Non‐Isotropic Contraction and Expansion of Samples in Diamond Anvil Cells: Implications for Thermal Conductivity at the Core‐Mantle Boundary

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