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1. Synthesis, Elasticity, and Spin State of an Intermediate MgSiO 3 ‐FeAlO 3 Bridgmanite: Implications for Iron in Earth's Lower Mantle

   https://doi.org/10.1029/2020JB019964  

   Zhu, Feng ; Liu, Jiachao ; Lai, Xiaojing ; Xiao, Yuming ; Prakapenka, Vitali ; Bi, Wenli ; Alp, E. Ercan ; Dera, Przemyslaw ; Chen, Bin ; Li, Jie ( July 2020 , Journal of Geophysical Research: Solid Earth)

   Fe‐Al‐bearing bridgmanite may be the dominant host for ferric iron in Earth's lower mantle. Here we report the synthesis of (Mg_0.5Fe³⁺_0.5)(Al_0.5Si_0.5)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) ų,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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2. A Cesium Rare‐Earth Silicate Cs 3 RESi 6 O 15 (RE=Dy–Lu, Y, In): The Parent of an Unusual Structural Class Featuring a Remarkable 57 Å Unit Cell Axis

   https://doi.org/10.1002/anie.201708798  

   Terry, Rylan ; Vinton, Daniel ; McMillen, Colin D. ; Kolis, Joseph W. ( February 2018 , Angewandte Chemie International Edition)

   The structure of Cs₃RESi₆O₁₅, where RE=Dy–Lu, Y, In, is unusual in that it contains octahedrally coordinated rare‐earth ions; their relative orientation dictates the structure, as they rotate about thec‐axis supported by the cyclic Si₆O₁₅framework. The repeat unit of the rotation is eight units generating a very long (ca. 57 Å) unit cell axis. This unusual repeat unit is created by the structural flexibility of the hexasilicate ring, which is in turn affected by the size of the rare earth ion as well as the size of alkali ion residing within the silicate layers. Previous work showed for the smaller Sc³⁺ion, the rotation of the octahedra is not sufficient to achieve closure at an integral repeat unit and an incommensurate structure results. The products are prepared as large, high quality single crystals using a high‐temperature (650 °C) hydrothermal method with CsOH and F⁻mineralizers. The presence of fluoride is essential to the formation of the product.

    

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3. Effect of electron- and hole-doping on properties of kagomé-lattice ferromagnet Fe 3 Sn 2

   https://doi.org/10.1088/1361-648X/acc91e  

   Adams, Milo ; Huang, Chen ; Shatruk, Michael ( April 2023 , Journal of Physics: Condensed Matter)

   We report a theoretical investigation of effects of Mn and Co substitution in the transition metal sites of the kagomé-lattice ferromagnet, Fe₃Sn₂. Herein, hole- and electron-doping effects of Fe₃Sn₂have been studied by density-functional theory calculations on the parent phase and on the substituted structural models of Fe_3−xM_xSn₂(M = Mn, Co;x= 0.5, 1.0). All optimized structures favor the ferromagnetic ground state. Analysis of the electronic density of states (DOS) and band structure plots reveals that the hole (electron) doping leads to a progressive decrease (increase) in the magnetic moment per Fe atom and per unit cell overall. The high DOS is retained nearby the Fermi level in the case of both Mn and Co substitutions. The electron doping with Co results in the loss of nodal band degeneracies, while in the case of hole doping with Mn emergent nodal band degeneracies and flatbands initially are suppressed in Fe_2.5Mn_0.5Sn₂but re-emerge in Fe₂MnSn₂. These results provide key insights into potential modifications of intriguing coupling between electronic and spin degrees of freedom observed in Fe₃Sn₂.

    

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4. Effect of R-site element on crystalline phase and thermal stability of Fe substituted Mn mullite-type oxides: R 2 (Mn 1−x Fe x ) 4 O 10−δ (R = Y, Sm or Bi; x = 0, 0.5, 1)

   https://doi.org/10.1039/C7RA09699B  

   Thampy, Sampreetha ; Ashburn, Nickolas ; Martin, Thomas J. ; Li, Chenzhe ; Zheng, Yongping ; Chan, Julia Y. ; Cho, Kyeongjae ; Hsu, Julia W. ( January 2018 , RSC Advances)

   Combining experimental and theoretical studies, we investigate the role of R-site (R = Y, Sm, Bi) element on the phase formation and thermal stability of R 2 (Mn 1−x Fe x ) 4 O 10−δ ( x = 0, 0.5, 1) mullite-type oxides. Our results show a distinct R-site dependent phase behavior for mullite-type oxides as Fe is substituted for Mn: 100% mullite-type phase was formed in (Y, Sm, Bi) 2 Mn 4 O 10 ; 55% and 18% of (Y, Sm) 2 Mn 2 Fe 2 O 10−δ was found when R = Y and Sm, respectively, for equal Fe and Mn molar concentrations in the reactants, whereas Bi formed 54% O10- and 42% O9-mixed mullite-type phases. Furthermore, when the reactants contain 100% Fe, no mullite-type phase was formed for R = Y and Sm, but a sub-group transition to Bi 2 Fe 4 O 9 O9-phase was found for R = Bi. Thermogravimetric analysis and density functional theory (DFT) calculation results show a decreasing thermal stability in O10-type structure with increasing Fe incorporation; for example, the decomposition temperature is 1142 K for Bi 2 Mn 2 Fe 2 O 10−δ vs. 1217 K for Bi 2 Mn 4 O 10 . On the other hand, Bi 2 Fe 4 O 9 O9-type structure is found to be thermally stable up to 1227 K. These findings are explained by electronic structure calculations: (1) as Fe concentration increases, Jahn–Teller distortion results in mid band-gap empty states from unstable Fe 4+ occupied octahedra, which is responsible for the decrease in O10 structure stability; (2) the directional sp orbital hybridization unique to Bi effectively stabilizes the mullite-type structure as Fe replaces Mn. 

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5. Spin Transitions and Compressibility of ε‐Fe 7 N 3 and γ′‐Fe 4 N: Implications for Iron Alloys in Terrestrial Planet Cores

   https://doi.org/10.1029/2020JB020660  

   Lv, Mingda ; Liu, Jiachao ; Zhu, Feng ; Li, Jie ; Zhang, Dongzhou ; Xiao, Yuming ; Dorfman, Susannah M. ( November 2020 , Journal of Geophysical Research: Solid Earth)

   Iron nitrides are possible constituents of the cores of Earth and other terrestrial planets. Pressure‐induced magnetic changes in iron nitrides and effects on compressibility remain poorly understood. Here we report synchrotron X‐ray emission spectroscopy (XES) and X‐ray diffraction (XRD) results for ε‐Fe₇N₃and γ′‐Fe₄N up to 60 GPa at 300 K. The XES spectra reveal completion of high‐ to low‐spin transition in ε‐Fe₇N₃and γ′‐Fe₄N at 43 and 34 GPa, respectively. The completion of the spin transition induces stiffening in bulk modulus of ε‐Fe₇N₃by 22% at ~40 GPa, but has no resolvable effect on the compression behavior of γ′‐Fe₄N. Fitting pressure‐volume data to the Birch‐Murnaghan equation of state yieldsV₀ = 83.29 ± 0.03 (ų),K₀ = 232 ± 9 GPa,K₀′ = 4.1 ± 0.5 for nonmagnetic ε‐Fe₇N₃above the spin transition completion pressure, andV₀ = 54.82 ± 0.02 (ų),K₀ = 152 ± 2 GPa,K₀′ = 4.0 ± 0.1 for γ′‐Fe₄N over the studied pressure range. By reexamining evidence for spin transition and effects on compressibility of other candidate components of terrestrial planet cores, Fe₃S, Fe₃P, Fe₇C₃, and Fe₃C based on previous XES and XRD measurements, we located the completion of high‐ to low‐spin transition at ~67, 38, 50, and 30 GPa at 300 K, respectively. The completion of spin transitions of Fe₃S, Fe₃P, and Fe₃C induces elastic stiffening, whereas that of Fe₇C₃induces elastic softening. Changes in compressibility at completion of spin transitions in iron‐light element alloys may influence the properties of Earth's and planetary cores.

    

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