New materials from high-pressure experiments

Mechanism of Pressure-Modulated Self-Trapped Exciton Emission in Cs2TeCl6 Double Perovskite

Pressure-modulated self-trapped exciton (STE) emission mechanism in all-inorganic lead-free metal halide double perovskites characterized by large Stokes-shifted broadband emission, has attracted much attention across various fields such as optics, optoelectronics, and biomedical sciences. Here, by employing the all-inorganic lead-free metal halide double perovskite Cs2TeCl6 as a paradigm, the authors elucidate that the performance of STE emission can be modulated by pressure, attributable to the pressure-induced evolution of the electronic state (ES). Two ES transitions happen at pressures of 1.6 and 5.8 GPa, sequentially. The electronic behaviors of Cs2TeCl6 can be jointly modulated by both pressure and ES transitions. When the pressure reaches 1.6 GPa, the Huang-Rhys factor S, indicative of the strength of electron-phonon coupling, attains an optimum value of ≈12.0, correlating with the pressure-induced photoluminescence (PL) intensity of Cs2TeCl6 is 4.8-fold that of its PL intensity under ambient pressure. Through analyzing the pressure-dependent STE dynamic behavioral changes, the authors have revealed the microphysical mechanism underlying the pressure-modulated enhancement and quenching of STE emission in Cs2TeCl6.

Ab-initio insights into the mechanical, phonon, bonding, electronic, optical and thermal properties of hexagonal W2N3 for prospective applications

We thoroughly investigated the structural, mechanical, electronic, vibrational, optical, thermodynamic, and a number of thermophysical properties of W2N3 compound through first-principles calculations using the DFT based formalism. The calculated structural parameters show very good agreement with the available theoretical and experimental results. The mechanical and dynamical stabilities of this compound have been investigated theoretically from the elastic constants and phonon dispersion curves. The Pugh's and Poisson's ratios of W2N3 are located quite close to the brittle/ductile borderline. W2N3 is elastically anisotropic. The calculated electronic band structure and density of states reveal that W2N3 is conducting in nature. The Fermi surface topology has also been explored. The analysis of charge density distribution map shows that W atoms have comparatively high electron density around compared to the N atoms. Presence of covalent bondings between W-N, W-W, and N-N atoms are anticipated. High melting temperature and high phonon thermal conductivity of W2N3 imply that the compound has potential to be used as a heat sink system. The optical characteristics show anisotropy. The compound can be used in optoelectronic devices due to its high absorption coefficient and low reflectivity in the visible to ultraviolet spectrum. Furthermore, the quasi-harmonic Debye model is used to examine temperature and pressure dependent thermal characteristics of W2N3 for the first time.

A brief review of characteristic luminescence properties of Eu3+ in mixed-anion compounds

Trivalent europium (Eu3+) ions show red luminescence with sharp spectral lines owing to the intraconfigurational 4f-4f transitions. Because of their characteristic luminescence properties, various Eu3+-doped inorganic compounds have been developed to meet the demands of optoelectronic devices. Regardless of shielding by the outer 5s and 5p orbitals, the properties of the Eu3+:4f-4f transition depend on the local environment, such as the shapes of the coordination polyhedra, site symmetry, nephelauxetic effects, crystal field effects, and bonding character. Mixed-anion coordination, where multiple types of anions surround a single Eu3+ ion, can directly affect the optical properties of Eu3+. We review the luminescence properties of Eu3+ ions in mixed-anion compounds of the oxynitride YSiO2N and oxyhalides YOX (X = Cl or Br). Oxynitride and oxyhalide coordination results in characteristic transition probabilities and branching ratios of the 5D0 → 7F0-6 transitions due to distorted structural environments and red-shifted charge transfer excitation bands due to an upward shift of the valence band. The expected and experimentally observed features of Eu3+ luminescence in mixed-anion compounds are outlined based on band and Judd-Ofelt theories. Future applications of the intense red luminescence at ∼620 nm under near-ultraviolet light illumination in Eu3+-doped mixed-anion compounds are introduced, and material design guidelines for new functional Eu3+-doped phosphors are presented.

Ultrafast Dynamics Across Pressure-Induced Electronic State Transitions, Fluorescence Quenching, and Bandgap Evolution in CsPbBr3 Quantum Dots

This work investigates the impact of pressure on the structural, optical properties, and electronic structure of CsPbBr3 quantum dots (QDs) using steady-state photoluminescence, steady-state absorption, and femtosecond transient absorption spectroscopy, reaching a maximum pressure of 3.38 GPa. The experimental results indicate that CsPbBr3 QDs undergo electronic state (ES) transitions from ES-I to ES-II and ES-II to ES-III at 0.38 and 1.08 GPa, respectively. Intriguingly, a mixed state of ES-II and ES-III is observed within the pressure range of 1.08-1.68 GPa. The pressure-induced fluorescence quenching in ES-II is attributed to enhanced defect trapping and reduced radiative recombination. Above 1.68 GPa, fluorescence vanishes entirely, attributed to the complete phase transformation from ES-II to ES-III in which radiative recombination becomes non-existent. Notably, owing to stronger quantum confinement effects, CsPbBr3 QDs exhibit an impressive bandgap tuning range of 0.497 eV from 0 to 2.08 GPa, outperforming nanocrystals by 1.4 times and bulk counterparts by 11.3 times. Furthermore, this work analyzes various carrier dynamics processes in the pressure-induced bandgap evolution and electron state transitions, and systematically studies the microphysical mechanisms of optical properties in CsPbBr3 QDs under pressure, offering insights for optimizing optical properties and designing novel materials.

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.

Time-resolved X-ray diffraction diagnostic development for the National Ignition Facility

We present the development of an experimental platform that can collect four frames of x-ray diffraction data along a single line of sight during laser-driven, dynamic-compression experiments at the National Ignition Facility. The platform is comprised of a diagnostic imager built around ultrafast sensors with a 2-ns integration time, a custom target assembly that serves also to shield the imager, and a 10-ns duration, quasi-monochromatic x-ray source produced by laser-generated plasma. We demonstrate the performance with diffraction data for Pb ramp compressed to 150 GPa and illuminated by a Ge x-ray source that produces ∼7 × 1011, 10.25-keV photons/ns at the 400 μm diameter sample.

Synthesis of tin(IV) nitride with spinel structure, γ-Sn3N4, from the elements and its Raman-spectroscopic examination at high pressures

We report on the synthesis of tin(IV) nitride with spinel structure, γ-Sn3N4, from the elements at high pressures and temperatures using a laser-heated diamond anvil cell, and on the Rietveld refinement of the product structure. The procedure described here is, in our opinion, the most reliable method of obtaining high-purity nitrides which are thermodynamically stable only at high pressures. Raman spectroscopy and powder X-ray diffraction were used to characterize the synthesis products. Pressure dependences of the Raman-band frequencies of γ-Sn3N4 were measured and used to determine its average mode Grüneisen parameter, 〈γ〉 = 0.95. Using this value, we estimated the thermal-shock resistance of γ-Sn3N4 to be about half that of γ-Si3N4, which, in turn, is moderately surpassed by β-Si3N4, known to be highly thermal-shock resistant. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.

Calibration of local chemical pressure by optical probe

Chemical stabilization of a high-pressure metastable state is a major challenge for the development of advanced materials. Although chemical pressure (Pchem) can effectively simulate the effect of physical pressure (Pphy), experimental calibration of the pressure passed to local structural motifs, denoted as local chemical pressure (Pchem-Δ) which significantly governs the function of solid materials, remains absent due to the challenge of probing techniques. Here we establish an innovative methodology to experimentally calibrate the Pchem-Δ and build a bridge between Pchem and Pphy via an optical probe strategy. Site-selective Bi3+-traced REVO4 (RE = Y, Gd) is adopted as a prototype to introduce Bi3+ optical probes and on-site sense of the Pchem-Δ experienced by the REO8 motif. The cell compression of RE0.98Bi0.02VO4 under Pphy is chemically simulated by smaller-ion substitution (Sc3+ → RE3+) in RE0.98-xScxBi0.02VO4. The consistent red shift (Δλ) of the emission spectra of Bi3+, which is dominated by locally pressure-induced REO8 dodecahedral variation in RE0.98Bi0.02VO4 (Pphy) and RE0.98-xScxBi0.02VO4 (Pchem-Δ), respectively, is evidence of their similar pressure-dependent local structure evolution. This innovative Δλ-based experimental calibration of Pchem-Δ in the crystal-field dimension portrays the anisotropic transmission of Pchem to the local structure and builds a bridge between Pchem-Δ and Pphy to guide a new perspective for affordable and practical interception of metastable states.

Theoretical and Experimental Advances in High-Pressure Behaviors of Nanoparticles

Using compressive mechanical forces, such as pressure, to induce crystallographic phase transitions and mesostructural changes while modulating material properties in nanoparticles (NPs) is a unique way to discover new phase behaviors, create novel nanostructures, and study emerging properties that are difficult to achieve under conventional conditions. In recent decades, NPs of a plethora of chemical compositions, sizes, shapes, surface ligands, and self-assembled mesostructures have been studied under pressure by in-situ scattering and/or spectroscopy techniques. As a result, the fundamental knowledge of pressure-structure-property relationships has been significantly improved, leading to a better understanding of the design guidelines for nanomaterial synthesis. In the present review, we discuss experimental progress in NP high-pressure research conducted primarily over roughly the past four years on semiconductor NPs, metal and metal oxide NPs, and perovskite NPs. We focus on the pressure-induced behaviors of NPs at both the atomic- and mesoscales, inorganic NP property changes upon compression, and the structural and property transitions of perovskite NPs under pressure. We further discuss in depth progress on molecular modeling, including simulations of ligand behavior, phase-change chalcogenides, layered transition metal dichalcogenides, boron nitride, and inorganic and hybrid organic-inorganic perovskites NPs. These models now provide both mechanistic explanations of experimental observations and predictive guidelines for future experimental design. We conclude with a summary and our insights on future directions for exploration of nanomaterial phase transition, coupling, growth, and nanoelectronic and photonic properties.

Microstructural Evolution and Mechanical Properties of SiC/Al-40Si Composites Fabricated by High Pressure Solidification

The microstructure and mechanical properties of SiC/Al-40Si composites prepared under high pressure were studied. As the pressure increases from 1 atm to 3 GPa, the primary Si phase in the Al-40Si alloy is refined. With increasing pressure, the composition of the eutectic point increases, the solute diffusion coefficient decreases exponentially, and the concentration of Si solute at the front of the solid-liquid interface of the primary Si is low, which contributes to the refining of the primary Si and inhibiting its faceted growth. The bending strength of SiC/Al-40Si composite prepared under 3 GPa was 334 MPa, which was 66% higher compared to the Al-40Si alloy prepared under the same pressure.

Ultrafast dynamics under high-pressure

High-pressure is a mechanical method to regulate the structure and internal interaction of materials. Therefore, observation of properties' change can be realized in a relatively pure environment. Furthermore, high-pressure affects the delocalization of wavefunction among materials' atoms and thus their dynamics process. Dynamics results are essential data for understanding the physical and chemical characteristics, which is valuable for materials application and development. Ultrafast spectroscopy is a powerful tool to investigate dynamics process and becoming a necessary characterization method for materials investigation. The combination of high-pressure with ultrafast spectroscopy in the nanocosecond∼femtosecond scale enables us to investigate the influence of the enhanced interaction between particles on the physical and chemical properties of materials, such as energy transfer, charge transfer, Auger recombination, etc. Base on this point of view, this review summarizes recent progress in the ultrafast dynamics under high-pressure for various materials, in which new phenomena and new mechanisms are observed. In this review, we describe in detail the principles ofin situhigh pressure ultrafast dynamics probing technology and its field of application. On this basis, the progress of the study of dynamic processes under high-pressure in different material systems is summarized. An outlook onin situhigh-pressure ultrafast dynamics research is also provided.

Synthesis and Post-Processing of Chemically Homogeneous Nanothreads from 2,5-Furandicarboxylic Acid

Compared with conventional, solution-phase approaches, solid-state reaction methods can provide unique access to novel synthetic targets. Nanothreads-one-dimensional diamondoid polymers formed through the compression of small molecules-represent a new class of materials produced via solid-state reactions, however, the formation of chemically homogeneous products with targeted functionalization represents a persistent challenge. Through careful consideration of molecular precursor stacking geometry and functionalization, we report here the scalable synthesis of chemically homogeneous, functionalized nanothreads through the solid-state polymerization of 2,5-furandicarboxylic acid. The resulting product possesses high-density, pendant carboxyl functionalization along both sides of the backbone, enabling new opportunities for the post-synthetic processing and chemical modification of nanothread materials applicable to a broad range of potential applications.

High-Pressure Bulk Synthesis of InN by Solid-State Reaction of Binary Oxide in a Multi-Anvil Apparatus

We present a new method to synthesize bulk indium nitride by means of a simple solid-state chemical reaction carried out under hydrostatic high-pressure/high-temperature conditions in a multi-anvil apparatus, not involving gases or solvents during the process. The reaction occurs between the binary oxide In₂O₃ and the highly reactive Li₃N as the nitrogen source, in the powder form. The formation of the hexagonal phase of InN, occurring at 350 °C and P ≥ 3 GPa, was successfully confirmed by powder X-ray diffraction, with the presence of Li₂O as a unique byproduct. A simple washing process in weak acidic solution followed by centrifugation allowed us to obtain pure InN polycrystalline powders as a precipitate. With an analogous procedure, it was possible to obtain pure bulk GaN, from Ga₂O₃ and Li₃N at T ≥ 600 °C and P ≥ 2.5 GPa. These results point out, particularly for InN, a clean, and innovative way to produce significant quantities of one of the most promising nitrides in the field of electronics and energy technologies.

A study of OH···O hydrogen bonds along various isolines in 2-ethyl-1-hexanol. Temperature or pressure - which parameter controls their behavior?

The nature of H-bonding interactions is still far from being understood despite intense experimental and theoretical studies on this subject carried out by the leading research centers. In this paper, by a combination of unique high-pressure infrared, dielectric and volumetric data, the intramolecular dynamics of hydroxyl moieties (which provides direct information about H-bonds) was studied along various isolines, i.e., isotherms, isobars, isochrones, and isochores, in a simple monohydroxy alcohol (2-ethyl-1-hexanol). This allowed us to discover that the temperature controls the intermolecular hydrogen bonds, which then affect the intramolecular dynamics of OH units. Although the role of density fluctuations gets stronger as temperature rises. We also demonstrated a clear connection between the intra- and intermolecular dynamics of the associating liquid at high pressure. The data reported herein open a new perspective to explore this important aspect of the glass transition phenomenon and understand H-bonding interactions at varying thermodynamic conditions.

Making the most of metastability

Researchers seek to preserve materials that are formed at high pressure.

Nanocrystals with metastable high-pressure phases under ambient conditions

The ambient metastability of the rock-salt phase in well-defined model systems comprising nanospheres or nanorods of cadmium selenide, cadmium sulfide, or both was investigated as a function of composition, initial crystal phase, particle structure, shape, surface functionalization, and ordering level of their assemblies. Our experiments show that these nanocrystal systems exhibit ligand-tailorable reversibility in the rock salt-to-zinc blende solid-phase transformation. Interparticle sintering was used to engineer kinetic barriers in the phase transformation to produce ambient-pressure metastable rock-salt structures in a controllable manner. Interconnected nanocrystal networks were identified as an essential structure that hosted metastable high-energy phases at ambient conditions. These findings suggest general rules for transformation-barrier engineering that are useful in the rational design of next-generation materials.

Geoinspired syntheses of materials and nanomaterials

The search for new materials is intimately linked to the development of synthesis methods. In the current urge for the sustainable synthesis of materials, taking inspiration from Nature's ways to process matter appears as a virtuous approach. In this review, we address the concept of geoinspiration for the design of new materials and the exploration of new synthesis pathways. In geoinspiration, materials scientists take inspiration from the key features of various geological systems and processes occurring in nature, to trigger the formation of artificial materials and nanomaterials. We discuss several case studies of materials and nanomaterials to highlight the basic geoinspiration concepts underlying some synthesis methods: syntheses in water and supercritical water, thermal shock syntheses, molten salt synthesis and high pressure synthesis. We show that the materials emerging from geoinspiration exhibit properties differing from materials obtained by other pathways, thus demonstrating that the field opens up avenues to new families of materials and nanomaterials. This review focuses on synthesis methodologies, by drawing connections between geosciences and materials chemistry, nanosciences, green chemistry, and environmental sciences.

Chemical Diversity for Tailoring Negative Thermal Expansion

Negative thermal expansion (NTE), referring to the lattice contraction upon heating, has been an attractive topic of solid-state chemistry and functional materials. The response of a lattice to the temperature field is deeply rooted in its structural features and is inseparable from the physical properties. For the past 30 years, great efforts have been made to search for NTE compounds and control NTE performance. The demands of different applications give rise to the prominent development of new NTE systems covering multifarious chemical substances and many preparation routes. Even so, the intelligent design of NTE structures and efficient tailoring for lattice thermal expansion are still challenging. However, the diverse chemical routes to synthesize target compounds with featured structures provide a large number of strategies to achieve the desirable NTE behaviors with related properties. The chemical diversity is reflected in the wide regulating scale, flexible ways of introduction, and abundant structure-function insights. It inspires the rapid growth of new functional NTE compounds and understanding of the physical origins. In this review, we provide a systematic overview of the recent progress of chemical diversity in the tailoring of NTE. The efficient control of lattice and deep structural deciphering are carefully discussed. This comprehensive summary and perspective for chemical diversity are helpful to promote the creation of functional zero-thermal-expansion (ZTE) compounds and the practical utilization of NTE.

Pressure-induced polymerization and bandgap-adjustment of TPEPA

Organic solar cells have become an important development direction in solar cell materials because of their low cost, light weight, and good flexibility. However, the size of their bandgap is difficult to continuously regulate, resulting in a low power conversion efficiency. In this work, an organic molecule TPEPA was synthesized, and its luminescence performance and polymerization under high pressure were studied by performing in situ Raman, IR, fluorescence, and UV-vis spectroscopy. The Raman and IR spectroscopic results show that single bonds (C–H, C–Ph) and long chains (C–C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C–C) are more unstable and prone to amorphization under high pressure. At 10 GPa, the TPEPA molecule undergoes a transition of amorphization accompanied by a few polymerizations in the CC bond structure. After holding pressure at 20 GPa for one day and releasing to ambient pressure, the other peaks almost disappeared, while the new peak of C(sp³)–H from the polymerization of the benzene ring was observed, indicating that the irreversible amorphization and polymerization did occur. UV-vis spectra results show that the bandgap is reduced from 2.9 eV to 1.3 eV, which is just in the maximum conversion efficiency bandgap range (1.3–1.4 eV) of p–n junction solar cell materials. This pressure is within the working pressure range of a large volume press, which is favorable in applications of large-scale synthesis. Our strategy may provide a method for the large-scale synthesis of novel organic solar cell materials.

Organic solar cells have become an important development direction in solar cell materials because of their low cost, light weight, and good flexibility.

Probing extreme states of matter using ultra-intense x-ray radiation

Extreme states of matter, that is, matter at extremes of density (pressure) and temperature, can be created in the laboratory either statically or dynamically. In the former, the pressure-temperature state can be maintained for relatively long periods of time, but the sample volume is necessarily extremely small. When the extreme states are generated dynamically, the sample volumes can be larger, but the pressure-temperature conditions are maintained for only short periods of time (ps toμs). In either case, structural information can be obtained from the extreme states by the use of x-ray scattering techniques, but the x-ray beam must be extremely intense in order to obtain sufficient signal from the extremely-small or short-lived sample. In this article I describe the use of x-ray diffraction at synchrotrons and XFELs to investigate how crystal structures evolve as a function of density and temperature. After a brief historical introduction, I describe the developments made at the Synchrotron Radiation Source in the 1990s which enabled the almost routine determination of crystal structure at high pressures, while also revealing that the structural behaviour of materials was much more complex than previously believed. I will then describe how these techniques are used at the current generation of synchrotron and XFEL sources, and then discuss how they might develop further in the future at the next generation of x-ray lightsources.

Energetics of polymeric carbon monoxide

The transformation of carbon monoxide (CO) from a molecular liquid to a polymeric solid under isothermal compression at room temperature is investigated using first principles theory. We report structural and thermodynamic properties from ambient density up to 2.45 g/cc obtained using density functional theory molecular dynamics simulations, including hybrid exchange corrections. The theoretical results are compared with newly obtained polymeric CO samples, synthesized in a large volume press. The explosive performance of polymeric CO is predicted and discussed. Under most favorable assumptions, it is found to be comparable to trinitrotoluene.

In Situ High-Pressure Synthesis of New Outstanding Light-Element Materials under Industrial P-T Range

High-pressure synthesis (which refers to pressure synthesis in the range of 1 to several GPa) adds a promising additional dimension for exploration of compounds that are inaccessible to traditional chemical methods and can lead to new industrially outstanding materials. It is nowadays a vast exciting field of industrial and academic research opening up new frontiers. In this context, an emerging and important methodology for the rapid exploration of composition-pressure-temperature-time space is the in situ method by synchrotron X-ray diffraction. This review introduces the latest advances of high-pressure devices that are adapted to X-ray diffraction in synchrotrons. It focuses particularly on the "large volume" presses (able to compress the volume above several mm3 to pressure higher than several GPa) designed for in situ exploration and that are suitable for discovering and scaling the stable or metastable compounds under "traditional" industrial pressure range (3-8 GPa). We illustrated the power of such methodology by (i) two classical examples of "reference" superhard high-pressure materials, diamond and cubic boron nitride c-BN; and (ii) recent successful in situ high-pressure syntheses of light-element compounds that allowed expanding the domain of possible application high-pressure materials toward solar optoelectronic and infra-red photonics. Finally, in the last section, we summarize some perspectives regarding the current challenges and future directions in which the field of in situ high-pressure synthesis in industrial pressure scale may have great breakthroughs in the next years.

Sound Velocities, Elasticity, and Mechanical Properties of Stoichiometric Submicron Polycrystalline δ-MoN at High Pressure

Acoustic velocities and elasticity of stoichiometric submicron polycrystalline δ-MoN have been reported at high pressure using ultrasonic measurements and first-principles calculations. Using the finite-strain equation-of-state approach, the bulk modulus and shear rigidity, as well as their pressure derivatives, are derived from the current experimental data, yielding B_S0 = 360.0(8) GPa, G₀ = 190.0(5) GPa, ∂B_S/∂P = 3.4(2), and ∂G/∂P = 1.4(1). Based on our experimental data and the velocity-elasticity correlated models, the mechanical/thermal properties (i.e., hardness, fracture toughness, Grüneisen parameter, Debye temperature, Poisson's ratio) are also derived. Interestingly, we find that hexagonal δ-MoN is almost as incompressible as superhard cubic boron nitride (cBN) (∼384 GPa) and its hexagonal ε-NbN (∼373 GPa) counterpart, and its shear rigidity (G = 190 GPa) is comparable to that of the superhard diamond composite (G = 204 GPa). Moreover, the fracture toughness of submicron δ-MoN polycrystals is achieved up to ∼4.3 MPa·m^1/2, which is comparable to superhard diamond (4-7 MPa·m^1/2) and cBN (2-5 MPa·m^1/2). The Vickers hardness of submicron δ-MoN is estimated to be H_v ≈ 17.4 GPa using Chen's model, which is found to be almost as hard as hexagonal ε-NbN and δ-WN, and may be very important for its applications in extreme environments.

Deep Learning Method to Accelerate Discovery of Hybrid Polymer-Graphene Composites

Interfacial encoded properties of polymer adlayers adsorbed on the graphene (GE) and silicon dioxide (SiO2) have been constituted a scaffold for the creation of new materials. The holistic understanding of nanoscale intermolecular interaction of 1D/2D polymer assemblies on substrate is the key to bottom-up design of molecular devices. We develop an integrated multidisciplinary approach based on electronic structure computation [density functional theory (DFT)] and big data mining [machine learning (ML)] in parallel with neural network (NN) and statistical analysis (SA) to design hybrid polymers from assembly on substrate. Here we demonstrate that interfacial pressure and structural deformation of polymer network adsorbed on GE and SiO2 offer unique directions for the fabrication of 1D/2D polymers using only a small number of simple molecular building blocks. Our findings serve as the platform for designing a wide range of typical inorganic heterostructures, involving noncovalent intermolecular interaction observed in many nanoscale electronic devices.

Novel Thermoelectric Character of Rhenium Carbonitride, ReCN

Nowadays, it is very important to study and propose new mechanisms for generating electricity that are environmentally friendly, in addition to using renewable resources. Thermoelectric (TE) devices are fabricated with materials that can convert a temperature difference into electricity, without the need for rotating parts. In this work, we report the TE properties of rhenium carbonitride (ReCN) as an important feature of a hard and thermodynamically stable material of band gap Δ_g = 0.626 eV. We use the electronic band structure behavior near the Fermi energy with the Seebeck coefficient to estimate the figure of merit ZT based on Boltzmann transport theory to characterize this property. Our results show that this compound has interesting TE properties among 300 and 1200 K for p- and n-type doping.

Hybrid x-ray laser-plasma/laser-synchrotron facility for pump-probe studies of the extreme state of matter at NRC "Kurchatov Institute"

We developed a hybrid optical pump-x-ray probe facility based on the "Kurchatov's synchrotron radiation source" and terawatt (TW) femtosecond laser. The bright x-ray photon source is based on either synchrotron radiation [up to 6 × 10¹⁴ photons/(s mm² mrad² 0.1% bandwidth)] or laser-plasma generators (up to 10⁸ photons/sr/pulse). The terawatt (TW) femtosecond laser pulse initiated phase transitions and a non-stationary "extreme" state of matter, while the delayed x-ray pulse acts as a probe. The synchronization between synchrotron radiation and laser pulses is achieved at 60.3 MHz using an intelligent field-programmable gate array-based phased locked loop. The timing jitter of the system is less than 30 ps. In laser-plasma sources, the x-ray and laser pulses are automatically synchronized because they are produced by using the same laser source (TW laser system). We have reached an x-ray yield of about 10⁶ photons/sr/pulse with 6-mJ sub-ps laser pulses and using helium as a local gas medium. Under vacuum conditions, the laser energy increase up to 40 mJ leads to the enhancement of the x-ray yield of up to 10⁸ photons/sr/pulse. The developed hybrid facility paves the way for a new class of time-resolved x-ray optical pump-probe experiments in the time interval from femtoseconds to microseconds and the energy spectrum from 3 to 30 keV.

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.

Phase stability analysis of shocked ammonium dihydrogen phosphate by X-ray and Raman scattering studies

Impact of shock waves on non-linear optical materials bring about a lot of unknown behaviors of materials and such kinds of shock wave recovery experiments are highly required for the better understanding of material-property relationship. In the present context, we have performed experiments on the impact of structural properties of ammonium dihydrogen phosphate (ADP) samples under shock wave loaded conditions and the results of the test samples have been evaluated by X-ray diffraction (XRD), Raman spectroscopy, diffused reflectance spectroscopy (DRS) and field emission scanning electron microscopic (FESEM) technique. Interestingly, prismatic face of ADP shows loss of degree of crystallinity whereas pyramidal face shows enhancement of crystalline nature with respect to number of shock pulses due to shock wave induced dynamic re-crystallization. Hence, the present problem is worthy enough to unearth and understand the anisotropic nature of the ADP crystal and their structural modifications at shock wave loaded conditions.

High-Pressure Synthesis and Crystal Structure of the Sulfur-Richest Chromium Sulfide CrS3 Composed of Cr(III) and Disulfide Ions

The new binary chromium sulfide CrS₃ has been synthesized by reaction of Cr₃S₄ and sulfur mixtures at 800 °C and 13 GPa. CrS₃ crystallizes in an orthorhombic unit cell with a = 4.6742(7) Å, b = 5.7315(8) Å, c = 10.603(2) Å, and V = 284.873(4) ų. It has a novel structure composed of Cr₂S₁₀ edge-shared octahedral dimers, which share all of their corners to form a three-dimensional structure. All of the sulfur atoms form S₂^2- disulfide ions with a S-S distance of 2.063(5) or 2.068(8) Å. The structure of CrS₃ is a derivative of the crystal structure of marcasite FeS₂, in which one in three metal sites of the marcasite structure is vacant in the CrS₃ structure.

Mechanical C-C Bond Formation by Laser Driven Shock Wave

Mechanically induced C-C bond formation was demonstrated by the laser driven shock wave generated in liquid normal alkanes at room temperature. Gas chromatography mass spectrometry analysis revealed the dehydrogenation condensation between two alkane molecules, for seven normal alkanes from pentane to undecane. Major products were identified to be linear and branched alkane molecules with double the number of carbons, and exactly coincided with the molecules predicted by supposing that a C-C bond was formed between two starting molecules. The production of the alkane molecules showed that the C-C bond formation occurred almost evenly at all the carbon positions. The dependence of the production on the laser pulse energy clearly indicated that the process was attributed to the shock wave. The C-C bond formation observed was not a conventional passive chemical reaction but an unprecedented active reaction.

Prediction of chlorine and fluorine crystal structures at high pressure using symmetry driven structure search with geometric constraints

The high-pressure properties of fluorine and chlorine are not yet well understood because both are highly reactive and volatile elements, which have made conducting diamond anvil cell and x-ray diffraction experiments a challenge. Here, we use ab initio methods to search for stable crystal structures of both elements at megabar pressures. We demonstrate how symmetry and geometric constraints can be combined to efficiently generate crystal structures that are composed of diatomic molecules. Our algorithm extends the symmetry driven structure search method [R. Domingos et al., Phys. Rev. B 98, 174107 (2018)] by adding constraints for the bond length and the number of atoms in a molecule while still maintaining generality. As a method of validation, we have tested our approach for dense hydrogen and reproduced the known molecular structures of Cmca-12 and Cmca-4. We apply our algorithm to study chlorine and fluorine in the pressure range of 10 GPa-4000 GPa while considering crystal structures with up to 40 atoms per unit cell. We predict chlorine to follow the same series of phase transformations as elemental iodine from Cmca to Immm to Fm3¯m, but at substantially higher pressures. We predict fluorine to transition from a C2/c to Cmca structure at 70 GPa, to a novel orthorhombic and metallic structure with P4₂/mmc symmetry at 2500 GPa, and finally to its cubic analog form with Pm3¯n symmetry at 3000 GPa.

First high-pressure XAFS results at the bending-magnet-based energy-dispersive XAFS beamline BL-8 at the Indus-2 synchrotron facility

The static focusing optics of the existing energy-dispersive XAFS beamline BL-8 have been advantageously exploited to initiate diamond anvil cell based high-pressure XANES experiments at the Indus-2 synchrotron facility, India. In the framework of the limited photon statistics with the 2.5 GeV bending-magnet source, limited focusing optics and 4 mm-thick diamond windows of the sample cell, a (non-trivial) beamline alignment method for maximizing photon statistics at the sample position has been designed. Key strategies include the selection of a high X-ray energy edge, the truncation of the smallest achievable focal spot size to target size with a slit and optimization of the horizontal slit position for transmission of the desired energy band. A motor-scanning program for precise sample centering has been developed. These details are presented with rationalization for every step. With these strategies, Nb K-edge XANES spectra for Nb₂O₅ under high pressure (0-16.9 GPa) have been generated, reproducing the reported spectra for Nb₂O₅ under ambient conditions and high pressure. These first HPXANES results are reported in this paper. The scope of extending good data quality to the EXAFS range in the future is addressed. This work should inspire and guide future high-pressure XAFS experiments with comparable infrastructure.

The Critical Raw Materials in Cutting Tools for Machining Applications: A Review

A variety of cutting tool materials are used for the contact mode mechanical machining of components under extreme conditions of stress, temperature and/or corrosion, including operations such as drilling, milling turning and so on. These demanding conditions impose a seriously high strain rate (an order of magnitude higher than forming), and this limits the useful life of cutting tools, especially single-point cutting tools. Tungsten carbide is the most popularly used cutting tool material, and unfortunately its main ingredients of W and Co are at high risk in terms of material supply and are listed among critical raw materials (CRMs) for EU, for which sustainable use should be addressed. This paper highlights the evolution and the trend of use of CRMs) in cutting tools for mechanical machining through a timely review. The focus of this review and its motivation was driven by the four following themes: (i) the discussion of newly emerging hybrid machining processes offering performance enhancements and longevity in terms of tool life (laser and cryogenic incorporation); (ii) the development and synthesis of new CRM substitutes to minimise the use of tungsten; (iii) the improvement of the recycling of worn tools; and (iv) the accelerated use of modelling and simulation to design long-lasting tools in the Industry-4.0 framework, circular economy and cyber secure manufacturing. It may be noted that the scope of this paper is not to represent a completely exhaustive document concerning cutting tools for mechanical processing, but to raise awareness and pave the way for innovative thinking on the use of critical materials in mechanical processing tools with the aim of developing smart, timely control strategies and mitigation measures to suppress the use of CRMs.

Nanoscale boron carbonitride semiconductors for photoredox catalysis

The conversion of solar energy to chemical energy achieved by photocatalysts comprising homogeneous transition-metal based systems, organic dyes, or semiconductors has received significant attention in recent years. Among these photocatalysts, boron carbon nitride (BCN) materials, as an emerging class of metal-free heterogeneous semiconductors, have extended the scope of photocatalysts due to their good performance and Earth abundance. The combination of boron (B), carbon (C), and nitrogen (N) constitutes a ternary system with large surface area and abundant activity sites, which together contribute to the good performance for reduction reactions, oxidation reactions and orchestrated both reduction and oxidation reactions. This Minireview reports the methods for the synthesis of nanoscale hexagonal boron carbonitride (h-BCN) and describes the latest advances in the application of h-BCN materials as semiconductor photocatalysts for sustainable photosynthesis, such as water splitting, reduction of CO₂, acceptorless dehydrogenation, oxidation of sp³ C-H bonds, and sp² C-H functionalization. h-BCN materials may have potential for applications in other organic transformations and industrial manufacture in the future.

Nanoarchitecture through Strained Molecules: Cubane-Derived Scaffolds and the Smallest Carbon Nanothreads

Relative to the rich library of small-molecule organics, few examples of ordered extended (i.e., nonmolecular) hydrocarbon networks are known. In particular, sp³ bonded, diamond-like materials represent appealing targets because of their desirable mechanical, thermal, and optical properties. While many covalent organic frameworks (COFs)-extended, covalently bonded, and porous structures-have been realized through molecular architecture with exceptional control, the design and synthesis of dense, covalent extended solids has been a longstanding challenge. Here we report the preparation of a sp³-bonded, low-dimensional hydrocarbon synthesized via high-pressure, solid-state diradical polymerization of cubane (C₈H₈), which is a saturated, but immensely strained, cage-like molecule. Experimental measurements show that the obtained product is crystalline with three-dimensional order that appears to largely preserve the basic structural topology of the cubane molecular precursor and exhibits high hardness (comparable to fused quartz) and thermal stability up to 300 °C. Among the plausible theoretical candidate structures, one-dimensional carbon scaffolds comprising six- and four-membered rings that pack within a pseudosquare lattice provide the best agreement with experimental data. These diamond-like molecular rods with extraordinarily small thickness are among the smallest members in the carbon nanothread family, and calculations indicate one of the stiffest one-dimensional systems known. These results present opportunities for the synthesis of purely sp³-bonded extended solids formed through the strain release of saturated molecules, as opposed to only unsaturated precursors.

Large-volume cubic press produces high temperatures above 4000 Kelvin for study of the refractory materials at pressures

The advent of a large-volume high-pressure apparatus has led to the discovery of many new materials with exceptional properties for widespread applications such as superhard materials (e.g., diamonds). However, for most conventional devices, the pressure and temperature capabilities are often limited to 6 GPa and 2300 K, which severely impedes the study of materials at extended pressures and temperatures. In this work, we present experimental optimizations of the high-pressure cell assembly for cubic press with a focus on the improvement of its temperature capability, leading to a record temperature value of ∼4050 K and largely extended pressure conditions up to ∼10 GPa with a centimeter-sized sample volume. Pressures of the new assembly at high temperatures are investigated by the melting-point method, giving rise to a series of parallel isoforce loading lines associated with thermally induced pressure. For the first time, the high-pressure melting curve of tungsten carbide is determined up to 3800 K and 8 GPa, and single-crystal refractory materials of Mo, Ta, and WC are also grown using the optimized cell.

Single-crystal diffraction at the high-pressure Indo-Italian beamline Xpress at Elettra, Trieste

In this study the first in situ high-pressure single-crystal X-ray diffraction experiments at Xpress, the Indo-Italian beamline of the Elettra synchrotron, Trieste (Italy), are reported. A description of the beamline experimental setup and of the procedures for single-crystal centring, data collection and processing, using diamond anvil cells, are provided. High-pressure experiments on a synthetic crystal of clinoenstatite (MgSiO₃), CaCO₃ polymorphs and a natural sample of leucophoenicite [Mn₇Si₃O₁₂(OH)₂] validated the suitability of the beamline experimental setup to: (i) locate and characterize pressure-induced phase transitions; (ii) solve ab initio the crystal structure of high-pressure polymorphs; (iii) perform fine structural analyses at the atomic scale as a function of pressure; (iv) disclose complex symmetry and structural features undetected using conventional X-ray sources.

Unique crystallization behavior in zeolite synthesis under external high pressures

Hydrothermal synthesis of zeolites under external high pressures generates unique crystallization behaviors that are different from the synthesis under autogenous pressure.

Pressure-Induced Phase Transitions in Sesquioxides

Pressure is an important thermodynamic parameter, allowing the increase of matter density by reducing interatomic distances that result in a change of interatomic interactions. In this context, the long range in which pressure can be changed (over six orders of magnitude with respect to room pressure) may induce structural changes at a much larger extent than those found by changing temperature or chemical composition. In this article, we review the pressure-induced phase transitions of most sesquioxides, i.e., A2O3 compounds. Sesquioxides constitute a big subfamily of ABO3 compounds, due to their large diversity of chemical compositions. They are very important for Earth and Materials Sciences, thanks to their presence in our planet’s crust and mantle, and their wide variety of technological applications. Recent discoveries, hot spots, controversial questions, and future directions of research are highlighted.

Electronic structure and optical properties of TaNO: An ab initio study

We performed ab initio calculations to study the structural and optoelectronic properties of simple and slab phase TaNO using density functional theory (DFT), in which the full potential augmented plane wave (FP-LAPW) method was implemented using the computational code Wien 2k. The modified Becke-Johnson potential (mBJ-GGA) was used for these calculations. The calculated band structure and electronic properties revealed an indirect bandgap for simple TaNO (3.2 eV) and a direct bandgap for slab TaNO (1.5 eV). The interband electronic transitions were investigated from the band structure, and transition peaks were observed from the imaginary part of the dielectric function. These transitions are due to Ta-p, N-p and O-p orbitals for simple TaNO and Ta-p, N-s as well as O-p orbitals for slab TaNO. The plasmon energy was related to the main peak of the energy loss function, which was approximately 10 eV. The static value of the dielectric constant and the refraction were close to the experimental values. In general, slab TaNO shows different properties and is more suitable for optoelectronic applications due to direct bandgap.

New stoichiometric compounds of Au-Hg system under high pressure

We have extensively explored Au-Hg system under high pressure in the range of 0-300 GPa with and without spin-orbit coupling (SOC) inclusion. The variable-composition methodology has been used as employed in Universal Structure Predictor: Evolutionary Xtallograpgy (USPEX) in combination with Vienna Ab initio Simulation Package (VASP). We have predicted several thermodynamically stable stoichiometric compounds of Au-Hg system, i.e. Au₃Hg (space group 194, P63/mmc), AuHg (space group 62, Pnma), AuHg₂ (space group 15, C2/c), AuHg₉ (space group 38, Amm2) at 50 GPa; AuHg₃ (space group 44, Imm2) at 100 GPa; Au₃Hg₂ (space group 15, C2/c) at 200 GPa and AuHg₄ (space group 15, C2/c) at 300 GPa. We also found a phase transition of AuHg₃ from Imm2 to P63/mmc at 248.5 GPa and 246.5 GPa with and without SOC, respectively.

Designing ultrastrong 5d transition metal diborides with excellent stability for harsh service environments

Much effort was devoted towards the rational design of ultrastrong transition metal borides (TMBs) with remarkable mechanical properties and excellent stabilities, owing to promising applications in machining, drilling tools and protective coatings for the aerospace industry. Although an enormous number of investigations have been performed on these TMBs under normal conditions, studies on the stability and mechanical strength in harsh high-pressure environments, which are critical for safe service behavior and a realistic understanding of stabilities and strengthening mechanisms, are yet nearly absent. In this work, taking 5d TMB2 (TM = Hf, Ta, W, Re, Os, Ir and Pt) as an illustration, we performed comprehensive high-throughput first-principles screening for thermodynamically stable and metastable structures under various pressures. Four experimentally observed structures are found to be thermodynamically feasible for most 5d TMB2 (TM = Hf, Ta, W, Re, Os and Ir) at 0 and 100 GPa. By exploiting orbital-decomposed electronic structures, we reveal that the pressure-induced stabilization and phase transitions of 5d TMB2 can be rationalized by the splitting of bonding and antibonding states around the Fermi level. Further investigations on the pressure-induced strengthening indicate that 5d TMB2 in the hP6[194] structure exhibit a profound strengthening effect under high pressure, which can be rationalized by the proposed strengthening factor η, but η fails in the oP6[59] structure due to the changed instability modes at different pressures. These findings suggest the necessity to explore the plasticity parameters for a realistic understanding of pressure-induced strengthening in TMBs, providing a strong argument for rules based on bond parameters at equilibrium in designing strong solids.

Pressure-Induced Large Volume Collapse, Plane-to-Chain, Insulator to Metal Transition in CaMn2Bi2

In situ high pressure single crystal X-ray diffraction study reveals that the quantum material CaMn₂Bi₂ undergoes a unique plane to chain structural transition between 2 and 3 GPa, accompanied by a large volume collapse. Puckered Mn-Mn honeycomb layer converts to quasi-one-dimensional (1D) zigzag chains above the phase transition pressure. Single crystal measurements reveal that the pressure-induced structural transformation is accompanied by a dramatic 2 orders of magnitude drop of resistivity. Although the ambient pressure phase displays semiconducting behavior at low temperatures, metallic temperature dependent resistivity is observed for the high pressure phase, as surprisingly, are two resistivity anomalies with opposite pressure dependences, while one of them could be a magnetic transition and the other originates from Fermi surface instability. Assessment of the total energies for hypothetical magnetic structures for high pressure CaMn₂Bi₂ indicates that ferrimagnetism is thermodynamically favored.

Viewpoint: the road to room-temperature conventional superconductivity

It is a honor to write a contribution on this memorial for Sandro Massidda. For both of us, at different stages in our lives, Sandro was first and foremost a friend. We both admired his humble, playful and profound approach to life and physics. In this contribution we describe the route which permitted to meet a long-standing challenge in solid state physics, i.e. room temperature superconductivity. In less than 20 years the critical temperature of conventional superconductors, which in the last century had been widely believed to be limited to 25 K, was raised from 40 K in MgB₂ to 265 K in LaH₁₀. This discovery was enabled by the development and application of computational methods for superconductors, a field in which Sandro Massidda played a major role.

Creation of two-dimensional layered Zintl phase by dimensional manipulation of crystal structure

The discovery of new families, beyond graphene, of two-dimensional (2D) layered materials has always attracted great attention. However, it has been challenging to artificially develop layered materials with honeycomb atomic lattice structure composed of multicomponents such as hexagonal boron nitride. Here, through the dimensional manipulation of a crystal structure from sp³-hybridized 3D-ZnSb, we create an unprecedented layered structure of Zintl phase, which is constructed by the staking of sp²-hybridized honeycomb ZnSb layers. Using structural analysis combined with theoretical calculation, it is found that the 2D-ZnSb has a stable and robust layered structure. The bidimensional polymorphism is a previously unobserved phenomenon at ambient pressure in Zintl families and can be a common feature of transition metal pnictides. This dimensional manipulation of a crystal structure thus provides a rational design strategy to search for new 2D layered materials in various compounds, enabling unlimited expansion of 2D libraries and corresponding physical properties.

Gold nanorods as a high-pressure sensor of phase transitions and refractive-index gauge

Gold nanorods (Au NRs), nanospheres and other nanoparticles display numerous superior physicochemical properties, such as resistance to oxidation and aggressive agents, strong enhancement of local electric field and a high absorption coefficient in the visible and near-infrared (NIR) range. The absorption peaks of surface plasmon resonance (SPR) in Au NRs are highly sensitive to their surrounding medium and to its refractive index (RI) changes. However, no applications of NRs for detecting phase transitions have been reported. Here, we show that Au NRs effectively detect phase transitions of compressed compounds, liquid and solid, by measuring their RI. Owing to the direct interaction of the NRs with their surrounding medium, its subtle RI changes can be observed by the use of high-pressure absorption vis-NIR spectroscopy. We have applied a Au NR-based sensor in a diamond anvil cell (DAC) for monitoring the phase transitions of compressed water, its freezing to ice VI and at the subsequent solid-solid phase transition to ice VII, and the monotonic compression and solid-solid phase transitions in urea and thiourea.

Novel pressure-induced topological phase transitions of supercooled liquid and amorphous silicene

This molecular dynamics (MD) simulation carries a detailed analysis of a pressure-induced structural transition supercooled liquid and amorphous silicene (a-silicene). Low-density models of supercooled liquid and a-silicene containing 10 000 atoms are obtained by rapid cooling processes from the melts. Then, an a-silicene model at T  =  1000 K, a supercooled liquid model at T  =  1500 K and a liquid silicon model at T  =  2000 K have been isothermally compressed step by step up to a high density in order to observe the pressure-induced structural changes. Specifically 'Cairo tiling' pentagonal and square lattices of silicene are discovered in our calculations. Structural properties of those penta-silicene and tetra-silicene models have been carefully analyzed through the radial distribution functions, interatomic distances, bond-angle distributions under high-pressure condition. The dependence of pressure on formation behaviors is calculated via pressure-volume and energy-density relationships. The first order transition from low-density supercooled liquid/amorphous silicene to high-density penta-silicene and continuous transition from low-density liquid to high-density tetra-silicene are discussed. Atomic mechanism and sp³/sp² hybridization evolution are inspected whereas the role of low-membered ring defects/boundary promises remarkable application and advanced research in future.