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We have performed in situ synchrotron X-ray diffraction experiments on TaC0.99 compressed in a diamond anvil cell along 3 isothermal paths to maximum pressure (P)-temperature (T) conditions of 38.8 GPa at 1073 K. By combining measurements performed in axial diffraction geometry at 296 K and in radial geometry at 673 K and 1073 K, we place constraints on the pressure-volume-temperature (P-V-T) equation of state of TaC in a wide range of conditions. A fit of the Birch-Murnaghan equation to the measurements performed in axial geometry at ambient temperature yields a value of the isothermal bulk modulus at ambient conditions K T 0 = 305 ± 5 (1 σ) GPa and its pressure derivative (∂ K T / ∂ P) T 0 = 6.1 ± 0.5. The fit of the Birch-Murnaghan-Debye model to our complete P-V-T dataset allows us to constrain the Grüneisen parameter at ambient pressure γ 0 = V (∂ P / ∂ E) V 0 to the value of 1.2 ± 0.1. [ABSTRACT FROM AUTHOR]

The elastic bulk modulus softening of (Mg,Fe)O ferropericlase across the iron spin crossover induces dramatic changes in its physical properties, including seismic P‐velocities and viscosity. Here, we performed compression of powders of (Mg0.8‐0.9Fe0.2‐0.1)O in a piezo‐driven dynamic Diamond Anvil Cell (dDAC) and derive the bulk modulus by differentiation of pressure and volume data, providing first data on the broadness of the elastic softening for ferropericlase with mantle‐relevant compositions. We complement our experimental results with theoretical calculations that extend previous studies by considering multiple random configurations of iron, and going beyond treating high‐ and low‐spin iron as an ideal solution. Both experiments and computations show a broad and asymmetric softening of the bulk modulus, and suggest that the softening is sensitive to the distribution of iron in the ferropericlase structure. Our high‐temperature calculations show that mixed‐spin (Mg,Fe)O dominates the lower mantle at all depths below 1,000 km. In contrast to most previous works, we find that ferropericlase will not exist in pure low‐spin state along a typical mantle geotherm. Based on our model, the physical properties of ferropericlase will show significant lateral variation at depths below 1,400 km, with the strongest effects expected between 2,000 and 2,600 km. Plain Language Summary: Understanding the physical properties of deep mantle minerals is pivotal to interpret geophysical observables and constrain large‐scale geodynamic models. (Mg,Fe)O ferropericlase is the second most abundant mineral in Earth's lower mantle, ranging from 660 to 2,890 km depths. Previous works have shown that the electronic configuration of iron in ferropericlase changes at pressures corresponding to the mid‐mantle. This process, called iron‐spin crossover, markedly affects a variety of physical properties, including seismic wave speeds and viscosity. Here we combine novel experiments and theoretical calculations to provide a coherent picture of the onset depth and broadness of the spin crossover in Earth's lower mantle. We show that the spin crossover happens throughout most of the lower mantle, altering physical properties at most depths up to the core‐mantle boundary. This quantitative understanding is key to model the impacts of the spin crossover on geophysical mantle properties and mantle dynamics. Key Points: We constrain the broadness of the iron spin crossover by a combination of novel experiments and computationsWe find a broad and asymmetric spin crossover range, which is sensitive to the distribution of iron in the ferropericlase structureFerropericlase is in mixed‐spin state throughout the lower mantle, but shows lateral spin state variations, impacting on mantle properties [ABSTRACT FROM AUTHOR]

Abstract: We experimentally determined the bulk modulus of (Mg0.8Fe0.2)O ferropericlase across the iron spin transition and in the low‐spin phase by employing a new experimental approach. In our measurements, we simulate the propagation of a compressional seismic wave (P wave) through our sample by employing a piezo‐driven dynamic diamond anvil cell that allows to oscillate pressure at seismic frequencies. During pressure oscillations, X‐ray diffraction images were continuously collected every 5–50 ms. The bulk modulus is directly calculated from these data at different pressures. Our experiments show a pronounced softening of the bulk modulus throughout the spin crossover, supporting previous single‐crystal measurements at very high frequencies and computations. Comparison of our results to previous data collected on (Mg,Fe)O with lower iron contents shows that the magnitude of softening strongly depends on iron content. Our experiments at seismic frequencies confirm that the iron spin crossover markedly affects the ratio of seismic compressional to shear wave velocities in Earth's lower mantle. [ABSTRACT FROM AUTHOR]