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Brillouin spectroscopy at room temperature and pressures up to 40 GPa documents nearly identical elasticity and refractive index of amorphous CaSiO3created by two different methods: temperature-quenching the melt at ambient pressure and pressure-amorphizing crystalline wollastonite at room temperature. We find reproducible hysteresis of 0 to 8% on pressure cycling that is small relative to the 30 to 60% changes in shear and longitudinal wave velocities over this pressure range. Together with observed changes in refractive index and previous results from Raman spectroscopy, these measurements reveal a continuous and reversible change in atomic packing induced by pressure. Unlike many other silicate glasses, amorphous CaSiO3exhibits highly reproducible properties, behaving smoothly and reversibly under pressure cycling and possessing similar structure and elasticity regardless of synthesis paths for the starting material, which suggests that the amorphous solid may mimic the liquid over the pressure range investigated.

Two new polyhydrides of calcium have been synthesized at high pressures and high temperatures and characterized by Raman spectroscopy, infrared spectroscopy, and synchrotron X-ray diffraction. Above 20 GPa and 700 K, we synthesize a phase having a monoclinic (C2/m) structure with Ca2H5composition, which is characterized by a distinctive vibration at 3789 cm–1at 25 GPa. The observed Raman spectrum is in close agreement with first-principles calculations of a Ca2H5structure characterized by a lattice containing a central layer of H2molecules oriented along the (100) direction. At higher pressures (e.g., 116 GPa and 1600 K), we synthesize another phase, which has the composition of CaH4and a denser body-centered tetragonal structure. This weakly metallic phase also contains molecular-like H2units, and its spectroscopic as well as diffraction signatures match closely with those predicted from first-principles calculations. This phase is observed to persist on decompression to 60 GPa at room temperature. The elongation of the H–H bond in these hydrides is a result of the Ca–H2interaction, analogous to what occurs in molecular compounds, where H2binds side-on to a d-element, such as in Kubas complex.

The thermal conductivities of mantle and core materials have a major impact on planetary evolution, but their experimental determination requires precise knowledge of sample thickness at high pressure. Despite its importance, thickness in most diamond anvil cell (DAC) experiments is not measured but inferred from equations of state, assuming isotropic contraction upon compression or assuming isotropic expansion upon decompression. Here we provide evidence that in DAC experiments both assumptions are invalid for a range of mechanically diverse materials (KCl, NaCl, Ar, MgO, silica glass, Al2O3). Upon compression, these samples are ∼30–50% thinner than expected from isotropic contraction. Most surprisingly, all the studied samples continue to thin upon decompression to 10–20 GPa. Our results partially explain some discrepancies among the highly controversial thermal conductivity values of iron at Earth's core conditions. More generally, we suggest that in situcharacterization of sample geometry is essential for conductivity measurements at high pressure. The thermal and electrical conductivities of the materials making up Earth's core and lowermost mantle are crucial inputs for modeling Earth's interior and the geodynamo mechanism. Yet, large disagreements between published values of conductivity are common, including a factor‐of‐seven discrepancy in the thermal conductivity of iron at core‐mantle boundary conditions. One possible source of systematic uncertainty is the estimate of sample thickness during high‐pressure experiments. Here we show that common materials in compression experiments tend to thin by much more than previously assumed. Surprisingly, the thinning continues upon decompression. These thinning trends could lead to ∼30–50% systematic error, partially explaining the discrepancy in iron conductivity. In situthickness measurements are thus crucial for accurate determination of conductivities of Earth's mantle and core. Samples in diamond anvil cell experiments contract and expand in a strongly non‐isotropic fashion upon compression and decompressionExperimental reports on iron conductivity at high pressure contain errors due to the assumption of isotropic contraction or expansionAccurate in situ determination of sample geometry is necessary for thermal and electrical conductivity measurements at high pressure Samples in diamond anvil cell experiments contract and expand in a strongly non‐isotropic fashion upon compression and decompression Experimental reports on iron conductivity at high pressure contain errors due to the assumption of isotropic contraction or expansion Accurate in situ determination of sample geometry is necessary for thermal and electrical conductivity measurements at high pressure

FeSi remains crystalline up to at least 2350 (±200) K at 23 GPa and 2770 (±200) K at 47 GPa in a laser-heated diamond-anvil cell, showing that addition of silicon does not cause a large amount of melting point depression; the melting temperature of pure iron ranges from 2300 (±100) K to 2700 (±150) K between 20 and 50 GPa. The transition between e (B20) and B2 (CsCl-structured) crystalline phases occurs at 30 (±2) GPa at all temperatures from 1200 to 2400 K. The resulting 5% density increase may cause an increase in the miscibility of silicon in iron at P > 30 GPa, with potential implications for the cores of small rocky planets such as Mars and Mercury.