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  1. Abstract High pressure is an effective tool to induce exotic quantum phenomena in magnetic topological insulators by controlling the interplay of magnetic order and topological state. This work presents a comprehensive high-pressure study of the crystal structure and magnetic ground state up to 62 GPa in an intrinsic topological magnet EuSn 2 P 2 . With a combination of high resolution X-ray diffraction, 151 Eu synchrotron Mössbauer spectroscopy, X-ray absorption spectroscopy, molecular orbital calculations, and electronic band structure calculations, it has been revealed that pressure drives EuSn 2 P 2 from a rhombohedral crystal to an amorphous phase at 36 GPa accompanied by a fourfold enhancement of magnetic ordering temperature. In the pressure-induced amorphous phase, Eu ions take an intermediate valence state. The drastic enhancement of magnetic ordering temperature from 30 K at ambient pressure to 130 K at 41.2 GPa resulting from Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions likely attributes to the stronger Eu–Sn interaction at high pressure. These rich results demonstrate that EuSn 2 P 2 is an ideal platform to study the correlation of the enhanced RKKY interactions, disordered lattice, intermediate valence, and topological state. 
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  2. Nanocrystalline olivine-structured Mg2SiO4 and MgCoSiO4, with an average particle size of 27 nm and 31 nm, respectively, were successfully synthesized from oxide precursors via mechanochemical methods. The two nanocrystalline products were obtained after milling for 360 min and displayed high concentrations of Mg2SiO4 (>94%) and MgCoSiO4 (>95%), together with minor amounts of WC (~3%) contaminant originating as debris abraded off milling balls and chambers. The macroscopic temperature monitoring of the grinding jars during milling trials recorded a peak temperature of 75 °C. A combination of analytical techniques that included XRD, TEM, SAED, and EDS were employed for the characterization of the synthesized products. 
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  3. null (Ed.)
    Abstract Olivine is the most abundant mineral in the Earth's upper mantle and subducting slabs. Studying the structural evolution and equation of state of olivine at high-pressure is of fundamental importance in constraining the composition and structure of these regions. Hydrogen can be incorporated into olivine and significantly influence its physical and chemical properties. Previous infrared and Raman spectroscopic studies indicated that local structural changes occur in Mg-rich hydrous olivine (Fo ≥ 95; 4883–9000 ppmw water) at high-pressure. Since water contents of natural olivine are commonly <1000 ppmw, it is inevitable to investigate the effects of such water contents on the equation of state (EoS) and structure of olivine at high-pressure. Here we synthesized a low water content hydrous olivine (Fo95; 1538 ppmw water) at low SiO2 activity and identified that the incorporated hydrogens are predominantly associated with the Si sites. We performed high-pressure single-crystal X-ray diffraction experiments on this olivine to 29.9 GPa. A third-order Birch-Murnaghan equation of state (BM3 EoS) was fit to the pressure-volume data, yielding the following EoS parameters: VT0 = 290.182(1) Å3, KT0 = 130.8(9) GPa, and K′T0 = 4.16(8). The KT0 is consistent with those of anhydrous Mg-rich olivine, which indicates that such low water content has negligible effects on the bulk modulus of olivine. Furthermore, we carried out the structural refinement of this hydrous olivine as a function of pressure to 29.9 GPa. The results indicate that, similar to the anhydrous olivine, the compression of the M1-O and M2-O bonds are comparable, which are larger than that of the Si-O bonds. The compression of M1-O and M2-O bonds of this hydrous olivine are comparable with those of anhydrous olivine, while the Si-O1 and Si-O2 bonds in the hydrous olivine are more compressible than those in the anhydrous olivine. Therefore, this study suggests that low water content has negligible effects on the EoS of olivine, though the incorporation of water softens the Si-O1 and Si-O2 bond. 
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  4. Abstract

    Single‐crystal X‐ray diffraction and Brillouin spectroscopy experiments were performed on a natural Cr‐pyrope (Prp71.0Alm12.6Sps0.7Grs3.5Uvr12.2) at high pressure and high temperature up to 11.0 GPa and 800 K. Fitting the collected data to the third‐order finite strain equation yields bulk modulus (KS0), shear modulus (G0), their pressure ((∂KS/∂P)Tand (∂G/∂P)T) and temperature ((∂KS/∂T)Pand(∂G/∂T)P) derivatives,KS0 = 167.7(8) GPa,G0 = 91.5(5) GPa, (∂KS/∂P)T = 4.3(1), (∂G/∂P)T = 1.4(1), (∂KS/∂T)P = 0.0175(1) GPa/K and (∂G/∂T)P = 0.0073(1) GPa/K. Using the obtained results, we examined whether the elastic properties of the Cr‐pyrope can be accurately calculated from those of endmembers including pyrope, almandine, grossular, and uvarovite assuming a linear relationship between elastic properties and composition (end‐member model). The results indicate that the end‐member model provides a sufficient approximation for the elastic properties of Cr‐pyrope in calculating the density and velocity of the subcontinental lithospheric mantle (SCLM). We modeled the densities and velocities of three typical types of SCLM (Archon, Proton, and Tecton) in order to investigate how the variation of chemical composition influences the SCLM. We obtained that the compositional change from the Archon to the Tecton increases the density of the SCLM significantly, which can be an important prerequisite for SCLM delamination. However, the compositional variation only slightly changes the velocity of the SCLM and the change is within the uncertainty of the calculation. Moreover, in comparison to the velocity,ρ/VPandρ/VSare much more sensitive to the compositional change of the SCLM.

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  5. Abstract

    (Mg, Fe)SiO3orthopyroxene is an abundant mineral of oceanic subducting slabs. In‐situ high‐pressure and high‐temperature single‐crystal X‐ray diffraction has been used to investigate the phase transition of orthopyroxene across the enstatite‐ferrosilite (En‐Fs) join (En70Fs30, En55Fs45, En44Fs56and Fs100) up to 24.3 GPa and 800 K, simulating conditions within the coldest part of a subduction zone consisting of an old and rapidly subducting slab. Instead of the orthopyroxene → high‐pressure clinopyroxene transition, the α‐opx → β‐opx and β‐opx → γ‐opx phase transition are observed at 7.2–15.3 and 11.6–21.1 GPa (depending on the Fs content), respectively. This study indicates that the pressure‐induced phase transition of (Mg, Fe)SiO3orthopyroxene under relatively low temperature (<800 K) could be different than those occurring under relatively high temperature (>800 K). Additionally, the α‐opx → β‐opx → γ‐opx phase transition could exist within the center of the extremely cold slabs (like Tonga), where such low temperature persists to ~600‐km depth.

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  6. Abstract

    Synchrotron‐based high‐pressure/high‐temperature single‐crystal X‐ray diffraction experiments to ~24 GPa and 700 K were conducted on eclogitic garnets (low‐Fe: Prp28Alm38Grs33Sps1and high‐Fe: Prp14Alm62Grs19Adr3Sps2) and omphacites (low‐Fe: Quad57Jd42Ae1and high‐Fe: Quad53Jd27Ae20), using an externally heated diamond anvil cell. Fitting the pressure‐volume‐temperature data to a third‐order Birch‐Murnaghan equation of state yields the thermoelastic parameters including bulk modulus (KT0), its pressure derivative (KT0), temperature derivative ((∂KT/∂T)P), and thermal expansion coefficient (αT). The densities of the high‐Fe and low‐Fe eclogites were then modeled along typical geotherms of the normal mantle and the subducted oceanic crust to the transition zone depth (550 km). The metastable low‐Fe eclogite could be a reason for the stagnant slabs within the upper range of the transition zone. Eclogite would be responsible for density anomalies within 100–200 km in the upper mantle of Asia.

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