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1. Phase Transitions and Thermal Equation of State of Fe‐9wt.%Si Applied to the Moon and Mercury

   https://doi.org/10.1029/2024JE008466  

   Berrada, Meryem; Chen, Bin; Chao, Keng‐Hsien; Peckenpaugh, Juliana; Wang, Siheng; Zhang, Dongzhou; Nguyen, Phuong; Li, Jie (November 2024, Journal of Geophysical Research: Planets)

   Abstract Accurate knowledge of the phase transitions and thermoelastic properties of candidate iron alloys, such as Fe‐Si alloys, is essential for understanding the nature and dynamics of planetary cores. The phase diagrams of some Fe‐Si alloys between 1 atm and 16 GPa have been back‐extrapolated from higher pressures, but the resulting phase diagram of Fe_83.6Si_16.4(9 wt.% Si) is inconsistent with temperature‐induced changes in its electrical resistivity between 6 and 8 GPa. This study reports in situ synchrotron X‐ray diffraction (XRD) measurements on pre‐melted and powder Fe_83.6Si_16.4samples from ambient conditions to 60 GPa and 900 K using an externally heated diamond‐anvil cell. Upon compression at 300 K, thebccphase persisted up to ∼38 GPa. Thehcpphase appeared near 8 GPa in the pre‐melted sample, and near 17 GPa in the powder sample. The appearance of thehcpphase in the pre‐melted sample reconciles the reported changes in electrical resistivity of a similar sample, thus resolving the low‐pressure region of the phase diagram. The resulting high‐temperature Birch‐Murnaghan equation of state (EoS) and thermal EoS based on the Mie‐Gruneisen‐Debye model of thebccandhcpstructures are consistent with, and complement the literature data at higher pressures. The calculated densities based on the thermal EoS of Fe‐9wt.%Si indicate that bothbccandhcpphases agree with the reported core density estimates for the Moon and Mercury. 

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2. Perturbation-tuned triple spiral metamagnetism and tricritical point in kagome metal ErMn6Sn6

   https://doi.org/10.1038/s43246-024-00552-x  

   Samatham, Satya Shanmukharao; Casey, Jacob; Szucs, Adrienn Maria; Yenugonda, Venkateswara; Burgio, Christopher; Siegrist, Theo; Pathak, Arjun K (December 2024, Communications Materials)

   Abstract Kagome materials are of topical interest for their diverse quantum properties linked with correlated magnetism and topology. Here, we report anomalous hydrostatic pressure (p) effect on ErMn₆Sn₆through isobaric and isothermal-isobaric magnetization measurements. Magnetic field (H) suppresses antiferromagneticT_Nwhile simultaneously enhancing the ferrimagneticT_Cby exhibiting dual metamagnetic transitions, arising from the triple-spiral-nature of Er and Mn spins. Counter-intuitively, pressure enhances bothT_CandT_Nwith a growth rate of 74.4 K GPa⁻¹and 14.4 K GPa⁻¹respectively. Pressure unifies the dual metamagnetic transitions as illustrated throughp-Hphase diagrams at 140 and 200 K. Temperature-field-pressure (T-H,T-p) phase diagrams illustrate distinct field- and pressure-induced critical points at (T_cr= 246 K,H_cr= 23.3 kOe) and (T_cr= 435.8 K,p_cr= 4.74 GPa) respectively. An unusual increase of magnetic entropy by pressure aroundT_crand a putative pressure-induced tricritical point pave a unique way of tuning the magnetic properties of kagome magnets through simultaneous application ofHandp. 

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3. Deep‐Learning‐Based Prediction of the Tetragonal → Cubic Transition in Davemaoite

   https://doi.org/10.1029/2023GL108012  

   Wu, Fulun; Sun, Yang; Wan, Tianqi; Wu, Shunqing; Wentzcovitch, Renata M (June 2024, Geophysical Research Letters)

   Abstract Davemaoite, that is, CaSiO₃perovskite (CaPv), is the third most abundant phase in the lower mantle and exhibits a tetragonal‐cubic phase transition at high pressures and temperatures. The phase boundary in CaPv has recently been proposed to be close to the cold slab adiabat and cause mid‐mantle seismic wave speed anomalies (Thomson et al., 2019,https://doi.org/10.1038/s41586‐019‐1483‐x). This study utilized accurate deep‐learning‐based simulations and thermodynamic integration techniques to compute free energies at temperatures ranging from 300 to 3,000 K and pressures up to 130 GPa. Our results indicate that CaPv exhibits a single cubic phase throughout lower‐mantle conditions. This suggests that the phase diagram proposed by Thomson et al. requires revision, and mid‐mantle seismic anomalies are likely attributable to other mechanisms. 

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4. Experimental Constraints on the Fate of Subducted Sedimentary Nitrogen in the Reduced Mantle

   https://doi.org/10.1029/2022JB025169  

   Huang, Shengxuan; Wu, Xiang; Chariton, Stella; Prakapenka, Vitali B.; Liang, Lin; Zhang, Dongzhou; Yang, Yan; Chen, Bin; Qin, Shan (December 2022, Journal of Geophysical Research: Solid Earth)

   Key Points Subducted N‐bearing silicates and fluids react with Fe or FeS to form different products at high P ‐ T conditions Stability of Fe‐LEs to 35 GPa and 1600 K is Fe‐O > Fe‐N > Fe‐S > Fe‐C, explaining the distribution of nitrides in diamonds The recycling efficiency of subducted sedimentary N is strongly related to the availability of the metallic Fe of the reduced mantle 

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5. Strong enhancement of magnetic ordering temperature and structural/valence transitions in EuPd ₃ S ₄ under high pressure

   https://doi.org/10.1073/pnas.2310779120  

   Huyan, Shuyuan; Ryan, Dominic H.; Slade, Tyler J.; Lavina, Barbara; Jose, Greeshma; Wang, Haozhe; Wilde, John M.; Ribeiro, Raquel A.; Zhao, Jiyong; Xie, Weiwei; et al (December 2023, Proceedings of the National Academy of Sciences)

   We present a comprehensive study of the inhomogeneous mixed-valence compound, EuPd₃S₄, by electrical transport, X-ray diffraction, time-domain¹⁵¹Eu synchrotron Mössbauer spectroscopy, and X-ray absorption spectroscopy measurements under high pressure. Electrical transport measurements show that the antiferromagnetic ordering temperature,T_N, increases rapidly from 2.8 K at ambient pressure to 23.5 K at ~19 GPa and plateaus between ~19 and ~29 GPa after which no anomaly associated withT_Nis detected. A pressure-induced first-order structural transition from cubic to tetragonal is observed, with a rather broad coexistence region (~20 GPa to ~30 GPa) that corresponds to theT_Nplateau. Mössbauer spectroscopy measurements show a clear valence transition from approximately 50:50 Eu²⁺:Eu³⁺to fully Eu³⁺at ~28 GPa, consistent with the vanishing of the magnetic order at the same pressure. X-ray absorption data show a transition to a fully trivalent state at a similar pressure. Our results show that pressure first greatly enhancesT_N, most likely via enhanced hybridization between the Eu 4fstates and the conduction band, and then, second, causes a structural phase transition that coincides with the conversion of the europium to a fully trivalent state. 

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