We have performed a series of co‐compression experiments on Au, Pt, Mo, MgO and NaCl to extend internally consistent pressure calibration and characterize shear strength of pressure media to 2.5 Mbar. Measured unit cell volumes of calibrants show differences between existing pressure scales of ∼10% above 2 Mbar. A new comprehensive pressure scale is proposed in good agreement with recent reduced shock isotherms for Au, Mo and Pt and with internal agreement to 3% at Mbar pressures. Deviatoric stresses were analyzed for each material based on diffraction line broadening and line shifts due to anisotropic strain. The measured strength of Ne at 2.5 Mbar is 8 GPa. Diffraction line width analysis suggests that deviatoric stress conditions at 2.5 Mbar are similar for He and Ne media.

Phase transitions and equations of state of the alkaline earth fluorides CaF2, SrF2, and BaF2 were examined by static compression to pressures as high as 146 GPa. Angle-dispersive x-ray diffraction experiments were performed on polycrystalline samples in the laser-heated diamond-anvil cell. We confirmed that at pressures less than 10 GPa all three materials undergo a phase transition from the cubic (Fm3\ifmmode\bar\else\textasciimacron\fi{}m) fluorite structure to the orthorhombic (Pnam) cotunnite-type structure. This work has characterized an additional phase transition in CaF2 and SrF2: these materials were observed to transform to a hexagonal (P63/mmc) Ni2In-type structure between 63\char21{}79 GPa and 28\char21{}29 GPa, respectively, upon laser heating. For SrF2, the Ni2In-type phase was confirmed by Rietveld refinement. Volumes were determined as a function of pressure for all high-pressure phases and fit to the third-order Birch-Murnaghan equation of state. For CaF2 and SrF2, the fluorite-cotunnite transition results in a volume decrease of 8\char21{}10\,%, while the bulk modulus of the cotunnite-type phase is the same or less than that of the fluorite phase within uncertainty. For all three fluorides, the volume reduction associated with the further transition to the Ni2In-type phase is $\sim${}5%. The percentage increase in the bulk modulus ($\Delta${}K) across the transition is greater when the cation is smaller. While for BaF2, $\Delta${}K is 10\char21{}30\,%, $\Delta${}K values for SrF2 and CaF2 are 45\char21{}65\,% and 20\char21{}40\,%. Although shock data for CaF2 have been interpreted to show a transition to a highly incompressible phase above 100 GPa, this is not consistent with our static equation of state data.

Heterogeneity in Earth’s mantle is a record of chemical and dynamic processes over Earth’s history. The geophysical signatures of heterogeneity can only be interpreted with quantitative constraints on effects of major elements such as iron on physical properties including density, compressibility, and electrical conductivity. However, deconvolution of the effects of multiple valence and spin states of iron in bridgmanite (Bdg), the most abundant mineral in the lower mantle, has been challenging. Here we show through a study of a ferric-iron-only (Mg_0.46Fe³⁺_0.53)(Si_0.49Fe³⁺_0.51)O₃Bdg that Fe³⁺in the octahedral site undergoes a spin transition between 43 and 53 GPa at 300 K. The resolved effects of the spin transition on density, bulk sound velocity, and electrical conductivity are smaller than previous estimations, consistent with the smooth depth profiles from geophysical observations. For likely mantle compositions, the valence state of iron has minor effects on density and sound velocities relative to major cation composition.

Heat flux from the core to the mantle provides driving energy for mantle convection thus powering plate tectonics, and contributes a significant fraction of the geothermal heat budget. Indirect estimates of core‐mantle boundary heat flow are typically based on petrological evidence of mantle temperature, interpretations of temperatures indicated by seismic travel times, experimental measurements of mineral melting points, physical mantle convection models, or physical core convection models. However, previous estimates have not consistently integrated these lines of evidence. In this work, an interdisciplinary analysis is applied to co‐constrain core‐mantle boundary heat flow and test the thermal boundary layer (TBL) theory. The concurrence of TBL models, energy balance to support geomagnetism, seismology, and review of petrologic evidence for historic mantle temperatures supports Q_CMB ∼15 TW, with all except geomagnetism supporting as high as ∼20 TW. These values provide a tighter constraint on core heat flux relative to previous work. Our work describes the seismic properties consistent with a TBL, and supports a long‐lived basal mantle molten layer through much of Earth's history.