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Abstract The influence of Al substitution on the elastic properties of stishovite and its transition to post-stishovite is of great importance for interpreting the seismic wave velocities of subducted mid-ocean ridge basalt (MORB) within the mantle transition zone and the lower mantle. However, atomistic mechanisms of Al substitution effects on the transition and its associated elasticity remain debated. Here synchrotron single-crystal X-ray diffraction measurements have been performed at room temperature on Al1.3-SiO2 (1.3 mol% Al in the chemical formula of Si0.965(3)Al0.041(1)O2H0.017(4)) and Al2.1-SiO2 (2.1 mol% Al in Si0.948(2)Al0.064(1)O2H 0.018(3)) crystals in diamond anvil cells with Boehler-Almax designed anvils up to 38.0 GPa and 28.5 GPa, respectively. Refinements of the diffraction patterns show that a transformation from stishovite (space group P42/mnm; No. 136) to CaCl2-typed post-stishovite (space group Pnnm; No. 58) is accompanied by splitting of O coordinates. The Al substitution in stishovite results in a faster decrease in the O coordinate, softer apical (Si,Al)-O bonds, and a softer and less distorted (Si,Al)O6 octahedron under compression. This leads to reduced adiabatic bulk modulus (KS), shear modulus (G), shear wave velocity (VS), and compressional wave velocity (VP) in the stishovite phase, explaining seismic wave perturbations in the mantle transition zone. Together with Raman data, Landau theory modeling shows that Al substitution increases the order parameter and excess free energy, stabilizing the post-stishovite phase at lower pressures. Correlation between elasticity and octahedral distortion index (D) reveals that at certain D, the Al substitution reduces KS, G, VS, and VP of the stishovite phase while increasing G, VS, and VP of the post-stishovite phase. Importantly, the maximum shear reduction is slightly enhanced at D = 0.00620(9) at the transition point. Our results help explain the seismically observed small-scale VS anomalies beneath subduction regions in the shallow lower mantle where Al,H-bearing stishovite undergoes the post-stishovite transition. 

Abstract Understanding hydrogen dissolution mechanisms in bridgmanite (Bgm), the most abundant mineral in the lower mantle, is essential for understanding water storage and rheological and transport properties in the region. However, interpretations of O‐H bands in Fourier transform infrared spectroscopy (FTIR) spectra of Bgm crystals remain uncertain. We conducted density functional theory (DFT) calculations on vibrational characteristics of O‐H dipoles and performed polarized FTIR measurements to address this issue. DFT calculations for four substitution models—Mg vacancies, Si vacancies, Al³⁺ + H⁺substitution for Si⁴⁺, and Al substitution with Mg vacancies—reveal distinct O‐H bands with different polarizations. Deconvolution of polarized FTIR spectra on Mg_0.88Fe²⁺_0.035Fe³⁺_0.065Al_0.14Si_0.90O₃and Mg_0.95Fe²⁺_0.033Fe³⁺_0.027Al_0.04Si_0.96O₃crystals shows five major O‐H bands with distinct polarizations along principal crystallographic axes. These experimental and calculated results attribute O‐H bands centered at 3,463–3,480, 2,913–2,924, and 2,452–2,470 cm⁻¹to Mg vacancies, Si vacancies, and Al³⁺ + H⁺substitution for Si⁴⁺, respectively. The total absorbance coefficient of bridgmanite was calculated to be 82,702(6,217) L/mol/cm². Mg and Si vacancies account for 43%–74% of the total water content, making them dominant hydrogen dissolution mechanisms in Bgm. The band frequencies for the Mg and Si vacancies in Bgm are drastically different from those in olivine and ringwoodite, corresponding to the significant changes in O‐H bond strengths and in the Si and Mg coordination environments from upper‐mantle to lower‐mantle minerals. These results highlight the need to incorporate hydrogen dissolution mechanisms in Bgm for understanding electrical conductivity and rheology of the lower mantle. 

Carbon materials display intriguing physical properties, including superconductivity and highly anisotropic thermal conductivity found in graphene. Compressive strain can induce structural and bonding transitions in carbon materials and create new carbon phases, but their interplay with thermal conductivity remains largely unexplored. We investigated the in situ high-pressure thermal conductivity of compressed graphitic phases using picosecond transient thermoreflectance and first-principles calculations. Our results show an anomalous thermal conductivity that peaks to 260  W/mK at 15–20 GPa but drops to 3.0  W/mK at ∼35  GPa. Together with complimentary in situ Raman and x-ray diffraction results, the abnormal thermal conductivity trend of compressed carbon is attributed to phonon-mediated conductivity influenced by interlayer buckling and 𝑠⁢𝑝2 to 𝑠⁢𝑝3 transition and, subsequently, the formation of 𝑀-carbon nanocrystals and amorphous carbon. Strain-induced structural and bonding variations provide a wide-range manipulation of thermal and mechanical properties in carbon materials. 

Abstract Here we have performed single-crystal X-ray diffraction (SCXRD) experiments on two high-quality crystal platelets of (Al,Fe)-bearing bridgmanite (Mg0.88Fe0.0653+Fe0.0352+Al0.03)(Al0.11Si0.90)O3 (Fe10-Al14-Bgm) up to 64.6(6) GPa at room temperature in a Boehler-Almax type diamond-anvil cell. Refinements on the collected SCXRD patterns reveal reliable structural information of single-crystal Fe10-Al14-Bgm, including unit-cell parameters, atomic coordinates, and anisotropic displacement parameters. Together with Mössbauer and electron microprobe analyses, our best single-crystal refinement model indicates that the sample contains ~6.5 mol% Fe3+, 3.5 mol% Fe2+, and 3 mol% Al3+ in the large pseudo-dodecahedral site (A site), and ~11 mol% Al3+ in the small octahedral site (B site). This may indicate that Al3+ in bridgmanite preferentially occupies the B site. Our results show that the compression of Fe10-Al14-Bgm with pressure causes monotonical decreases in the volumes of AO12 pseudo-dodecahedron and BO6 octahedron (VA and VB, respectively) as well as the associated A-O and B-O bond lengths. The interatomic angles of B-O1-B and B-O2-B decrease from 145.2–145.8° at 4.2(1) GPa to 143.3–143.5° at 64.6(6) GPa. Quantitative calculations of octahedral tilting angles (Ф) show that Ф increases smoothly with pressure. We found a linear relationship between the polyhedral volume ratio and the Ф in the bridgmanite with different compositions: VA/VB = –0.049Φ + 5.549. Our results indicate an increased distortion of the Fe10-Al14-Bgm structure with pressure, which might be related to the distortion of A-site Fe2+. The local environmental changes of A-site Fe2+ in bridgmanite could explain previous results on the hyperfine parameters, abnormal lattice thermal conductivity, mean force constant of iron bonds and other physical properties, which in turn provide insights into our understanding on the geophysics and geochemistry of the planet. 

Abstract Bridgmanite, the most abundant mineral in the lower mantle, can play an essential role in deep-Earth hydrogen storage and circulation processes. To better evaluate the hydrogen storage capacity and its substitution mechanism in bridgmanite occurring in nature, we have synthesized high-quality single-crystal bridgmanite with a composition of (Mg0.88Fe0.052+Fe0.053+Al0.03)(Si0.88Al0.11H0.01)O3 at nearly water-saturated environments relevant to topmost lower mantle pressure and temperature conditions. The crystallographic site position of hydrogen in the synthetic (Fe,Al)-bearing bridgmanite is evaluated by a time-of-flight single-crystal neutron diffraction scheme, together with supporting evidence from polarized infrared spectroscopy. Analysis of the results shows that the primary hydrogen site has an OH bond direction nearly parallel to the crystallographic b axis of the orthorhombic bridgmanite lattice, where hydrogen is located along the line between two oxygen anions to form a straight geometry of covalent and hydrogen bonds. Our modeled results show that hydrogen is incorporated into the crystal structure via coupled substitution of Al3+ and H+ simultaneously exchanging for Si4+, which does not require any cation vacancy. The concentration of hydrogen evaluated by secondary-ion mass spectrometry and neutron diffraction is ~0.1 wt% H2O and consistent with each other, showing that neutron diffraction can be an alternative quantitative means for the characterization of trace amounts of hydrogen and its site occupancy in nominally anhydrous minerals. 

Abstract 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. 

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