Seven tesla knee MRI T2*-mapping detects intrasubstance meniscus degeneration in patients with posterior root tears.

Background: Medial meniscus root tears often lead to knee osteoarthritis. The extent of meniscal tissue changes beyond the localized root tear is unknown. Purpose: To evaluate if 7 Tesla 3D T2*-mapping can detect intrasubstance meniscal degeneration in patients with arthroscopically verified medial meniscus posterior root tears (MMPRTs), and assess if tissue changes extend beyond the immediate site of the posterior root tear detected on surface examination by arthroscopy. Methods: In this prospective study we acquired 7 T knee MRIs from patients with MMPRTs and asymptomatic controls. Using a linear mixed model, we compared T2* values between patients and controls, and across different meniscal regions. Patients underwent arthroscopic assessment before MMPRT repair. Changes in pain levels before and after repair were calculated using Knee Injury & Osteoarthritis Outcome Score (KOOS). Pain changes and meniscal extrusion were correlated with T2* using Pearson correlation (r). Results: Twenty patients (mean age 53 ± 8; 16 females) demonstrated significantly higher T2* values across the medial meniscus (anterior horn, posterior body and posterior horn: all P < .001; anterior body: P = .007), and lateral meniscus anterior (P = .024) and posterior (P < .001) horns when compared to the corresponding regions in ten matched controls (mean age 53 ± 12; 8 females). Elevated T2* values were inversely correlated with the change in pain levels before and after repair. All patients had medial meniscal extrusion of ≥2 mm. Arthroscopy did not reveal surface abnormalities in 70% of patients (14 out of 20). Conclusions: Elevated T2* values across both medial and lateral menisci indicate that degenerative changes in patients with MMPRTs extend beyond the immediate vicinity of the posterior root tear. This suggests more widespread meniscal degeneration, often undetected by surface examinations in arthroscopy.

Retraction: Carbon content drives high temperature superconductivity in a carbonaceous sulfur hydride below 100 GPa.

Retraction of 'Carbon content drives high temperature superconductivity in a carbonaceous sulfur hydride below 100 GPa' by G. Alexander Smith et al., Chem. Commun., 2022, 58, 9064-9067, https://doi.org/10.1039/D2CC03170A.

Novel Class of Rhenium Borides Based on Hexagonal Boron Networks Interconnected by Short B2 Dumbbells.

Transition metal borides are known due to their attractive mechanical, electronic, refractive, and other properties. A new class of rhenium borides was identified by synchrotron single-crystal X-ray diffraction experiments in laser-heated diamond anvil cells between 26 and 75 GPa. Recoverable to ambient conditions, compounds rhenium triboride (ReB3) and rhenium tetraboride (ReB4) consist of close-packed single layers of rhenium atoms alternating with boron networks built from puckered hexagonal layers, which link short bonded (∼1.7 Å) axially oriented B2 dumbbells. The short and incompressible Re-B and B-B bonds oriented along the hexagonal c-axis contribute to low axial compressibility comparable with the linear compressibility of diamond. Sub-millimeter samples of ReB3 and ReB4 were synthesized in a large-volume press at pressures as low as 33 GPa and used for material characterization. Crystals of both compounds are metallic and hard (Vickers hardness, H V = 34(3) GPa). Geometrical, crystal-chemical, and theoretical analysis considerations suggest that potential ReB x compounds with x > 4 can be based on the same principle of structural organization as in ReB3 and ReB4 and possess similar mechanical and electronic properties.

Carbon content drives high temperature superconductivity in a carbonaceous sulfur hydride below 100 GPa.

We report a previously unobserved superconducting state of the photosynthesized carbonaceous sulfur hydride (C-S-H) system with a maximum TC of 191(1) K below 100 GPa. The properties of C-S-H are dependent on carbon content, and X-ray diffraction and simulations reveal the system remains molecular-like up to 100 GPa.

The high-pressure lithium-palladium and lithium-palladium-hydrogen systems.

The lithium-palladium and lithium-palladium-hydrogen systems are investigated at high pressures at and above room temperature. Two novel lithium-palladium compounds are found below [Formula: see text]. An ambient temperature phase is tentatively assigned as [Formula: see text], with [Formula: see text] Å at 8.64 GPa, isostructural with [Formula: see text]. The other phase occurs at high-temperature and is [Formula: see text], [Formula: see text] Å at 3.88 GPa and 200 [Formula: see text], similar to [Formula: see text], which is also known at high pressure. The presence of hydrogen in the system results in an [Formula: see text] structure with [Formula: see text] Å at 9.74 GPa. This persists up to [Formula: see text], the highest pressure studied. Below [Formula: see text] an fcc phase with a large unit cell, [Formula: see text] Å at 0.39 GPa, is also observed in the presence of hydrogen. On heating the hydrogen containing system at 4 GPa the [Formula: see text] phases persists to the melting point of lithium. In both systems melting the lithium results in the loss of crystalline diffraction from palladium containing phases. This is attributed to dissolution of the palladium in the molten lithium, and on cooling the palladium remains dispersed.

Realization of an Ideal Cairo Tessellation in Nickel Diazenide NiN2: High-Pressure Route to Pentagonal 2D Materials.

Most of the studied two-dimensional (2D) materials are based on highly symmetric hexagonal structural motifs. In contrast, lower-symmetry structures may have exciting anisotropic properties leading to various applications in nanoelectronics. In this work we report the synthesis of nickel diazenide NiN2 which possesses atomic-thick layers comprised of Ni2N3 pentagons forming Cairo-type tessellation. The layers of NiN2 are weakly bonded with the calculated exfoliation energy of 0.72 J/m2, which is just slightly larger than that of graphene. The compound crystallizes in the space group of the ideal Cairo tiling (P4/mbm) and possesses significant anisotropy of elastic properties. The single-layer NiN2 is a direct-band-gap semiconductor, while the bulk material is metallic. This indicates the promise of NiN2 to be a precursor of a pentagonal 2D material with a tunable direct band gap.

High-Pressure Synthesis of Dirac Materials: Layered van der Waals Bonded BeN_{4} Polymorph.

High-pressure chemistry is known to inspire the creation of unexpected new classes of compounds with exceptional properties. Here, we employ the laser-heated diamond anvil cell technique for synthesis of a Dirac material BeN_{4}. A triclinic phase of beryllium tetranitride tr-BeN_{4} was synthesized from elements at ∼85 GPa. Upon decompression to ambient conditions, it transforms into a compound with atomic-thick BeN_{4} layers interconnected via weak van der Waals bonds and consisting of polyacetylene-like nitrogen chains with conjugated π systems and Be atoms in square-planar coordination. Theoretical calculations for a single BeN_{4} layer show that its electronic lattice is described by a slightly distorted honeycomb structure reminiscent of the graphene lattice and the presence of Dirac points in the electronic band structure at the Fermi level. The BeN_{4} layer, i.e., beryllonitrene, represents a qualitatively new class of 2D materials that can be built of a metal atom and polymeric nitrogen chains and host anisotropic Dirac fermions.

Temperature- and Rate-Dependent Pathways in Formation of Metastable Silicon Phases under Rapid Decompression.

High-pressure metallic ß-Sn silicon (Si-II), depending on temperature, decompression rate, stress, etc., may transform to diverse metastable forms with promising semiconducting properties under decompression. However, the underlying mechanisms governing the different transformation paths are not well understood. Here, two distinctive pathways, viz., a thermally activated crystal-crystal transition and a mechanically driven amorphization, were characterized under rapid decompression of Si-II at various temperatures using in situ time-resolved x-ray diffraction. Under slow decompression, Si-II transforms to a crystalline bc8/r8 phase in the pressure range of 4.3-9.2 GPa through a thermally activated process where the overdepressurization and the onset transition strain are strongly dependent on decompression rate and temperature. In comparison, Si-II collapses structurally to an amorphous form at around 4.3 GPa when the volume expansion approaches a critical strain via rapid decompression beyond a threshold rate. The occurrence of the critical strain indicates a limit of the structural metastability of Si-II, which separates the thermally activated and mechanically driven transition processes. The results show the coupled effect of decompression rate, activation barrier, and thermal energy on the adopted transformation paths, providing atomistic insight into the competition between equilibrium and nonequilibrium pathways and the resulting metastable phases.

Nitrogen in black phosphorus structure.

Group V elements in crystal structure isostructural to black phosphorus with unique puckered two-dimensional layers exhibit exciting physical and chemical phenomena. However, as the first element of group V, nitrogen has never been found in the black phosphorus structure. Here, we report the synthesis of the black phosphorus-structured nitrogen at 146 GPa and 2200 K. Metastable black phosphorus-structured nitrogen was retained after quenching it to room temperature under compression and characterized in situ during decompression to 48 GPa, using synchrotron x-ray diffraction and Raman spectroscopy. We show that the original molecular nitrogen is transformed into extended single-bonded structure through gauche and trans conformations. Raman spectroscopy shows that black phosphorus-structured nitrogen is strongly anisotropic and exhibits high Raman intensities in two A g normal modes. Synthesis of black phosphorus-structured nitrogen provides a firm base for exploring new type of high-energy-density nitrogen and a new direction of two-dimensional nitrogen.

High-Pressure Melt Curve and Phase Diagram of Lithium.

We investigate the phase diagram of lithium at temperatures of 200 to 400 K, to pressures over 100 GPa using x-ray diffraction in diamond anvil cells, covering the region in which the melting curve is disputed. To overcome degradation of the diamond anvils by dense lithium we utilize a rapid compression scheme taking advantage of the high flux available at modern synchrotrons. Our results show the hR1 and cI16 phases to be stable to higher temperature than previously reported. The melting minima of lithium is found to be close to room temperature between 40 and 60 GPa, below which the solid is crystalline. Analysis of the stability fields of the cI16 and oC88 phases suggest the existence of a triple point between these and an undetermined solid phase at 60 GPa between 220 and 255 K.

Ultrahigh-pressure isostructural electronic transitions in hydrogen.

High-pressure transitions are thought to modify hydrogen molecules to a molecular metallic solid and finally to an atomic metal1, which is predicted to have exotic physical properties and the topology of a two-component (electron and proton) superconducting superfluid condensate2,3. Therefore, understanding such transitions remains an important objective in condensed matter physics4,5. However, measurements of the crystal structure of solid hydrogen, which provides crucial information about the metallization of hydrogen under compression, are lacking for most high-pressure phases, owing to the considerable technical challenges involved in X-ray and neutron diffraction measurements under extreme conditions. Here we present a single-crystal X-ray diffraction study of solid hydrogen at pressures of up to 254 gigapascals that reveals the crystallographic nature of the transitions from phase I to phases III and IV. Under compression, hydrogen molecules remain in the hexagonal close-packed (hcp) crystal lattice structure, accompanied by a monotonic increase in anisotropy. In addition, the pressure-dependent decrease of the unit cell volume exhibits a slope change when entering phase IV, suggesting a second-order isostructural phase transition. Our results indicate that the precursor to the exotic two-component atomic hydrogen may consist of electronic transitions caused by a highly distorted hcp Brillouin zone and molecular-symmetry breaking.

Probing disorder in high-pressure cubic tin (IV) oxide: a combined X-ray diffraction and absorption study.

The transparent conducting oxide, SnO2, is a promising optoelectronic material with predicted tailorable properties via pressure-mediated band gap opening. While such electronic properties are typically modeled assuming perfect crystallinity, disordering of the O sublattice under pressure is qualitatively known. Here a quantitative approach is thus employed, combining extended X-ray absorption fine-structure (EXAFS) spectroscopy with X-ray diffraction, to probe the extent of Sn-O bond anharmonicities in the high-pressure cubic (Pa\bar{3}) SnO2 - formed as a single phase and annealed by CO2 laser heating to 2648 ± 41 K at 44.5 GPa. This combinational study reveals and quantifies a large degree of disordering in the O sublattice, while the Sn lattice remains ordered. Moreover, this study describes implementation of direct laser heating of non-metallic samples by CO2 laser alongside EXAFS, and the high quality of data which may be achieved at high pressures in a diamond anvil cell when appropriate thermal annealing is applied.

In situ x-ray diffraction study of polyamorphism in H2O under isothermal compression and decompression.

Amorphous-amorphous transformations in H2O have been studied under rapid isothermal compression and decompression in a diamond anvil cell together with in situ x-ray diffraction measurements using brilliant synchrotron radiation. The experimental pathways provide a density-driven approach for studying polyamorphic relations among low-, high-, and very high-density amorphs (LDA, HDA, VHDA) in a pressure range of 0-3.5 GPa at temperatures of 145-160 K. Our approach using rapid (de)compression allows for studying the polyamorphic transformations at higher temperatures than the conditions previously studied under slow (de)compression or isobaric annealing. Multiple compression-decompression cycles can be integrated with in situ x-ray measurements, thus facilitating the study of repeatability and reversibility of the polyamorphic transformations. Fast in situ x-ray diffraction measurements allow for obtaining detailed insight into the structural changes across polyamorphic transformations regarding the (dis)continuity, reversibility, and possible intermediate forms. As demonstrated at isothermal conditions of 145 K and 155 K, the polyamorphic transformations are characterized by a sharp and reversible LDA-VHDA transformation, with an HDA-like form (referred to as HDA') appearing as an intermediate state. The LDA-VHDA transformation is found to occur in two steps: a discontinuous transition between LDA and HDA' and a continuous change within HDA' involving structural reconfigurations and finally converging to VHDA.

Multimode scanning X-ray diffraction microscopy for diamond anvil cell experiments.

We have designed and implemented a new experimental system for fast mapping of crystal structures and other structural features of materials under high pressure at the High Pressure Collaborative Access Team, Sector 16 of the Advanced Photon Source. The system utilizes scanning X-ray diffraction microscopy (SXDM) and is optimized for use with diamond anvil cell devices. In SXDM, the X-ray diffraction (XRD) is collected in a forward scattering geometry from points on a two-dimensional grid by fly-scanning the sample with respect to a micro-focused X-ray beam. The recording of XRD is made during the continuous motion of the sample using a fast (millisecond) X-ray area detector in synchrony with the sample positioners, resulting in a highly efficient data collection for SXDM. A new computer program, X-ray Diffractive Imaging (XDI), has been developed with the SXDM system. The XDI program provides a graphical interface for constructing and displaying the SXDM images in several modes: (1) phase mapping based on structural information, (2) pressure visualization based on the equation of state, (3) microstructural features mapping based on peak shape parameters, and (4) grain size and preferred-orientation based on peak shape parameters. The XDI is a standalone program and can be generally used for displaying SXDM images. Two examples of iron and zirconium samples under high pressure are presented to demonstrate the applications of SXDM.

Fly scan apparatus for high pressure research using diamond anvil cells.

The hardware and software used to execute fly scans at Sector 16 of the Advanced Photon Source are described. The system design and capabilities address dimensions and time scales relevant to samples in high pressure diamond anvil cells. The time required for routine sample positioning and centering is significantly reduced, and more importantly, the time savings associated with fly scanning make it feasible for users to routinely generate two-dimensional x-ray transmission and x-ray diffraction maps. Consequently, this facilitates an important shift in high pressure research as experimentalists embrace the study of heterogeneous and minute sample volumes in the diamond anvil cell.

Venture into Water's No Man's Land: Structural Transformations of Solid H_{2}O under Rapid Compression and Decompression.

Pressure-induced formation of amorphous ices and the low-density amorphous (LDA) to high-density amorphous (HDA) transition have been believed to occur kinetically below a crossover temperature (T_{c}) above which thermodynamically driven crystalline-crystalline (e.g., ice I_{h}-to-II) transitions and crystallization of HDA and LDA are dominant. Here we show compression-rate-dependent formation of a high-density noncrystalline (HDN) phase transformed from ice I_{c} above T_{c}, bypassing crystalline-crystalline transitions under rapid compression. Rapid decompression above T_{c} transforms HDN to a low-density noncrystalline (LDN) phase which crystallizes spontaneously into ice I_{c}, whereas slow decompression of HDN leads to direct crystallization. The results indicate the formation of HDA and the HDN-to-LDN transition above T_{c} are results of competition between (de)compression rate, energy barrier, and temperature. The crossover temperature is shown to have an exponential relationship with the threshold compression rate. The present results provide important insight into the dynamic property of the phase transitions in addition to the static study.

A CO2 laser heating system for in situ high pressure-temperature experiments at HPCAT.

We present a CO2 laser heating setup for synchrotron x-ray diffraction inside a diamond anvil cell, situated at HPCAT (Sector 16, Advanced Photon Source, Argonne National Lab, Illinois, USA), which is modular and portable between the HPCAT experiment hutches. The system allows direct laser heating of wide bandgap insulating materials to thousands of degrees at static high pressures up to the Mbar regime. Alignment of the focused CO2 laser spot is performed using a mid-infrared microscope, which addressed past difficulties with aligning the invisible radiation. The implementation of the mid-infrared microscope alongside a mirror pinhole spatial filter system allows precise alignment of the heating laser spot and optical pyrometry measurement location to the x-ray probe. A comparatively large heating spot (∼50 µm) relative to the x-ray beam (<10 µm) reduces the risk of temperature gradients across the probed area. Each component of the heating system and its diagnostics have been designed with portability in mind and compatibility with the various experimental hutches at the HPCAT beamlines. We present measurements on ZrO2 at 5.5 GPa which demonstrate the improved room-temperature diffraction data quality afforded by annealing with the CO2 laser. We also present in situ measurements at 5.5 GPa up to 2800 K in which we do not observe the postulated fluorite ZrO2 structure, in agreement with recent findings.

Pressure-Tuneable Visible-Range Band Gap in the Ionic Spinel Tin Nitride.

The application of pressure allows systematic tuning of the charge density of a material cleanly, that is, without changes to the chemical composition via dopants, and exploratory high-pressure experiments can inform the design of bulk syntheses of materials that benefit from their properties under compression. The electronic and structural response of semiconducting tin nitride Sn3 N4 under compression is now reported. A continuous opening of the optical band gap was observed from 1.3 eV to 3.0 eV over a range of 100 GPa, a 540 nm blue-shift spanning the entire visible spectrum. The pressure-mediated band gap opening is general to this material across numerous high-density polymorphs, implicating the predominant ionic bonding in the material as the cause. The rate of decompression to ambient conditions permits access to recoverable metastable states with varying band gaps energies, opening the possibility of pressure-tuneable electronic properties for future applications.

Covalency is Frustrating: La2Sn2O7 and the Nature of Bonding in Pyrochlores under High Pressure-Temperature Conditions.

Natural specimens of the pyrochlore (A2B2O7) compounds have been found to retain foreign actinide impurities within their parent framework, undergoing metamictization to a fully amorphous state. The response to radionuclide decay identifies pyrochlore systems with having high radiation tolerance and tailored use in radioactive waste applications and radionuclide sequestration. High pressure is a powerful pathway to high density states and amorphization with parallels to radiation-induced processes. Here, La2Sn2O7 is evaluated under extreme conditions via the combination of laser heating in a diamond anvil cell with X-ray diffraction and Raman spectroscopy. The measurements are supported by ab initio random structure searching and molecular dynamics calculations. A new ground state at 70 GPa is revealed, and high temperature annealing is fundamental to access its crystalline ground state and fully determine the structure. This crystalline phase ( P21/ c) retains its structural integrity during decompression and is fully recoverable to ambient conditions. The final state of the system is shown to be highly pathway dependent due to the covalent nature of the Sn-O bonding. The Tc pyrochlore, La2Tc2O7, is analyzed for similarities in the bonding to determine the likelihood of an analogous pathway dependency to a final state.

Experimental evidence of low-density liquid water upon rapid decompression.

Water is an extraordinary liquid, having a number of anomalous properties which become strongly enhanced in the supercooled region. Due to rapid crystallization of supercooled water, there exists a region that has been experimentally inaccessible for studying deeply supercooled bulk water. Using a rapid decompression technique integrated with in situ X-ray diffraction, we show that a high-pressure ice phase transforms to a low-density noncrystalline (LDN) form upon rapid release of pressure at temperatures of 140-165 K. The LDN subsequently crystallizes into ice-Ic through a diffusion-controlled process. Together with the change in crystallization rate with temperature, the experimental evidence indicates that the LDN is a low-density liquid (LDL). The measured X-ray diffraction data show that the LDL is tetrahedrally coordinated with the tetrahedral network fully developed and clearly linked to low-density amorphous ices. On the other hand, there is a distinct difference in structure between the LDL and supercooled water or liquid water in terms of the tetrahedral order parameter.