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Abstract Fe‐Al‐bearing bridgmanite may be the dominant host for ferric iron in Earth's lower mantle. Here we report the synthesis of (Mg0.5Fe3+0.5)(Al0.5Si0.5)O3bridgmanite (FA50) with the highest Fe3+‐Al3+coupled substitution known to date. X‐ray diffraction measurements showed that at ambient conditions, the FA50 adopted the LiNbO3structure. Upon compression at room temperature to 18 GPa, it transformed back into the bridgmanite structure, which remained stable up to 102 GPa and 2,600 K. Fitting Birch‐Murnaghan equation of state of FA50 bridgmanite yieldsV0 = 172.1(4) Å3,K0 = 229(4) GPa withK0′ = 4(fixed). The calculated bulk sound velocity of the FA50 bridgmanite is ~7.7% lower than MgSiO3bridgmanite, mainly because the presence of ferric iron increases the unit‐cell mass by 15.5%. This difference likely represents the upper limit of sound velocity anomaly introduced by Fe3+‐Al3+substitution. X‐ray emission and synchrotron Mössbauer spectroscopy measurements showed that after laser annealing, ~6% of Fe3+cations exchanged with Al3+and underwent the high‐ to low‐spin transition at 59 GPa. The low‐spin proportion of Fe3+increased gradually with pressure and reached 17–31% at 80 GPa. Since the cation exchange and spin transition in this Fe3+‐Al3+‐enriched bridgmanite do not cause resolvable unit‐cell volume reduction, and the increase of low‐spin Fe3+fraction with pressure occurs gradually, the spin transition would not produce a distinct seismic signature in the lower mantle. However, it may influence iron partitioning and isotopic fractionation, thus introducing chemical heterogeneity in the lower mantle.
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Structure and Oxidation States of Giant Unit Cell Compound Dy 117+x Fe 57–y Sn 112–z
Motivated by the complex structure and properties of giant unit cell intermetallic compounds, a new isostructural Fe analogue of the Dy117Co57Sn112structure type was synthesized. Single crystals of Dy122Fe55Sn101were grown at 1260 °C via a Dy–Fe eutectoid flux. The Fe analogue also adopts the space groupFm3mwith lattice parametersa= 29.914(9) Å,V= 26769(23) Å3, andZ= 4. Dy122Fe55Sn101has a large cell volume, structural complexity, and consists of seven Dy, eight Fe, and ten Sn unique crystallographic sites. There are fifteen fully occupied atomic positions, three unique pairs of alternating atomic positions with positional disorder, and seven partially occupied atomic sites. Within this complex unit cell, only approximately half the unique atomic positions are fully occupied with the remainder of the atoms either positionally or occupationally disordered. X‐ray photoelectron spectroscopy indicates that the compound contains Dy3+, Fe0, Fe2+, Sn0, and Sn4+.
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- Award ID(s):
- 1700030
- PAR ID:
- 10046257
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Zeitschrift für anorganische und allgemeine Chemie
- Volume:
- 643
- Issue:
- 23
- ISSN:
- 0044-2313
- Page Range / eLocation ID:
- p. 2038-2044
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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