Design of high-temperature superconductors at moderate pressures by alloying AlH3 or GaH3

Since the discovery of hydride superconductors,a significant challenge has been to reduce the pressure required for their stabilization.In this context,we propose that alloying could be an effective strategy to achieve this.We focus on a series of alloyed hydrides with the AMH_(6)composition,which can be made via alloying A15 AH_(3)(A=Al or Ga)with M(M=a group IIIB or IVB metal),and study their behavior under pressure.Seven of them are predicted to maintain the A15-type structure,similar to AH_(3)under pressure,providing a platform for studying the effects of alloying on the stability and superconductivity of AH_(3).Among these,the A15-type phases of AlZrH_(6)and AlHfH_(6)are found to be thermodynamically stable in the pressure ranges of 40–150 and 30–181 GPa,respectively.Furthermore,they remain dynamically stable at even lower pressures,as low as 13 GPa for AlZrH_(6)and 6 GPa for AlHfH_(6).These pressures are significantly lower than that required for stabilizing A15 AlH3.Additionally,the introduction of Zr or Hf increases the electronic density of states at the Fermi level compared with AlH3.This enhancement leads to higher critical temperatures(Tc)of 75 and 76 K for AlZrH_(6)and AlHfH_(6)at 20 and 10 GPa,respectively.In the case of GaMH_(6)alloys,where M represents Sc,Ti,Zr,or Hf,these metals reinforce the stability of the A15-type structure and reduce the lowest thermodynamically stable pressure for GaH_(3) from 160 GPa to 116,95,80,and 85 GPa,respectively.Particularly noteworthy are the A15-type GaMH_(6)alloys,which remain dynamically stable at low pressures of 97,28,5,and 6 GPa,simultaneously exhibiting high Tc of 88,39,70,and 49 K at 100,35,10,and 10 GPa,respectively.Overall,these findings enrich the family of A15-type superconductors and provide insights for the future exploration of high-temperature hydride superconductors that can be stabilized at lower pressures.

Pressure-Induced Structural Transition and Enhanced Photoelectric Properties of Tm_(2)S_(3)

Rare earth sesquisulfides have drawn growing attention in photoelectric applications because of their excellent electronic and photoelectric properties upon compression.We investigate the structural,electrical,and photoelectric properties of Tm_(2)S_(3) under high pressure through electrical impedance,UV-vis absorption,Raman spectroscopy,x-ray diffraction,and photoelectric measurements.It is found that δ-Tm_(2)S_(3) transforms into high-pressure𝛼-phase around 5GPa,accompanied by a substantial reduction in atomic distance,bandgap,and resistivity.Consequently,the photocurrent density and responsivity of Tm_(2)S_(3) exhibit dramatic increase behavior,achieving five orders of magnitude enhancement in α-phase compared with the initial δ-Tm_(2)S_(3).Moreover,α-phase maintains a high photocurrent responsivity of three orders of magnitude after unloading.This work demonstrates significant enhancement of the photoelectric properties of Tm_(2)S_(3) by applying pressure,which paves the way for improving the performance of future photoelectric devices.

Diamond gets harder,tougher,and more deformable

Diamonds may not be forever,but research interest in diamond has never ebbed.Owing to its highly symmetric crystal structure and strong covalentC–Cbonds,diamond possesses an exceptional combination of physical properties.Its hardness and thermal conductivity are the highest among covalent materials.It also has a large bandgap and electric breakdown field,as well as optical transparency over a wide range of wavelengths.All of these are essential for a wide range of applications in both industrial and scientific areas.Despite these outstanding advantages,however,diamond is extremely brittle,with inferior toughness and poor deformability.These shortcomings have caused undesired tool breakage and have imposed severe constraints on technological innovations.To surmount these intrinsic deficiencies,tremendous research effort has been dedicated to developing advanced diamond products,with great progress being achieved in the past few years.

Nanocrystalline high-entropy hexaboride ceramics enable remarkable performance as thermionic emission cathodes

The development of high-entropy borides with combined structural and functional performance holds untold scientific and technological potential,yet relevant studies have been rarely reported.In this work,we report nanocrystalline(La_(0.25)Ce_(0.25)Nd_(0.25)Eu_(0.25))B6 high-entropy rare-earth hexaboride(HEReB6-1)ceramics fabricated through the high-pressure sintering of self-synthesized nanopowders for the first time.The as-fabricated samples exhibited a highly dense(96.3%)nanocrystalline(94 nm)microstructure with major(001)fiber textures and good grain boundaries without any impurities,resulting in a remarkable mechanical,electrical,and thermionic emission performance.The results showed that the samples possessed outstanding comprehensive mechanical properties and a high electrical resistivity from room temperature to high temperatures;these were greater than the average values of corresponding binary rare-earth hexaborides,such as a Vickers hardness of 23.4±0.6 GPa and a fracture toughness of 3.0±0.4 MPa•m^(1/2)at room temperature.More importantly,they showed high emission current densities at elevated temperatures,which were higher than the average values of the corresponding binary rare-earth hexaborides.For instance,the maximum emission current density reached 48.3 A•cm^(−2)at 1873 K.Such superior performance makes the nanocrystalline HEReB6-1 ceramics highly suitable for potential applications in thermionic emission cathodes.

Effects of Spin Transition and Cation Substitution on the Optical Properties and Iron Partitioning in Carbonate Minerals

The high-pressure behavior of deep carbonate dictates the state and dynamics of oxidized carbon in the Earth's mantle,playing a vital role in the global carbon cycle and potentially influencing long-term climate change.Optical absorption and Raman spectroscopic measurements were carried out on two natural carbonate samples in diamond-anvil cells up to 60 GPa.Mg-substitution in high-spin siderite FeCO_(3)increases the crystal field absorption band position by approximately 1000 cm^(-1),but such an effect is marginal at>40 GPa when entering the low-spin state.The crystal field absorption band of dolomite cannot be recognized upon compression to 45.8 GPa at room temperature but,in contrast,the high-pressure polymorph of dolomite exhibits a strong absorption band at frequencies higher than(Mg,Fe)CO_(3)in the lowspin state by 2000–2500 cm^(-1).Additionally,these carbonate minerals show more complicated features for the absorption edge,decreasing with pressure and undergoing a dramatic change through the spin crossover.The optical and vibrational properties of carbonate minerals are highly correlated with iron content and spin transition,indicating that iron is preferentially partitioned into low-spin carbonates.These results shed new light on how carbonate minerals evolve in the mantle,which is crucial to decode the deep carbon cycle.

Hard nanocrystalline gold materials prepared via high-pressure phase transformation

As one of the important materials,nanocrystalline Au(n-Au)has gained numerous interests in recent decades owing to its unique properties and promising applications.However,most of the current n-Au thin films are supported on substrates,limiting the study on their mechanical properties and applications.Therefore,it is urgently desired to develop a new strategy to prepare nAu materials with superior mechanical strength and hardness.Here,a hard n-Au material with an average grain size of~40 nm is prepared by cold-forging of the unique Au nanoribbons(NRBs)with unconventional 4H phase under high pressure.Systematic characterizations reveal the phase transformation from 4H to face-centered cubic(fcc)phase during the cold compression.Impressively,the compressive yield strength and Vickers hardness(HV)of the prepared n-Au material reach~140.2 MPa and~1.0 GPa,which are 4.2 and 2.2 times of the microcrystalline Au foil,respectively.This work demonstrates that the combination of high-pressure cold-forging and the in-situ 4H-to-fcc phase transformation can effectively inhibit the grain growth in the obtained n-Au materials,leading to the formation of novel hard n-Au materials.Our strategy opens up a new avenue for the preparation of nanocrystalline metals with superior mechanical property.

Tunable electrical properties of C_(60)·m-xylene and the formation of semiconducting ordered amorphous carbon clusters under pressure

Ordered amorphous carbon clusters(OACC)transformed from m-xylene solvated C_(60)(C_(60)·m-xylene)are known as the first crystalline material constructed from amorphous building blocks and have attracted a lot of attention.The formation mechanism and physical properties of this material are of great importance for the design of more materials with such structural characteristics.In this article,the transport and structural properties of C_(60)·m-xylene are systematically investigated under pressure using impedance spectroscopy,four-probe resistance measurements,and Raman spectroscopy.It is found that C_(60)·mxylene is an insulator at ambient pressure.The resistance decreases sharply starting at the pressure around 8 GPa due to the pressure-induced dimerization of C_(60)verified by the Raman study.The presence of solvent hinders further polymerization of C_(60)under higher pressures.The temperature-dependence of resistance exhibits a semiconducting characteristic at>8-26.9 GPa,and is well described by Mott's three-dimensional variable-range hopping model(3D-VRH),indicating an insulating-tosemiconducting transition accompanied with pressure-induced dimerization.The resistance and hopping energy are both found to decrease monotonically with pressure and reach the minimum near 24 GPa.Above the pressure,resistance and hopping energy values start to rise,suggesting a transition to another semiconducting state,which is attributed to the pressure-induced formation of OACC.The conductivity shows a large hysteresis during decompression from higher than 24 GPa,confirming a different transport behavior of the sample with retained fullerenes versus OACC.The findings of our study suggest that the transport property of fullerene is tunable by introducing solvates and further enhance our understanding of the OACC.

Novel Boron Nitride Polymorphs with Graphite-Diamond Hybrid Structure

Both boron nitride(BN)and carbon(C)have sp,sp^(2)and sp^(3)hybridization modes,thus resulting in a variety of BN and C polymorphs with similar structures,such as hexagonal BN(hBN)and graphite,cubic BN(cBN)and diamond.Here,five types of BN polymorph structures are proposed theoretically,inspired by the graphite-diamond hybrid structures discovered in a recent experiment.These BN polymorphs with graphite-diamond hybrid structures possess excellent mechanical properties with combined high hardness and high ductility,and also exhibit various electronic properties such as semi-conductivity,semi-metallicity,and even one-and two-dimensional conductivity,differing from known insulators hBN and cBN.The simulated diffraction patterns of these BN hybrid structures could account for the unsolved diffraction patterns of intermediate products composed of so-called“compressed hBN”and diamond-like BN,caused by phase transitions in previous experiments.Thus,this work provides a theoretical basis for the presence of these types of hybrid materials during phase transitions between graphite-like and diamond-like BN polymorphs.

Stimuli-responsive cyclometalated platinum complex bearing bent molecular geometry for highly efficient solution-processable OLEDs

Smart materials,such as stimuli-responsive luminescence,have attracted much attentions due to their potential application in semiconductor filed.In this context,platinum complexes of(dfppy-DC)Pt(acac) and(dfppy-O-DC)Pt(acac) were prepared and characterized,in which(2-(4',6'-difluorophenyl)pyridinato-N,C2')(2,4-pentanedionato-0,0)Pt(Ⅱ) was used as the planar emission core and 9-(4-(phenylsulfonyl)phenyl)-9 H-carbazole(DC) was regard as the bent pendent.Both platinum complexes showed bright emission in solution and solid state,concomitant with charming external-stimuli-responsive emission under mechanical grinding,organic solvent vapors and pressure.The change emission color spanned from yellow to near-infrared region.Using the platinum complexes as the dopant,solution processable organic light-emitting diodes(OLEDs) were fabricated and a maximum external quantum efficiency of ~18% was achieved,which is the highest value among the reported solution-processable OLEDs based on externalstimuli-responsive luminescence.This research demonstrated that platinum complex can show promising stimuli responsive emission via ingenious molecular design,indicating a novel way for developing the smart materials in semiconductor filed.

Ultrafine-grained high-entropy zirconates with superior mechanical and thermal properties

Ultrafine-grained(Sm_(0.2)Gd_(0.2)Dy_(0.2)Er_(0.2)Yb_(0.2))_(2)Zr_(2)O_(7)high-entropy zirconates with single fluorite structure have been fabricated by high-pressure sintering of the self-synthesized nanopowders for the first time.The as-sintered samples exhibit a good microstructure with a grain size of 220 nm and a relative density of 96.8%,which yield excellent comprehensive mechanical properties with a high Vickers hardness of 12.5 GPa and a high fracture toughness of 3.4 MPa·m1/2.In addition,the as-sintered samples possess a good thermostability with the grain growth rate of 30 nm/h,and a low thermal conductivity of 1.57 W·m^(-1)·℃^(-1)at room temperature.The superior mechanical and thermal properties are primarily attributed to the“high-entropy”and grain-refinement effects and good interface bonding.

Discovery of superhard materials via CALYPSO methodology

The study of superhard materials plays a critical role in modern industrial applications due to their widespread applications as cutting tools, abrasives, exploitation drills, and coatings. The search for new superhard materials with superior performance remains a hot topic and is mainly considered as two classes of materials:(i) the light-element compounds in the B-C-N-O(-Si) system with strong and short covalent bonds, and(ii) the transition-element light-element compounds with strong covalent bonds frameworks and high valence electron density. In this paper, we review the recent achievements in the prediction of superhard materials mostly using the advanced CALYPSO methodology. A number of novel, superhard crystals of light-element compounds and transition-metal borides, carbides, and nitrides have been theoretically identified and some of them account well for the experimentally mysterious phases. To design superhard materials via CALYPSO methodology is independent of any known structural and experimental data, resulting in many remarkable structures accelerating the development of new superhard materials.

Design of a Class of New sp^(2)-sp^(3)Carbons Constructed by Graphite and Diamond Building Blocks

The sp^(2)–sp^(3)-hybridized carbon allotropes with the advantage of two hybrid structures possess rich and fascinating electronic and mechanical properties and they have received long-standing attention.We design a class of versatile sp^(2)–sp^(3)carbons composed of graphite and diamond structural units with variable sizes.This class of sp^(2)–sp^(3)carbons is energetically more favorable than graphite under high pressure,and their mechanical and dynamical stabilities are further confirmed at ambient pressure.The calculations of band structure and mechanical properties indicate that this class of sp^(2)–sp^(3)carbons not only exhibits peculiar electronic characteristics adjusted from semiconducting to metallic nature but also presents excellent mechanical characteristics,such as superhigh hardness and high ductility.These sp^(2)–sp^(3)carbons have desirable properties across a broad range of potential applications.

The rise of plastic deformation in boron nitride ceramics

Ceramics are bonded by ionic or covalent bonds,with very limited slip systems for dislocation nucleation and movement[1].The poor deformability and natural brittleness are the major drawbacks of ceramics,especially when compared with metals.Under stress,ceramics tend to fracture before noticeable plastic deformation takes place.Cracks occur and propagate rapidly in ceramics subjected to stress much lower than the theoretical strength[2].As a result,ceramics can only endure very small strains(<1%),absorb limited mechanical energy,and display poor toughness[3].Moreover,microstructure imperfections in ceramics may decrease the toughness even further.Due to the lack of significant plastic deformation capacity for ceramic materials,the catastrophic failures without warning are easy to happen under stress which critically increases the unreliability of ceramics in the applications as structural materials.

Extraordinary high-temperature mechanical properties in binder-free nanopolycrystalline WC ceramic

From the perspective of high-temperature applications,materials with excellent high-temperature mechanical properties are always desirable.The present work demonstrates that the binder-free nanopolycrystalline WC ceramic with an average grain size of 103 nm obtained by high-pressure and hightemperature sintering exhibits excellent mechanical properties at both room temperature and high temperature up to 1000℃.Specifically,the binder-free nanopolycrystalline WC ceramic still maintains a considerably high Vicker hardness H_(V)of 23.4 GPa at 1000℃,which is only 22%lower than the room temperature H_(V).This outstanding thermo-mechanical stability is superior to that of typical technical ceramics,e.g.SiC,Si_(3)N_(4),Al_(2)O_(3),etc.Nanocrystalline grains with many dislocations,numerous low-energy,highly stableΣ2 grain boundaries,and a relatively low thermal expansion coefficient,are responsible for the observed outstanding high-temperature mechanical properties.

Heat-treated glassy carbon under pressure exhibiting superior hardness,strength and elasticity

Glassy carbon(GC)is a type of non-graphitizing disordered carbon material at ambient pressure and high temperatures,which has been widely used due to its excellent mechanical properties.Here we report the changes in the microstructure and mechanical properties of GC treated at high pressures(up to 5 GPa)and high temperatures.The formation of intermediate sp2-sp3 phases is identified at moderate treatment temperatures before the complete graphitization of GC,by analyzing synchrotron X-ray diffraction,Raman spectra,and transmission electron microscopy images.The intermediate metastable carbon materials exhibit superior mechanical properties with hardness reaching up to 10 GPa and compressive strength reaching as high as 2.5 GPa,nearly doubling those of raw GC,and improving elasticity and thermal stability.The synthesis pressure used in this study can be achieved in the industry on a commercial scale,enabling the scalable synthesis of this type of strong,hard,and elastic carbon materials.

Novel superhard boron-rich nitrides under pressure

Boron and its compounds have attracted much attention due to their interesting and complex structures[1-4].In particular,boron-rich compounds containing icosahedral structures have excellent properties,such as low density,high hardness,high melting point and low wear coffi-cient.

Greatly enhanced mechanical properties of thermoelectric SnSe through microstructure engineering

The outstanding thermoelectric material, SnSe, is also known for its inferior mechanical properties, which bring great inconvenience for its application in thermoelectric devices. In this work, SnSe bulks were prepared via a sequential procedure of high-pressure synthesis (HPS), ball milling, and spark plasma sintering (SPS). The produced polycrystalline samples with a unique microstructure of tightly-bound quasi-equiaxed grains exhibited excellent mechanical properties. The Vickers hardness (HV), compressive strength (σ_(c)), and bending strength (σ_(b)) reached 1.1 GPa, 300 MPa, and 90 MPa, respectively, all of which are far superior to those of ordinary polycrystalline SnSe. Furthermore, the microstructures did not deteriorate thermoelectric performance. This work demonstrated an effective procedure to prepare polycrystalline microstructure-engineered SnSe materials, which not only show advantages in device applications but also shed light on property enhancement for other layer-structured thermoelectric materials.

3C-SiC各向异性行为的原位纳米划痕研究

立方碳化硅(3C-SiC)是一种在极端条件下具有优异机械和物理性能的理想材料.然而,由于其生长深度有限且脆性较高,研究其材料去除和摩擦性能具有一定的挑战性.在本研究中,我们使用扫描电子显微镜对厚度约为20μm的3C-SiC单晶进行了原位纳米划痕研究,并观察了其各向异性行为.随后在(100)平面上沿[110]和[100]方向分别进行了纳米划痕实验.与[100]方向相比,[110]方向在划痕过程中表现出更高的硬度,导致材料去除率较低和摩擦系数较高.通过对划痕沟槽的原子级分辨观察,我们发现3C-SiC的塑性去除是通过位错滑移和显著的晶格畸变实现的.在塑性变形阶段,两个划痕方向的亚表面主要经历了全位错滑移.此外,强烈的应变导致了多晶化,这是3C-SiC中的一个重要变形机制.

基于二十面体的新型B_(12)CN和B_(13)CN结构的第一性原理研究

以二十面体为基元的富硼化合物具有复杂多样的电子和机械性能.在本工作中,我们采用粒子群优化结构预测方法结合第一性原理计算,首次对二十面体基元的三元B_(12)CN和B_(13)CN化合物的晶体结构和性质进行了全面系统的研究.我们搜索得到了B_(13)CN和B_(12)CN化合物的新结构,其空间群均为Cmc21,与α-B的变体结构相比,新结构具有更优异的热力学稳定性.B_(12)CN的热力学稳定性和机械性能均稍逊于B_(13)CN.此外,B含量的微小差异造成了B_(12)CN与B_(13)CN两种三元化合物迥异的电学特性,即B_(13)CN具有半导体特性,而B_(12)CN具有空穴型导电特性.此外,在B_(12)CN与B_(13)CN系列新结构的拉伸过程中,由于二十面体的连续破坏和二十面体之间的连续断键造成的应力再增强、结构多级破坏和类蠕变变形等特殊变形机制也同样被揭示.

Small onion-like BN leads to ultrafine-twinned cubic BN

Nanotwinned cubic boron nitride(nt-cBN) with remarkable hardness, toughness, and stability has attracted widespread attention due to its distinct scientific and industrial importance. The key for nt-cBN synthesis is to adopt an onion-like BN(oBN) nano-precursor and induce phase transition under high pressure. Here, we found that the size change of oBN used greatly affected the mechanical performance of products. With the precursor size decreasing from^320 to 90 nm, the Vickers hardness of nanostructured products improved from 61 to 108 GPa, due to the fact that large oBN nanoparticles possessed more flattened, orderly and graphite-like shell layers, in sharp contrast to the highly wrinkled and imperfect layers in small-diameter nanoparticles, thus resulting in the apparent reduction of ultrafinetwin substructure in the synthetic products. This study reveals that only small oBN precursor could produce complete ultrafine nt-cBN with outstanding performance. A practical route was proposed to further improve the performance of this important material.