Incommensurate Spiral Spin Order in CaMn2Bi2 Observed via High-Pressure Neutron Diffraction.

High-pressure neutron diffraction is employed to investigate the magnetic behavior of CaMn2Bi2 in extreme conditions. In contrast to antiferromagnetic ordering on Mn atoms reported at ambient pressure, our results reveal that at high pressure, incommensurate spiral spin order emerges due to the interplay between magnetism on the Mn atoms and strong spin-orbit coupling on the Bi atoms: sinusoidal spin order is observed at pressures as high as 7.4 GPa. First-principles calculations with a noncollinear spin orientation demonstrate band crossing behavior near the Fermi level as a result of strong hybridization between the d orbitals of Mn and the p orbitals of Bi atoms. Competing antiferromagnetic order is observed at different temperatures in the partially frustrated lattice. Theoretical models have been developed to investigate spin dynamics. This research provides a unique toolbox for conducting experimental and theoretical magnetic and spin dynamics studies of magnetic quantum materials via high-pressure neutron diffraction.

Decoupling Lattice and Magnetic Instabilities in Frustrated CuMnO2.

The AMnO2 delafossites (A = Na, Cu) are model frustrated antiferromagnets, with triangular layers of Mn3+ spins. At low temperatures (TN = 65 K), a C2/m → P1̅ transition is found in CuMnO2, which breaks frustration and establishes magnetic order. In contrast to this clean transition, A = Na only shows short-range distortions at TN. Here, we report a systematic crystallographic, spectroscopic, and theoretical investigation of CuMnO2. We show that, even in stoichiometric samples, nonzero anisotropic Cu displacements coexist with magnetic order. Using X-ray/neutron diffraction and Raman scattering, we show that high pressures act to decouple these degrees of freedom. This manifests as an isostuctural phase transition at ∼10 GPa, with a reversible collapse of the c-axis. This is shown to be the high-pressure analogue of the c-axis negative thermal expansion seen at ambient pressure. Density functional theory (DFT) simulations confirm that dynamical instabilities of the Cu+ cations and edge-shared MnO6 layers are intertwined at ambient pressure. However, high pressure selectively activates the former, before an eventual predicted reemergence of magnetism at the highest pressures. Our results show that the lattice dynamics and local structure of CuMnO2 are quantitatively different from nonmagnetic Cu delafossites and raise questions about the role of intrinsic inhomogeneity in frustrated antiferromagnets.

Thermal Expansion and Response to Pressure of Double-ReO3-Type Fluorides NaMVF6 (M = Nb, Ta).

Several II-IV double-ReO3-type (DROT) fluorides are known to exhibit strong negative thermal expansion (NTE) over a wide temperature range while retaining a cubic structure down to 120 K or lower. CaZrF6, CaNbF6, CaTiF6, and MgZrF6, embody these properties. In contrast to the behavior of these II-IV materials, the I-V DROT material, NaSbF6, has been reported to display a phase transition from rhombohedral to cubic above 300 K and positive thermal expansion both above and below the transition. In this work, NaNbF6 and NaTaF6 are shown to undergo first-order cubic-to-rhombohedral transitions on cooling to ∼130 K. Above this transition, NaNbF6 shows modest NTE between 160 and 250 K, whereas NaTaF6 exhibits near-zero thermal expansion over the range 210-270 K. These I-V systems are elastically softer than their II-IV counterparts, with a zero pressure bulk modulus, K0, of 14.6(8) GPa and first derivative of the bulk modulus with respect to pressure, K0', of -18(3) for cubic NaNbF6, and K0 = 14.47(3) GPa and K0'= -21.56(7) for cubic NaTaF6. When subject to ∼0.3 GPa at 300 K, both compounds exhibit a phase transition from Fm3̅m to R3̅. The R3̅ phases exhibit negative linear compressibility over a limited pressure range. A further transition with phase coexistence occurs at ∼2.5-3.0 GPa for NaNbF6 and ∼4.5 GPa for NaTaF6. Compression of NaNbF6 in helium at room temperature and below provides no evidence for helium penetration into the structure to form a perovskite with helium on the A-site, as was previously reported for CaZrF6.

3D scanning and 3D printing AlSi10Mg single crystal mounts for neutron scattering.

We present methods to quantify sample shapes and generate sample mounts as motivated by the needs of neutron scattering experiments. The 3D sample scanning was performed using photogrammetry and laser scanning, and a comparison is made between the two techniques. The aluminum alloy AlSi10Mg is shown to have favorable properties for many types of mounts used in neutron scattering. Parts were first prototyped with 3D plastic printers, and then, 3D AlSi10Mg prints were made. The final additively manufactured part holds the sample with more points of contact than is possible with traditional manufacturing. The goodness of fit between the mount and sample was measured by x-ray tomography.

Giant Barocaloric Effect at the Spin Crossover Transition of a Molecular Crystal.

The first experimental evidence for a giant, conventional barocaloric effect (BCE) associated with a pressure-driven spin crossover transition near room temperature is provided. Magnetometry, neutron scattering, and calorimetry are used to explore the pressure dependence of the SCO phase transition in polycrystalline samples of protonated and partially deuterated [FeL2 ][BF4 ]2 [L = 2,6-di(pyrazol-1-yl)pyridine] at applied pressures of up to 120 MPa (1200 bar). The data indicate that, for a pressure change of only 0-300 bar (0-30 MPa), an adiabatic temperature change of 3 K is observed at 262 K or 257 K in the protonated and deuterated materials, respectively. This BCE is equivalent to the magnetocaloric effect (MCE) observed in gadolinium in a magnetic field change of 0-1 Tesla. The work confirms recent predictions that giant, conventional BCEs will be found in a wide range of SCO compounds.

Next-generation diamond cell and applications to single-crystal neutron diffraction.

A diamond cell optimized for single-crystal neutron diffraction is described. It is adapted for work at several of the single-crystal diffractometers of the Spallation Neutron Source and the High Flux Isotope Reactor at the Oak Ridge National Laboratory (ORNL). A simple spring design improves portability across the facilities and affords load maintenance from offline pressurization and during temperature cycling. Compared to earlier prototypes, pressure stability of polycrystalline diamond (Versimax®) has been increased through double-conical designs and ease of use has been improved through changes to seat and piston setups. These anvils allow ∼30%-40% taller samples than possible with comparable single-crystal anvils. Hydrostaticity and the important absence of shear pressure gradients have been established with the use of glycerin as a pressure medium. Large single-crystal synthetic diamonds have also been used for the first time with such a clamp-diamond anvil cell for pressures close to 20 GPa. The cell is made from a copper beryllium alloy and sized to fit into ORNL's magnets for future ultra-low temperature and high-field studies. We show examples from the Spallation Neutron Source's SNAP and CORELLI beamlines and the High Flux Isotope Reactor's HB-3A and IMAGINE beamlines.

Direct observation of symmetrization of hydrogen bond in δ-AlOOH under mantle conditions using neutron diffraction.

At ambient pressure, the hydrogen bond in materials such as ice, hydrates, and hydrous minerals that compose the Earth and icy planets generally takes an asymmetric O-H···O configuration. Pressure significantly affects this configuration, and it is predicted to become symmetric, such that the hydrogen is centered between the two oxygen atoms at high pressure. Changes of physical properties of minerals relevant to this symmetrization have been found; however, the atomic configuration around this symmetrization has remained elusive so far. Here we observed the pressure response of the hydrogen bonds in the aluminous hydrous minerals δ-AlOOH and δ-AlOOD by means of a neutron diffraction experiment. We find that the transition from P21nm to Pnnm at 9.0 GPa, accompanied by a change in the axial ratios of δ-AlOOH, corresponds to the disorder of hydrogen bond between two equivalent sites across the center of the O···O line. Symmetrization of the hydrogen bond is observed at 18.1 GPa, which is considerably higher than the disorder pressure. Moreover, there is a significant isotope effect on hydrogen bond geometry and transition pressure. This study indicates that disorder of the hydrogen bond as a precursor of symmetrization may also play an important role in determining the physical properties of minerals such as bulk modulus and seismic wave velocities in the Earth's mantle.

Lomentospora prolificans fungemia in hematopoietic stem cell transplant patients: First report in South America and literature review.

Lomentospora prolificans is a filamentous fungus and an emerging pathogen in immunocompromised patients. It is encountered most commonly in Australia, Spain, and USA. We described the first case of Lomentospora prolificans fungemia in South America. The patient was a hematopoietic stem cell transplantation (HSCT) recipient who developed the infection 37 days after stem cells infusion. In addition, we performed a literature review of invasive lomentosporiosis in HSCT patients.

Synthesis of Defect Perovskites (He2-x□x)(CaZr)F6 by Inserting Helium into the Negative Thermal Expansion Material CaZrF6.

Defect perovskites (He2-x□x)(CaZr)F6 can be prepared by inserting helium into CaZrF6 at high pressure. They can be recovered to ambient pressure at low temperature. There are no prior examples of perovskites with noble gases on the A-sites. The insertion of helium gas into CaZrF6 both elastically stiffens the material and reduces the magnitude of its negative thermal expansion. It also suppresses the onset of structural disorder, which is seen on compression in other media. Measurements of the gas released on warming to room temperature and Rietveld analyses of neutron diffraction data at low temperature indicate that exposure to helium gas at 500 MPa leads to a stoichiometry close to (He1□1)(CaZr)F6. Helium has a much higher solubility in CaZrF6 than silica glass or crystobalite. An analogue with composition (H2)2(CaZr)F6 would have a volumetric hydrogen storage capacity greater than current US DOE targets. We anticipate that other hybrid perovskites with small neutral molecules on the A-site can also be prepared and that they will display a rich structural chemistry.

Adsorption and molecular siting of CO2, water, and other gases in the superhydrophobic, flexible pores of FMOF-1 from experiment and simulation.

FMOF-1 is a flexible, superhydrophobic metal-organic framework with a network of channels and side pockets decorated with -CF3 groups. CO2 adsorption isotherms measured between 278 and 313 K and up to 55 bar reveal a maximum uptake of ca. 6.16 mol kg-1 (11.0 mol L-1) and unusual isotherm shapes at the higher temperatures, suggesting framework expansion. We used neutron diffraction and molecular simulations to investigate the framework expansion behaviour and the accessibility of the small pockets to N2, O2, and CO2. Neutron diffraction in situ experiments on the crystalline powder show that CO2 molecules are favourably adsorbed at three distinct adsorption sites in the large channels of FMOF-1 and cannot access the small pockets in FMOF-1 at 290 K and oversaturated pressure at 61 bar. Stepped adsorption isotherms for N2 and O2 at 77 K can be explained by combining Monte Carlo simulations in several different crystal structures of FMOF-1 obtained from neutron and X-ray diffraction under different conditions. A similar analysis is successful for CO2 adsorption at 278 and 283 K up to ca. 30 bar; however, at 298 K and pressures above 30 bar, the results suggest even more substantial expansion of the FMOF-1 framework. The measured contact angle for water on an FMOF-1 pellet is 158°, demonstrating superhydrophobicity. Simulations and adsorption measurements also show that FMOF-1 is hydrophobic and water is not adsorbed in FMOF-1 at room temperature. Simulated mixture isotherms of CO2 in the presence of 80% relative humidity predict that water does not influence the CO2 adsorption in FMOF-1, suggesting that hydrophobic MOFs could hold promise for CO2 capture from humid gas streams.

Deuterium Isotope Effects in Polymerization of Benzene under Pressure.

The enormous versatility in the properties of carbon materials depends on the content of the sp2 and sp3 covalent bonds. Under compression, if intermolecular distances cross a critical threshold, then unsaturated hydrocarbons gradually transform to saturated carbon polymers. However, the mechanism of polymerization, even for benzene, the simplest aromatic hydrocarbon, is still not understood. We used high-pressure synchrotron X-ray, neutron diffraction, and micro-Raman spectroscopy together with density functional calculations to investigate the isotope effects in benzene isotopologues C6H6 and C6D6 up to 46.0 GPa. Raman spectra of polymeric products recovered from comparable pressures show the progression of polymerization exhibiting a pronounced kinetic isotope effect. Kinetically retarded reactions in C6D6 shed light on the mechanism of polymerization of benzene. We find that C6D6-derived products recovered from P < 35 GPa actively react with moisture, forming polymers with higher sp3 hydrogen contents. Significant isotopic shift (≥7 GPa) in persistence of Bragg reflections of C6D6 is observed.

Boundaries for martensitic transition of (7)Li under pressure.

Physical properties of lithium under extreme pressures continuously reveal unexpected features. These include a sequence of structural transitions to lower symmetry phases, metal-insulator-metal transition, superconductivity with one of the highest elemental transition temperatures, and a maximum followed by a minimum in its melting line. The instability of the bcc structure of lithium is well established by the presence of a temperature-driven martensitic phase transition. The boundaries of this phase, however, have not been previously explored above 3 GPa. All higher pressure phase boundaries are either extrapolations or inferred based on indirect evidence. Here we explore the pressure dependence of the martensitic transition of lithium up to 7 GPa using a combination of neutron and X-ray scattering. We find a rather unexpected deviation from the extrapolated boundaries of the hR3 phase of lithium. Furthermore, there is evidence that, above ∼3 GPa, once in fcc phase, lithium does not undergo a martensitic transition.

Charge disproportionation and the pressure-induced insulator-metal transition in cubic perovskite PbCrO3.

The perovskite PbCrO3 is an antiferromagnetic insulator. However, the fundamental interactions leading to the insulating state in this single-valent perovskite are unclear. Moreover, the origin of the unprecedented volume drop observed at a modest pressure of P = 1.6 GPa remains an outstanding problem. We report a variety of in situ pressure measurements including electron transport properties, X-ray absorption spectrum, and crystal structure study by X-ray and neutron diffraction. These studies reveal key information leading to the elucidation of the physics behind the insulating state and the pressure-induced transition. We argue that a charge disproportionation 3Cr(4+) → 2Cr(3+) + Cr(6+) in association with the 6s-p hybridization on the Pb(2+) is responsible for the insulating ground state of PbCrO3 at ambient pressure and the charge disproportionation phase is suppressed under pressure to give rise to a metallic phase at high pressure. The model is well supported by density function theory plus the correlation energy U (DFT+U) calculations.

Neutron diffraction observations of interstitial protons in dense ice.

The motif of distinct H2O molecules in H-bonded networks is believed to persist up to the densest molecular phase of ice. At even higher pressures, where the molecule dissociates, it is generally assumed that the proton remains localized within these same networks. We report neutron-diffraction measurements on D2O that reveal the location of the D atoms directly up to 52 GPa, a pressure regime not previously accessible to this technique. The data show the onset of a structural change at ∼13 GPa and cannot be described by the conventional network structure of ice VII above ∼26 GPa. Our measurements are consistent with substantial deuteron density in the octahedral, interstitial voids of the oxygen lattice. The observation of this "interstitial" ice VII form provides a framework for understanding the evolution of hydrogen bonding in ice that contrasts with the conventional picture. It may also be a precursor for the superionic phase reported at even higher pressure with important consequences for our understanding of dense matter and planetary interiors.

Neutron diffraction and electrical transport studies on the incommensurate magnetic phase transition in holmium at high pressures.

Neutron diffraction and electrical transport measurements have been made on the heavy rare earth metal holmium at high pressures and low temperatures in order to elucidate its transition from a paramagnetic (PM) to a helical antiferromagnetic (AFM) ordered phase as a function of pressure. The electrical resistance measurements show a change in the resistance slope as the temperature is lowered through the antiferromagnetic Néel temperature. The temperature of this antiferromagnetic transition decreases from approximately 122 K at ambient pressure at a rate of -4.9 K GPa(-1) up to a pressure of 9 GPa, whereupon the PM-to-AFM transition vanishes for higher pressures. Neutron diffraction measurements as a function of pressure at 89 and 110 K confirm the incommensurate nature of the phase transition associated with the antiferromagnetic ordering of the magnetic moments in a helical arrangement and that the ordering occurs at similar pressures as determined from the resistance results for these temperatures.