Monoclinic EuSn_{2}As_{2}: A Novel High-Pressure Network Structure

Single-crystal growth, structure and thermal transport properties of the metallic antiferromagnet Zintl-phase β-EuIn2As2

Zintl-phase materials have attracted significant research interest owing to the interplay of magnetism and strong spin-orbit coupling, providing a prominent material platform for axion electrodynamics. Here, we report the single-crystal growth, structure, magnetic and electrical/thermal transport properties of the antiferromagnet layer Zintl-phase compound β-EuIn2As2. Importantly, the new layered structure of β-EuIn2As2, in rhombohedral (R3̄m) symmetry, contains triangular layers of Eu2+ ions. The in-plane resistivity ρ(H, T) measurements reveal metal behavior with an antiferromagnetic (AFM) transition (TN ∼ 23.5 K), which is consistent with the heat capacity Cp(H, T) and magnetic susceptibility χ(H, T) measurements. Negative MR was observed in the temperature range from 2 K to 20 K with a maximum MR ratio of 0.06. Unique 4f7J = S = 7/2 Eu2+ spins were supposed magnetically order along the c-axis. The Seebeck coefficient shows a maximum thermopower |Smax| of about 40 μV K-1. The kink around 23 K in the Seebeck coefficient originates from the effect of the antiferromagnetic phase on the electron band structure, while the pronounced thermal conductivity peak at around 10 K is attributed to the phonon-phonon Umklapp scattering. The results suggest that the Eu2+ spin arrangement plays an important role in the magnetic, electrical, and thermal transport properties in β-EuIn2As2, which might be helpful for future potential technical applications.

Strong enhancement of magnetic ordering temperature and structural/valence transitions in EuPd3S4 under high pressure

We present a comprehensive study of the inhomogeneous mixed-valence compound, EuPd3S4, by electrical transport, X-ray diffraction, time-domain 151Eu 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 with TN is 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 the TN plateau. 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 enhances TN, most likely via enhanced hybridization between the Eu 4f states 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.

Superconductivity in an Orbital-Reoriented SnAs Square Lattice: A Case Study of Li0.6 Sn2 As2 and NaSnAs

Searching for functional square lattices in layered superconductor systems offers an explicit clue to modify the electron behavior and find exotic properties. The trigonal SnAs₃ structural units in SnAs-based systems are relatively conformable to distortion, which provides the possibility to achieve structurally topological transformation and higher superconducting transition temperatures. In the present work, the functional As square lattice was realized and activated in Li_0.6 Sn₂ As₂ and NaSnAs through a topotactic structural transformation of trigonal SnAs₃ to square SnAs₄ under pressure, resulting in a record-high T_c among all synthesized SnAs-based compounds. Meanwhile, the conductive channel transfers from the out-of-plane p_z orbital to the in-plane p_x +p_y orbitals, facilitating electron hopping within the square 2D lattice and boosting the superconductivity. The reorientation of p-orbital following a directed local structure transformation provides an effective strategy to modify layered superconducting systems.

Effects of high pressure on the lattice structure and electrical transport properties of BiOI

To reveal the pressure effects on BiOX semiconductors, we performed in-situ Raman spectroscopy and electrical transport measurements on BiOI up to 26.1 GPa and 19.2 GPa. BiOI showed good structural stability, while the electron conduction characteristics maintained dominance throughout the pressure range. The influence of grain boundary conduction disappeared at pressures above 9.2 GPa. With pressure elevation, the pressure-induced lattice fragmentation and grain refinement introduced a large number of relevant levels in the energy gap and resulted in a significant increase in the conductivity of BiOI under compression. The conductivity increased by 10⁶ at 19.2 GPa from the initial value and maintained an increase of 10² after depressurization until ambient conditions were attained. At the same time, the space charge polarization of the crystal interface layer became weaker with pressure elevation resulting in a decrease in the relative permittivity of BiOI. The calculation results of the complex permittivity showed that the frequency of orientation polarization response decreases with pressure elevation, and the complex permittivity becomes constant in the high-frequency region. Our work proves that pressure could significantly increase the carrier concentration and mobility, thus effectively improving the conductivity of BiOX semiconductors.

Application of impedance spectroscopy in exploring electrical properties of dielectric materials under high pressure

Impedance spectroscopy (IS) is an indispensable method of exploring electrical properties of materials. In this review, we provide an overview on the specific applications of IS measurement in the investigations of various electrical properties of materials under high pressure, including electric conduction in bulk and grain boundary, dielectric properties, ionic conduction, and electrostrictive effect. Related studies are summarized to demonstrate the method of analyzing different electrical transport processes with various designed equivalent circuits of IS and reveal some interesting phenomena of electrical properties of materials under high pressure.

The Zintl phases compound AEIn2As2 (AE=Ca, Sr, Ba): topological phase transition under pressure

AEIn2As2 (AE=Ca, Sr, Ba), as a new crucial nonmagnetic thermoelectric candidate, is desired to be understood in terms of its potential physical properties and controversial structural phases in both experimental...

Structural, magnetic, and electronic properties of EuSi2 thin films on Si(111) surface

Searching for magnetic silicide thin films has long been a hot topic in condensed matter physics and materials science based on its fundamental physics and promising device applications. Here we...