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.

Extensively Microtwinned Diamond with Nanolaminates of Lonsdaleite Formed by Flash Laser Heating of Glassy Carbon.

Diamond's unique properties on the nanoscale make it one of the most important materials for use in biosensors and quantum computing and for components that can withstand the harsh environments of space. We synthesize oriented, faceted diamond particles by flash laser heating of glassy carbon at 16 GPa and 2300 K. Detailed transmission electron microscopy shows them to consist of a mosaic of diamond nanocrystals frequently joined at twin boundaries forming microtwins. Striking 3-fold translational periodicity was observed in both imaging and diffraction. This periodicity was shown to originate from nanodimensional wedge-shaped overlapping regions of twinned diamond and not from a possible 9R polytype, which has also been reported in other group IVa elements and water ice. Extended bilayers of hexagonal layer stacking were observed, forming lonsdaleite nanolaminates. The particles exhibited optical fluorescence with a rapid quench time (<1 ns) attributed to their unique twinned microstructure.

Neutron scattering study of polyamorphic THF·17(H2O) - toward a generalized picture of amorphous states and structures derived from clathrate hydrates.

From crystalline tetrahydrofuran clathrate hydrate, THF-CH (THF·17H2O, cubic structure II), three distinct polyamorphs can be derived. First, THF-CH undergoes pressure-induced amorphization when pressurized to 1.3 GPa in the temperature range 77-140 K to a form which, in analogy to pure ice, may be called high-density amorphous (HDA). Second, HDA can be converted to a densified form, VHDA, upon heat-cycling at 1.8 GPa to 180 K. Decompression of VHDA to atmospheric pressure below 130 K produces the third form, recovered amorphous (RA). Results from neutron scattering experiments and molecular dynamics simulations provide a generalized picture of the structure of amorphous THF hydrates with respect to crystalline THF-CH and liquid THF·17H2O solution (∼2.5 M). Although fully amorphous, HDA is heterogeneous with two length scales for water-water correlations (less dense local water structure) and guest-water correlations (denser THF hydration structure). The hydration structure of THF is influenced by guest-host hydrogen bonding. THF molecules maintain a quasiregular array, reminiscent of the crystalline state, and their hydration structure (out to 5 Å) constitutes ∼23H2O. The local water structure in HDA is reminiscent of pure HDA-ice featuring 5-coordinated H2O. In VHDA, the hydration structure of HDA is maintained but the local water structure is densified and resembles pure VHDA-ice with 6-coordinated H2O. The hydration structure of THF in RA constitutes ∼18 H2O molecules and the water structure corresponds to a strictly 4-coordinated network, as in the liquid. Both VHDA and RA can be considered as homogeneous.

Synthesis of Novel Phases in Si Nanowires Using Diamond Anvil Cells at High Pressures and Temperatures.

Silicon has several technologically promising allotropes that are formed via high-pressure synthesis. One of these phases (hd) has been predicted to have a direct band gap under tensile strain, whereas other (r8 and bc8) phases are predicted to have narrow band gaps and good absorption across the solar spectrum. Pure volumes of these phases cannot be made using conventional nanowire growth techniques. In this work, Si nanowires were compressed up to ∼20 GPa and then decompressed using a diamond anvil cell in the temperature range of 25-165 °C. It was found that at intermediate temperatures, near-phase-pure bc8-Si nanowires were produced, whereas amorphous Si (a-Si) dominated at lower temperatures, and a direct transformation to the diamond cubic phase (dc-Si) occurred at higher temperatures under compression. Thus this study has opened up a new pressure-temperature pathway for the synthesis of novel Si nanowires consisting of designed phase components with transformative properties.

Investigation of Room Temperature Formation of the Ultra-Hard Nanocarbons Diamond and Lonsdaleite.

Diamond is an attractive material due to its extreme hardness, high thermal conductivity, quantum optical, and biomedical applications. There is still much that is not understood about how diamonds form, particularly at room temperature and without catalysts. In this work, a new route for the formation of nanocrystalline diamond and the diamond-like phase lonsdaleite is presented. Both diamond phases are found to form together within bands with a core-shell structure following the high pressure treatment of a glassy carbon precursor at room temperature. The crystallographic arrangements of the diamond phases revealed that shear is the driving force for their formation and growth. This study gives new understanding of how shear can lead to crystallization in materials and helps elucidate how diamonds can form on Earth, in meteorite impacts and on other planets. Finally, the new shear induced formation mechanism works at room temperature, a key finding that may enable diamond and other technically important nanomaterials to be synthesized more readily.

Evidence for the R8 phase of germanium.

The formation of R8 germanium is reported. The ß-Sn phase is first induced by the indentation of amorphous germanium (a-Ge) and the resultant phases on pressure release are characterized by Raman scattering. The expected Raman line frequencies for the various phases of Ge are determined from first-principles calculations using density functional perturbation theory of the zone-center phonons in the diamond, ST12, BC8, and R8 Ge phases. In addition to the R8 phase, traces of BC8 may also be present following pressure release.

Advancing neutron diffraction for accurate structural measurement of light elements at megabar pressures.

Over the last 60 years, the diamond anvil cell (DAC) has emerged as the tool of choice in high pressure science because materials can be studied at megabar pressures using X-ray and spectroscopic probes. In contrast, the pressure range for neutron diffraction has been limited due to low neutron flux even at the strongest sources and the resulting large sample sizes. Here, we introduce a neutron DAC that enables break-out of the previously limited pressure range. Key elements are ball-bearing guides for improved mechanical stability, gem-quality synthetic diamonds with novel anvil support and improved in-seat collimation. We demonstrate a pressure record of 1.15 Mbar and crystallographic analysis at 1 Mbar on the example of nickel. Additionally, insights into the phase behavior of graphite to 0.5 Mbar are described. These technical and analytical developments will further allow structural studies on low-Z materials that are difficult to characterize by X-rays.

Behavior of Long-Chain Hydrocarbons at High Pressures and Temperatures.

Although long-chain aliphatic hydrocarbons are documented in meteorites, their origin is poorly understood. A key question is whether they are pristine or a byproduct of terrestrial alteration? To understand if these long-chain hydrocarbons are indigenous, it will be important to explore their thermodynamic and mechanical stability at conditions experienced by extraterrestrial objects during atmospheric entry and passage. Extreme pressures and temperatures experienced by meteorites are likely to alter the molecular organization of these long-chain hydrocarbons. These structural changes associated with extreme conditions are often documented via laboratory-based Raman spectroscopic measurements. So far, Raman spectroscopic measurements have investigated the effect of static compression on the aliphatic hydrocarbons. The effect of temperature on the structural changes remains poorly explored. To bridge this gap, in this study, we have explored the behavior of two aliphatic hydrocarbons at simultaneously high pressures and temperatures. We have used a resistively heated diamond anvil cell. On compression to moderate pressures, the appearance of new vibrational modes in the low-energy region confirms prior studies and is related to the bending of the linear chains. Upon heating to ∼220 °C, we note that the new low-energy mode undergoes softening. The mode softening is likely due to the combination of unbending of the alkane chain and mode anharmonicity.

3D-printed B4C collimation for neutron pressure cells.

A design for an incident-beam collimator for the Paris-Edinburgh pressure cell is described here. This design can be fabricated from reaction-bonded B4C but also through fast turnaround, inexpensive 3D-printing. 3D-printing thereby also offers the opportunity of composite collimators whereby the tip closest to the sample can exhibit even better neutronic characteristics. Here, we characterize four such collimators: one from reaction-bonded B4C, one 3D-printed and fully infiltrated with cyanoacrylate, a glue, one with a glue-free tip, and one with a tip made from enriched 10B4C. The collimators are evaluated on the Spallation Neutrons and Pressure Diffractometer of the Spallation Neutron Source and the Wide-Angle Neutron Diffractometer at the High Flux Isotope Reactor, both at Oak Ridge National Laboratory. This work clearly shows that 3D-printed collimators perform well and also that composite collimators improve performance even further. Beyond use in the Paris-Edinburgh cell, these findings also open new avenues for collimator designs as clearly more complex shapes are possible through 3D printing. An example of such is shown here with a collimator made for single-crystal samples measured inside a diamond anvil cell. These developments are expected to be highly advantageous for future experimentation in high pressure and other extreme environments and even for the design and deployment of new neutron scattering instruments.

Tunable multiferroic order parameters in Sr1- x Ba x Mn1- y Ti y O3.

Responding to the rapidly increasing demand for efficient energy usage and increased speed and functionality of electronic and spintronic devices, multiferroic oxides have recently emerged as key materials capable of tackling this multifaceted challenge. In this paper, we describe the development of single-site manganese-based multiferroic perovskite materials with modest amounts of nonmagnetic Ti substituted at the magnetic Mn site in Sr1- x Ba x Mn1- y Ti y O3 (SBMTO). Significantly enhanced properties were achieved with ferroelectric-type structural transition temperatures boosted to ∼430K. Ferroelectric distortions with large spontaneous polarization values of ∼30µC/cm2, derived from a point charge model, are similar in magnitude to those of the prototypical nonmagnetic BaTiO3. Temperature dependence of the system's properties was investigated by synchrotron x-ray powder diffraction and neutron powder diffraction at ambient and high pressures. Various relationships were determined between the structural and magnetic properties, Ba and Ti contents, and T N and T C. Most importantly, our results demonstrate the large coupling between the magnetic and ferroelectric order parameters and the wide tunability of this coupling by slight variations of the material's stoichiometry.

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.

Nanocrystalline hexagonal diamond formed from glassy carbon.

Carbon exhibits a large number of allotropes and its phase behaviour is still subject to significant uncertainty and intensive research. The hexagonal form of diamond, also known as lonsdaleite, was discovered in the Canyon Diablo meteorite where its formation was attributed to the extreme conditions experienced during the impact. However, it has recently been claimed that lonsdaleite does not exist as a well-defined material but is instead defective cubic diamond formed under high pressure and high temperature conditions. Here we report the synthesis of almost pure lonsdaleite in a diamond anvil cell at 100 GPa and 400 °C. The nanocrystalline material was recovered at ambient and analysed using diffraction and high resolution electron microscopy. We propose that the transformation is the result of intense radial plastic flow under compression in the diamond anvil cell, which lowers the energy barrier by "locking in" favourable stackings of graphene sheets. This strain induced transformation of the graphitic planes of the precursor to hexagonal diamond is supported by first principles calculations of transformation pathways and explains why the new phase is found in an annular region. Our findings establish that high purity lonsdaleite is readily formed under strain and hence does not require meteoritic impacts.