Crystal structures of (Mg _{1- x} ,Fe _x )SiO ₃ postperovskite at high pressures

Abstract

X-ray diffraction experiments on postperovskite (ppv) with compositions (Mg 0.9 Fe 0.1 )SiO 3 and (Mg 0.6 Fe 0.4 )SiO 3 at Earth core-mantle boundary pressures reveal different crystal structures. The former adopts the CaIrO 3 -type structure with space group Cmcm , whereas the latter crystallizes in a structure with the Pmcm ( Pmma ) space group. The latter has a significantly higher density ( ρ = 6.119(1) g/cm 3 ) than the former ( ρ = 5.694(8) g/cm 3 ) due to both the larger amount of iron and the smaller ionic radius of Fe 2+ as a result of an electronic spin transition observed by X-ray emission spectroscopy (XES). The smaller ionic radius for low-spin compared to high-spin Fe 2+ also leads to an ordered cation distribution in the M1 and M2 crystallographic sites of the higher density ppv structure. Rietveld structure refinement indicates that approximately 70% of the total Fe 2+ in that phase occupies the M2 site. XES results indicate a loss of 70% of the unpaired electronic spins consistent with a low spin M2 site and high spin M1 site. First-principles calculations of the magnetic ordering confirm that Pmcm with a two-site model is energetically more favorable at high pressure, and predict that the ordered structure is anisotropic in its electrical and elastic properties. These results suggest that interpretations of seismic structure in the deep mantle need to treat a broader range of mineral structures than previously considered.

Single-crystal structure determination of (Mg,Fe)SiO3postperovskite

“…The consistency of the structural parameters between the Fe-bearing ppv in our experiments and the Fe-free ppv from the theoretically calculated structural model (2) suggests that the Fe content in the mantle has a negligible effect on the crystal structure of the ppv phase, in agreement with the previous theoretical calculations suggesting a minor effect of 6.25 mol% Fe content on the thermodynamic properties of the ppv phase at P-T conditions of the lowermost mantle (20). However, if the ultralow-velocity zones at the very base of the lower mantle are highly enriched in Fe from reacting with the core (6), the ppv phase might alter its space group from Cmcm to Pmcm depending on the degree of Fe enrichment (8). Structures from powder diffraction are typically less accurate than those from single crystals.…”

“…Table 3 lists the selected bond lengths and interatomic angles in the structure of ppv at high P and room T in comparison with the results from powder diffraction data in (Mg 1-x , Fe x )SiO 3 (0≤ x ≤ 0.1) (2,8,10,19). The Fe content in the ppv phase was refined to be x = 0.07 when the Fe-Mg ratio was used as a variable in the refinement.…”

“…Furthermore, compositional changes in (Mg, Fe)SiO 3 may result in subtle changes in the crystal structure of the ppv phase. Recent powder X-ray diffraction (XRD) experiments on ppv with different Fe content suggest that (Mg 0.6 Fe 0.4 )SiO 3 ppv adopts a structure with the Pmcm space group, indicating a structural change in ppv due to Fe-enrichment (8). Precise structure analysis for the ppv phase is required to understand subtle effects of pressure and/or composition on its crystal structure.…”

Composition dependence of spin transition in (Mg,Fe)SiO3bridgmanite

“…An additional complication for deep lower mantle seismic structure is the possibility of multiple post-perovskite phases in Fe-bearing compositions (Yamanaka et al 2012;Zhang et al 2014). Zhang et al (2014) have observed that at ~95 GPa, Fe partitions strongly out of bridgmanite into a new "H-phase," of unknown structure and assumed (Mg,Fe)SiO 3 stoichiometry.…”

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

“…The XES technique has been extended to high P-T conditions with laser heated DAC techniques [137,239]. Examples of HP XES include the study of predicted high-spin/low-spin transitions in iron oxides and sulfides, such as(Mg,Fe)O and Fe2O3 [240][241][242], silicate bridgmanites [243,244], and silicate post-bridgmanite [245] to above 100 GPa . 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 [Insert Fig.…”