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57Fe nuclear resonance vibrational spectroscopy (NRVS) is used to study the tetranuclear iron clusters bearing a terminal Fe(iii)–O/OH moiety. The redox states of the three remote basal iron sites modulate the Fe(iii)–O/OH vibrational frequencies. 

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. 

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. 

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. 

Engineering structures that bridge between elements with disparate mechanical properties are a significant challenge. Organisms reap synergy by creating complex shapes that are intricately graded. For instance, the wear-resistant cusp of the chiton radula tooth works in concert with progressively softer microarchitectural units as the mollusk grazes on and erodes rock. Herein, we focus on the stylus that connects the ultrahard and stiff tooth head to the flexible radula membrane. Using techniques that are especially suited to probe the rich chemistry of iron at high spatial resolution, in particular synchrotron Mössbauer and X-ray absorption spectroscopy, we find that the upper stylus ofCryptochiton stelleriis in fact a mineralized tissue. Remarkably, the inorganic phase is nano disperse santabarbaraite, an amorphous ferric hydroxyphosphate that has not been observed as a biomineral. The presence of two persistent polyamorphic phases, amorphous ferric phosphate and santabarbaraite, in close proximity, is a unique aspect that demonstrates the level of control over phase transformations inC. stelleridentition. The stylus is a highly graded material in that its mineral content and mechanical properties vary by a factor of 3 to 8 over distances of a few hundred micrometers, seamlessly bridging between the soft radula and the hard tooth head. The use of amorphous phases that are low in iron and high in water content may be key to increasing the specific strength of the stylus. Finally, we show that we can distill these insights into design criteria for inks for additive manufacturing of highly tunable chitosan-based composites.