Structural and transport properties of (Mg,Fe)SiO₃ at high temperature and high pressure

Zhang R., Guo R., Li Q., Li S., Long H., He D.

Abstract Cubic boron nitride and hexagonal boron nitride are the two predominant crystalline structures of boron nitride. They can interconvert under varying pressure and temperature conditions. However, this transformation requires overcoming significant potential barriers in dynamics, which poses great difficulty in determining the c-BN/h-BN phase boundary. This study used high-pressure in situ differential thermal measurements to ascertain the temperature of h-BN/c-BN conversion within the commonly used pressure range (3–6 GPa) for the industrial synthesis of c-BN to constrain the P–T phase boundary of h-BN/c-BN in the pressure–temperature range as much as possible. Based on the analysis of the experimental data, it is determined that the relationship between pressure and temperature conforms to the following equation: P = a + 1 b T . Here, P denotes the pressure (GPa) and T is the temperature (K). The coefficients are a = −3.8±0.8 GPa and b = 229.8±17.1 GPa/K. These findings call into question existing high-pressure and high-temperature phase diagrams of boron nitride, which seem to overstate the phase boundary temperature between c-BN and h-BN. The BN phase diagram obtained from this study can provide critical temperature and pressure condition guidance for the industrial synthesis of c-BN, thus optimizing synthesis efficiency and product performance.

Guo Z., Xing D., Xi X., Liang C., Hao B., Zeng X., Tang H., Chen H., Yin W., Zhang P., Zhou K., Zheng Q., Ma P.

Many countries and commercial organizations have shown great interest in constructing a Martian base. In situ resource utilization (ISRU) provides a cost-effective way to achieve this ambitious goal. In this article, we proposed to use Martian soil simulant to produce a fiber to satisfy material requirement for the construction of Martian base. The composition, melting behavior, and fiber forming process of the soil simulant was studied, and continuous fiber with maximum strength of 1320 MPa and elastic modulus of 99 GPa was obtained on a spinning facility. The findings of this study demonstrate the feasibility of ISRU to prepare Martian fiber from the soil on the Mars, offering a new way to obtain key materials for the construction of a Martian base.

Wang D., Wu Z., Deng X.

The thermal conductivity of the lowermost mantle controls the energy transfer from the core to the mantle, and provides insight into the thermal history, evolution, and magnetic field of the Earth. Bridgmanite (Brg) and post-perovskite (PPv) are the most abundant minerals in the lowermost mantle. However, their thermal conductivities, as well as the impact of Fe impurities, are highly controversial. In this study, the thermal conductivities of Fe-free and Fe-bearing Brg and PPv at high pressure and temperature were predicted, through a combination of non-equilibrium molecular dynamics and machine learning potential (MLP) trained with data from first-principles calculations. The thermal conductivities of Fe-free Brg and PPv are in agreement with those calculated by the Green-Kubo method based on the MLP. We found that the presence of 12.5 mol% Fe in the lowermost mantle decreases the thermal conductivities of Brg and PPv by ∼10% and ∼14%, respectively. Furthermore, the phase transition from Brg to PPv increases the thermal conductivity of pyrolite by ∼22%. Incorporating the distribution of minerals, temperature, and iron content obtained through the inversion based on mineral elasticity and seismic tomography models, a global heat flow with substantial lateral variation at the core-mantle boundary was reported. The total heat flux from the core was found to be 7.1 ± 0.5 TW, implying a geologically young inner core of 0.75 ± 0.35 Ga.

Liu Y., Fan Q., Yang J., Wang L., Zhang W., Yao G.

Hydrides offer an opportunity to study high critical temperature (high-T c) superconductivity at experimentally achievable pressures. However, the pressure needed remains extremely high. Using density functional theory calculations, herein we demonstrate that a new rare earth hydride ErH2 could be superconducting with T c ∼ 80 K at 14.5 GPa, the lowest reported value for compressed hydrides to date. Intriguingly, due to Kondo destruction, superconductivity was prone to exist at 15 GPa. We also reveal an energy gap at 20 GPa on the background of normal metallic states. At 20 GPa, this compressed system could act as a host of superconductor judged from a sharp jump of spontaneous magnetic susceptibility with an evanescent spin density of state at Fermi level. Finally, electron pairing glue for ErH2 at these three typical pressures was attributed to the antiferromagnetic spin fluctuation.

Yang F., Zeng Q., Chen B., Kang D., Zhang S., Wu J., Yu X., Dai J.

Lattice thermal conductivity (κ lat) of MgSiO3 perovskite and post-perovskite is an important parameter for the thermal dynamics in the Earth. Here, we develop a deep potential of density functional theory quality under entire thermodynamic conditions in the lower mantle, and calculate the κ lat by the Green–Kubo relation. Deep potential molecular dynamics captures full-order anharmonicity and considers ill-defined phonons in low-κ lat materials ignored in the phonon gas model. The κ lat shows negative temperature dependence and positive linear pressure dependence. Interestingly, the κ lat undergos an increase at the phase boundary from perovskite to post-perovskite. We demonstrate that, along the geotherm, the κ lat increases by 18.2% at the phase boundary. Our results would be helpful for evaluating Earth’s thermal dynamics and improving the Earth model.

Chen Q., Wu J., Chen T., Wang X., Ding C., Huang T., Lu Q., Sun J.

Pressure is an effective and clean way to modify the electronic structures of materials, cause structural phase transitions and even induce the emergence of superconductivity. Here, we predicted several new phases of the ZrXY family at high pressures using the crystal structures search method together with first-principle calculations. In particular, the ZrGeS compound undergoes an isosymmetric phase transition from P4/nmm-I to P4/nmm-II at approximately 82 GPa. Electronic band structures show that all the high-pressure phases are metallic. Among these new structures, P4/nmm-II ZrGeS and P4/mmm ZrGeSe can be quenched to ambient pressure with superconducting critical temperatures of approximately 8.1 K and 8.0 K, respectively. Our study provides a way to tune the structure, electronic properties, and superconducting behavior of topological materials through pressure.

Guo Z., Xing D., Xi X., Yue X., Liang C., Hao B., Zheng Q., Gutnikov S.I., Lazoryak B.I., Ma P.

The construction of a lunar base is considered to be an important step towards deep-space exploration by humanity, and will rely on the utilisation of in situ lunar resources. In this paper, we discuss the current knowledge on the feasibility of converting lunar soil to high-performance fibres that can be used for the construction of a lunar base. This fibre would be combined with further portions of lunar soil to generate fibre-reinforced composites, which is utilized as multi-functional materials for lunar base construction. We discuss and analyse the latest findings regarding the composition of lunar soil simulants and their fibrisation properties, and techniques for fibre spinning and system integration. Finally, we suggest how the achievements made so far could be applied to the construction of a lunar base.

Thompson A.P., Aktulga H.M., Berger R., Bolintineanu D.S., Brown W.M., Crozier P.S., in 't Veld P.J., Kohlmeyer A., Moore S.G., Nguyen T.D., Shan R., Stevens M.J., Tranchida J., Trott C., Plimpton S.J.

Since the classical molecular dynamics simulator LAMMPS was released as an open source code in 2004, it has become a widely-used tool for particle-based modeling of materials at length scales ranging from atomic to mesoscale to continuum. Reasons for its popularity are that it provides a wide variety of particle interaction models for different materials, that it runs on any platform from a single CPU core to the largest supercomputers with accelerators, and that it gives users control over simulation details, either via the input script or by adding code for new interatomic potentials, constraints, diagnostics, or other features needed for their models. As a result, hundreds of people have contributed new capabilities to LAMMPS and it has grown from fifty thousand lines of code in 2004 to a million lines today. In this paper several of the fundamental algorithms used in LAMMPS are described along with the design strategies which have made it flexible for both users and developers. We also highlight some capabilities recently added to the code which were enabled by this flexibility, including dynamic load balancing, on-the-fly visualization, magnetic spin dynamics models, and quantum-accuracy machine learning interatomic potentials. Program Title: Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) CPC Library link to program files: https://doi.org/10.17632/cxbxs9btsv.1 Developer's repository link: https://github.com/lammps/lammps Licensing provisions: GPLv2 Programming language: C++, Python, C, Fortran Supplementary material: https://www.lammps.org Nature of problem: Many science applications in physics, chemistry, materials science, and related fields require parallel, scalable, and efficient generation of long, stable classical particle dynamics trajectories. Within this common problem definition, there lies a great diversity of use cases, distinguished by different particle interaction models, external constraints, as well as timescales and lengthscales ranging from atomic to mesoscale to macroscopic. Solution method: The LAMMPS code uses parallel spatial decomposition, distributed neighbor lists, and parallel FFTs for long-range Coulombic interactions [1]. The time integration algorithm is based on the Størmer-Verlet symplectic integrator [2], which provides better stability than higher-order non-symplectic methods. In addition, LAMMPS supports a wide range of interatomic potentials, constraints, diagnostics, software interfaces, and pre- and post-processing features. Additional comments including restrictions and unusual features: This paper serves as the definitive reference for the LAMMPS code. [1] S. Plimpton, Fast parallel algorithms for short-range molecular dynamics. J. Comp. Phys. 117 (1995) 1–19. [2] L. Verlet, Computer experiments on classical fluids: I. Thermodynamical properties of Lennard–Jones molecules, Phys. Rev. 159 (1967) 98–103.

Koppers A.A., Becker T.W., Jackson M.G., Konrad K., Müller R.D., Romanowicz B., Steinberger B., Whittaker J.M.

The existence of mantle plumes was first proposed in the 1970s to explain intra-plate, hotspot volcanism, yet owing to difficulties in resolving mantle upwellings with geophysical images and discrepancies in interpretations of geochemical and geochronological data, the origin, dynamics and composition of plumes and their links to plate tectonics are still contested. In this Review, we discuss progress in seismic imaging, mantle flow modelling, plate tectonic reconstructions and geochemical analyses that have led to a more detailed understanding of mantle plumes. Observations suggest plumes could be both thermal and chemical in nature, can attain complex and broad shapes, and that more than 18 plumes might be rooted in regions of the lowermost mantle. The case for a deep mantle origin is strengthened by the geochemistry of hotspot volcanoes that provide evidence for entrainment of deeply recycled subducted components, primordial mantle domains and, potentially, materials from Earth’s core. Deep mantle plumes often appear deflected by large-scale mantle flow, resulting in hotspot motions required to resolve past tectonic plate motions. Future research requires improvements in resolution of seismic tomography to better visualize deep mantle plume structures at smaller than 100-km scales. Concerted multi-proxy geochemical and dating efforts are also needed to better resolve spatiotemporal and chemical evolutions of long-lived mantle plumes. Mantle plumes are an integral aspect of Earth’s convection system, yet, difficulty in imaging mantle upwellings led to controversies surrounding their origin, dynamics and composition. This Review synthesizes geophysical, geodynamic and geochemical constraints on mantle plumes and their importance in the Earth system.

Gillet P., McCammon C.A., Cava R.J., Dutton S.E., Magrez A., Liu J., Alp E.E., Bi W., Chumakov A.I., Kupenko I., Greenberg E., Lv M., Potapkin V., Dorfman S.M.

Abstract Electronic states of iron in the lower mantle's dominant mineral, (Mg,Fe,Al)(Fe,Al,Si)O3 bridgmanite, control physical properties of the mantle including density, elasticity, and electrical and thermal conductivity. However, the determination of electronic states of iron has been controversial, in part due to different interpretations of Mössbauer spectroscopy results used to identify spin state, valence state, and site occupancy of iron. We applied energy-domain Mössbauer spectroscopy to a set of four bridgmanite samples spanning a wide range of compositions: 10–50% Fe/total cations, 0–25% Al/total cations, 12–100% Fe3+/total Fe. Measurements performed in the diamond-anvil cell at pressures up to 76 GPa below and above the high to low spin transition in Fe3+ provide a Mössbauer reference library for bridgmanite and demonstrate the effects of pressure and composition on electronic states of iron. Results indicate that although the spin transition in Fe3+ in the bridgmanite B-site occurs as predicted, it does not strongly affect the observed quadrupole splitting of 1.4 mm/s, and only decreases center shift for this site to 0 mm/s at ~70 GPa. Thus center shift can easily distinguish Fe3+ from Fe2+ at high pressure, which exhibits two distinct Mössbauer sites with center shift ~1 mm/s and quadrupole splitting 2.4–3.1 and 3.9 mm/s at ~70 GPa. Correct quantification of Fe3+/total Fe in bridgmanite is required to constrain the effects of composition and redox states in experimental measurements of seismic properties of bridgmanite. In Fe-rich, mixed-valence bridgmanite at deep-mantle-relevant pressures, up to ~20% of the Fe may be a Fe2.5+ charge transfer component, which should enhance electrical and thermal conductivity in Fe-rich heterogeneities at the base of Earth's mantle.

Xie L., Yoneda A., Yamazaki D., Manthilake G., Higo Y., Tange Y., Guignot N., King A., Scheel M., Andrault D.

Thermochemical heterogeneities detected today in the Earth’s mantle could arise from ongoing partial melting in different mantle regions. A major open question, however, is the level of chemical stratification inherited from an early magma-ocean (MO) solidification. Here we show that the MO crystallized homogeneously in the deep mantle, but with chemical fractionation at depths around 1000 km and in the upper mantle. Our arguments are based on accurate measurements of the viscosity of melts with forsterite, enstatite and diopside compositions up to ~30 GPa and more than 3000 K at synchrotron X-ray facilities. Fractional solidification would induce the formation of a bridgmanite-enriched layer at ~1000 km depth. This layer may have resisted to mantle mixing by convection and cause the reported viscosity peak and anomalous dynamic impedance. On the other hand, fractional solidification in the upper mantle would have favored the formation of the first crust. Following the impact of the protoplanet Theia, planet Earth likely transformed into a magma ocean. New high temperature and pressure experiments by Xie et al. suggest that a layer enriched in bridgmanite formed during the magma ocean phase of Earth–remnants of this ancient layer today may be responsible for the viscosity peak between 660 and 1500 km in present solid mantle.

Solomatova N.V., Caracas R.

With ab initio molecular dynamics simulations on a Na-, Ca-, Fe-, Mg-, and Al-bearing silicate melt of pyrolite composition, we examine the detailed changes in elemental coordination as a function of pressure and temperature. We consider the average coordination as well as the proportion and distribution of coordination environments at pressures and temperatures encompassing the conditions at which molten silicates may exist in present-day Earth and those of the Early Earth's magma ocean. At ambient pressure and 2,000 K, we find that the average coordination of cations with respect to oxygen is 4.0 for Si-O, 4.0 for Al-O, 3.7 for Fe-O, 4.6 for Mg-O, 5.9 for Na-O, and 6.2 for Ca-O. Although the coordination for iron with respect to oxygen may be underestimated, the coordination number for all other cations are consistent with experiments. By 15 GPa (2,000 K), the average coordination for Si-O remains at 4.0 but increases to 4.1 for Al-O, 4.2 for Fe-O, 4.9 for Mg-O, 8.0 for Na-O, and 6.8 for Ca-O. The coordination environment for Na-O remains approximately constant up to core-mantle boundary conditions (135 GPa and 4000 K) but increases to about 6 for Si-O, 6.5 for Al-O, 6.5 for Fe-O, 8 for Mg-O, and 9.5 for Ca-O. We discuss our results in the context of the metal-silicate partitioning behavior of siderophile elements and the viscosity changes of silicate melts at upper mantle conditions. Our results have implications for melt properties, such as viscosity, transport coefficients, thermal conductivities, and electrical conductivities, and will help interpret experimental results on silicate glasses.

Zheng Y.

Abstract Crustal recycling at convergent plate boundaries is essential to mantle heterogeneity. However, crustal signatures in the mantle source of basaltic rocks above subduction zones were primarily incorporated in the form of liquid rather than solid phases. The physicochemical property of liquid phases is determined by the dehydration behavior of crustal rocks at the slab-mantle interface in subduction channels. Because of the significant fractionation in incompatible trace elements but the full inheritance in radiogenic isotopes relative to their crustal sources, the production of liquid phases is crucial to the geochemical transfer from the subducting crust into the mantle. In this process, the stability of specific minerals in subducting crustal rocks exerts a primary control on the enrichment of given trace elements in the liquid phases. For this reason, geochemically enriched oceanic basalts can be categorized into two types in terms of their trace element distribution patterns in the primitive mantle-normalized diagram. One is island arc basalts (IAB), showing enrichment in LILE, Pb and LREE but depletion in HFSE such as Nb and Ta relative to HREE. The other is ocean island basalts (OIB), exhibiting enrichment in LILE and LREE, enrichment or non-depletion in HFSE but depletion in Pb relative to HREE. In either types, these basalts show the enhanced enrichment of LILE and LREE with increasing their incompatibility relative to normal mid-ocean ridge basalts (MORB). The thermal regime of subduction zones can be categorized into two stages in both time and space. The first stage is characterized by compressional tectonism at low thermal gradients. As a consequence, metamorphic dehydration of the subducting crust prevails at forearc to subarc depths due to the breakdown of hydrous minerals such as mica and amphibole in the stability field of garnet and rutile, resulting in the liberation of aqueous solutions with the trace element composition that is considerably enriched in LILE, Pb and LREE but depleted in HFSE and HREE relative to normal MORB. This provides the crustal signature for the mantle sources of IAB. The second stage is indicated by extensional tectonism at high thermal gradients, leading to the partial melting of metamorphically dehydrated crustal rocks at subarc to postarc depths. This involves not only the breakdown of hydrous minerals such as amphibole, phengite and allanite in the stability field of garnet but also the dissolution of rutile into hydrous melts. As such, the hydrous melts can acquire the trace element composition that is significantly enriched in LILE, HFSE and LREE but depleted in Pb and HREE relative to normal MORB, providing the crustal signature for the mantle sources of OIB. In either case, these liquid phases would metasomatize the overlying mantle wedge peridotite at different depths, generating ultramafic metasomatites such as serpentinized and chloritized peridotites, and olivine-poor pyroxenites and hornblendites. As a consequence, the crustal signatures are transferred by the liquid phases from the subducting slab into the mantle.

Sun Y., Zhou H., Yin K., Zhao M., Xu S., Lu X.

Karki B.B., Ghosh D.B., Maharjan C., Karato S., Park J.