Ultrahard bulk amorphous carbon from collapsed fullerene.

Amorphous materials inherit short- and medium-range order from the corresponding crystal and thus preserve some of its properties while still exhibiting novel properties1,2. Due to its important applications in technology, amorphous carbon with sp2 or mixed sp2-sp3 hybridization has been explored and prepared3,4, but synthesis of bulk amorphous carbon with sp3 concentration close to 100% remains a challenge. Such materials inherit the short-/medium-range order of diamond and should also inherit its superior properties5. Here, we successfully synthesized millimetre-sized samples-with volumes 103-104 times as large as produced in earlier studies-of transparent, nearly pure sp3 amorphous carbon by heating fullerenes at pressures close to the cage collapse boundary. The material synthesized consists of many randomly oriented clusters with diamond-like short-/medium-range order and possesses the highest hardness (101.9 ± 2.3 GPa), elastic modulus (1,182 ± 40 GPa) and thermal conductivity (26.0 ± 1.3 W m-1 K-1) observed in any known amorphous material. It also exhibits optical bandgaps tunable from 1.85 eV to 2.79 eV. These discoveries contribute to our knowledge about advanced amorphous materials and the synthesis of bulk amorphous materials by high-pressure and high-temperature techniques and may enable new applications for amorphous solids.

Tuning the Flat Band in Bi2O2Se by Pressure to Induce Superconductivity.

The discovery of superconductivity in twisted bilayer graphene has reignited enthusiasm in the field of flat-band superconductivity. However, important challenges remain, such as constructing a flat-band structure and inducing a superconducting state in materials. Here, we successfully achieved superconductivity in Bi2O2Se by pressure-tuning the flat-band electronic structure. Experimental measurements combined with theoretical calculations reveal that the occurrence of pressure-induced superconductivity at 30 GPa is associated with a flat-band electronic structure near the Fermi level. Moreover, in Bi2O2Se, a van Hove singularity is observed at the Fermi level alongside pronounced Fermi surface nesting. These remarkable features play a crucial role in promoting strong electron-phonon interactions, thus potentially enhancing the superconducting properties of the material. These findings demonstrate that pressure offers a potential experimental strategy for precisely tuning the flat band and achieving superconductivity.

Anomalous Pressure Response of Temperature-Induced Spin Transition and a Pressure-Induced Spin Transition in Two-Dimensional Hofmann Coordination Polymers.

Spin transition (ST) compounds have been extensively studied because of the changes in rich physicochemical properties accompanying the ST process. The study of ST mainly focuses on the temperature-induced spin transition (TIST). To further understand the ST, we explore the pressure response behavior of TIST and pressure-induced spin transition (PIST) of the 2D Hofmann-type ST compounds [Fe(Isoq)2M(CN)4] (Isoq-M) (M = Pt, Pd, Isoq = isoquinoline). The TISTs of both Isoq-Pt and Isoq-Pd compounds exhibit anomalous pressure response, where the transition temperature (T1/2) exhibits a nonlinear pressure dependence and the hysteresis width (ΔT1/2) exhibits a nonmonotonic behavior with pressure, by the synergistic influence of the intermolecular interaction and the distortion of the octahedral coordination environment. And the distortion of the octahedra under critical pressures may be the common behavior of 2D Hofmann-type ST compounds. Moreover, ΔT1/2 is increased compared with that before compression because of the partial irreversibility of structural distortion after decompression. At room temperature, both compounds exhibit completely reversible PIST. Because of the greater change in mechanical properties before and after ST, Isoq-Pt exhibits a more abrupt ST than Isoq-Pd. In addition, it is found that the hydrostatic properties of the pressure transfer medium (PTM) significantly affect the PIST due to their influence on spin-domain formation.

The Preparation and Performance Study of Polyamide Film Based on PDA@MWCNTs/PVDF Porous Support Layer.

It is urgent to develop a polyamide (PA) thin-film composite (TFC) membrane with a new method in this study by designing and constructing a new nanomaterial support layer instead of the conventional support layer. Polydopamine-wrapped single-walled carbon nanotubes (PDA@MWCNTs) as the place of the polymerization reaction can optimize the PA film structure and performance. The resulting composite membrane presents a higher water flux of 15.8 L·m-2·h-1·bar-1 and a rejection rate of 97% to Na2SO4, simultaneously maintaining this high separation performance in 300 min. It is a new ideal to construct novel support layer by using inorganic nanoparticles and organic polymer nanofiber membranes.

Abnormal Metal-Semiconductor-Like Transition and Exceptional Enhanced Superconducting State in Pressurized Restacked TaS2.

Interlayer coupling and stacking order play essential roles in shaping the exotic electronic properties of two-dimensional materials. Here, we employ restacked TaS2─a novel transition metal dichalcogenide (TMD) with weak vdW bonding and twisted angles─to investigate the strain effects of interlayer modulation on the electronic properties. Under pressure, an unexpected transition from metallic to semiconducting-like states occurs. Superconductivity coexists with the semiconducting-like state over a wide pressure range, which has never before been observed in TMDs. Upon further compression, a new superconducting SC-II state emerges without structural evolution and gradually replaces the initial SC-I state. The emerging SC-II state exhibits robust zero-resistance superconductivity and an ultrahigh upper critical field. The abundant electronic state changes in RS-TaS2 are strongly related to band-structure engineering resulting from pressure-induced interlayer stacking angle modulation. Our results reveal the remarkable effect of interlayer rearrangement on electronic properties and provide a special way to explore the unique properties of 2D materials.

Superconductivity in Layered van der Waals Hydrogenated Germanene at High Pressure.

The emergence of superconductivity in two-dimensional (2D) materials has attracted tremendous research efforts because the origins and mechanisms behind the unexpected and fascinating superconducting phenomena remain unclear. In particular, the superconductivity can survive in 2D systems even with weakened disorder and broken spatial inversion symmetry. Here, structural and superconducting transitions of 2D van der Waals (vdW) hydrogenated germanene (GeH) are observed under compression and decompression processes. GeH possesses a superconducting transition with a critical temperature (Tc) of 5.41 K at 8.39 GPa. A crystalline to amorphous transition occurs at 16.80 GPa, while superconductivity remains. An abnormal increase of Tc up to 6.11 K was observed during the decompression process, while the GeH remained in the 2D amorphous phase. A combination study of in situ high-pressure synchrotron X-ray diffraction, in situ high-pressure Raman spectroscopy, transition electron microscopy, and density functional theory simulations suggests that the superconductivity in 2D vdW GeH is attributed to the increased density of states at the Fermi level as well as the enhanced electron-phonon coupling effect under high pressure even in the form of an amorphous phase. The unique pressure-induced phase transition of GeH from 2D crystalline to 2D amorphous metal hydride provides a promising platform to study the mechanisms of amorphous hydride superconductivity.

Pressure-Induced Mixed States Caused by Spin-Elastic Interactions during First-Order Spin Phase Transition in Spin Crossover Compounds.

Recently, the possibility of exploiting the phenomenon of spin transition (ST) has been intensively investigated; therefore, it is particularly important to study the behavior of ST under various stimuli. Here, the shape and content of the intermediate phase of ST in Hoffmann-like compounds [Fe(Fpz)2M(CN)4] (M = Pt, Pd) under external stimuli are studied. For this purpose, magnetic and Raman spectroscopy studies were carried out. In pressure-induced spin transition (PIST), a mixture of high-spin and low-spin states appears, while in temperature-induced spin transition (TIST), a homogeneous state occurs. The first-order ST induced by pressure has a hysteresis but is not abrupt. However, the temperature-induced spin transition at ambient pressure is hysteretic and abrupt. To investigate this difference, we discuss using a thermodynamic model that considers elastic interactions, showing that the slope of the hysteresis loop is related to the appearance of internal pressure, which is related to the difference in sample compressibility under high-spin and low-spin states.

Pressure-Driven Abnormal Emission Blue-Shift of Lead-Free Halide Double Perovskite Cs2AgInCl6 Nanocrystals.

Lead-free halide double perovskites (DPs) have been proposed as stable and promising alternatives to lead halide perovskites. Understanding the structural-optical properties of halide DPs is important for their applications. In this study, Cs2AgInCl6 DP nanocrystals, with a direct band gap, were synthesized and studied. Because of a strong electron-phonon coupling leading to exciton self-trapping, a broad emission with a large Stokes shift of Cs2AgInCl6 DP nanocrystals is observed. We observed an abnormal blue-shifted emission accompanied by a red-shifted direct absorption edge because of the reduced electron-phonon coupling under compression in the cubic phase Cs2AgInCl6 DP nanocrystals. Our study clarified the basic structural-optical correlation of halide DPs and may promote their application in related fields.

Expansion of the multifunctionality in off-stoichiometric manganites using post-annealing and high pressure: physical and electrochemical studies.

Prospects for the use of manganites in various areas of modern technologies require comprehensive studies of their physical and chemical properties. La0.9Mn1.1O3 (LMO) ceramics have been synthesized at an annealing temperature tann of 1150 °C with further post-annealing at 1250, 1350, and 1450 °C. As tann increases, the structure symmetry changes, and both the crystallite size and chemical defects increase. The post-annealing, on one hand, leads to a dramatic reduction of the magnetocaloric effect (MCE) |-ΔSmaxM| from 3.50 to 0.75 J (kg K)-1 at 2 T and a Curie temperature TC from 227 to 113 K with increasing tann. On the other hand, an external hydrostatic high-pressure P works oppositely enhancing ferromagnetic interactions. The saturation of -ΔSmaxM and TC is already achieved at a relatively low P of ≈ 0.4 GPa. LMO-1150 exhibits the best magnetocaloric characteristics compared with other studied samples. Moreover, the electrochemical characteristics of the LMO materials as electrocatalysts for overall water splitting (OER process) and features of their transformation in different 0.5 M K2SO4, 0.5 M K2HPO4, and 0.1 M K2B4O7 electrolytes have been studied thoroughly. After electrocatalysis of LMO, the magnetization M decreases and TC remains, which makes it possible to control the depletion of electrodes and predict their working time based on the magnetic measurements. All samples show the best OER activity in the 0.5 M K2HPO4 media. The obtained results demonstrate the ways for controlling the MCE of LMO under changing internal and external conditions, and an evaluation of the possibilities for their OER applications in electrocatalysts.

Pressure Tunable Electronic Bistability in Fe(II) Hofmann-like Two-Dimensional Coordination Polymer [Fe(Fpz)2Pt(CN)4]: A Comprehensive Experimental and Theoretical Study.

A comprehensive experimental and theoretical study of both thermal-induced spin transition (TIST) as a function of pressure and pressure-induced spin transition (PIST) at room temperature for the two-dimensional Hofmann-like SCO polymer [Fe(Fpz)2Pt(CN)4] is reported. The TIST studies at different fixed pressures have been carried out by magnetic susceptibility measurements, while PIST studies have been performed by means of powder X-ray diffraction, Raman, and visible spectroscopies. A combination of the theory of elastic interactions and numerical Monte Carlo simulations has been used for the analysis of the cooperative interactions in TIST and PIST studies. A complete (T, P) phase diagram for the compound [Fe(Fpz)2Pt(CN)4] has been constructed. The critical temperature of the spin transition follows a lineal dependence with pressure, meanwhile the hysteresis width shows a nonmonotonic behavior contrary to theoretical predictions. The analysis shows the exceptional role of the total entropy and phonon contribution in setting the temperature of the spin transition and the width of the hysteresis. The anomalous behavior of the thermal hysteresis width under pressure in [Fe(Fpz)2Pt(CN)4] is a direct consequence of a local distortion of the octahedral geometry of the Fe(II) centers for pressures higher than 0.4 GPa. Interestingly, there is not a coexistence of the high- and low-spin (HS and LS, respectively) phases in TIST experiments, while in PIST experiments, the coexistence of the HS and LS phases in the metastable region of the phase transition induced by pressure is observed for a first time in a first-order gradual spin transition with hysteresis.

Retainable Superconductivity and Structural Transition in 1T-TaSe2 Under High Pressure.

As a prominent platform possessing the properties of superconductivity (SC) and charge density wave (CDW), transition-metal dichalcogenides (TMDCs) have attracted considerable attention for a long time. Moreover, extensive efforts have been devoted for exploring the SC and/or the interplay between SC and CDW in TMDCs in the past few decades. Here, we systematically investigate the electronic properties and structural evolution of 1T-TaSe2 under pressure. With increasing pressure, pressure-induced superconductivity is observed at ∼2.6 GPa. The superconductive transition temperature (Tc) increases with the suppression of the CDW state to the maximum value of ∼5.1 K at 21.8 GPa and then decreases monotonously up to the highest pressure of 57.8 GPa. 1T-TaSe2 transforms into a monoclinic C2/m structure above 19 GPa. The monoclinic phase coexists with the original phase as the pressure is released under ambient conditions and the retainable superconductivity with Tc = 2.9 K is observed in the released sample. We suggest that the retained superconductivity can be ascribed to the retention of the superconductive high-pressure monoclinic phase in the released sample. Our findings demonstrate that both the structure and CDW order are related to the superconductivity of TaSe2.

Pressure-Induced Electronic and Structural Transition in Nodal-Line Semimetal ZrSiSe.

The nodal-line semimetals have recently gained attention as a promising material due to their exotic electronic structure and properties. Here, we investigated the structural evolution and physical properties of nodal-line semimetal ZrSiSe under pressure via experiments and theoretical calculations. An isostructural electronic transition is observed at ∼6 GPa. Upon further compression, the original tetragonal phase starts to transform into an orthorhombic phase at ∼13 GPa and the two phases coexist until the maximal experimental pressure. By analysis of the electronic band structure, we suggest that the significant changes in the Fermi surface contribute to the occurrence of the isostructural electronic transition. The results provide a new insight into the structure and properties of ZrSiSe.

Variable Cooperative Interactions in the Pressure and Thermally Induced Multistep Spin Transition in a Two-Dimensional Iron(II) Coordination Polymer.

Two types of experiments conducted to investigate the effect of pressure on the spin crossover (SCO) properties of the 2D Fe(II) coordination polymer formulated {Fe[bipy(ttr)2]}n are reported, namely, (1) magnetic measurements performed at variable temperature and at fixed pressure and (2) visible spectroscopy at variable pressure and fixed temperature. The magnetic experiments carried out under a hydrostatic pressure constraint of 0.04, 0.08, and 0.8 GPa reveal a two-step spin transition behavior. The characteristic critical temperatures of the spin transition are shifted upward in temperature as pressure increases. The slope of the straight-line of the Tc vs P plot, dTc/dP, is 775 K/GPa and 300 K/GPa, for the high temperature and the low temperature steps, respectively. These values are remarkably large and denote the extreme sensitivity of the material to the application of pressure. Indeed, the visible spectroscopic measurements performed at 293 K show that a complete spin transition is induced at pressures as low as 0.4 GPa. Moreover, the pressure-induced spin transition is reversible and shows an asymmetric hysteresis. An analysis of the cooperative interactions of the thermal- and pressure-induced spin transition in the framework of the model of elastic interactions reveals that the elastic energy of the lattice as well as the interaction parameter between the SCO centers change during the course of the spin transition. Consequently, the character of the spin transition varies from abrupt for the high temperature step to continuous for the low temperature step.

Multifunctionality of lanthanum-strontium manganite nanopowder.

Manganites are multifunctional materials which are widely used in both technology and devices. In this article, new prospects of their use as nanoparticles for various types of applications are demonstrated. For that, the ferromagnetic nanopowder of La0.6Sr0.4MnO3 has been synthesized by the sol-gel method with a subsequent annealing at 700-900 °C. The crystal structure, phase composition and morphology of nanoparticles as well as magnetic, magnetothermal and electrocatalytic properties have been studied comprehensively. The critical sizes of superparamagnetic, single-domain, and multi-domain states have been determined. It has been established that an anomalously wide temperature range of magnetocaloric properties is associated with an additional contribution to the magnetocaloric effect from superparamagnetic nanoparticles. The maximum values of the specific loss power are observed in the relaxation hysteresis region near the magnetic phase transition temperature. The electrochemical stability and features of the decomposition of nanoparticles in 1 M KOH and Na2SO4 electrolytes have been determined. A decrease in the particle size contributes to an increase in electrocatalytic activity for overall water splitting. Magnetocaloric and electrocatalytic results of the work indicate the prospects for obtaining the possibility of changing the temperature regime of electrocatalysis using contactless heating or cooling.

A Facile Method to Control the Diameter of Monoclinic Vanadium Dioxide Rods.

Nano/submicro vanadium dioxide rods in monoclinic phase (VO2 (M)) were synthesized through hydrothermal reaction combined with subsequent calcinations. The morphology and structure of samples were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The average diameter of VO2 (M) dioxide rods from 210 nm to 1 µm were successfully controlled by adjusting the synthesis conditions including the concentration of Vanadium pentoxide (V2O5) solution and the molar ratio of V2O5 and oxalic acid. Our results reveal that the concentration of V2O5 is the key factor to determine the diameter of VO2 (M) rods, while higher molar ratio favors formation of VO2 rods with narrow diameter distributions. The growth mechanism of vanadium dioxide rods was discussed.

High-pressure behavior of bromine confined in the one-dimensional channels of zeolite AlPO4-5 single crystals.

We present a joint experimental and theoretical study on the high-pressure behavior of bromine confined in the one-dimensional (1D) nanochannels of zeolite AlPO4-5 (AFI) single crystals. Raman scattering experiments indicate that loading bromine into AFI single crystals can lead to the formation of bromine molecular chains inside the nanochannels of the crystals. High-pressure Raman and X-ray diffraction studies demonstrate that high pressure can increase the length of the confined bromine molecular chains and modify the inter- and intramolecular interactions of the molecules. The confined bromine shows a considerably different high-pressure behavior to that of bulk bromine. The pressure-elongated bromine molecular chains can be preserved when the pressure is reduced to ambient pressure. Theoretical simulations explain the experimental results obtained from the Raman spectroscopy and X-ray diffraction studies. Furthermore, we find that the intermolecular distance between confined bromine molecules gradually becomes comparable to the intramolecular bond length in bromine molecules upon compression. This may result in the dissociation of the bromine molecules and the formation of 1D bromine atomic chains at pressures above 24 GPa. Our study suggests that the unique nanoconfinement has a considerable effect on the high-pressure behavior of bromine, and the confined bromine species concomitantly enhance the structural stability of the host AFI single crystals.

Electron Microscope Study of the Pressure-Induced Phase Transformation and Microstructure Change of TiO2 Nanocrystals.

Microstructure transformation of materials under compression is crucial to understanding their high-pressure phase transformation. However, direct observation of the microstructure of compressive materials is a considerable challenge, which impedes the understanding of the relations among phase transformation, microstructure, and material properties. In this study, we used transmission Kikuchi diffraction and transmission electron microscopy to intuitively characterize pressure-induced phase transformation and microstructure of TiO2. We observed the changes of twin boundaries with increasing pressure and intermediate phase TiO2-I of anatase transformed into TiO2-II (α-PbO2 phase) for the first time. The following changes occur during this transformation: anatase (diameter of ∼100 nm) → anatase twins 60° along the [110] zone axis → intermediate TiO2-I twins 60° along the [010] zone axis → TiO2-II twins 90° along the [010] zone axis. These results directly reveal the crystallographic relation among these structures, enhancing our understanding of the phase transformation in TiO2 nanocrystals.

The Multifunctionality of Lanthanum-Strontium Cobaltite Nanopowder: High-Pressure Magnetic Studies and Excellent Electrocatalytic Properties for OER.

Simultaneous study of magnetic and electrocatalytic properties of cobaltites under extreme conditions expands the understanding of physical and chemical processes proceeding in them with the possibility of their further practical application. Therefore, La0.6Sr0.4CoO3 (LSCO) nanopowders were synthesized at different annealing temperatures tann = 850-900 °C, and their multifunctional properties were studied comprehensively. As tann increases, the rhombohedral perovskite structure of the LSCO becomes more single-phase, whereas the average particle size and dispersion grow. Co3+ and Co4+ are the major components. It has been found that LSCO-900 shows two main Curie temperatures, TC1 and TC2, associated with a particle size distribution. As pressure P increases, average ⟨TC1⟩ and ⟨TC2⟩ increase from 253 and 175 K under ambient pressure to 268 and 180 K under P = 0.8 GPa, respectively. The increment of ⟨dTC/dP⟩ for the smaller and bigger particles is sufficiently high and equals 10 and 13 K/GPa, respectively. The magnetocaloric effect in the LSCO-900 nanopowder demonstrates an extremely wide peak δTfwhm > 50 K that can be used as one of the composite components, expanding its working temperature window. Moreover, all LSCO samples showed excellent electrocatalytic performance for the oxygen evolution reaction (OER) process (overpotentials of only 265-285 mV at a current density of 10 mA cm-2) with minimal η10 for LSCO-900. Based on the experimental data, it was concluded that the formation of a dense amorphous layer on the surface of the particles ensures high stability as a catalyst (at least 24 h) during electrolysis in 1 M KOH electrolyte.

Structural phase transition and photoluminescence properties of YF3 and YF3:Eu3+ under high pressure.

We investigate high-pressure induced phase transitions of YF3 and Eu-doped YF3 (YF3:Eu(3+)) by using the angular dispersive synchrotron X-ray diffraction technique at room temperature. It is found that the starting orthorhombic phase transforms into a new high-pressure phase which is identified as hexagonal structure in both YF3 and YF3:Eu(3+). The high-pressure structure of YF3 and YF3:Eu(3+) returned to the orthorhombic phase after release of pressure. The photoluminescence properties of YF3:Eu(3+) have also been studied under high pressure up to 25 GPa. The Eu(3+) ion luminescence lines of (5)D0→(7)F1,2,3,4 transition originating from the orthorhombic phase transform into another group of luminescence lines of hexagonal phase under high pressure, which reveals the pressure-induced structural transition of YF3:Eu(3+). The relative luminescence intensity ratio of (5)D0→(7)F2 to (5)D0→(7)F1 transitions of the Eu(3+) ions is found to increase with increasing pressure before phase transition and decrease after transition finished, indicating reducing and enhancing of the symmetry around the Eu(3+) ions, respectively.

Doping of charge-transfer molecules in cocrystals for the design of materials with novel piezo-activated luminescence.

A novel piezo-activated luminescent material with wide range modulation of the luminescence wavelength and a giant intensity enhancement upon compression was prepared using a strategy of molecular doping. The doping of THT molecules into TCNB-perylene cocrystals results in the formation of a weak but pressure-enhanced emission center in the material at ambient pressure. Upon compression, the emissive band from the undoped component TCNB-perylene undergoes a normal red shift and emission quenching, while the weak emission center shows an anomalous blue shift from 615 nm to 574 nm and a giant luminescence enhancement up to 16 GPa. Further theoretical calculations show that doping by THT could modify intermolecular interactions, promote molecular deformation, and importantly, inject electrons into the host TCNB-perylene upon compression, which contributes to the novel piezochromic luminescence behavior. Based on this finding, we further propose a universal approach to design and regulate the piezo-activated luminescence of materials by using other similar dopants.