Origin of the near-room temperature resistance transition in lutetium with H2/N2 gas mixture under high pressure.

The recent report of room-temperature superconductivity at near-ambient pressure in nitrogen-doped lutetium hydride (Lu-H-N) by Dasenbrock-Gammon et al. [Nature 615, 244-250 (2023)] has attracted tremendous attention due to its anticipated great impact on technology. However, the results could not be independently reproduced by other groups worldwide in follow-up studies, which elicited intense controversy. Here, we develop a reliable experimental protocol to minimize the extensively concerned extrinsic influences on the sample by starting the reaction from pure lutetium loaded with an H2/N2 gas mixture in a diamond anvil cell under different pressures and temperatures and simultaneously monitoring the entire chemical reaction process using in situ four-probe resistance measurements. Therefore, we could repeatedly reproduce the near-room temperature upsurge of electrical resistance at a relatively early stage of the chemical reaction. However, the mechanism is suggested to be a metal-to-semiconductor/insulator transition associated with the structural modulation in the non-stoichiometric Lu-H-N, rather than superconductivity.

Ultrahigh Permeability at High Frequencies via A Magnetic-Heterogeneous Nanocrystallization Mechanism in an Iron-Based Amorphous Alloy.

The prevalence of wide-bandgap (WBG) semiconductors allows modern electronic devices to operate at much higher frequencies. However, development of soft magnetic materials with high-frequency properties matching the WBG-based devices remains challenging. Here, a promising nanocrystalline-amorphous composite alloy with a normal composition Fe75.5 Co0.5 Mo0.5 Cu1 Nb1.5 Si13 B8 in atomic percent is reported, which is producible under industrial conditions, and which shows an exceptionally high permeability at high frequencies up to 36 000 at 100 kHz, an increase of 44% compared with commercial FeSiBCuNb nanocrystalline alloy (25 000 ± 2000 at 100 kHz), outperforming all existing nanocrystalline alloy systems and commercial soft magnetic materials. The alloy is obtained by a unique magnetic-heterogeneous nanocrystallization mechanism in an iron-based amorphous alloy, which is different from the traditional strategy of nanocrystallization by doping nonmagnetic elements (e.g., Cu and Nb). The induced magnetic inhomogeneity by adding Co atoms locally promotes the formation of highly ordered structures acting as the nuclei of nanocrystals, and Mo atoms agglomerate around the interfaces of the nanocrystals, inhibiting nanocrystal growth, resulting in an ultrafine nanocrystalline-amorphous dual-phase structure in the alloy. The exceptional soft magnetic properties are shown to be closely related to the low magnetic anisotropy and the unique spin rotation mechanism under alternating magnetic fields.

Pressure-induced nonmonotonic cross-over of steady relaxation dynamics in a metallic glass.

Relaxation dynamics, as a key to understand glass formation and glassy properties, remains an elusive and challenging issue in condensed matter physics. In this work, in situ high-pressure synchrotron high-energy X-ray photon correlation spectroscopy has been developed to probe the atomic-scale relaxation dynamics of a cerium-based metallic glass during compression. Although the sample density continuously increases, the collective atomic motion initially slows down as generally expected and then counterintuitively accelerates with further compression (density increase), showing an unusual nonmonotonic pressure-induced steady relaxation dynamics cross-over at ~3 GPa. Furthermore, by combining in situ high-pressure synchrotron X-ray diffraction, the relaxation dynamics anomaly is evidenced to closely correlate with the dramatic changes in local atomic structures during compression, rather than monotonically scaling with either sample density or overall stress level. These findings could provide insight into relaxation dynamics and their relationship with local atomic structures of glasses.

Ultrastrong yet Ductile 2D Titanium Nanomaterial for On-Skin Conformal Triboelectric Sensing.

Conventional titanium (e.g., bulk or thin films) is well-known for its relatively high mechanical strength, excellent corrosion resistance, and superior biocompatibility, which are suitable for biomedical engineering and wearable devices. However, the strength of conventional titanium often trades off its ductility, and their use in wearable devices has not been explored yet. In this work, we fabricated a series of large-sized 2D titanium nanomaterials with the method of polymer surface buckling enabled exfoliation (PSBEE), which possess a unique heterogeneous nanostructure containing nanosized titanium, titanium oxide, and MXene-like phases. As a result, these 2D titaniums exhibit both superb mechanical strength (6-13 GPa) and remarkable ductility (25-35%) at room temperature, outperforming all other titanium-based materials reported so far. More interestingly, we demonstrate that the 2D titanium nanomaterials also showed good performance in triboelectric sensing and can be used to fabricate self-powered, on-skin conformal triboelectric sensors with good mechanical reliability.

Solution epitaxy of polarization-gradient ferroelectric oxide films with colossal photovoltaic current.

Solution growth of single-crystal ferroelectric oxide films has long been pursued for the low-cost development of high-performance electronic and optoelectronic devices. However, the established principles of vapor-phase epitaxy cannot be directly applied to solution epitaxy, as the interactions between the substrates and the grown materials in solution are quite different. Here, we report the successful epitaxy of single-domain ferroelectric oxide films on Nb-doped SrTiO3 single-crystal substrates by solution reaction at a low temperature of ~200 oC. The epitaxy is mainly driven by an electronic polarization screening effect at the interface between the substrates and the as-grown ferroelectric oxide films, which is realized by the electrons from the doped substrates. Atomic-level characterization reveals a nontrivial polarization gradient throughout the films in a long range up to ~500 nm because of a possible structural transition from the monoclinic phase to the tetragonal phase. This polarization gradient generates an extremely high photovoltaic short-circuit current density of ~2.153 mA/cm2 and open-circuit voltage of ~1.15 V under 375 nm light illumination with power intensity of 500 mW/cm2, corresponding to the highest photoresponsivity of ~4.306×10-3 A/W among all known ferroelectrics. Our results establish a general low-temperature solution route to produce single-crystal gradient films of ferroelectric oxides and thus open the avenue for their broad applications in self-powered photo-detectors, photovoltaic and optoelectronic devices.

Research on Chemical Mechanical Polishing Technology for Zirconium-Based Amorphous Alloys.

Crystallization often occurs in the processing of amorphous alloys, causing the materials lose their excellent properties. The study adopts chemical mechanical polishing of amorphous alloys, presenting the effect of the rotational speed of the polishing turntable, size of abrasive, polishing pressure, and oxidant concentration. The Taguchi method is used to find the best processing parameters, and AFM is used to characterize the machined material surface. At the same time, XPS is used to detect the change of oxide film composition with the addition of oxidant. The results indicate the optimum process parameters: rotational speed of the polishing turntable is 75 r/min, polishing pressure is 28.3 kPa, the size of abrasive is 0.5 µm, and the size of abrasive is a significant factor affecting surface roughness Sa. In addition, as the size of abrasive increases, the material removal rate increases while the surface roughness Sa increases. At pH 10, with an abrasive particle size of 0.5 µm, as the H2O2 concentration increases, the MRR first rapidly decreases at 0.21 wt.% H2O2, and then gradually increases, while the Sa decreases. Furthermore, with the addition of oxidant, the main composition of the surface oxide film changes from oxide to hydroxide, and the contents of Zr4+ and Cu0/Cu1+ elements increase. The findings can provide a feasible chemical mechanical polishing process for zirconium-based amorphous alloys to obtain a satisfactory polishing effect.

Probing FeSi, a d-electron topological Kondo insulator candidate, with magnetic field, pressure, and microwaves.

Recently, evidence for a conducting surface state (CSS) below 19 K was reported for the correlated d-electron small gap semiconductor FeSi. In the work reported herein, the CSS and the bulk phase of FeSi were probed via electrical resistivity ρ measurements as a function of temperature T, magnetic field B to 60 T, and pressure P to 7.6 GPa, and by means of a magnetic field-modulated microwave spectroscopy (MFMMS) technique. The properties of FeSi were also compared with those of the Kondo insulator SmB6 to address the question of whether FeSi is a d-electron analogue of an f-electron Kondo insulator and, in addition, a "topological Kondo insulator" (TKI). The overall behavior of the magnetoresistance of FeSi at temperatures above and below the onset temperature TS = 19 K of the CSS is similar to that of SmB6. The two energy gaps, inferred from the ρ(T) data in the semiconducting regime, increase with pressure up to about 7 GPa, followed by a drop which coincides with a sharp suppression of TS. Several studies of ρ(T) under pressure on SmB6 reveal behavior similar to that of FeSi in which the two energy gaps vanish at a critical pressure near the pressure at which TS vanishes, although the energy gaps in SmB6 initially decrease with pressure, whereas in FeSi they increase with pressure. The MFMMS measurements showed a sharp feature at TS ≈ 19 K for FeSi, which could be due to ferromagnetic ordering of the CSS. However, no such feature was observed at TS ≈ 4.5 K for SmB6.

Strong yet Ductile High Entropy Alloy Derived Nanostructured Cermet.

Cermet is an important composite composed of ceramics and metals, which has widespread engineering applications but is notoriously known for its poor ductility. This work synthesizes freestanding cermet nanosheets through the reaction of high entropy alloy nanocrystals with the molecular structure of poly-vinyl alcohol, which results in a unique nanostructure consisting of metallic nanocrystals surrounded by complex amorphous ceramics. Atomic force microscopy indentation shows that such a high entropy alloy derived cermet is strong and tough at ambient temperature with a superb strength (∼3.2 GPa) and excellent ductility (∼50%), therefore overcoming the long-standing issue of brittleness facing conventional cermets.

Spatial Resolution Limit for Nanoindentation Mapping on Metallic Glasses.

Spatial heterogeneity, as a crucial structural feature, has been intensively studied in metallic glasses (MGs) using various techniques, including two-dimensional nanoindentation mapping. However, the limiting spatial resolution of nanoindentation mapping on MGs remains unexplored. In this study, a comprehensive study on four representative MGs using nanoindentation mapping with a Berkovich indenter was carried out by considering the influence of a normalized indentation spacing d/h (indentation spacing/maximum indentation depth). It appeared to have no significant correlation with the measured hardness and elastic modulus when d/h > 10. The hardness and elastic modulus started to increase slightly (up to ~5%) when d/h < 10 and further started to decrease obviously when d/h < 5. The mechanism behind these phenomena was discussed based on a morphology analysis of residual indents using scanning electron microscopy and atomic force microscopy. It was found that the highest spatial resolution of ~200 nm could be achieved with d/h = 10 using a typical Berkovich indenter for nanoindentation mapping on MGs, which was roughly ten times the curvature radius of the Berkovich indenter tip (not an ideal triangular pyramid) used in this study. These results help to promote the heterogeneity studies of MGs using nanoindentation that are capable of covering a wide range of length scales with reliable and consistent results.

Preservation of high-pressure volatiles in nanostructured diamond capsules.

High pressure induces dramatic changes and novel phenomena in condensed volatiles1,2 that are usually not preserved after recovery from pressure vessels. Here we report a process that pressurizes volatiles into nanopores of type 1 glassy carbon precursors, converts glassy carbon into nanocrystalline diamond by heating and synthesizes free-standing nanostructured diamond capsules (NDCs) capable of permanently preserving volatiles at high pressures, even after release back to ambient conditions for various vacuum-based diagnostic probes including electron microscopy. As a demonstration, we perform a comprehensive study of a high-pressure argon sample preserved in NDCs. Synchrotron X-ray diffraction and high-resolution transmission electron microscopy show nanometre-sized argon crystals at around 22.0 gigapascals embedded in nanocrystalline diamond, energy-dispersive X­ray spectroscopy provides quantitative compositional analysis and electron energy-loss spectroscopy details the chemical bonding nature of high-pressure argon. The preserved pressure of the argon sample inside NDCs can be tuned by controlling NDC synthesis pressure. To test the general applicability of the NDC process, we show that high-pressure neon can also be trapped in NDCs and that type 2 glassy carbon can be used as the precursor container material. Further experiments on other volatiles and carbon allotropes open the possibility of bringing high-pressure explorations on a par with mainstream condensed-matter investigations and applications.

Configurational Entropy Effects on Glass Transition in Metallic Glasses.

Configurational entropy (Sconf) is known to be a key thermodynamic factor governing a glass transition process. However, this significance remains speculative because Sconf is not directly measurable. In this work, we demonstrate the role of Sconf theoretically and experimentally by a comparative study of a Zr-Ti-Cu-Ni-Be high-entropy metallic glass (HE-MG) with one of its conventional MG counterparts. It is revealed that the higher Sconf leads to a glass that is energetically more stable and structurally more ordered. This is manifested by ab initio molecular dynamics simulations, showing that ∼60% fewer atoms are agitated above Tg, and experimental results of smaller heat capacity jump, inconspicuous stiffness loss, insignificant structural change during glass transition, and a more depressed boson peak in the HE-MG than its counterpart. We accordingly propose a model to explain that a higher Sconf promotes a faster degeneracy-dependent kinetics for exploration of the potential energy landscape upon glass transition.

Nested order-disorder framework containing a crystalline matrix with self-filled amorphous-like innards.

Solids can be generally categorized by their structures into crystalline and amorphous states with different interactions among atoms dictating their properties. Crystalline-amorphous hybrid structures, combining the advantages of both ordered and disordered components, present a promising opportunity to design materials with emergent collective properties. Hybridization of crystalline and amorphous structures at the sublattice level with long-range periodicity has been rarely observed. Here, we report a nested order-disorder framework (NOF) constructed by a crystalline matrix with self-filled amorphous-like innards that is obtained by using pressure to regulate the bonding hierarchy of Cu12Sb4S13. Combined in situ experimental and computational methods demonstrate the formation of disordered Cu sublattice which is embedded in the retained crystalline Cu framework. Such a NOF structure gives a low thermal conductivity (~0.24 W·m-1·K-1) and a metallic electrical conductivity (8 × 10-6 Ω·m), realizing the collaborative improvement of two competing physical properties. These findings demonstrate a category of solid-state materials to link the crystalline and amorphous forms in the sublattice-scale, which will exhibit extraordinary properties.

High-entropy induced a glass-to-glass transition in a metallic glass.

Glass-to-glass transitions are useful for us to understand the glass nature, but it remains difficult to tune the metallic glass into significantly different glass states. Here, we have demonstrated that the high-entropy can enhance the degree of disorder in an equiatomic high-entropy metallic glass NbNiZrTiCo and elevate it to a high-energy glass state. An unusual glass-to-glass phase transition is discovered during heating with an enormous heat release even larger than that of the following crystallization at higher temperatures. Dramatic atomic rearrangement with a short- and medium-range ordering is revealed by in-situ synchrotron X-ray diffraction analyses. This glass-to-glass transition leads to a significant improvement in the modulus, hardness, and thermal stability, all of which could promote their applications. Based on the proposed high-entropy effect, two high-entropy metallic glasses are developed and they show similar glass-to-glass transitions. These findings uncover a high-entropy effect in metallic glasses and create a pathway for tuning the glass states and properties.

Strong, Ductile, and Tough Nanocrystal-Assembled Freestanding Gold Nanosheets.

The structural and mechanical properties of low-dimensional nanostructured metals have been attracting tremendous interest in the fast-growing fields of nanosciences and nanotechnologies. However, it still remains a challenge today to develop strong yet ductile low-dimensional metals that can support the further development of nanodevices. Here, through the polymer-assisted assembly of gold nanocrystals, we successfully fabricated the freestanding, ultrathin gold nanomaterial. Unlike conventional bulk gold or other low-dimensional gold nanostructures (i.e., nanowires and nanosheets), these gold nanosheets are composed of highly distorted gold nanocrystals that are 3-5 nm in size, which are joined together through nanosized amorphous carbon interphases. As a result, the gold nanosheets exhibit superb strength (up to 1.2 GPa), excellent ductility (>50%), and superior fracture toughness (>100 J/m2), outperforming various gold nanostructures hitherto reported.

Origin of Plasticity in Nanostructured Silicon.

The mechanism of plasticity in nanostructured Si has been intensively studied over the past decade but still remains elusive. Here, we used in situ high-pressure radial x-ray diffraction to simultaneously monitor the deformation and structural evolution of a large number of randomly oriented Si nanoparticles (SiNPs). In contrast to the high-pressure ß-Sn phase dominated plasticity observed in large SiNPs (∼100 nm), small SiNPs (∼9 nm) display a high-pressure simple hexagonal phase dominated plasticity. Meanwhile, dislocation activity exists in all of the phases, but significantly weakens as the particle size decreases and only leads to subtle plasticity in the initial diamond cubic phase. Furthermore, texture simulations identify major active slip systems in all of the phases. These findings elucidate the origin of plasticity in nanostructured Si under stress and provide key guidance for the application of nanostructured Si.

Two-way tuning of structural order in metallic glasses.

Metallic glasses are expected to have quite tunable structures in their configuration space, without the strict constraints of a well-defined crystalline symmetry and large energy barriers separating different states in crystals. However, effectively modulating the structure of metallic glasses is rather difficult. Here, using complementary in situ synchrotron x-ray techniques, we reveal thermal-driven structural ordering in a Ce65Al10Co25 metallic glass, and a reverse disordering process via a pressure-induced rejuvenation between two states with distinct structural order characteristics. Studies on other metallic glass samples with different compositions also show similar phenomena. Our findings demonstrate the feasibility of two-way structural tuning states in terms of their dramatic ordering and disordering far beyond the nearest-neighbor shells with the combination of temperature and pressure, extending accessible states of metallic glasses to unexplored configuration spaces.

High-Pressure Induced Phase Transitions in High-Entropy Alloys: A Review.

High-entropy alloys (HEAs) as a new class of alloy have been at the cutting edge of advanced metallic materials research in the last decade. With unique chemical and topological structures at the atomic level, HEAs own a combination of extraordinary properties and show potential in widespread applications. However, their phase stability/transition, which is of great scientific and technical importance for materials, has been mainly explored by varying temperature. Recently, pressure as another fundamental and powerful parameter has been introduced to the experimental study of HEAs. Many interesting reversible/irreversible phase transitions that were not expected or otherwise invisible before have been observed by applying high pressure. These recent findings bring new insight into the stability of HEAs, deepens our understanding of HEAs, and open up new avenues towards developing new HEAs. In this paper, we review recent results in various HEAs obtained using in situ static high-pressure synchrotron radiation x-ray techniques and provide some perspectives for future research.

Publisher Correction: Enhanced strength and ductility in a high-entropy alloy via ordered oxygen complexes.

Change history: In this Letter, owing to a production error, all the data points (except the two points for O-2 and N-2, respectively) were missing in Fig. 1b. The figure has been corrected online.

High Pressure Induced in Situ Solid-State Phase Transformation of Nonepitaxial Grown Metal@Semiconductor Nanocrystals.

Considering the large lattice mismatch induced interface strain between nonepitaxial grown monocrystalline semiconductor shell and metal core, we studied the solid-state phase transformation of such nonepitaxial grown Au@CdS core/shell NCs under high pressure in this paper. Consistent with HRTEM characterizations, the high resolution Raman spectra and synchrotron angle-dispersive X-ray diffraction (ADXRD) spectra evolution were utilized to investigate the hydrostatic pressure (0-24 GPa) induced gradual phase transformation. Due to the strong lattice interactions between Au core and CdS shell, the different behavior and improved stability under high pressure appeared compared to single quantum dots (QDs).