Crystal structures of group IVa metals at ultrahigh pressures

Room-Temperature Synthesis of Thioether-Stabilized Ruthenium Nanocubes and Their Optical Properties

Controlling the nucleation and growth processes for nanoparticle synthesis allows the development of well-defined structures that offer unique chemical and physical properties. Here, we report a wet chemical reduction method for synthesizing ruthenium nanocubes (Ru NCs) that display plasmonic properties at room temperature (RT). The growth of the particles to form nanostructured cubes was established by varying the carbon chain length of the thioether stabilizing ligands and the reaction time to produce stable and controlled growth. In this study, we found that the longer the thioether chain length, the less isotropic the shape of the particles. Short chain lengths of thioethers (ethyl sulfide and butyl sulfide) produced spherical nanoparticles, whereas longer chain lengths (hexyl sulfide and octyl sulfide) produced cubic nanoparticles. In addition, parameters such as the ligand to precursor ratio also played an important role in the homogeneity of the nanocubes. The Ru NCs were characterized by UV-visible absorbance spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), which supported a face-centered cubic (fcc) structure. Moreover, to demonstrate catalytic efficiency, we studied their ability to reduce benzaldehyde to benzyl alcohol, and the Ru NCs demonstrated an overall 78% efficiency at room temperature.

Record high Tc element superconductivity achieved in titanium

It is challenging to search for high T_c superconductivity (SC) in transition metal elements wherein d electrons are usually not favored by conventional BCS theory. Here we report experimental discovery of surprising SC up to 310 GPa with T_c above 20 K in wide pressure range from 108 GPa to 240 GPa in titanium. The maximum T_c^onset above 26.2 K and zero resistance T_c^zero of 21 K are record high values hitherto achieved among element superconductors. The H_c2(0) is estimated to be ∼32 Tesla with coherence length 32 Å. The results show strong s-d transfer and d band dominance, indicating correlation driven contributions to high T_c SC in dense titanium. This finding is in sharp contrast to the theoretical predications based on pristine electron-phonon coupling scenario. The study opens a fresh promising avenue for rational design and discovery of high T_c superconductors among simple materials via pressure tuned unconventional mechanism.

Superconductivity at megabar pressures has recently attracted interest in the context of hydrides. Here, the authors demonstrate superconductivity up to 26 K at high pressure in elemental titanium, and further suggest that electron correlations contribute to the high T_c.

Athermal ω Phase and Lattice Modulation in Binary Zr-Nb Alloys

To further explore the potential of Zr-based alloys as a biomaterial that will not interfere with magnetic resonance imaging (MRI), the microstructural characteristics of Zr-xat.% Nb alloys (10 ≤ x ≤ 18), particularly the athermal ω phase and lattice modulation, were investigated by conducting electrical resistivity and magnetic susceptibility measurements and transmission electron microscopy observations. The 10 Nb alloy and 12 Nb alloys had a positive temperature coefficient of electrical resistivity. The athermal ω phase existed in 10 Nb and 12 Nb alloys at room temperature. Alternatively, the 14 Nb and 18 Nb alloys had an anomalous negative temperature coefficient of the resistivity. The selected area diffraction pattern of the 14 Nb alloy revealed the co-occurrence of ω phase diffraction and diffuse satellites. These diffuse satellites were represented by gβ + q when the zone axis was [001] or [113], but not [110]. These results imply that these diffuse satellites appeared because the transverse waves consistent with the propagation and displacement vectors were q = <ζ ζ¯ 0>* for the ζ~1/2 and <110> directions. It is possible that the resistivity anomaly was caused by the formation of the athermal ω phase and transverse wave. Moreover, control of the athermal ω-phase transformation and occurrence of lattice modulation led to reduced magnetic susceptibility, superior deformation properties, and a low Young’s modulus in the Zr-Nb alloys. Thus, Zr-Nb alloys are promising MRI-compatible metallic biomaterials.

Multi-phase equation of state of ultrapure hafnium to 120 GPa

Hafnium (Hf) is an industrially important material due to its large neutron absorption cross-section and its high corrosion resistance. When subjected to high pressure, Hf phase transforms from its hexagonal close packed α-Hf phase to the hexagonal ω-Hf phase. Upon further compression, ω-Hf phase transforms to the body centered cubic β-Hf phase. In this study, the high pressure phase transformations of Hf are studied by compressing and decompressing a well-characterized Hf sample in diamond anvil cells up to 120 GPa while collecting x-ray diffraction data. The phase transformations of Hf were compared in both a He pressure transmitting medium (PTM) and no PTM over several experiments. It was found that the α-Hf to ω-Hf phase transition occurs at a higher pressure during compression and lower pressure during decompression with a helium (He) PTM compared to using no PTM. There was little difference in the ω-Hf to β-Hf phase transition pressure between the He PTM and no PTM. The equation of state was fit for all three phases of Hf and under both PTM and no-PTM.

The phase diagram of Ti-6Al-4V at high-pressures and high-temperatures

We report results from a series of diamond-anvil-cell synchrotron x-ray diffraction and large-volume-press experiments, and calculations, to investigate the phase diagram of commercial polycrystalline high-strength Ti-6Al-4V alloy in pressure-temperature space. Up to ∼30 GPa and 886 K, Ti-6Al-4V is found to be stable in the hexagonal-close-packed, orαphase. The effect of temperature on the volume expansion and compressibility ofα-Ti-6Al-4V is modest. The martensiticα→ω(hexagonal) transition occurs at ∼30 GPa, with both phases coexisting until at ∼38-40 GPa the transition to theωphase is completed. Between 300 K and 844 K theα→ωtransition appears to be independent of temperature.ω-Ti-6Al-4V is stable to ∼91 GPa and 844 K, the highest combined pressure and temperature reached in these experiments. Pressure-volume-temperature equations-of-state for theαandωphases of Ti-6Al-4V are generated and found to be similar to pure Ti. A pronounced hysteresis is observed in theω-Ti-6Al-4V on decompression, with the hexagonal structure reverting back to theαphase at pressures below ∼9 GPa at room temperature, and at a higher pressure at elevated temperatures. Based on our data, we estimate the Ti-6Al-4Vα-β-ωtriple point to occur at ∼900 K and 30 GPa, in good agreement with our calculations.

Hydrogen Pentagraphenelike Structure Stabilized by Hafnium: A High-Temperature Conventional Superconductor

The recent discovery of H_{3}S and LaH_{10} superconductors with record high superconducting transition temperatures T_{c} at high pressure has fueled the search for room-temperature superconductivity in the compressed superhydrides. Here we introduce a new class of high T_{c} hydrides with a novel structure and unusual properties. We predict the existence of an unprecedented hexagonal HfH_{10}, with remarkably high value of T_{c} (around 213-234 K) at 250 GPa. As concerns the novel structure, the H ions in HfH_{10} are arranged in clusters to form a planar "pentagraphenelike" sublattice. The layered arrangement of these planar units is entirely different from the covalent sixfold cubic structure in H_{3}S and clathratelike structure in LaH_{10}. The Hf atom acts as a precompressor and electron donor to the hydrogen sublattice. This pentagraphenelike H_{10} structure is also found in ZrH_{10}, ScH_{10}, and LuH_{10} at high pressure, each material showing a high T_{c} ranging from 134 to 220 K. Our study of dense superhydrides with pentagraphenelike layered structures opens the door to the exploration of a new class of high T_{c} superconductors.

Contributed Review: Culet diameter and the achievable pressure of a diamond anvil cell: Implications for the upper pressure limit of a diamond anvil cell

Recently, static pressures of more than 1.0 TPa have been reported, which raises the question: what is the maximum static pressure that can be achieved using diamond anvil cell techniques? Here we compile culet diameters, bevel diameters, bevel angles, and reported pressures from the literature. We fit these data and find an expression that describes the maximum pressure as a function of the culet diameter. An extrapolation of our fit reveals that a culet diameter of 1 μm should achieve a pressure of ∼1.8 TPa. Additionally, for pressure generation of ∼400 GPa with a single beveled diamond anvil, the most commonly reported parameters are a culet diameter of ∼20 μm, a bevel angle of 8.5°, and a bevel diameter to culet diameter ratio between 14 and 18. Our analysis shows that routinely generating pressures more than ∼300 GPa likely requires diamond anvil geometries that are fundamentally different from a beveled or double beveled anvil (e.g., toroidal or double stage anvils) and culet diameters that are ≤20 μm.

PRESSURE INDUCED STRUCTURAL PHASE TRANSITION AND SUPERCONDUCTIVITY IN TITANIUM METAL

The electronic band structure, density of states, structural phase transition, and superconducting transition temperature under normal and high pressures are reported for titanium ( Ti ). The normal pressure band structure and density of states of hcp- Ti agree well with the previous calculations. The high pressure band structure exhibits significant deviations from the normal pressure band structure due to s, p → d transition. On the basis of band structure and total energy results obtained using full potential linear muffin-tin orbital method (FP LMTO), we predict a phase transformation sequence of α (hcp) → ω (hexagonal) → γ (distorted hcp) → β (bcc) in titanium under pressure. From our analysis we predict a δ (distorted bcc) phase which is not stable at any high pressures. According to the present calculation, at normal pressure, the superconducting transition of hcp- Ti occurs at 0.36 K which is in agreement with the experimental observation of 0.4 K. When the pressure is increased, it is predicted that, Tc increases at a rate of 3.123 K/Mbar in hcp- Ti . On further increase of pressure Tc begins to decrease at a rate of 1.464 K/Mbar.

High-pressure structures and phase transformations in elemental metals

At ambient conditions the great majority of the metallic elements have simple crystal structures, such as face-centred or body-centred cubic, or hexagonal close-packed. However, when subjected to very high pressures, many of the same elements undergo phase transitions to low-symmetry and surprisingly complex structures, an increasing number of which are being found to be incommensurate. The present critical review describes the high-pressure behaviour of each of the group 1 to 16 metallic elements in detail, summarising previous work and giving the best present understanding of the structures and transitions at ambient temperature. The principal results and emerging systematics are then summarised and discussed.

New mechanism for the alpha to omega martensitic transformation in pure titanium

We propose a new direct mechanism for the pressure driven alpha-->omega martensitic transformation in pure titanium. A systematic algorithm enumerates all possible pathways whose energy barriers are evaluated. A new, homogeneous pathway emerges with a barrier at least 4 times lower than other pathways. The pathway is shown to be favorable in any nucleation model.

New delta (distorted-bcc) titanium to 220 GPa

Structural phase transitions of a 3d transition element, titanium, have been investigated at pressures up to 220 GPa at room temperature using a monochromatic synchrotron x-ray diffraction technique. At 140 GPa, the hexagonal (omega) phase was transformed into an orthorhombic (delta) phase with a distorted bcc structure via an intermediate (gamma) phase, which has recently been proposed by Vohra and Spencer [Phys. Rev. Lett. 86, 3068 (2001)]. Both the delta and the gamma phases had a unique zigzag-chain-like structure, which resulted from an orthorhombic distortion. The delta-gamma transition could be explained as a rearrangement of the packing between the zigzag chains.

Novel gamma-phase of titanium metal at megabar pressures

Group IV transition metals titanium, zirconium, and hafnium are expected to transform from an ambient hexagonal close packed (hcp, alpha-phase) to a body centered cubic (bcc, beta-phase) at high pressures. This transition path is usually facilitated by the occurrence of an intermediate hexagonal phase (distorted bcc, omega-phase). The existence of a bcc phase in zirconium and hafnium at high pressures has been known for the past ten years; however, its occurrence in titanium has been theoretically predicted but never observed. We report a novel unexpected transformation in titanium metal from an omega phase to an orthorhombic phase (distorted hcp, gamma-phase) at a pressure of 116+/-4 GPa.