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Thermoelastic properties of mantle candidate minerals are essential to our understanding of geophysical phenomena, geochemistry, and geodynamic evolutions of the silicate Earth. However, the lower-mantle mineralogy remains much debated due to the lack of single-crystal elastic moduli (Cij) and aggregate sound velocities of (Al,Fe)-bearing bridgmanite, the most abundant mineral of the planet, at the lower mantle pressure-temperature (P-T) conditions. Here we report single-crystal Cij of (Al,Fe)-bearing bridgmanite, Mg0.88Fe0.1Al0.14Si0.90O3 (Fe10-Al14-Bgm) with Fe3+/ΣFe = ~0.65, up to ~82 GPa using X-ray diffraction (XRD), Brillouin light scattering (BLS), and impulsive stimulated light scattering (ISLS) measurements in diamond-anvil cells (DACs). Two crystal platelets with orientations of (–0.50, 0.05, –0.86) and (0.65, –0.59, 0.48), that are sensitive to deriving all nine Cij, are used for compressional and shear wave velocity (νP and νS) measurements as a function of azimuthal angles over 200° at each experimental pressure. Our results show that all Cij of singe-crystal Fe10-Al14-Bgm increase monotonically with pressure with small uncertainties of 1–2% (±1σ), except C55 and C23, which have uncertainties of 3–4%. Using the third-order Eulerian finite-strain equations to model the elasticity data yields the aggregate adiabatic bulk and shear moduli and respective pressure derivatives at the reference pressure of 25 GPa: KS = 326 ± 4 GPa, µ = 211 ± 2 GPa, KS′ = 3.32 ± 0.04, and µ′ = 1.66 ± 0.02 GPa. The high-pressure aggregate νS and νP of Fe10-Al14-Bgm are 2.6–3.5% and 3.1–4.7% lower than those of MgSiO3 bridgmanite end-member, respectively. These data are used with literature reports on bridgmanite with different Fe and Al contents to quantitatively evaluate pressure and compositional effects on their elastic properties. Comparing with one-dimensional seismic profiles, our modeled velocity profiles of major lower-mantle mineral assemblages at relevant P-T suggest that the lower mantle could likely consist of about 89 vol% (Al,Fe)-bearing bridgmanite. After considering uncertainties, our best-fit model is still indistinguishable from pyrolitic or chondritic models.

The post-stishovite transition is a classic pseudo-proper typed ferroelastic transition with a symmetry-breaking spontaneous strain. This transition has been studied using high-pressure spontaneous strains, optic modes, and elastic moduli (Cij) based on the Landau modeling, but its atomistic information and structural distortion remain poorly understood. Here we have conducted synchrotron single-crystal X-ray diffraction measurements on stishovite crystals up to 75.3 GPa in a diamond-anvil cell. Analysis of the data reveals atomic positions, bond lengths, bond angles, and variations of SiO6 octahedra across the transition at high pressure. Our results show that the O coordinates split at ~51.4 GPa, where the apical and equatorial Si-O bond lengths cross over, the SiO6 octahedral distortion vanishes, and the SiO6 octahedra start to rotate about the c axis. Moreover, distortion mode analysis shows that an in-plane stretching distortion (GM1+ mode) occurs in the stishovite structure at high pressure while a rotational distortion (GM2+ mode) becomes dominant in the post-stishovite structure. These results are used to correlate with elastic moduli and Landau parameters (symmetry-breaking strain e1–e2 and order parameter Q) to provide atomistic insight into the ferroelastic transition. When the bond lengths of two Si-O bonds are equal due to the contribution from the GM1+ stretching mode, C11 converges with C12, and the shear wave VS1[110] polarizing along [110] and propagating along [110] vanishes. Values of e1–e2 and Q are proportional to the SiO6 rotation angle from the occurrence of the GM1+ rotational mode in the post-stishovite structure. Our results on the pseudo-proper type transition are also compared with that for the proper type in albite and improper type in CaSiO3 perovskite. The symmetry-breaking strain, in all these types of transitions, arises as the primary effect from the structural angle (such as SiO6 rotation or lattice constant angle) and its relevant distortion mode in the low-symmetry ferroelastic phase.

Phase Egg and δ-AlOOH are two typical hydrous phases that might exist in the wet sedimentary layer of subducted slabs under mantle conditions. They are thus regarded as potential water carriers to Earth’s deep mantle. In this report, we report the full elastic constants of both phases determined by Brillouin scattering and X-ray diffraction measurements under ambient conditions. Our results indicate that the hydrogen-bond configurations in the crystal structures of the two phases have a profound effect on their principal elastic constants. The adiabatic bulk modulus (KS) and shear modulus (G) calculated from the obtained elastic constants using the Voigt-Reuss-Hill averaging scheme are 158.3(201) GPa and 123.0(60) GPa for phase Egg and 162.9(31) GPa and 145.2(13) GPa for δ-AlOOH, respectively. These results allow us to evaluate elastic moduli and sound velocities of hydrous minerals in the Al2O3-H2O-SiO2 ternary system (simplified composition of subducted wet sedimentary layer) at ambient conditions, including the contrast of the acoustic velocities VP and VS for the reaction AlSi3OH = δ-AlOOH + SiO2 (stishovite) and the evolution in the elastic moduli and sound velocities of hydrous minerals as a function of density.

In this study, we have investigated the crystal structure and equation of state of tetragonal CaSiO3-perovskite up to 200 GPa using synchrotron X-ray diffraction in laser-heated diamond-anvil cells. X-ray diffraction patterns of the quenched CaSiO3-perovskite above 148 GPa clearly show that 200, 211, and 220 peaks of the cubic phase split into 004+220, 204+312, and 224+400 peak pairs, respectively, in the tetragonal structure, and their calculated full-width at half maximum (FWHM) exhibits a substantial increase with pressure. The distribution of diffraction peaks suggests that the tetragonal CaSiO3-perovskite most likely has an I4/mcm space group at 300 K between 148 and 199 GPa, although other possibilities might still exist. Using the Birch-Murnaghan equations, we have determined the equation of state of tetragonal CaSiO3-perovskite, yielding the bulk modulus K0T = 227(21) GPa with the pressure derivative of the bulk modulus, K0T′ = 4.0(3). Modeled sound velocities at 580 K and around 50 GPa using our results and literature values show the difference in the compressional (VP) and shear-wave velocity (VS) between the tetragonal and cubic phases to be 5.3 and 6.7%, respectively. At ~110 GPa and 1000 K, this phase transition leads to a 4.3 and 9.1% jump in VP and VS, respectively. Since the addition of Ti can elevate the transition temperature, the transition from the tetragonal to cubic phase may have a seismic signature compatible with the observed mid-lower mantle discontinuity around the cold subduction slabs, which needs to be explored in future studies.

Excitonic properties in 2D heterobilayers are closely governed by charge transfer (CT) and excitonic energy transfer (ET) at van der Waals interfaces. Various means have been employed to modulate the interlayer CT and ET, including electrical gating and modifying interlayer spacing, but with limited extent in their controllability. Here, we report a novel method to modulate these transfers in the MoS

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 (H2Ot) 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 ±1σ. 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– 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.

Tailoring interlayer coupling has emerged as a powerful tool to tune the electronic structure of van der Waals (vdW) bilayers. One example is the usage of the moire pattern to create controllable two-dimensional electronic superlattices through the configurational dependence of interlayer electronic couplings. This approach has led to some remarkable discoveries in twisted graphene bilayers, and transition metal dichalcogenide (TMD) homo- and hetero-bilayers. However, a largely unexplored factor is the interlayer distance, d, which can impact the interlayer coupling strength exponentially. In this letter, we quantitatively determine the coupling strengths as a function of interlayer spacing at various critical points of the Brillouin zone in bilayer MoS2. The exponential dependence of the coupling parameter on the gap distance is demonstrated. Most significantly, we achieved a 280% enhancement of K-valley coupling strength with an 8% reduction of the vdW gap, pointing to a new strategy in designing a novel electronic system in vdW bilayers.

Silicate and oxide glasses are often chemically doped with a variety of cations to tune for desirable properties in technological applications, but their performances are often limited by relatively lower mechanical and elastic properties. Finding a new route to synthesize silica-based glasses with high elastic and mechanical properties needs to be explored. Here, we report a dense SiO2-glass with ultra-high elastic moduli using sound velocity measurements by Brillouin scattering up to 72 GPa at 300 K. High-temperature measurements were performed up to 63 GPa at 750 K and 59 GPa at 1000 K. Compared to compression at 300 K, elevated temperature helps compressed SiO2-glass effectively overcome the kinetic barrier to undergo permanent densification with enhanced coordination number and connectivity. This hot compressed SiO2-glass exhibits a substantially high bulk modulus of 361–429 GPa which is at least 2–3 times greater than the metallic, oxide, and silicate glasses at ambient conditions. Its Poisson’s ratio, an indicator for the packing efficiency, is comparable to the metallic glasses. Even after temperature quench and decompression to ambient conditions, the SiO2-glass retains some of its unique properties at compression and possesses a Poisson’s ratio of 0.248(11). In addition to chemical alternatives in glass syntheses, coupled compression and heating treatments can be an effective means to enhance mechanical and elastic properties in high-performance glasses.

The isotopic compositions of iron in major mantle minerals may record chemical exchange between deep-Earth reservoirs as a result of early differentiation and ongoing plate tectonics processes. Bridgmanite (Bdg), the most abundant mineral in the Earth’s lower mantle, can incorporate not only Al but also Fe with different oxidation states and spin states, which in turn can influence the distribution of Fe isotopes between Bdg and ferropericlase (Fp) and between the lower mantle and the core. In this study, we combined first-principles calculations with high-pressure nuclear resonant inelastic X-ray scattering measurements to evaluate the effects of Fe site occupancy, valence, and spin states at lower-mantle conditions on the reduced Fe partition function ratio (β-factor) of Bdg. Our results show that the spin transition of octahedral-site (B-site) Fe3+ in Bdg under mid-lower-mantle conditions generates a +0.09‰ increase in its β-factor, which is the most significant effect compared to Fe site occupancy and valence. Fe2+-bearing Bdg varieties have smaller β-factors relative to Fe3+-bearing varieties, especially those containing B-site Fe3+. Our models suggest that Fe isotopic fractionation between Bdg and Fp is only significant in the lowermost mantle due to the occurrence of low-spin Fe2+ in Fp. Assuming early segregation of an iron core from a deep magma ocean, we find that neither core formation nor magma ocean crystallization would have resulted in resolvable Fe isotope fractionation. In contrast, Fe isotopic fractionation between low-spin Fe3+-bearing Bdg/Fe2+-bearing Fp and metallic iron at the core-mantle boundary may have enriched the lowermost mantle in heavy Fe isotopes by up to +0.20‰.

The stacking order of 2D materials can result in fascinating physical properties. Two different stacking orders (AA and AB) were recently identified in ReS2, a rising star in the transition metal d...

The relation between antifungal susceptibility and treatment outcomes is not well-characterized. There is paucity of surveillance data for cerebrospinal fluid (CSF) isolates of cryptococcus investigated with YEASTONE colorimetric broth microdilution susceptibility testing. A retrospective study of laboratory-confirmed cryptococcus meningitis (CM) patients was conducted. The antifungal susceptibility of CSF isolates was determined using YEASTONE colorimetric broth microdilution. Clinical parameters, CSF laboratory indices, and antifungal susceptibility results were analyzed to identify risk factors for mortality. High rates of resistance to fluconazole and flucytosine were observed in this cohort. Voriconazole had the lowest MIC (0.06 µg/mL) and lowest rate of resistance (3.8%). In a univariate analysis, hematological malignancy, concurrent cryptococcemia, high Sequential Organ Failure Assessment (SOFA) score, low Glasgow coma scale (GCS) score, low CSF glucose level, high CSF cryptococcal antigen titer, and high serum cryptococcal antigen burden were associated with mortality. In a multivariate analysis, meningitis with concurrent cryptococcemia, GCS score, and high CSF cryptococcus burden, were independent predictors of poor prognosis. Both early and late mortality rates were not significantly different between CM wild type and non-wild type species.

During the decompression of plastically deformed glasses at room temperature, some aspects of irreversible densification may be preserved. This densification has been primarily attributed to topological changes in glass networks. The changes in short-range structures like cation coordination numbers are often assumed to be relaxed upon decompression. Here the NMR results for aluminosilicate glass upon permanent densification up to 24 GPa reveal noticeable changes in the Al coordination number under pressure conditions as low as ∼6 GPa. A drastic increase in the highly coordinated Al fraction is evident over only a relatively narrow pressure range of up to ∼12 GPa, above which the coordination change becomes negligible up to 24 GPa. In contrast, Si coordination environments do not change, highlighting preferential coordination transformation during deformation. The observed trend in the coordination environment shows a remarkable similarity to the pressure-induced changes in the residual glass density, yielding a predictive relationship between the irreversible densification and the detailed structures under extreme compression. The results open a way to access the nature of plastic deformation in complex glasses at room temperature.

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)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 VP and VS 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 Fe-enriched 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.

Boron arsenide (BAs) is a covalent semiconductor with a theoretical intrinsic thermal conductivity approaching 1300 W/m K. The existence of defects not only limits the thermal conductivity of BAs significantly but also changes its pressure-dependent thermal transport behavior. Using both picosecond transient thermoreflectance and femtosecond time-domain thermoreflectance techniques, we observed a non-monotonic dependence of thermal conductivity on pressure. This trend is not caused by the pressure-modulated phonon–phonon scattering, which was predicted to only change the thermal conductivity by 10%–20%, but a result of several competing effects, including defect–phonon scattering and modification of structural defects under high pressure. Our findings reveal the complexity of the defect-modulated thermal behavior under pressure.

Iron crystallization in super-Earth interiors plays a key role in their habitability

In this study, we investigated the single-crystal elasticity of (Al,Fe)-bearing bridgmanite (Bgm) with chemical compositions of Mg 0.95 Fe 0.033 2 + Fe 0.027 3 + Al0.04Si0.96O3 (Fe6-Al4-Bgm) and Mg 0.89 Fe 0.024 2 + Fe 0.096 3 + Al0.11Si0.89O3 (Fe12-Al11-Bgm) using combined experimental results from Brillouin light scattering (BLS), impulsive stimulated light scattering (ISLS), and X-ray diffraction (XRD) measurements in diamond anvil cells at 25 and 35 GPa. Based on experimentally measured compressional and shear wave velocities ( V P , V S ) as a function of azimuthal angles within selected crystal platelets that are sensitive to derivation of nine elastic constants for each composition, we reliably derived the full elastic constants of Fe6-Al4-Bgm and Fe12-Al11-Bgm at the two experimental pressures. Our results show that the combined Fe and Al substitution results in a reduction of both V S and V P in Fe12-Al11-Bgm up to 2.6(±0.5)% and 1.5(±0.3)%, respectively, compared with those in Fe6-Al4-Bgm at the experimental pressures. In particular, we observed strong combined Fe and Al effects on V S splitting anisotropy of (Al,Fe)-bearing Bgm at the two experimental pressures: Fe6-Al4-Bgm exhibits the highest V S splitting anisotropy of ∼8.23-9.0% along the [001] direction, while the direction shifts to the midway between [100] and [001] directions for Fe12-Al11-Bgm with V S splitting anisotropy of ∼7.68-11.06%. Combining the single-crystal elasticity data of Fe6-Al4-Bgm and Fe12-Al11-Bgm with the crystallographic preferred orientation (CPO) results of deformed Bgm at relevant lower-mantle pressure-temperature (P-T) conditions from literature, we modeled the seismic V S radial anisotropy of deformed (Al,Fe)-bearing Bgm near a subducting slab at conditions relevant to the topmost lower mantle. Taking into account the Fe and Al contents in (Al,Fe)-bearing Bgm with depth in the Earth's topmost lower mantle, the results of our model show that the deformation of Fe6-Al4-Bgm and Fe12-Al11-Bgm crystals would produce ∼0.9% and ∼0.8% V S radial anisotropy at depths of ∼670 and ∼920 km, respectively. These findings provide mineral physics explanations to the distinct seismically-detected V S radial anisotropies at the topmost lower mantle near subducted slabs, especially in the Tonga-Kermadec subduction region.

Iron is the most abundant transition metal in the Earth's interior, yet considerable uncertainties remain as to why mantle-derived rocks have diverse iron isotopic compositions. In particular, the isotopic fractionation behavior of iron in the lower-mantle minerals bridgmanite and ferropericlase are largely unexplored. The reason is that it is challenging to study isotopic fractionation at the high pressures relevant to the deep mantle. Here we report in situ measurements of the mean force constants of iron bonds in these minerals pressurized in diamond anvil cells using the technique of nuclear resonant inelastic X-ray scattering (NRIXS). We find that the transition from high- to low-spin iron in ferropericlase ((Mg0.75Fe0.25)O) at approximately 60 GPa drastically stiffens its iron bonds in the low-spin state. The mean force constant of iron bonds in both Fe-bearing and (Fe,Al)-bearing bridgmanite exhibits softening by 21% at approximately 40–60 GPa, which seems to be associated with changes in the iron local environment during the transition from low to high quadrupole splitting states. These results indicate that in the lower mantle, low-spin ferropericlase is enriched in heavy iron isotopes relative to bridgmanite and metallic iron by +0.15‰ and +0.12‰, respectively. Based on these results, we investigate whether terrestrial magma ocean crystallization could have fractionated iron isotopes. We conclude that this process cannot be responsible for the heavy iron isotope enrichment measured in terrestrial basalts.

Few-layer black phosphorus (BP) with an in-plane puckered crystalline structure has attracted intense interest for strain engineering due to both its significant anisotropy in mechanical and electrical properties and its high intrinsic strain limit. Here, we investigated the phonon response of few layer BP under uniaxial tensile strain (∼7%) with in situ polarized Raman spectroscopy. Together with the first-principles density functional theory (DFT) analysis, the anisotropic Poisson's ratio in few-layer BP was verified as one of the primary factors that caused the large discrepancy in the trend of reported Raman frequency shift for strained BP, armchair (AC) direction in particular. By carefully including and excluding the anisotropic Poisson's ratio in the DFT emulations, we rebuilt both trends reported for Raman mode shifts. Furthermore, the angle-resolved Raman spectroscopy was conducted in situ under tensile strain for systematic investigation of the in-plane anisotropy of BP phonon response. The experimentally observed thickness and crystallographic orientation dependence is elaborated using DFT theory as having a strong correlation between the strain-perturbated electronic-band structure and the phonon vibration modes. This study provides insight, both experimentally and theoretically, for the complex electron-phonon interaction behavior in strained BP, which enables diverse possibilities for the strain engineering of electrical and optical properties in BP and similar two-dimensional nanomaterials.

Preferential iron partitioning into melt during melting and crystallization of lower mantle minerals – bridgmanite and ferropericlase – can play a critical role in our understanding of the origin of the early Earth and its evolution to form chemically and seismically distinct regions in the present lowermost mantle. Of particular interest is the consequence of iron spin crossover in ferropericlase on the physical and chemical properties of the molten materials under relevant pressure–temperature (P–T) conditions of the lowermost mantle. However, the spin crossover in liquid (Mg, Fe)O and its effects on melting curves, iron partitioning, melt density – and thus the evolution of an early basal magma ocean – remain poorly studied. Here we conducted high P–T melting experiments on ferropericlase with a starting composition of (Mg0.86Fe0.14)O using synchrotron X-ray diffraction up to ∼120 GPa and ∼5400 K in laser-heated diamond anvil cells, together with chemical analyses on quenched samples using focused ion beam and energy dispersive spectroscopy technique. An ideal solid solution model could be satisfactorily used to fit the experimental data of the liquidus and solidus of (Mg, Fe)O for pure high-spin (HS, below ∼83 GPa), and low-spin (LS, above ∼120 GPa) states, respectively. The experimental solidus and liquidus at 99 GPa and ∼4000–5200 K strongly deviate from ideal solid solution behavior for pure HS and LS states alone, but can be qualitatively explained using a thermodynamics model for a mixture of HS and LS states across the spin crossover. We found that LS (Mg, Fe)O exhibits ∼6–8% lower solidus and liquidus temperature than its HS counterpart. Furthermore, our results show that iron preferentially partitions into melt within the spin crossover to generate iron-rich LS melt. Such iron-rich LS (Mg, Fe)O is ∼27(±5)% denser than materials expected for lowermost mantle and could potentially persist as residual melt in the lowermost mantle at the late stage of magma ocean crystallization. Modeled results indicate that the existence of the dense, iron-rich LS (Mg, Fe)O melt in the lowermost mantle could provide plausible explanations for characteristic seismological signatures of ultra-low velocity zones (ULVZs).

We examine electronic and crystal structures of iron-based superconductors

Earth’s mantle composition is essential to our understanding of its physics and dynamics. Here we report single-crystal elasticity (Cij) of (Al,Fe)-bearing bridgmanite, Mg0.88Fe0.1Al0.14Si0.90O3 with Fe3+/∑Fe=~0.65, up to ~82 GPa measured in diamond anvil cells. Together with heat capacity measurements on bridgmanite and ferropericlase, we develop a fully internally-consistent thermoelastic model to simultaneously evaluate lower-mantle mineralogy and geotherm via comparisons of P-wave, S-wave velocities, and density (VP, VS, and ρ) with one-dimensional seismic profiles. Our best-fit model demonstrates the lower mantle consists of ~89 vol% (Al,Fe)-bearing bridgmanite, ~4 vol% ferropericlase, and ~7 vol% CaSiO3 perovskite. A chemically layered mantle with pyrolitic upper mantle and bridgmanite-predominant lower mantle would display ~3.2(±1.5)%, ~5.2(±1.5)%, and ~5.0(±1.0)% jumps in VP, VS, and ρ, respectively, across the 660-km discontinuity, which are well consistent with seismic reflection observations. The lower mantle could have become bridgmanite-predominant via accumulations of ancient silica-rich materials, which helps explain current deep-Earth seismic and geochemical signatures.

Experiments in laser-heated diamond anvil cells (LH DACs) are conducted to assess phase diagrams of planetary materials at high pressure-temperature (P-T) conditions; thus, reliable determination of temperature in LH DAC experiments is essential. Radiometric temperature determination in LH DACs relies on the assumption of sample's wavelength-independent optical properties (graybody assumption), which is not justified for major lower mantle materials. The result is that experimental phase diagrams contain systematic unconstrained errors. Here we estimate the systematic error in radiometric temperature of nongray polycrystalline bridgmanite (Bgm; Mg0.96Fe2+0.036Fe3+0.014Si0.99O3) in a LH DAC by modeling emission and absorption of thermal radiation in a sample with experimentally-constrained optical properties. A comparison to experimental data validates the models and reveals that thermal spectra measured in LH DAC experiments record the interaction of radiation with the hot nongray sample. The graybody assumption in the experiments on translucent Bgm (light extinction coefficient, k < ∼250 cm-1 at 500–900 nm) yields temperatures ∼5% higher than the maximum temperature in the sample heated to ∼1900 K. In contrast, the graybody temperature of dark Bgm (k > ∼1500 cm−1), such as that produced upon melt quenching in LH DACs, underestimates the maximum temperature by ∼10%. Our experimental results pose quantitative constraints on the effect of nongray optical properties on the uncertainty of radiometric temperature determination in Bgm in the LH DACs. Evaluating nongray temperature in the future would enable a revision of the Bgm to post-perovskite phase transition and the high-pressure melting curve of Bgm.

The melting curve of vanadium at high pressure and temperature (P-T) is of great interest to our understanding of $d$-orbital transition metals with simple crystal structures at extreme P-T conditions. Here we have investigated the melting curve and crystal structures of polycrystalline vanadium at high P-T using synchrotron x-ray diffraction (XRD) in laser-heated diamond anvil cells (LH DACs) up to \ensuremath{\sim}100 GPa and \ensuremath{\sim}4400 K, a two-stage light-gas gun with in situ shock temperature measurements up to \ensuremath{\sim}256 GPa and \ensuremath{\sim}6200 K, and ab initio molecular dynamics (AIMD) with density functional theory computations up to \ensuremath{\sim}200 GPa. The occurrence of the diffuse scattering signals in high P-T XRD patterns is used as the primary criterion to determine the melting curve of body-centered cubic (bcc) vanadium up to \ensuremath{\sim}100 GPa in LH DACs. Analysis of thermal radiation spectra of shocked vanadium using a quasispectral pyrometer constrains the melting curve up to \ensuremath{\sim}246 GPa and \ensuremath{\sim}5830 K, which is consistent with our static results using the Simon equation. The present static and dynamic experiments on the melting curve of vanadium are consistent with our AIMD simulations with the two-phase melting modeling, and are overall consistent with other theoretical simulations using the Z method. The results reconcile the recently reported theoretical discrepancy, and refute a higher melting curve report given by self-consistent ab initio lattice dynamics calculations. The consistencies among our studies indicate that one does not have to invoke superheating as a hypothesis to describe the solid-liquid equilibrium boundary of vanadium as an explanation for static vs dynamic experimental results. Our static and dynamic results with in situ diagnostics of melting and two-phase AIMD simulation have implications for studying melting curves of other $d$-orbital transition metals and their alloys at extreme P-T conditions.

${\mathrm{WN}}_{6}$ phase discovered at 126--165 GPa after heating of W in nitrogen. XRD refinements reveal a unit cell in space group $R\overline{3}m$ which is consistent with the ${\mathrm{WN}}_{6}$ structure with armchairlike hexazine (${\mathrm{N}}_{6}$) rings, while strong ${A}_{1g}$ Raman mode confirms its $\mathrm{N}─\mathrm{N}$ single bonds. Density functional theory (DFT) calculations reveal balanced contributions of attractive interactions between W and covalent ${\mathrm{N}}_{6}$ rings, and repulsions between ${\mathrm{N}}_{6}$ rings that make ${\mathrm{WN}}_{6}$ ultrastiff and tough. The ${\mathrm{WN}}_{6}$ phase displays long bond lengths in the nearest N-N and pressure-enhanced electronic band gap, which pave the way for finding novel nitrides.

The amount of heat transport from the core, which constrains the dynamics and thermal evolution of the region, depends on the transport properties of iron. Ohta et al.(2016) and Konopkova et al.(2016) measured electrical resistivity and thermal conductivity of iron, respectively, in laser-heated diamond anvil cells (DACs) at relevant Earth's core pressure-temperature (P-T) conditions, and obtained dramatically contradictory results. Here we measure the electrical resistivity of hcp-iron up to ~170 GPa and ~3,000 K using a four-probe van der Pauw method coupled with homogeneous flat-top laser-heating in a DAC. We also compute its electrical and thermal conductivity by first-principles methods including electron-phonon and electron-electron scattering. We find that the measured resistivity of hcp-iron increases almost linearly with increasing temperature, and is consistent with current first-principles computations. The proportionality coefficient between resistivity and thermal conductivity (the Lorenz number) in hcp-iron differs from the ideal value (2.44*10^-8 W Omega K^-2), so a non-ideal Lorenz number of ~(2.0-2.1)*10^-8 W Omega K^-2 is used to convert the experimental resistivity to the thermal conductivity of hcp-Fe at high P-T. The results constrain the resistivity and thermal conductivity of hcp-iron to ~80(5) u Omega cm and ~100(10) W/mK, respectively, at conditions near core-mantle boundary. Our results indicate an adiabatic heat flow of ~10(1) TW through the core-mantle boundary for a liquid Fe alloy outer core, supporting a present-day geodynamo driven by thermal convection through the core's secular cooling and by compositional convection through the latent heat and gravitational energy during the inner core's solidification., Comment: 39 pages, 14 figures

Elastic moduli (${C}_{ij}$) of single-crystal stishovite and post-stishovite are determined using Brillouin light scattering, impulsive stimulated light scattering, and x-ray diffraction up to 70 GPa. The ${C}_{12}$ of stishovite converges with the ${C}_{11}$ at $\ensuremath{\sim}55\text{ }\text{ }\mathrm{GPa}$, where the transverse wave ${V}_{S1}$ propagating along [110] also vanishes. Landau modeling of the ${C}_{ij}$, ${B}_{1g}$ optic mode, and lattice parameters reveals a pseudoproper type ferroelastic post-stishovite transition. The transition would cause peculiar anomalies in ${V}_{S}$ and Poisson's ratio in silica-bearing subducting slabs in the mid-lower mantle.

$\mathrm{LiOs}{\mathrm{O}}_{3}$ synthesized under high pressure in recent years is a rare metal since it undergoes a nonpolar to polar phase transition at ${T}_{s}=140\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Forming a polar axis through a phase transition in a metal seems against common sense. It is also not clear whether the transition to a polar phase in the oxide fits the mechanism predicted by Anderson and Blount in 1965. As monitored by an anomaly of resistivity in $\mathrm{LiOs}{\mathrm{O}}_{3}$ at ${T}_{s}$ reported recently, ${T}_{s}$ increases under pressure. The structural study under high pressure could give us a useful clue for understanding how dipoles form in this metallic oxide. Here, we report the identification of a polar phase of $\mathrm{LiOs}{\mathrm{O}}_{3}$ at room temperature under high pressure by using in situ probes of Raman and synchrotron x-ray diffraction. In the Raman study, the pressure-induced modes and their responses to polarized light, the linewidth change, the peak profile change, and mode softening have been directly compared with the corresponding changes of $\mathrm{LiOs}{\mathrm{O}}_{3}$ on cooling through ${T}_{s}$ at ambient pressure. Whereas a complete set of Raman modes from the $R3c$ phase can be found at $P\ensuremath{\ge}15.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, a Raman mode of the $R3c$ phase appears in the $R\overline{3}c$ phase at 4.11 GPa. A significant drop in the linewidth occurs at 12.6 GPa that coincides with the critical pressure for the phase transition to the polar phase detected by x-ray diffraction. Fitting the peak profile of a Raman mode to the Fano formula also indicates a clear change of electron-phonon coupling at 16 GPa. In contrast to a sharp structural transition to the polar phase at ${T}_{s}$ under ambient pressure, our results reveal all the structural ingredients to facilitate the polar phase over a broad range of pressure. A bond valence sum analysis has been introduced to reveal the local structural instability under pressure. The transition to the polar phase in metallic $\mathrm{LiOs}{\mathrm{O}}_{3}$ is solely caused by optimizing the local structure in order to make the bond valence sum close to the formal valence of the Li ion.

Crystal structure prediction (CSP) methods recently proposed a series of new rare-earth (RE) hydrides at high pressures with novel crystal structures, unusual stoichiometries, and intriguing features such as high

The recent observation of unusually high thermal conductivity exceeding 1000 W m-1 K-1 in single-crystal boron arsenide (BAs) has led to interest in the potential application of this semiconductor for thermal management. Although both the electron/hole high mobilities have been calculated for BAs, there is a lack of experimental investigation of its electronic properties. Here, a photoluminescence (PL) measurement of single-crystal BAs at different temperatures and pressures is reported. The measurements reveal an indirect bandgap and two donor-acceptor pair (DAP) recombination transitions. Based on first-principles calculations and time-of-flight secondary-ion mass spectrometry results, the two DAP transitions are confirmed to originate from Si and C impurities occupying shallow energy levels in the bandgap. High-pressure PL spectra show that the donor level with respect to the conduction band minimum shrinks with increasing pressure, which affects the release of free carriers from defect states. These findings suggest the possibility of strain engineering of the transport properties of BAs for application in electronic devices.

Two distinct stacking orders in ReS