction to free electron conduction. Compared with bridgmanite, ferropericlase and conventional mantle compositional models, the electrical conductivity of the metallic ε-FeOOH is 1-3 orders of magnitude higher. Minor or moderate incorporation of metallic ε-FeOOH into the ambient lower mantle could reproduce the observed electrical heterogeneity derived from geomagnetic data at 900-1400 km depth.

Metallization of hydrogen as a key problem in modern physics is the pressure-induced evolution of the hydrogen electronic band from a wide-gap insulator to a closed gap metal. However, due to its remarkably high energy, the electronic band gap of insulating hydrogen has never before been directly observed under pressure. Using high-brilliance, high-energy synchrotron radiation, we developed an inelastic x-ray probe to yield the hydrogen electronic band information in situ under high pressures in a diamond-anvil cell. The dynamic structure factor of hydrogen was measured over a large energy range of 45 eV. The electronic band gap was found to decrease linearly from 10.9 to 6.57 eV, with an 8.6 times densification ($\ensuremath{\rho}/{\ensuremath{\rho}}_{0}\ensuremath{\sim}8.6$) from zero pressure up to 90 GPa.

The structural stability of Ti

Understanding the mineralogy of the Earth's interior is a prerequisite for unravelling the evolution and dynamics of our planet. Here, we conducted high pressure-temperature experiments mimicking the conditions of the deep lower mantle (DLM, 1800–2890 km in depth) and observed surprising mineralogical transformations in the presence of water. Ferropericlase, (Mg, Fe)O, which is the most abundant oxide mineral in Earth, reacts with H2O to form a previously unknown (Mg, Fe)O2Hx (x ≤ 1) phase. The (Mg, Fe)O2Hx has a pyrite structure and it coexists with the dominant silicate phases, bridgmanite and post-perovskite. Depending on Mg content and geotherm temperatures, the transformation may occur at 1800 km for (Mg0.6Fe0.4)O or beyond 2300 km for (Mg0.7Fe0.3)O. The (Mg, Fe)O2Hx is an oxygen excess phase that stores an excessive amount of oxygen beyond the charge balance of maximum cation valences (Mg2+, Fe3+ and H+). This important phase has a number of far-reaching implications including extreme redox inhomogeneity, deep-oxygen reservoirs in the DLM and an internal source for modulating oxygen in the atmosphere.

As the reaction product of subducted water and the iron core, FeO2 with more oxygen than hematite (Fe2O3) has been recently recognized as an important component in the D” layer just above the Earth's core-mantle boundary. Here, we report a new oxygen-excess phase (Mg, Fe)2O3+δ (0 < δ < 1, denoted as ‘OE-phase’). It forms at pressures greater than 40 gigapascal when (Mg, Fe)-bearing hydrous materials are heated over 1500 kelvin. The OE-phase is fully recoverable to ambient conditions for ex situ investigation using transmission electron microscopy, which indicates that the OE-phase contains ferric iron (Fe3+) as in Fe2O3 but holds excess oxygen through interactions between oxygen atoms. The new OE-phase provides strong evidence that H2O has extraordinary oxidation power at high pressure. Unlike the formation of pyrite-type FeO2Hx which usually requires saturated water, the OE-phase can be formed with under-saturated water at mid-mantle conditions, and is expected to be more ubiquitous at depths greater than 1000 km in the Earth's mantle. The emergence of oxygen-excess reservoirs out of primordial or subducted (Mg, Fe)-bearing hydrous materials may revise our view on the deep-mantle redox chemistry.

The clay minerals are good indicators of rock history because their wide transformation variations are sensitive to the formation conditions, like temperature (T), pressure (P) and fluid composition. Accordingly, many geothermometers and fluid-rock interaction indicators based on mineral assemblages, illite polytypism and composition of chlorite have been proposed during the last 30 years. In this study, dioctahedral smectite dominated mudstone was tested hydrothermally in the 250–550 °C temperature range and 5–280 MPa pressure range, with reactions lasting 24 h, with a liquid/solid ratio of 1:1, and in saturated KCl solution. The mineralogical and chemical evolution of the clay minerals was characterized by using X-ray powder diffraction (XRD), X-ray fluorescence spectroscopy (XRF), field-emission scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and high resolution transmission electron microscopy (HRTEM). Molecular simulation was performed to determine the structure and behavior of interlayer cations and interlayer water changes of Na and Ca-rich smectite. We then obtained the transformation series as follows: from dioctahedral smectite to dioctahedral randomly interstratified illite/smectite to dioctahedral ordered illite/smectite to coexistence of dioctahedral and trioctahedral ordered illite/smectite and finally to trioctahedral chlorite and illite/smectite. This work illustrated the dioctahedral smectite alteration and transformation process and showed a continuous octahedral sheet transformation from dioctahedral smectite to trioctahedral chlorite. The series of clay minerals transformation provided an important experimental basis for understanding the geochemical conditions to which clay minerals were exposed to in the geological environment and will offer basic parameters for establishing a quantitative link between composition and formation conditions.

Silica (SiO2) is a major component of many minerals on the Earth. Under ambient conditions, silica does not react with carbon dioxide (CO2). However, at high pressure and temperature, the stability of silica may be affected by CO2, which becomes supercritical fluid CO2 under extreme conditions and can percolate into the Earth’s mantle to react with silica. Here, we investigated the chemical reactions between zeolite SSZ-56 as a model silicate and CO2 under temperature and pressure conditions close to those orf the mantle transition zone using density functional theory and molecular dynamics calculations. The reactions occurred on the SiO2 surface forming the zeolite’s cavities. In the melt, CO2 and SiO2 mixed closely and, upon cooling, formed a solid with disordered Si and C sites similar to a cristobalite SiO2–CO2 solid-solution structure. This structure was thermodynamically stable with respect to α-cristobalite and solid CO2 above 9 GPa.

Significance Diamond anvil cell (DAC) could be one of the most popular and versatile devices to generate static high pressure. By using beveled anvils, researchers can study materials up to 4 Mbar, which is considered as a limit pressure of conventional DAC; however, very few experiments have visited this pressure region. Here, we have systematically studied the behaviors of three DACs with three typical culet sizes, one of which reached 4 Mbar. In addition, by using submicrometer synchrotron X-ray probe, double cupping of beveled diamond anvils up to 4 Mbar is reported experimentally. Our results provide important guidance for future DAC studies to extreme high pressures.

Using global structure searches, we have explored the structural stability of CaB$_3$N$_3$, a compound analogous to CaC$_6$, under pressure. There are two high-pressure phases with space groups $\textit{R3c}$ and $\textit{Amm2}$ that were found to be stable between 29 and 42 GPa, and above 42 GPa, respectively. The two phases show different structural frameworks, analogous to graphitic CaC$_6$. Phonon calculations confirm that both structures are also dynamically stable at high pressures. The electronic structure calculations show that the $\textit{R3c}$ phase is a semiconductor with a band gap of 2.21 eV and that the $\textit{Amm2}$ phase is a semimetal. These findings help advance our understanding of the Ca-B-N ternary system.

High-pressure single-crystal/powder synchrotron X-ray diffraction was carried out on a hydrous pure magnesium pyrope (Mg 3 Al 2 Si 3 O 12 ) containing 900 ppmw H 2 O, synthesized at 4.0 GPa and 1300 K. The pressure-volume ( P - V ) single-crystal data from room pressure to 9.81 GPa at ambient temperature were fitted by a third-order Birch-Murnaghan equation of state (BM-EoS) yielding a unit-cell volume of V 0 = 1505.14 ± 0.38 A 3 , an isothermal bulk modulus of K 0 = 160 ± 3 GPa and its pressure derivative K ′ 0 = 5.2 ± 0.4. When fixing K' 0 = 4.0, the data yielded V 0 = 1504.58 ± 0.32 A 3 and K 0 = 166 ± 2 GPa. The pressure-volume-temperature ( P - V - T ) EoS of the synthetic hydrous pyrope was also measured at temperatures up to 900 K and pressures up to 16.75 GPa, using a diamond anvil cell in conjunction with in situ synchrotron angle-dispersive powder X-ray diffraction. The P - V data at room temperature and in a pressure range of 0.0001–14.81 GPa were then analyzed by a third-order BM-EoS and yielded V 0 = 1505.35 ± 0.25 A 3 , K 0 = 161 ± 2 GPa, K ′ 0 = 5.0 ± 0.3. With K' 0 fixed to 4.0, we also obtained V 0 = 1505.04 ± 0.29 A 3 and K 0 = 167 ± 1 GPa. Consequently, we fitted the P - V - T data with the high-temperature third-order BM-EoS approach and obtained the thermoelastic parameters of V 0 = 1505.4 ± 0.3 A 3 , K 0 = 162 ± 1 GPa, K ′ 0 = 4.9 ± 0.2, the temperature derivative of the bulk modulus ( ∂K 0 / ∂T ) P = −0.018 ± 0.004 GPa K −1 , and the thermal expansion coefficient at ambient conditions α 0 = (3.2 ± 0.1) × 10 −5 K −1 . These properties were consistent with the thermal pressure EoS analysis. These new results on hydrous pyrope were also compared with previous studies of anhydrous pyrope. The main effect of hydration on pyrope is to decrease K 0 and increase K' 0 by increasing the vacancies or unoccupied volume in the structure. The entire dataset enabled us to examine the thermoelastic properties of important mantle garnets and this data has further applications for modeling the P-T conditions in the upper mantle of the Earth’s interior using deep mineral assemblages.

Earth's water cycle enables the incorporation of water (hydration) in mantle minerals that can influence the physical properties of the mantle. Lattice thermal conductivity of mantle minerals is critical for controlling the temperature profile and dynamics of the mantle and subducting slabs. However, the effect of hydration on lattice thermal conductivity remains poorly understood and has often been assumed to be negligible. Here we have precisely measured the lattice thermal conductivity of hydrous San Carlos olivine (Mg0.9Fe0.1)2SiO4 (Fo90) up to 15 gigapascals using an ultrafast optical pump-probe technique. The thermal conductivity of hydrous Fo90 with ∼7,000 wt ppm water is significantly suppressed at pressures above ∼5 gigapascals, and is approximately 2 times smaller than the nominally anhydrous Fo90 at mantle transition zone pressures, demonstrating the critical influence of hydration on the lattice thermal conductivity of olivine in this region. Modeling the thermal structure of a subducting slab with our results shows that the hydration-reduced thermal conductivity in hydrated oceanic crust further decreases the temperature at the cold, dry center of the subducting slab. Therefore, the olivine-wadsleyite transformation rate in the slab with hydrated oceanic crust is much slower than that with dry oceanic crust after the slab sinks into the transition zone, extending the metastable olivine to a greater depth. The hydration-reduced thermal conductivity could enable hydrous minerals to survive in deeper mantle and enhance water transportation to the transition zone.

The present study reports effects of annealing treatment on the hardness of Ce75Al23Si2 rare earth-based metallic glasses (REMG). Then specimens were annealed at 100°C, 200°C, 250°C, 270°C, and 290°C five different temperatures for 30 minutes. After that, three different characteristic methods, including microindentation, X-ray Diffraction (XRD), and scanning electrical microscope (SEM) were conducted on the as received REMG sample and five annealed samples. XRD data demonstrate that the crystallization occurs in the sample at the annealing temperature as low as 200°C. The microindentation measurement shows that hardness of the REMG sample does not change at all before crystallization occurs in the sample and increases with the annealing temperature in the range of 200°C to 290°C. The average crystal size in the annealed samples was estimated using Debye-Scherrer equation to be 28-42 nm, in consistent with the SEM observation, indicating that nano-crystalline domains may give rise to the enhancement of hardness.

Core-shell architectures with hollow micro-structures have exhibited many potential applications in electrochemical energy storage fields. In this study, hierarchical Fe2O3@NiCo2O4 core-shell tubular arrays have been synthesized by a multistep chemical bath deposition method. The obtained samples have been fully characterized using powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. The results show that NiCo2O4 nanosheets have been uniformly coated on the Fe2O3 tubular array obtained by ZnO array template. When tested as an anode for lithium ion battery, the Fe2O3@NiCo2O4 tubular arrays have been found to exhibit an improved specific capacity of 587 mAh g−1 at a current density of 1000 mA g−1, good rate capability and cycling stability.

The critical secondary phases along with critical temperatures in regard to the creep resistance of Grade 91 steel alloy were evaluated in order to optimize the composition of the alloy to improve the creep resistance. The critical temperatures which are highlighted in this paper are Ac1 (the threshold temperature in which austinite begins to form), and Ac3 (the threshold temperature at which ferrite is fully transformed into austenite). Furthermore, optimization was presented through various Cr, V, Nb, and N concentrations in Gr.91 in relation to these critical temperatures and the mole fraction of M23C6, MX, and Z-Phase as critical secondary phases. The results indicate that increasing V and Nb will increase MX stability, whereas decreasing Cr and N lead to a decrease in Z-Phase and M23C6 stability, which may lead to an increased creep resistance of the material. Upon further analyzing the V, Nb, and N concentrations in Gr.91, results indicate that increasing Nb content in the steel can lead to an increased stability of Nb-rich MX2 carbide which can be used to replace Z-Phase in low N steels. Simulation results indicate that elimination of N with a dramatic increase in Nb concentrations result in the stability increase of only M23C6 and MX2 phases at temperature regions between 600 °C and 1370 °C and a complete removal of Z-Phase precipitation. Overall, with the elimination of N, which destabilizes Z-Phase precipitation, and an increase in Nb for NbC carbide stability, a possible solution to increasing both long-term and short-term creep resistance for Gr.91 can be achieved, though further optimization and creep experiments need to be conducted for final conclusions.

Recently we are witnessing the boom of high-pressure science and technology from a small niche field to becoming a major dimension in physical sciences. One of the most important technological advances is the integration of synchrotron nanotechnology with the minute samples at ultrahigh pressures. Applications of high pressure have greatly enhanced our understanding of the electronic, phonon, and doping effects on the newly emerged graphene and related 2D layered materials. High pressure has created exotic stoichiometry even in common Group 17, 15, and 14 compounds and drastically altered the basic σ and π bonding of organic compounds. Differential pressure measurements enable us to study the rheology and flow of mantle minerals in solid state, thus quantitatively constraining the geodynamics. They also introduce a new approach to understand defect and plastic deformations of nano particles. These examples open new frontiers of high-pressure research. PACS: 74.62.Fj, 07.85.Qe, 31.15.ae, 83.50.-v, Keywords: High pressure science and technology, Static high pressure, Synchrotron X-ray probe, Equation of state

High pressure behavior of ammonia borane after thermal decomposition was studied by Raman spectroscopy at high pressure up to 10 GPa using diamond anvil cell (DAC). The ammonia borane was decomposed at around 140 °C under the pressure at ∼0.7 GPa. Raman spectra show the hydrogen was desorbed within 1 h. The hydrogen was sealed in DAC well and cooled down to room temperature. Applying higher pressure up to ∼10 GPa indicates interactions between the products and loss of dihydrogen bonding. No rehydrogenation was detected in the pressure range investigated.

Electrodeposition of nickel sulfide (NieS) on NiCo2O4 (NiCo2O4@NieS) nanoplate arrays produces a new hierarchical coreeshell functional-material. Weak crystalline NieS nanosheets have been uniformly coated on the NiCo2O4 nanoplate array obtained by hydrothermal growth. When tested as an electrode for supercapacitors, the NiCo2O4@NieS nanoplate arrays have been found to exhibit a significantly improved areal capacitance of 1.85 F cm � 2 at a current density of 8 mA cm � 2 , good rate capability and cycling stability. In addition, the capacitance fading of the NiCo2O4@NieS arrays are attributed to the surface coarsening of the NieS nanosheets. Published by Elsevier B.V.

Using global structure searches, we have explored the structural stability of CaB

Goethite is a major iron-bearing sedimentary mineral on Earth. In this study, we conducted in situ high-pressure x-ray diffraction, Raman, and electrical impedance spectroscopy measurements of goethite using a diamond anvil cell (DAC) at room temperature and high pressures up to 32 GPa. We observed feature changes in both the Raman spectra and electrical resistance at about 5 and 11 GPa. However, the x-ray diffraction patterns show no structural phase transition in the entire pressure range of the study. The derived pressure-volume (P-V) data show a smooth compression curve with no clear evidence of any second-order phase transition. Fitting the volumetric data to the second-order Birch&ndash, Murnaghan equation of state yields V0 = 138.9 0.5 &Aring, 3 and K0 = 126 5 GPa.

Uniform solid solution of (1-x-y)Li[Li1/3Mn2/3]O2-xLiFeO2-yLiNiO2 (x+y≤0.5) were prepared by citric acid assisted sol-gel process including three-step of “preparation of wet gel—formation of spray-dried gel—annealing at high temperature”, and the effects of composition on the Li-storage properties of as-prepared samples were investigated by XRD, XPS and charge-discharge tests. The results indicate that all samples prepared at 650 °C ~800 °C show pure hexagonal phase (or monoclinic phase). Obvious cooperative effects are observed for the samples with suitable amount of composited LiNiO2 and LiFeO2 (x+y≈0.3~0.4), which can present high capacity of >210 mAhg-1, good structural stability and cycling performance. In view of capacity, cycling performance and cost, the samples with x+y≈0.3~0.4 and x/y=1.6~3.0 are considered to be more promising new cobalt-free cathode materials with high capacity of more than 240 mAhg-1.

The combined effects of Fe and Al on the electronic spin and valence states as well as the equation of state (EoS) of post-perovskite have been investigated using synchrotron X-ray diffraction and Mossbauer spectroscopy in high-pressure diamond anvil cells. Two post-perovskite samples (Mg0.6Fe0.15Al0.5Si0.75O3 and Mg0.66Fe0.13Al0.28Si0.86O3) were synthesized at approximately 165 GPa and 2200–2500 K, and were subsequently investigated for these properties at 114–170 GPa and 300 K. Analyses of the high-pressure Mossbauer spectra show that Fe2+ and Fe3+ occupy the large bipolar prismatic sites in both of our samples and remain in the high-spin state at ∼165–168 GPa∼165–168 GPa and 300 K. Combining the Mossbauer results with the obtained pressure–volume relationship from X-ray diffraction, we have found that the unit cell volume of post-perovskite can be significantly affected by the spin and valence states of Fe and the Al substitution. Mg0.6Fe0.15Al0.5Si0.75O3–PPv with the predominantly high-spin Fe2+ (∼95%∼95%) and a greater amount of Al has a unit cell volume similar to that of Mg0.66Fe0.13Al0.28Si0.86O3–PPv in which ∼65%∼65% of Fe is in the high-spin Fe3+ state. Our results are used together with previous results regarding the EoS parameters in Fe-bearing perovskite and post-perovskite to model the density and bulk sound velocity variation between perovskite and post-perovskite in the D″D″ layer, in which the enrichment of Fe and Al can produce an increase in density but substantially reduce the bulk sound velocity across the phase transition. That is, the combined effect of Fe and Al leads to an anti-correlation between the enhanced density and the reduced bulk sound velocity at the pressure condition of the lowermost mantle. Our results indicate that (Fe,Al)(Fe,Al)-rich silicate post-perovskite existing in the D″D″ region would be shown as a relatively high-density and low-velocity region in deep-mantle seismic observations.

Ammonia borane has received tremendous research attention in the past decade because of its potential for chemical hydrogen storage. This paper reviews recent studies about the behavior of ammonia borane at high pressures. While much work is still needed to comprehensively understand the pressure influence on this molecular crystal, a phase diagram based on the available experimental and theoretical data is constructed. Raman spectroscopy studies indicate five transitions upon compression up to 65 GPa at ambient temperature. Diffraction experiments and theoretical studies demonstrate that three of these transitions are first-order phase transformations in the sequence of I4mm-Cmc21-P21-(different) P21, and two are iso-structural. A low-temperature phase (Pmn21) and a high-temperature high-pressure phase (Pmna) are also recognized.

We conducted in situ Raman spectroscopy study on ammonia borane loaded in diamond anvil cell (DAC). The ammonia borane was decomposed at around 140 degree Celsius under the pressure ~0.7 GPa. Raman spectra show the hydrogen was desorbed within 1 hour at 140 degree Celsius. The hydrogen was sealed in the DAC well and cooled down near to room temperature. Applying higher pressure up to ~10 GPa indicates interactions between the products and loss of dihydrogen bonding. No rehydrogenation was detected in the pressure range investigated.Keywords: Ammonia borane; Diamond anvil cell; High pressure; Phase transition

The pressure-induced tetragonal (I4mm) to orthorhombic (Cmc21) phase transition in ammonia borane has been studied in a diamond anvil cell (DAC) using in situ Raman spectroscopy. At near ambient pressure in DAC, ammonia borane started decomposing as low as ∼88 °C, but at higher pressure, ammonia borane is stable up to 100 °C above 0.5 GPa. In the pressure–temperature phase diagram, the boundary between tetragonal (I4mm) and orthorhombic (Cmc21) phase was determined by the characteristic change in Raman spectra during compression and decompression at 3 constant temperatures: 20, 60 and 100 °C, respectively. The result shows that the phase transition is reversible with a small hysteresis, while the phase boundary shifts towards higher pressure with increasing temperature and a positive Clapeyron-slope. The positive Clapeyron-slope indicates that the phase transition is exothermic and it is predicted that the rehydrogenation of the high pressure orthorhombic phase would be easier due to its lower enthalpy.

Density of liquid FeS was measured at 1650 K and pressures up to 5.6 GPa using the X-ray absorption radiograph system of the X17B2 beamline at the National Synchrotron Light Source at the Brookhaven National Laboratory. The experimental data were fitted to the third-order Birch–Murnaghan equation of state to calculate the isothermal bulk modulus (K0) of the liquid, yielding K0 = 11 ± 3 GPa when the pressure derivative of bulk modulus is fixed at 5. Combining this result with those from previous studies on Fe–S liquid system, we suggest an exponential relation between the liquid Fe–S alloy bulk modulus and its sulfur content.

The pressure–volume relations of three synthetic garnet samples along pyrope–almandine (Py–Alm) join were measured at ambient temperature and high pressures up to 7, 21 and 19 GPa for Py83Alm17, Py54Alm46 and Py30Alm70, respectively. The obtained P–V data were fitted to the third order Birch–Murnaghan equation of state (EOS). The ambient cell volumes V0 of the three samples were measured to be 1511(1), 1515(2), and 1526(1) A3 respectively. With fixed pressure derivative of the isothermal bulk modulus K′ at 4.3, isothermal bulk moduli K0 of the three samples were determined to be 172(4), 174(2), and 183(2) GPa respectively. These results confirm that almandine content (iron substitution) increase the bulk modulus of the garnet join following a nearly ideal mixing model. The relation between bulk modulus and almandine mole fraction (n) in this garnet join is derived to be K0 = 170 + 15 n. These data can be used to contribute to construction of compositional model of earth mantle.

Iron-bearing pyrope, an abundant silicate mineral in the Earth’s upper mantle, exhibits the largest quadrupole splitting (QS) in Mossbauer spectra among all common Fe-bearing rock-forming silicate minerals at ambient conditions, with a value of ~3.5 mm/s. Knowledge regarding the hyperfine QS of mantle minerals at relevant pressures and temperatures ( P-T ) is needed to aid our understanding of the electronic spin and valence states of iron and local site distortion in major mantle silicate minerals. The latter, in turn, is relevant for coherent model of electronic and mechanical properties of the Earth’s mantle. Here we have measured synchrotron Mossbauer spectra (SMS) of the high-spin Fe2+ in Fe-bearing pyrope garnet with two distinct compositions, (Mg0.8Fe0.2)3Al2Si3O12 (py80alm20) and Fe3Al2Si3O12 (alm100), up to 30 GPa and 750 K. Analyses of the SMS spectra revealed that the high-spin Fe2+ ions in the distorted dodecahedral site exhibit extremely high QS of ~3.4–3.6 mm/s and relatively high chemical shifts (CS) of ~1.2–1.3 mm/s at high P-T , indicating that the Fe2+ ions remain in the high-spin state. An increase in the Fe content in the pyrope-almandine series only slightly decreases the QS and CS of the Fe2+ ions. To decipher the energy separation (Δ) between the two lowest energy levels of the 3 d electrons of the Fe2+ ions in the sample, the d xy and d z 2 orbitals, the QS values of py80alm20 at high P-T were further evaluated using Huggins model. Our modeled results show that the Δ of the Fe2+ ions in py80alm20 is ~156 meV at high P-T , and may be correlated to the change of the crystal-field energy splitting (ΔC). Comparison of the QS, Δ, and ΔC values of Fe2+ ions in the distorted dodecahedral sites of pyrope and silicate perovskite indicates that the high-spin Fe2+ with the extremely high QS can remain stable at high P-T conditions, consistent with recent theoretical predictions. Our results thus contribute to our current understanding of the hyperfine parameters and spin and valence states of iron in the mantle silicate minerals at high P-T .

We have studied the phase transformation behavior of the potential hydrogen storage compound ammonia borane at low temperature (from room temperature down to 90 K) and high pressure (from ambient pressure to 9.5 GPa at room temperature and up to 15 GPa at 90 K) region using the diamond anvil cell. This material shows four new phase transitions in this pressure and temperature region. The phase transition phenomenon is evidenced by the splitting of the peak and/or the appearance of the new peak in the Raman spectra as well as by the change of the pressure coefficient of the Raman modes. The phase boundaries between these phases are also established from the data collected during different cooling cycles. These results provide the information about the stability of the bonding characteristics of this potential hydrogen storage material at low temperature and high pressure region.

To date, most of the experiments performed to quantify the hydrolytic weakening of olivine single crystals have been performed at low confining pressure (and low water fugacities). In order to determine if the degree of hydrolytic weakening changes at higher water fugacities (higher water contents), we have performed a series of high pressure ( P ) and high temperature ( T ) deformation experiments in hydrous condition on San Carlos olivine (Fo92) and forsterite (Fo100) single crystals, in a Deformation-DIA apparatus (D-DIA). Deformation was carried out in axial compression along the [1 1 0] c crystallographic reference frame direction in order to activate [1 0 0](0 1 0) dislocation slip system alone, at P ranging from 4 to 7 GPa and T = 1200 °C. Water was supplied during the experiments through dehydration of a talc sleeve (talc → enstatite + coesite + H 2 O) around the sample, while the differential stress and strain rate of the specimen were calculated from in situ X-ray diffraction and time-resolved imaging, respectively. Run products were investigated using Fourier transform infrared (FTIR) spectroscopy, and optical/electron microscopy (SEM and TEM). Structurally bound hydroxyl content (C OH ) derived from absorbance in FTIR spectra, based on the Paterson (1982) relation, were typically between 290 and 720 ppm H/Si in the run products and the majority of these hydroxyls were observed as distinct peaks in the range of wavenumber from 3620 to 3500 cm −1 . TEM reveals that the [1 0 0](0 1 0) slip system was effectively activated and responsible for crystal deformation, and that dislocation glide was significantly assisted by recovering processes such as dislocation climb and dipole annihilation. Combining the experimental data from this study and Mackwell et al. (1985) , we estimate the activation volume V ∗ [110]c for San Carlos olivine in order to produce a reasonable water-fugacity exponent r in the flow law, and obtain V ∗ [110]c = 12.1 cm 3 /mol for r = 0 and V ∗ [110]c = 17.3 cm 3 /mol for r = 1.2. Comparisons of the strengths of the wet single crystals to the strengths of dry single crystals calculated from the rheological laws ( Bai et al., 1991, Darot and Gueguen, 1981, Raterron et al., 2009 ) indicate that the wet single crystal cylinders were between 1.3 and 1.6 times weaker than their dry counterpart at same conditions, which is consistent with previous studies of [1 0 0](0 1 0) olivine rheology performed at low confining pressure (300 MPa).

We studied the crystal structure and spin state of MnP under high pressure with synchrotron X-ray diffraction and X-ray emission spectroscopy. MnP has an exceedingly strong anisotropy in compressibility, with the primary compressible direction along the b axis of the Pnma structure. X-ray emission spectroscopy reveals a pressure-driven quenching of the spin state in MnP. Firstprinciples calculations suggest that the strongly anisotropic compression behavior significantly enhances the dispersion of the Mn d-orbitals and the splitting of the d orbital levels compared to the hypothetical isotropic compression behavior. Thus, we propose spin quenching results mainly from the significant enhancement of the itinerancy of d electrons and partly from spin rearrangement occurring in the split d-orbital levels near the Fermi level. This explains the fast suppression of magnetic ordering in MnP under high pressure. The spin quenching lags behind the occurrence of superconductivity at ~8 GPa implying that spin fluctuations govern the electron pairing for superconductivity., Comment: 17 pages, 6 figures, accepted by New Journal of Physics

Raman spectroscopy, synchrotron angle-dispersive X-ray diffraction (ADXRD), first-principles calculations, and electrical resistivity measurements were carried out under high pressure to investigate the structural stability and electrical transport properties of metavanadate MgV2O6. The results have revealed the coordination change of vanadium ions (from 5+1 to 6) at around 4 GPa. In addition, a pressure-induced structure transformation from the C2/m phase to the C2 phase in MgV2O6 was detected above 20 GPa, and both phases coexisted up to the highest pressure. This structural phase transition was induced by the enhanced distortions of MgO6 octahedra and VO6 octahedra under high pressure. Furthermore, the electrical resistivity decreased with pressure but exhibited different slope for these two phases, indicating that the pressure-induced structural phase transitions of MgV2O6 was also accompanied by the obvious changes in its electrical transport behavior.

One of the major obstacles to the use of hydrogen as an energy carrier is the lack of proper hydrogen storage material. Lithium amidoborane has attracted significant attention as hydrogen storage material. It releases ∼10.9 wt% hydrogen, which is beyond the Department of Energy target, at remarkably low temperature (∼90 °C) without borazine emission. It is essential to study the bonding behavior of this potential material to improve its dehydrogenation behavior further and also to make rehydrogenation possible. We have studied the high-pressure behavior of lithium amidoborane in a diamond anvil cell using in situ Raman spectroscopy. We have discovered that there is no dihydrogen bonding in this material, as the N—H stretching modes do not show redshift with pressure. The absence of the dihydrogen bonding in this material is an interesting phenomenon, as the dihydrogen bonding is the dominant bonding feature in its parent compound ammonia borane. This observation may provide guidance to the improvement of the hydrogen storage properties of this potential material and to design new material for hydrogen storage application. Also two phase transitions were found at high pressure at 3.9 and 12.7 GPa, which are characterized by sequential changes of Raman modes.