Jie Li

Lightly Cu‐doped BiMn _{7‐ x} Cu _x O ₁₂ ( x = 0.05, 0.10, and 0.15) was investigated using high‐pressure synthesis, single‐crystal X‐ray diffraction, pair distribution function (PDF) analysis, Scanning Transmission Electron Microscope (STEM), magnetic measurements, and first‐principles calculations. All compositions retain an average monoclinic I 2/ m structure, while Cu substitution progressively suppresses the monoclinic distortion and drives the lattice toward a pseudo‐cubic metric symmetry. PDF analysis reveals increasing local structural disorder and reduced medium‐range coherence with increasing Cu concentration, despite preservation of the overall quadruple‐perovskite framework. Comparative single‐crystal refinements qualitatively favor Cu association with the octahedral Mn _B sublattice over the square‐planar Mn _A’ sites. However, due to the low Cu concentration and substantial systematic residual density, the Cu content was determined by EDS, and the local Cu distribution cannot be quantified independently of the X‐ray data. Magnetic measurements reveal an intrinsic transition near T _M ∼ 50–60 K, accompanied by magnetic irreversibility, field‐dependent hysteresis, and unsaturated magnetization. An additional broad feature near T _L ∼ 30K becomes weaker and less distinct with increasing Cu concentration. First‐principles calculations favor Cu occupation at the square‐planar sites, in contrast to the qualitative preference of the average B‐site refinement model.

Some low-density exoplanets are thought to be water-rich worlds that formed beyond the snow line of their protoplanetary disk, possibly accreting coequal portions of rock and water. However, the compositions of bodies within the solar system and the stability of volatile-rich solids in accretionary disks suggest that a planet rich in water should also acquire as much as 40% refractory organic carbon (“soot”). This would reduce the water mass fraction well below 50%, making the composition of these planets similar to those of solar system comets. Here we show that soot-rich planets, with or without water, can account for the low average densities of exoplanets that were previously attributed to a binary combination of rock and water. Formed in locations beyond the soot and/or snow lines in disks, these planets are likely common in our galaxy and already observed by JWST. The surfaces and interiors of soot-rich planets will be influenced by the chemical and physical properties of carbonaceous phases, and the atmospheres of such planets may contain plentiful methane and other hydrocarbons, with implications for photochemical haze generation and habitability.

The incorporation of ferric iron in mantle silicates stabilizes different crystal structures and changes phase transition conditions, thus impacting seismic wave speeds and discontinuities. Recent experiments of MgSiO3-Fe2O3 mixtures indicate the coexistence of fully oxidized iron-rich (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 with Fe-poor silicate (wadsleyite or bridgmanite) and stishovite at 15 to 27 GPa and 1773 to 2000 K, conditions relevant to subducted lithosphere in the Earth’s transition zone and uppermost lower mantle. X-ray diffraction (XRD) shows that (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 recovered from these conditions adopts the R3c LiNbO3-type structure, which transforms to the bridgmanite structure again between 18.3 and 24.7 GPa at 300 K. XRD data are used to obtain the equation of state of the LiNbO3-type phase up to 18.3 GPa. Combined with multi-anvil experiments, these observations suggest that the stable phase of (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 is bridgmanite at 15–27 GPa, which transforms on decompression to LiNbO3-type structure. Our calculation revealed that ordering of the ferric ion reduces the kinetic energy barrier of the transition between (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 LiNbO3 structure and bridgmanite relative to the MgSiO3 akimotoite-bridgmanite system. A dense Fe3+-rich bridgmanite structure is thus stable at substantially shallower depths than MgSiO3 bridgmanite and would promote subduction.

Melting of carbonated rocks in the mantle influences the Earth’s deep carbon cycle and the long-term evolution of the atmosphere. Previous studies of the high-pressure melting curve of K₂CO₃ have yielded inconsistent results, with discrepancies of nearly 200 °C at 3 GPa and more than 400 °C at 12 GPa. Here, we report constraints on the melting curve of K₂CO₃ at pressures up to 20 GPa from in situ ionic conduction experiments and Pt sphere experiments. To help resolve the large discrepancies, we tested the ionic conduction method against the well-established differential thermal analysis (DTA) method and conventional Pt sphere method at the ambient pressure of 1 bar. Furthermore, ionic conduction experiments were conducted on sodium chloride (NaCl) to reduce uncertainties in pressure calibration of the multi-anvil press. We also modified the configuration of the in situ ionic conduction experiments to minimize the influence of thermal gradient on melting point determination. Finally, we inspected the effect of water by varying the initial sample state and container wall thickness in the Pt sphere experiments and applied X-ray radiography as a reliable and efficient method to examine the products. Compared with the results from the ionic conduction experiments, the melting point of K₂CO₃ from the Pt sphere experiments was found to be 200~400 °C lower, likely due to a small amount of water trapped by hygroscopic K₂CO₃ in closed platinum (Pt) capsules. We find that anhydrous K₂CO₃ remains more refractory than Na₂CO₃ at elevated pressures.

Subducted carbonates dominate the global carbon fluxes from Earth's surface to its interior, hence their fate ultimately modulates the planet's long-term climate dynamics. Calcium carbonate is a major component of subducted slabs, yet little is known about its stability under relevant conditions. Here, we show that aragonite decomposes and forms graphite at 1.8-3.0 gigapascals and 230-900 °C, even under relatively oxidizing conditions. Interestingly, graphite forms exclusively from aragonite but not calcite, revealing that crystal structure influences calcium carbonate instability and graphite formation as much as oxygen fugacity. Graphite formation in the vast regions of shallow subduction zones would help retain carbon, thus facilitating its transportation to greater depths.

Quantifying how grain size and/or deviatoric stress impact (Mg,Fe)₂SiO₄ phase stability is critical for advancing our understanding of subduction processes and deep-focus earthquakes. Here, we demonstrate that well-resolved X-ray diffraction patterns can be obtained on nano-grained thin films within laser-heated diamond anvil cells (DACs) at hydrostatic pressures up to 24 GPa and temperatures up to 2300 K. Combined with well-established literature processes for tuning thin film grain size, biaxial stress, and substrate identity, these results suggest that DAC-loaded thin films can be useful for determining how grain size, deviatoric stress, and/or the coexistence of other phases influence high-pressure phase stability. As such, this novel DAC-loaded thin film approach may find use in a variety of earth science, planetary science, solid-state physics, and materials science applications.

We explore terrestrial planet formation with a focus on the supply of solid-state organics as the main source of volatile carbon. For the water-poor Earth, the water ice line, or ice sublimation front, within the planet-forming disk has long been a key focal point. We posit that the soot line, the location where solid-state organics are irreversibly destroyed, is also a key location within the disk. The soot line is closer to the host star than the water snow line and overlaps with the location of the majority of detected exoplanets. In this work, we explore the ultimate atmospheric composition of a body that receives a major portion of its materials from the zone between the soot line and water ice line. We model a silicate-rich world with 0.1% and 1% carbon by mass with variable water content. We show that as a result of geochemical equilibrium, the mantle of these planets would be rich in reduced carbon but have relatively low water (hydrogen) content. Outgassing would naturally yield the ingredients for haze production when exposed to stellar UV photons in the upper atmosphere. Obscuring atmospheric hazes appear common in the exoplanetary inventory based on the presence of often featureless transmission spectra. Such hazes may be powered by the high volatile content of the underlying silicate-dominated mantle. Although this type of planet has no solar system counterpart, it should be common in the galaxy with potential impact on habitability.

Earth’s precursor material lost most carbon through sublimation in the nebular disk shortly after the Sun’s birth.