Recent astrophysical observations have shown that some stars have sufficiently high carbon‐to‐oxygen ratios and may host planets composed mainly of carbides instead of silicates and oxides. From the low thermal expansion of SiC at 1 bar, it can be inferred that the buoyancy force of thermal anomalies is much lower in the carbide planets than in the silicate planets. However, numerous studies have shown that high pressure in planetary interiors can fundamentally change the physical properties of materials. We have measured the pressure‐volume‐temperature relations of two SiC polymorphs (3C and 6H) at pressures and temperatures up to 80 GPa and 1900 K and 65 GPa and 1920 K, respectively, in the laser‐heated diamond anvil cell combined with synchrotron X‐ray diffraction. We found no evidence of dissociations of these phases up to our maximum pressure condition, supporting the stability of SiC to 1900 km depth in Earth‐size Si‐rich carbide planets. Following the Mie‐Grüneisen approach, we fit our data to the Birch‐Murnaghan or the Vinet equations of state combined with the Debye approach. We found that the pressure‐induced change in the thermal expansion parameter of SiC is much smaller than that of Mg silicate perovskite (bridgmanite). Our new measurements suggest that the thermal buoyancy force may be stronger in the deep interiors of Si‐rich carbide exoplanets than in the “Earth‐like” silicate planets.

Sub-Neptunes are common among the discovered exoplanets. However, lack of knowledge on the state of matter in H2O-rich setting at high pressures and temperatures (P−T) places important limitations on our understanding of this planet type. We have conducted experiments for reactions between SiO2 and H2O as archetypal materials for rock and ice, respectively, at high P−T. We found anomalously expanded volumes of dense silica (up to 4%) recovered from hydrothermal synthesis above ∼24 GPa where the CaCl2-type (Ct) structure appears at lower pressures than in the anhydrous system. Infrared spectroscopy identified strong OH modes from the dense silica samples. Both previous experiments and our density functional theory calculations support up to 0.48 hydrogen atoms per formula unit of (Si1−xH4x)O2 (x=0.12). At pressures above 60 GPa, H2O further changes the structural behavior of silica, stabilizing a niccolite-type structure, which is unquenchable. From unit-cell volume and phase equilibrium considerations, we infer that the niccolite-type phase may contain H with an amount at least comparable with or higher than that of the Ct phase. Our results suggest that the phases containing both hydrogen and lithophile elements could be the dominant materials in the interiors of water-rich planets. Even for fully layered cases, the large mutual solubility could make the boundary between rock and ice layers fuzzy. Therefore, the physical properties of the new phases that we report here would be important for understanding dynamics, geochemical cycle, and dynamo generation in water-rich planets.

Although it has previously been considered to be essentially anhydrous, Al‐free stishovite can contain up to ∼1.3 wt % of H2O, perhaps through the direct substitution ( Si4+→4normalH+), according to recent studies. Yet the stability of such substitution and its impact on the properties of silica and rutile‐structured hydrous phases (such as δ‐AlOOH and phase H) are unknown at the conditions of the deeper mantle. We have synthesized hydrous and anhydrous Al‐free stishovite samples at 723 K and 9 GPa, and 1473 K and 10 GPa, respectively. Synchrotron X‐ray diffraction patterns show that the unit cell volume of hydrous stishovite is 1.3% greater than that of anhydrous stishovite at 1 bar, suggesting significant incorporation of OH in the crystal structure (3.2 ± 0.5 wt % H2O). At 300 K, we found a lower and broader transition pressure from rutile type to CaCl2 type (28–42 GPa) in hydrous dense silica. We also found that hydrous silica polymorphs are more compressible than their anhydrous counterparts. After the phase transition, the unit cell volume of hydrous silica becomes the same as that of anhydrous silica, showing that the proton incorporation through a direct substitution can be further stabilized at high pressure. The lower pressure transition and the pressure stabilization of the proton incorporation in silica would provide ways to transport and store water in the lower mantle in silica‐rich heterogeneities, such as subducted oceanic crust.

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[1] MgGeO 3 post-perovskite (pPv) is a lower pressure analogue of MgSiO 3 -pPv, which is believed to be the main constituent of the Earth's D″ layer. Understanding the physical properties of this phase is critical to explain seismological observations as seismic anisotropy is likely linked to lattice preferred orientation in post-perovskite, which is governed by the motion of defects such as dislocations. Here, we apply in-situ three-dimensional X-ray diffraction to a polycrystalline sample of MgGeO 3 -pPv at 90 GPa. We demonstrate how the method can be used to follow individual grains within the material, including their individual orientations, positions, and strain tensors. We then use X-ray line profile analysis to characterize dislocations in the grains. The most probable slip systems are