Spin crossover equation of state and sound velocities of (Mg_0.65Fe_0.35)O ferropericlase to 140 GPa

Abstract

[1] We have determined the elastic and vibrational properties of periclase-structured (Mg 0.65 Fe 0.35 )O ("FP35"), a composition representative of deep mantle "pyrolite" or chondrite-pyroxenite models, from nuclear resonant inelastic x-ray scattering (NRIXS) and x-ray diffraction (XRD) measurements in diamond-anvil cells at 300 K. Combining with in situ XRD measurements, the Debye sound velocity of FP35 was determined from the low-energy region of the partial phonon density of states (DOS) obtained from NRIXS measurements in the pressure range of 70 to 140 GPa. In order to obtain an accurate description of the equation of state (EOS) for FP35, separate XRD measurements were performed up to 126 GPa at 300 K. A new spin crossover EOS was introduced and applied to the full P-V data set, resulting in a zero-pressure volume V 0 = 77.24 AE 0.17 Å 3 , bulk modulus K 0 = 159 AE 8 GPa and its pressure-derivative K′ 0 = 4.12 AE 0.42 for high-spin FP35 and K 0,LS = 72.9 AE 1.3 Å 3 , K 0,LS = 182 AE 17 GPa with K′ 0,LS fixed to 4 for low-spin FP35. The high-spin to low-spin transition occurs at 64 AE 3 GPa. Using the spin crossover EOS and Debye sound velocity, we derived the shear (V S ) and compressional (V P ) velocities for FP35. Comparing our data with previous results on (Mg,Fe)O at similar pressures, we find that the addition of iron decreases both V P and V S , while elevating their ratio (V P /V S ). Small amounts (<10%) of low-spin FP35 mixed with silicates could explain moderate reductions in wave speeds near the core mantle boundary (CMB), while a larger amount of FP35 near the CMB would not allow a large structure to maintain neutral buoyancy.

Critical behavior of Mg1–xFexO at the pressure-induced iron spin-state crossover

“…There are numerous X-ray diffraction studies addressing the compression of Fp, many investigating the effect of pressure on the cubic unit cell and on the onset of the HS-LS transition [5][6][7]. However, most of the studies were conducted on powder material, where a strong discrepancy is observed even between results obtained on similar materials at seemly the same hydrostatic conditions (e.g.…”

High-pressure studies with x-rays using diamond anvil cells

“…Preferred orientation develops during dislocation glide which can be used to infer deformation mechanisms [90,308,493,557]. Radial XRD offers a good measure of lattice preferred orientation.…”

“…With the orientation distribution information, we can calculate polycrystal physical properties based on our knowledge of single-crystal physical properties (e.g., see a review article [558] and references therein). Pole figure analysis also provides important information in understanding the mechanisms of texture, such as the dislocation glide during deformation [89,[559][560][561], dislocation mechanism of nanomaterials [308,493], recrystallization (e.g., [562]), grain boundary effect (e.g., [563]), and inherited textures through phase transformations (e.g., [564]). …”

“…The NFS hyperfine signals are very sensitive to internal magnetic fields, electric field gradients, and isomer shifts [303][304][305], and has been used to study HP behavior of materials at megabar pressures, such as magnetic collapse [172,243,306], site occupancy [244,307], and valence and spin state [308][309][310][311]. Fast NFS experiments have been performed to determine the HP melting temperatures of iron [312], by measuring the LambMössbauer factor which describes the probability of recoilless absorption.…”

Sound velocity and density of magnesiowüstites: Implications for ultralow‐velocity zone topography

“…Together with a reliable equation of state for this material [Wicks et al, 2015], we report the Debye sound velocities as a function of density and pressure. (Mg 0.058(1) Fe 0.942(1) )O (Mw94) and Fe 0.941(2) O (FeO, wüstite) samples were synthesized at ambient pressure using 95% enriched 57 Fe (previously described in Chen et al [2012] and Wicks et al [2015]). The chemical composition of wüstite was computed based upon its lattice parameter (a = 4.306(1) Å) and the relationship reported in McCammon and Liu [1984]: a = 3.856 + 0.478x for Fe x O.…”