Deformation of lower-mantle ferropericlase (Mg,Fe)O across the electronic spin transition

Recent high-pressure studies have shown that an electronic spin transition of iron in ferropericlase, an expected major phase of Earth's lower mantle, results in changes in its properties, including density, incompressibility, radiative thermal conductivity, electrical conductivity, and sound velocities. To understand the rheology of ferropericlase across the spin transition, we have used in situ radial X-ray diffraction techniques to examine ferropericlase, (Mg_0.83,Fe_0.17)O, deformed non-hydrostatically in a diamond cell up to 81 GPa at room temperature. Compared with recent quasi-hydrostatic studies, the range of the spin transition is shifted by approximately 20 GPa as a result of the presence of large differential stress in the sample. We also observed a reduction in incompressibility and in the unit cell volume of 3% across the spin transition. Our radial X-ray diffraction results show that the {0 0 1} texture is the dominant lattice preferred orientation in ferropericlase across the spin transition and in the low-spin state. Viscoplastic self-consistent polycrystal plasticity simulations suggest that this preferred orientation pattern is produced by {1 1 0}<1-10> slip. Analyzing our radial X-ray diffraction patterns using lattice strain theory, we evaluated the lattice d-spacings of ferropericlase and Mo as a function of the ψ angle between the compression direction and the diffracting plane normal. These analyses give the ratio between the uniaxial stress component (t) and the shear modulus (G) under constant stress condition, which represents a proxy for the supported differential stress and elastic strength. This ratio in the mixed-spin and low-spin states is lower than what is expected from previous studies of high-spin ferropericlase, indicating that the spin transition results in a reduced differential stress and elastic strength along with the volume reduction. The influence of the spin transition on the differential stress and strength of ferropericlase is expected to be less dominant across the wide spin transition zone at high pressure-temperature conditions relevant to the lower mantle.