We present a resonant-mode, transverse-field, radio-frequency (rf) neutron spin flipper design that uses high-temperature superconducting films to ensure sharp transitions between uniform magnetic field regions. Resonant mode allows for low-power, high-frequency operation but requires strict homogeneity of the magnetic fields inside the device. This design was found to efficiently flip neutrons at 96.6 ± 0.6% at an effective frequency of 4 MHz in bootstrap configuration with a beam size of 2.4 × 2.5 cm2 and a wavelength of 0.4 nm. The high frequency and efficiency enable this device to perform high-resolution neutron spectroscopy with comparable performance with currently implemented rf flipper designs. The limitation of the maximum frequency was found due to the field homogeneity of the device. We numerically analyze the maximum possible efficiency of this design using a Bloch solver simulation with magnetic fields generated from finite-element simulations. We also discuss future improvements of the efficiency and frequency to the design based on the experimental and simulation results.

EuCd2As2 has been proposed to be one of the ideal platforms as an intrinsic topological magnetic system, potentially hosting a single pair of Weyl points when it is tuned into the ferromagnetic state with spins aligned out of plane by either external pressure or chemical doping. To investigate the possible realization of an ideal topological state, we have systematically investigated pressure control of the magnetic state, valence, and crystal structure using synchrotron-based time-domain Mossbauer spectroscopy, x-ray absorption spectroscopy, and powder x-ray diffraction. Our experimental results show that the magnetic configuration remains mostly in plane under pressure up to 42.8 GPa and pressure effectively enhances the magnetic ordering temperature. Meanwhile, Eu ions remain divalent when subjected to pressure up to 35.9 GPa, and the trigonal crystal lattice is maintained up to 34.6 GPa. Our work provides valuable experimental data to benchmark future theoretical studies in magnetic topological materials.

The study of high-entropy materials has attracted enormous interest since they could show new functional properties that are not observed in their related parent phases. Here, we report single crystal growth, structure, thermal transport, and magnetic property studies on a novel high-entropy oxide with the spinel structure (MgMnFeCoNi)Al2O4. We have successfully grown high-quality single crystals of this high-entropy oxide using the optical floating zone growth technique for the first time. The sample was confirmed to be a phase pure high-entropy oxide using x-ray diffraction and energy-dispersive spectroscopy. Through magnetization measurements, we found (MgMnFeCoNi)Al2O4 exhibits a cluster spin glass state, though the parent phases show either antiferromagnetic ordering or spin glass states. Furthermore, we also found that (MgMnFeCoNi)Al2O4 has much greater thermal expansion than its CoAl2O4 parent compound using high resolution neutron Larmor diffraction. We further investigated the structure of this high-entropy material via Raman spectroscopy and extended x-ray absorption fine structure spectroscopy (EXAFS) measurements. From Raman spectroscopy measurements, we observed (MgMnFeCoNi)Al2O4 to display a combination of the active Raman modes in its parent compounds with the modes shifted and significantly broadened. This result, together with the varying bond lengths probed by EXAFS, reveals severe local lattice distortions in this high-entropy phase. Additionally, we found a substantial decrease in thermal conductivity and suppression of the low temperature thermal conductivity peak in (MgMnFeCoNi)Al2O4, consistent with the increased lattice defects and strain. These findings advance the understanding of the dependence of thermal expansion and transport on the lattice distortions in high-entropy materials.

Neutron resonance spin echo (NRSE) technique has the potential to increase the Fourier time and energy resolution in neutron scattering by using radio frequency (rf) neutron spin-flippers. However, aberrations arising from variations in the neutron path length between the rf flippers reduce the polarization. Here, we develop and test a transverse static-field magnet, a series of which are placed between the rf flippers, to correct for these aberrations. The prototype correction magnet was both simulated in an NRSE beamline using McStas, a Monte Carlo neutron ray-tracing software package, and measured using neutrons. The results from the prototype demonstrate that this static-field design corrects for transverse-field NRSE aberrations.

Tantalum (Ta) is a metal that has useful properties that make it useful in extreme environments. It is, therefore, important to understand how Ta performs in such extreme conditions by accurately measuring its properties. In this work, the yield strength of tantalum has been measured at pressures up to 276 GPa using axial and radial x-ray diffraction (XRD) methods in diamond anvil cells (DACs). We measured strength using XRD in a radial DAC to 50 GPa, in an axial DAC to 60 GPa using diamonds with standard flat culets, and in a final experiment to 276 GPa using toroidal diamond anvils. The radial XRD data were refined using the Material Analysis Using Diffraction Rietveld software package to extract lattice strain and the yield strength. The axial data were refined using the General Structure Analysis System II and a linewidth method was used to calculate the yield strength. The yield strength measured near ambient pressure was found to be 0.5 GPa and increased with a pressure of up to 50 GPa, where the yield strength plateaued at a value of 2.4 GPa. At pressures above 60 GPa, the strength increased again to a maximum value of 9 GPa at the highest pressure of 276 GPa. The data from the three experiments show good agreement between the methods and previously reported experimental data. This agreement illustrates the value of axial diffraction data for material strength determination and allows for measurements at multi-hundreds of GPa using toroidal DACs.

The compression behavior of the hexagonal AlB2 phase of Hafnium Diboride (HfB2) was studied in a diamond anvil cell to a pressure of 208 GPa by axial X-ray diffraction employing platinum as an internal pressure standard. The deformation behavior of HfB2 was studied by radial X-ray diffraction technique to 50 GPa, which allows for measurement of maximum differential stress or compressive yield strength at high pressures. The hydrostatic compression curve deduced from radial X-ray diffraction measurements yielded an ambient-pressure volume V0 = 29.73 Å3/atom and a bulk modulus K0 = 282 GPa. Density functional theory calculations showed ambient-pressure volume V0 = 29.84 Å3/atom and bulk modulus K0 = 262 GPa, which are in good agreement with the hydrostatic experimental values. The measured compressive yield strength approaches 3% of the shear modulus at a pressure of 50 GPa. The theoretical strain-stress calculation shows a maximum shear stress τmax~39 GPa along the (1−10) [110] direction of the hexagonal lattice of HfB2, which thereby can be an incompressible high strength material for extreme-environment applications.

ABSTRACT Thermoelastic behavior of transition metal boride Os2B3 was studied under quasi-hydrostatic and isothermal conditions in a Paris-Edinburgh cell to 5.4 GPa and 1273 K. In-situ Energy Dispersive X-ray diffraction was used to determine interplanar spacings of the hexagonal crystal structure and the P-V-T data were fitted to a 3rd Order Birch–Murnaghan equation of state with a temperature modification to determine thermal elastic constants. The bulk modulus was shown to be K0  = 402 ± 21 GPa when the first pressure derivative was held to K0’ = 4.0 from the room temperature P-V curve. Under a quadratic fit , the thermal expansion coefficients were determined to be  K−1,  K−2, and  K. Density functional theory (DFT) with the quasi-harmonic approximation (QHA) were employed to study Os2B3, including its P-V-T curves, phonon spectra, bulk modulus, specific heat, thermal expansion, and the Grüneisen parameter. A good agreement between the first-principle theory and experimental observations was achieved, highlighting the success of the Armiento-Mattsson 2005 generalized gradient approximation functional employed in this study and QHA for describing thermodynamic properties of Os2B3.

The compression behavior of osmium metal was investigated up to 280 GPa (volume compression V/V o = 0.725) under nonhydrostatic conditions at ambient temperature using angle dispersive axial x-ray diffraction (A-XRD) with a diamond anvil cell (DAC). In addition, shear strength of osmium was measured to 170 GPa using radial x-ray diffraction (R-XRD) technique in DAC. Both diffraction techniques in DAC employed platinum as an internal pressure standard. Density functional theory (DFT) calculations were also performed, and the computed lattice parameters and volumes under compression are in good agreement with the experiments. DFT predicts a monotonous increase in axial ratio ( c/a ) with pressure and the structural anomalies of less than 1% in ( c/a ) ratio reported below 150 GPa were not reproduced in theoretical calculations and hydrostatic measurements. The measured value of shear strength of osmium ( τ ) approaches a limiting value of 6 GPa above a pressure of 50 GPa in contrast to theoretical predictions of 24 GPa and is likely due to imperfections in polycrystalline samples. DFT calculations also enable the studies of shear and tensile deformations. The theoretical ideal shear stress is found along the (001)[1–10] shear direction with the maximal shear stress ∼24 GPa at critical strain ∼0.13.

Shear strength measurements have been carried out on rhenium diboride, ReB2, to a pressure of 74 GPa using a Radial X-ray Diffraction (R-XRD) technique in a diamond anvil cell using platinum as an internal x-ray pressure standard. The R-XRD result has provided a unique insight into the deformation of hexagonal crystal lattice under non-hydrostatic compression and variation of shear strength with increasing pressure. From R-XRD data, we have estimated hydrostatic component of compression to determine an equation of state of rhenium diboride yielding a bulk modulus of K0 = 366 ± 25 GPa with a pressure derivative K0′ = 4.3 ± 0.5 in good agreement with hydrostatic density functional theory calculations. The average lower bound of shear strength (τ) from various diffraction planes was then calculated using the measured interplanar d-spacing (dm) and hydrostatic component of d-spacing (dp) to be shown to approach 6.7 ± 0.4 GPa at 70 GPa. Our results show that the anisotropic compression effects observed in ReB2 under hydrostatic compression are correlated to electronic structure changes under compression as predicted by theoretical calculations.

A high-entropy transition metal boride (Hf0.2 Ti0.2 Zr0.2 Ta0.2 Mo0.2)B2 sample was synthesized under high-pressure and high-temperature starting from ball-milled oxide precursors (HfO2, TiO2, ZrO2, Ta2O5, and MoO3) mixed with graphite and boron-carbide. Experiments were conducted in a large-volume Paris–Edinburgh press combined with in situ energy dispersive x-ray diffraction. The hexagonal AlB2 phase with an ambient pressure volume V0 = 27.93 ± 0.03 Å3 was synthesized at a pressure of 0.9 GPa and temperatures above 1373 K. High-pressure high-temperature studies on the synthesized high-entropy transition metal boride sample were performed up to 7.6 GPa and 1873 K. The thermal equation of state fitted to the experimental data resulted in an ambient pressure bulk-modulus K0 = 344 ± 39 GPa, dK/dT = −0.108 ± 0.027 GPa/K, and a temperature dependent volumetric thermal expansion coefficient α = α0 + α1T + α2 T−2. The thermal stability combined with a high bulk-modulus establishes this high-entropy transition metal boride as an ultrahard high-temperature ceramic material.

High pressure study on ultra-hard transition-metal boride Os2B3was carried out in a diamond anvil cell under isothermal and non-hydrostatic compression with platinum as an x-ray pressure standard. The ambient-pressure hexagonal phase of Os2B3is found to be stable with a volume compressionV/V0= 0.670 ± 0.009 at the maximum pressure of 358 ± 7 GPa. Anisotropic compression behavior is observed in Os2B3to the highest pressure, with thec-axis being the least compressible. The measured equation of state using the 3rd-order Birch-Murnaghan fit reveals a bulk modulusK0=397 GPa and its first pressure derivativeK0'=4.0. The experimental lattice parameters and bulk modulus at ambient conditions also agree well with our density-functional-theory (DFT) calculations within an error margin of ∼1%. DFT results indicate that Os2B3becomes more ductile under compression, with a strong anisotropy in the axial bulk modulus persisting to the highest pressure. DFT further enables the studies of charge distribution and electronic structure at high pressure. The pressure-enhanced electron density and repulsion along the Os and B bonds result in a high incompressibility along the crystalc-axis. Our work helps to elucidate the fundamental properties of Os2B3under ultrahigh pressure for potential applications in extreme environments.

An emerging class of superhard materials for extreme environment applications are compounds formed by heavy transition metals with light elements. In this work, ultrahigh pressure experiments on transition metal rhenium diboride (ReB2) were carried out in a diamond anvil cell under isothermal and non-hydrostatic compression. Two independent high-pressure experiments were carried out on ReB2 for the first time up to a pressure of 241 GPa (volume compression V/V0 = 0.731 ± 0.004), with platinum as an internal pressure standard in X-ray diffraction studies. The hexagonal phase of ReB2 was stable under highest pressure, and the anisotropy between the a-axis and c-axis compression increases with pressure to 241 GPa. The measured equation of state (EOS) above the yield stress of ReB2 is well represented by the bulk modulus K0 = 364 GPa and its first pressure derivative K0´ = 3.53. Corresponding density-functional-theory (DFT) simulations of the EOS and elastic constants agreed well with the experimental data. DFT results indicated that ReB2 becomes more ductile with enhanced tendency towards metallic bonding under compression. The DFT results also showed strong crystal anisotropy up to the maximum pressure under study. The pressure-enhanced electron density distribution along the Re and B bond direction renders the material highly incompressible along the c-axis. Our study helps to establish the fundamental basis for anisotropic compression of ReB2 under ultrahigh pressures.

The isothermal compression of transition metal tantalum (Ta) was studied in a diamond anvil cell by X-ray diffraction utilizing rhenium (Re) and gold (Au) as internal X-ray pressure standards. The Re pressure marker was employed during non-hydrostatic compression to pressures up to 310 GPa while the Au pressure marker was used during quasi-hydrostatic compression in a neon pressure-transmitting medium to 80 GPa. Two ultra-high pressure experiments were conducted on Ta and Re mixtures utilizing focused-ion beam machined toroidal diamond anvils with central flats varying from 8 microns to 16 microns in diameter. The Ta metal was observed to be stable in the body-centered-cubic phase to a volume compression V/V-0 = 0.581. The measured equations of state (EOS) of Ta using two different calibrations of the Re pressure marker are compared with the ambient temperature isotherm derived from shock compression data. We provide a detailed analysis of EOS fit parameters for Ta under quasi-hydrostatic and non-hydrostatic conditions.