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  1. Free, publicly-accessible full text available May 20, 2025
  2. Abstract

    Dirac materials offer exciting opportunities to explore low-energy carrier dynamics and novel physical phenomena, especially their interaction with magnetism. In this context, this work focuses on studies of pressure control on the magnetic state of EuMnBi2, a representative magnetic Dirac semimetal, through time-domain synchrotron Mössbauer spectroscopy in151Eu. Contrary to the previous report that the antiferromagnetic order is suppressed by pressure above 4 GPa, we have observed robust magnetic order up to 33.1 GPa. Synchrotron-based x-ray diffraction experiment on a pure EuMnBi2sample shows that the tetragonal crystal lattice remains stable up to at least 31.7 GPa.

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  3. 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,TN, 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 withTNis 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 theTNplateau. 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 enhancesTN, 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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    Free, publicly-accessible full text available December 19, 2024
  4. 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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    We demonstrate the synthesis and phase stability of TcN, Tc 2 N, and a substoichiometric TcN x from 0 to 50 GPa and to 2500 K in a laser-heated diamond anvil cell. At least potential recoverability is demonstrated for each compound. TcN adopts a previously unpredicted structure identified via crystal structure prediction. 
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  9. Abstract

    The transport of hydrogen into Earth's deep interior may have an impact on lower mantle dynamics as well as on the seismic signature of subducted material. Due to the stability of the hydrous phasesδ‐AlOOH (delta phase), MgSiO2(OH)2(phase H), andε‐FeOOH at high temperatures and pressures, their solid solutions may transport significant amounts of hydrogen as deep as the core‐mantle boundary. We have constrained the equation of state, including the effects of a spin crossover in the Fe3+atoms, of (Al, Fe)‐phase H: Al0.84Fe3+0.07Mg0.02Si0.06OOH, using powder X‐ray diffraction measurements to 125 GPa, supported by synchrotron Mössbauer spectroscopy measurements on (Al, Fe)‐phase H andδ‐(Al, Fe)OOH. The changes in spin state of Fe3+in (Al, Fe)‐phase H results in a significant decrease in bulk sound velocity and occurs over a different pressure range (48–62 GPa) compared withδ‐(Al, Fe)OOH (32–40 GPa). Changes in axial compressibilities indicate a decrease in the compressibility of hydrogen bonds in (Al, Fe)‐phase H near 30 GPa, which may be associated with hydrogen bond symmetrization. The formation of (Al, Fe)‐phase H in subducted oceanic crust may contribute to scattering of seismic waves in the mid‐lower mantle (∼1,100–1,550 km). Accumulation of 1–4 wt.% (Al, Fe)‐phase H could reproduce some of the seismic signatures of large, low seismic‐velocity provinces. Our results suggest that changes in the electronic structure of phases in the (δ‐AlOOH)‐(MgSiO2(OH)2)‐(ε‐FeOOH) solid solution are sensitive to composition and that the presence of these phases in subducted oceanic crust could be seismically detectable throughout the lower mantle.

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