Extraordinary Indentation Strain Stiffening Produces Superhard Tungsten Nitrides

Structural, strength and fracture mechanisms of superconducting transition metal nitrides TM3N5 (TM = W and Mo)

Transition metal (TM) nitrides are recognized for their outstanding and highly desirable properties, categorizing them as a class of multifunctional materials with diverse industrial applications. In particular, the newly synthesized W3N5 is notable for its exceptional ultra-incompressibility (406 GPa for bulk modulus), remarkable hardness (34 GPa), and superconductivity (9.4 K), positioning it as a potential ultra-hard superconductor. We performed a comprehensive study of the structural, electronic, and mechanical properties of TM3N5 (TM = W and Mo), emphasizing their behavior under shear deformation and lattice instability. The distinct ionic TM-N and covalent N-N bonding characteristics in TM3N5 were characterized through a topological analysis of charge density. Compared to W3N5, the superconducting transition temperature of Mo3N5 at ambient pressure was estimated to be 14.8 K. Both compounds demonstrate impressive uniaxial compressive strengths of -265.7 GPa for W3N5 and -216.5 GPa for Mo3N5, which are comparable to that of diamond (-223.1 GPa) along the [100] direction. The superior mechanical strength of TM3N5, especially in W3N5, was manifested by the calculated ideal tensile strengths exceeding 40 GPa along the main crystal axes of [100], [010], [001], and [101]. However, W3N5 shows a considerably lower Vickers indentation shear strength of 16.2 GPa along the (110)[11̄0] direction when compared to the well-known WNx, indicating a limitation in its shear fracture resistance and hardness, as suggested by the determined Vickers hardness of 22.0-22.5 GPa. Finally, the lattice instability and fracture mechanisms of W3N5 under indentation shear deformation were clarified through in-depth analyses of atomic bonding and electronic structure evolutions.

Theoretical Calculation and Experimental Studies of Boron Phosphide Polycrystalline Synthesized at High Pressure and High Temperature

In this study, a combination of theoretical calculations and experiments were carried out to analyze boron phosphide materials. Amorphous boron powder and amorphous red phosphorus were used as raw materials to directly synthesize the target samples in one step under high-pressure and high-temperature (HPHT) conditions. Theoretical calculations were then carried out based on the XRD spectra of boron phosphide at 4 GPa and 1200 °C. The experimental results show that the target samples can be successfully prepared at HPHT. The electrical properties of the samples were characterized, and it was found that their conductivity increased with the increase in temperature, and they have a semiconducting nature, which is consistent with the theoretical calculations. Its Seebeck coefficient is positive at different temperatures, indicating that the synthesized boron phosphide is a P-type semiconductor. The combination of theoretical calculations and experiments shows that high pressure can reduce the lattice constant of boron phosphide, thus reducing its forbidden bandwidth, which improves its electrical properties. EDS shows a homogeneous distribution of the elements in the samples. Successful synthesis of BP crystals will probably stimulate more research into its semiconductor properties. It may also provide some assistance in the application of BP in aero-engine high-temperature monitoring systems as well as thermally controlled coatings for deep-space probes.

Structural evolution and electronic properties of anionic carbon-nitrogen clusters

Carbon-doped nitrogen clusters have represented a fascinating area of materials science in recent years. However, achieving precise control of the doping level and distribution of carbon within nitrogen clusters is challenging. Advanced techniques are required to elucidate their exact structures and electronic properties. Here, we perform systematic structure searches of anionic carbon-doped nitrogen clusters by the Crystal structure AnaLYsis by Particle Swarm Optimization method combined with density-functional theory calculations. The ground state structures of CNn- (n = 4-16) clusters for each cluster size are determined. The structural evolution, photoelectron energy spectra, electronic properties, and chemical bonding modes of anionic CNn- (n = 4-16) clusters are discussed. The calculated results indicate that the two-dimensional planar geometry of the anionic CN6- cluster with the C1 symmetry exhibits robust stability. The molecular orbitals and the adaptive natural density partitioning calculations show that the high stability of the anionic CN6- cluster is attributed to the localized σ-bonding formed by the 2p orbitals of carbon and nitrogen atoms. The four delocalized π chemical bonds contribute to the stability of the anionic CN6- cluster, resulting in a stable ring geometry. The present results enrich the database of geometric structures of carbon-nitrogen clusters and provide valuable insights for the experimental synthesis and characterization of nitrogen-based clusters.

Hardness and electronic properties of Si-C-N structures

Three novel hexagonal Si-C-N structures, namely SiC3N3, SiC7N6, and SiC13N14, were constructed on the basis of the α-Si3N4 crystal structure. The stability of the three structures is demonstrated by analyzing their elastic constants and phonon dispersion spectra and by calculating their formation energies. The calculated band structures and partial densities of states suggest that the SiC3N3 and SiC7N6 structures possess hole conductivity. The electron orbital analyses indicate that the SiC3N3 and SiC7N6 crystals possess three-dimensional and one-dimensional conductivity, respectively. SiC13N14 is a semiconductor with a wide bandgap of 4.39 eV. Based on two different hardness models and indentation shear stress calculations, the Vickers hardness values of SiC3N3, SiC7N6, and SiC13N14 are estimated to be 28.04/28.45/16.18 GPa, 31.17/34.19/20.24 GPa, and 40.60/41.59/36.40 GPa. This result indicates that SiC3N3 and SiC7N6 are conductive hard materials while SiC13N14 is a quasi superhard material.

Photoluminescence and energy transfer mechanisms of Tm3+ doped Y2O3 laser crystals: experimental and theoretical insights

Rare-earth thulium (Tm3+) doped yttrium oxide (Y2O3) host single crystals are promising "eye-safe" laser materials. However, the mechanisms of photoluminescence and energy transfer in Tm3+ doped Y2O3 crystals are not yet understood at the fundamental level. Here, we synthetize a series of Y2O3:Tm3+ samples by the sol-gel method. Our experimental results show that the most intensive absorption line of the 3H6 → 1D2 transition occurs at 358 nm, and the strongest emission line of the 1D2 → 3F4 transition is located at 453 nm, which are in good agreement with the calculations of 363 nm and 458 nm, respectively. By using the CALYPSO structural search method, the ground state structure of Y2O3:Tm3+ with P2 space group symmetry is uncovered. The complete energy levels, including free-ion LS terms and crystal-field LSJ multiplet manifolds, of Y2O3:Tm3+ are obtained based on our developed WEPMD method. The present findings show that our WEPMD method can be used in experiments to elucidate the underlying mechanisms of photoluminescence and energy transfer in Tm3+ doped Y2O3 crystals, which offer insights for further understanding of other rare-earth doped laser materials.

Structures, Electronic, and Magnetic Properties of CoKn (n = 2-12) Clusters: A Particle Swarm Optimization Prediction Jointed with First-Principles Investigation

Transition-metal-doped clusters have long been attracting great attention due to their unique geometries and interesting physical and/or chemical properties. In this paper, the geometries of the lowest- and lower-energy CoKn (n = 2-12) clusters have been screened out using particle swarm optimization and first principles relaxation. The results show that except for CoK2 the other CoKn (n = 3-12) clusters are all three-dimensional structures, and CoK7 is the transition structure from which the lowest energy structures are cobalt atom-centered cage-like structures. The stability, the electronic structures, and the magnetic properties of CoKn clusters (n = 2-12) clusters are further investigated using the first principles method. The results show that the medium-sized clusters whose geometries are cage-like structures are more stable than smaller-sized clusters. The electronic configuration of CoKn clusters could be described as 1S1P1D according to the spherical jellium model. The main components of petal-shaped D molecular orbitals are Co-d and K-s states or Co-d and Co-s states, and the main components of sphere-like S molecular orbitals or spindle-like P molecular orbitals are K-s states or Co-s states. Co atoms give the main contribution to the total magnetic moments, and K atoms can either enhance or attenuate the total magnetic moments. CoKn (n = 5-8) clusters have relatively large magnetic moments, which has a relation to the strong Co-K bond and the large amount of charge transfer. CoK4 could be a magnetic superatom with a large magnetic moment of 5 μB.

Solving Strength–Toughness Dilemma in Superhard Transition-Metal Diborides via a Distinct Chemically Tuned Solid Solution Approach

Solid solution strengthening enhances hardness of metals by introducing solute atoms to create local distortions in base crystal lattice, which impedes dislocation motion and plastic deformation, leading to increased strength but reduced ductility and toughness. In sharp contrast, superhard materials comprising covalent bonds exhibit high strength but low toughness via a distinct mechanism dictated by brittle bond deformation, showcasing another prominent scenario of classic strength–toughness tradeoff dilemma. Solving this less explored and understood problem presents a formidable challenge that requires a viable strategy of tuning main load-bearing bonds in these strong but brittle materials to achieve concurrent enhancement of the peak stress and related strain range. Here, we demonstrate a chemically tuned solid solution approach that simultaneously enhances hardness and toughness of superhard transition-metal diboride Ta 1− x Zr x B 2 . This striking phenomenon is achieved by introducing solute atom Zr that has lower electronegativity than solvent atom Ta to reduce the charge depletion on the main load-bearing B–B bonds during indentation, leading to prolonged deformation that gives rise to notably higher strain range and the corresponding peak stress. This finding highlights the crucial role of properly matched contrasting relative electronegativity of solute and solvent atoms in creating concurrent strengthening and toughening and opens a promising avenue for rational design of enhanced mechanical properties in a large class of transition-metal borides. This strategy of concurrent strength–toughness optimization via solute-atom-induced chemical tuning of the main load-bearing bonding charge is expected to work in broader classes of materials, such as nitrides and carbides.

High-pressure phases of a Mn-N system

Highly compressed extended states of light elemental solids have emerged recently as a novel group of energetic materials. The application of these materials is seriously limited by the energy-safety contradiction, because the material with high energy density is highly metastable and can hardly be recovered under ambient conditions. Recently, it has been found that high-energy density transition metal polynitrides could be synthesized at ∼100 GPa and recovered at ∼20 GPa. Inspired by these findings, we have studied a high-pressure Mn-N system from the aspects of structure, stability, phase transition, energy density and electronic structure theoretically for the first time. The results reveal that MnN₄_P1̄ consisting of [N₄]_∞^2- is thermodynamically stable at 36.9-100 GPa, dynamically stable at 0 GPa and has a noticeably high volumetric energy density of 15.71 kJ cm^-3. Upon decompression, this structure will transform to MnN₄_C2/m with the transition barrier declining sharply at 5-10 GPa due to the switching of transition pathways. Hence, we propose MnN₄_P1̄ as a potential energetic material that is synthesizable above 40 GPa and recoverable until 10 GPa.

Theoretically evaluating two-dimensional tetragonal Si2Se2 and SiSe2 nanosheets as anode materials for alkali metal-ion batteries

In this work, based on first-principles calculations, we theoretically predict two kinds of two-dimensional tetragonal Si–Se compounds, Si2Se2 and SiSe2, as the anode materials for alkali metal-ion batteries.

Insights into the Structures and Bonding of Medium-Sized Cerium-Doped Boron Clusters

Since the discovery of metal-doped boron clusters attracted great significance to create a new class of materials, research interests have been directed to chemical bonding and structural evolution of lanthanide boride clusters. Here, we perform an extensive ground-state structure search for the CeB_n and CeB_n^- clusters in the size range from 9 to 18 using the Crystal structure AnaLYsis by Particle Swarm Optimization method and density functional theory optimization. It is found that the ground-state structures in both neutral and anionic series possess half-sandwich geometry. The host boron moiety in neutral series has a tendency to form borophene-like geometry. The pentagonal and hexagonal holes are more common in the larger anionic CeB_n^- series. The theoretical photoelectron spectroscopy has been simulated by applying time-dependent density functional theory calculations. The neutral CeB₁₄ cluster is identified as a magic cluster on the basis of its robust relative stability with respect to its neighbors. Electronic structure and chemical bonding analyses reveal that the CeB₁₄ cluster possesses a large HOMO-LUMO gap and enhanced stability with strong delocalized π and δ bonding via interactions between the Ce 5d- and B 2p-AOs.

Propagating Chiral Phonons in Three-Dimensional Materials

Chiral phonons were initially proposed and experimentally verified in two-dimensional (2D) systems. Their intriguing effects have generated profound impacts on multiple research fields. However, all chiral phonons reported to date are constrained to be local, in the sense that their group velocities vanish identically. Here, we propose the concept of propagating 3D chiral phonons, which can transport the information on chirality and angular momentum. Guided by the necessary conditions and using first-principles calculations, we demonstrate their existence in WN₂. The chirality, group velocity, and pseudoangular momentum are analyzed. Based on their selective coupling with valley electrons and photons, we propose an experimental setup to detect the unique feature of propagating chiral phonons. Our work endows chiral phonons with a crucial character-the ability to propagate and transport quantized information, which creates a new research direction and opens up the possibility to design novel phononic quantum devices.

Indentation Strengths of Zirconium Diboride: Intrinsic versus Extrinsic Mechanisms

Zirconium diboride (ZrB₂) is an important ultra-high-temperature ceramic, which exhibits outstanding mechanical properties and is widely used in extreme environments. Extensive experimental studies, however, have found that synthesized ZrB₂ specimens show widely scattered indentation hardness values ranging from 8.7 to 26 GPa. We have performed comprehensive stress-strain calculations of ZrB₂ to explore its structural and stress responses and found that ZrB₂ possesses an intrinsic indentation strength of 32.7 GPa, which is on par with those of other transition-metal borides that exhibit higher indentation hardness values of ∼30 GPa. This result suggests that large variations in measured hardness are driven by extrinsic factors, and an analysis of available experimental data indicates that the quality of the crystallinity of specimens holds the key to realizing improved hardness corresponding to the predicted intrinsic indentation strength. These findings offer insights into the origin of the previously reported lower hardness values of ZrB₂ and raise the prospects of achieving superior strengths in well-crystallized ZrB₂ that approach or match those of other ultrahard transition-metal compounds.

Insights into the Microstructures and Energy Levels of Pr3+-Doped YAlO3 Scintillating Crystals

Trivalent praseodymium (Pr3+)-doped materials have been extensively used in high-resolution laser spectroscopy, owing to their outstanding conversion efficiencies of plentiful transitions in the visible laser region. However, to clarify the microstructure and energy transfer mechanism of Pr3+-doped host crystals is a challenging topic. In this work, the stable structures of Pr3+-doped yttrium orthoaluminate (YAlO3) have been widely searched based on the CALYPSO method. A novel monoclinic structure with the Pm group symmetry is successfully identified. The Pr3+ impurity can precisely occupy the Y3+ position and get incorporated into the YAlO3 (YAP) host crystal with a Pr3+ concentration of 6.25%. The result of the electronic band structure reveals a 3.62 eV band gap, which suggests a semiconductor character of YAP:Pr. Using our developed well-established parametrization matrix diagonalization (WEPMD) method, we have systematically analyzed the energy level scheme and proposed a set of newly improved parameters. Additionally, the energy transfer mechanism of YAP:Pr is clarified by deciphering the numerical electric dipole and magnetic dipole transitions. The popular red emission at 653 nm is assigned to the transition 3P0 → 3F2, while the transition 3P0 → 3H4 with a large branching ratio is predicted to be a good laser channel. Many promising emission lines for laser actions are also obtained in the visible light region. Our results not only provide important insights into the energy transfer mechanisms of rare-earth ion-doped materials but also pave the way for the implementation of new types of laser devices.

Pressure-Induced Evolution of Crystal and Electronic Structure of Ammonia Borane

Ammonia borane (NH₃BH₃) has long attracted considerable interest for its high hydrogen content and easy dehydrogenation conditions which make it a promising hydrogen storage material. Here, we report on a computational study of the structural stability and phase transition sequence of NH₃BH₃ and associated lattice dynamics and electronic properties in a wide pressure range up to 300 GPa. The results confirm previously reported structures, including the experimentally observed orthorhombic Pmn2₁ structure at low temperature and ambient pressure, and predict the phase transition sequence Pmn2₁ → Pc → P2₁ → P1̅ for NH₃BH₃. Our calculations also reveal systematic trends of monotonically decreasing band gap with rising pressure in the three high-pressure NH₃BH₃ phases, which nevertheless all remain nonconducting up to the highest pressure of 300 GPa examined in this work. The present findings elucidate structural and electronic properties of NH₃BH₃ over an extensive pressure range, providing knowledge essential to further study of NH₃BH₃ in an expanded pressure-temperature phase space.

Atomistic Mechanisms for Contrasting Stress-Strain Relations of B13CN and B13C2

Boron-rich compounds comprise intricate bonding structures and possess excellent mechanical properties. Here, we report on a comparative study of B₁₃CN and B₁₃C₂, which are isostructural but differ in electron fillings, with the former being electron-precise and the latter electron-deficient. Our results show that the different electron fillings in B₁₃CN and B₁₃C₂ have profound effects on the bonding features despite their shared crystal structure, generating distinct structural deformation modes and the accompanying stress responses under diverse loading strain conditions. The most striking phenomena include a creeplike stress response under a tensile strain and superior strength under the vast majority of loading conditions for B₁₃CN compared to B₁₃C₂. Such enhanced stability of the B₁₂ icosahedra in B₁₃CN by N-induced electron compensation may be effective for structural and mechanical enhancement of other boron-rich compounds and offers improved understanding of a broader class of covalent crystals with complex bonding networks.

High-Pressure Phases and Properties of the Mg3Sb2 Compound

Pressure always plays an important role in influencing the structure configuration and electronic properties of materials. Here, combining density function theory and structure prediction algorithm, we systematically studied the Mg3Sb2 system from its phase transition to thermodynamic and electronic properties under high pressure. We find that two novel phases, namely Cm and C2/m, are stable under high pressure. Calculation results of phonon dispersions showed that both novel phases have no imaginary frequency, which indicates that the novel phases are thermodynamically stable. Due to the larger ionic radius of Sb compared to N, P, and As elements, the Mg3Sb2 compound shows a different electronic property at high pressure. The electronic calculations show that the novel phases of Cm and C2/m of Mg3Sb2 possess metallic behavior under high pressure. These results provide new insights for understanding the Mg3Sb2 compound.

Negative Poisson Ratio in Two-Dimensional Tungsten Nitride: Synergistic Effect from Electronic and Structural Properties

Low-dimensional materials with high stabilities and outstanding mechanical properties are essential for next generation microelectromechanical systems (MEMS). The successful synthesis of two-dimensional (2D) tungsten nitride makes it a promising candidate for the MEMS application. Here, we have confirmed the existence of experimentally synthesized W₂N₃ and predicted three additional new 2D monolayer tungsten nitrides: WN₂, WN₄, and W₃N based on extensively structural searches by CALYPSO method and first-principle calculations. The calculations indicate that the nitrogen-rich WN₄ monolayer possesses large in-plane negative Poisson ratios attributed to the 4-fold-coordinated WN₄ ν_x = -0.103 and ν_y = -0.113, which are tetrahedron combined with the strong coupling between the 2p orbitals of N and 5d orbitals of W. Our findings not only enrich the family of 2D transition metal nitrides with excellent mechanical properties but also open avenues for design and synthesis of other novel 2D layered materials.

Elucidating Stress-Strain Relations of ZrB12 from First-Principles Studies

Transition-metal boron-rich compounds exhibit favorable synthesis conditions and mechanical properties that hold great promise for wide-ranging applications. However, the complex bonding networks of these compounds produce diverse structural and mechanical behaviors that require in-depth studies. A notable case is ZrB₁₂, which has been reported to possess high Vickers hardness comparable to those of ReB₂ and WB₄. Surprisingly, first-principles calculations of stress-strain relations reveal unexpected low indentation strengths of ZrB₁₂ well below those of ReB₂ and WB₄. Such contrasting results are reconciled by noting that the additional presence of a boron-rich phase of ZrB₅₀ in the experimental synthesis process likely plays a key role in the extrinsic strengthening. These findings uncover mechanisms for the higher measured strength of ZrB₁₂ and offer insights for elucidating extrinsic hardening phenomena that may exist in other transition-metal compounds.

Plasma-Assisted Synthesis of Platinum Nitride Nanoparticles under HPHT: Realized by Carbon-Encapsulated Ultrafine Pt Nanoparticles

Noble metal nitrides (NMNs) have important theoretical significance and potential application prospects due to their high bulk modulus and remarkable electrical properties. However, NMNs can only be synthesized under extreme conditions of ultrahigh pressure and temperature, and nanoscaled NMNs have not been reported. In this work, as typical NMNs, PtN_x nanoparticles were synthesized at 5 GPa and 750 K by the method of plasma-assisted laser-heating diamond anvil cell. The significantly reduced synthesis condition benefited from the ingenious design of the precursor and the remarkable chemical activity of the ultrafine Pt nanoparticles. This study, combining nanomaterials with high-pressure and -temperature (HPHT) techniques, provides a novel process for the preparation of NMN nanomaterials, and a new direction for the synthesis of superhard materials.

Searching new structures of beryllium-doped in small-sized magnesium clusters: Be2 Mgn Q (Q = 0, -1; n = 1-11) clusters DFT study

Using CALYPSO method to search new structures of neutral and anionic beryllium-doped magnesium clusters followed by density functional theory (DFT) calculations, an extensive study of the structures, electronic and spectral properties of Be₂ Mg_n ^Q (Q = 0, -1; n = 2-11) clusters is performed. Based on the structural optimization, it is found that the Be₂ Mg_n ^Q (Q = 0, -1) clusters are shown by tetrahedral-based geometries at n = 2-6 and tower-like-based geometries at n = 7-11. The calculations of stability indicate that Be₂ Mg₅ ^Q=0 , Be₂ Mg₅ ^Q=-1 , and Be₂ Mg₈ ^Q=-1 clusters are "magic" clusters with high stability. The NCP shows that the charges are transferred from Mg atoms to Be atoms. The s- and p-orbitals interactions of Mg and Be atoms are main responsible for their NEC. In particular, chemical bond analysis including molecular orbitals (MOs) and chemical bonding composition for magic clusters to further study their stability. The results confirmed that the high stability of these clusters is due to the interactions between the Be atom and the Mg₅ or Mg₈ host. Finally, theoretical calculations of infrared and Raman spectra of the ground state of Be₂ Mg_n ^Q (Q = 0, -1; n = 1-11) clusters were performed, which will be absolutely useful for future experiments to identify these clusters.

Crystal structure and properties of iodine monofluoride compounds at high pressure

The pressure-induced structural phase transitions of halogen compound IF under high pressure were studied by using the unbiased CALYPSO structure prediction method. The phase transition sequence of IF under high pressure is determined to be P1 → P21/c-I → P21/c-II → I4/mmm, and the corresponding phase transition pressures are 3 GPa, 16.5 GPa and 46 GPa, respectively. The physical properties of each predicted phase were thus fully studied, and it was found that the P1 phase was unstable which is in excellent agreement with the experiment result. Furthermore, our exploration of the high pressure phases for IF will provide fundamental insights for further exploration the structural phase transition of other halogen compounds under high pressure.

Structural, mechanical and electronic properties and hardness of ionic vanadium dihydrides under pressure from first-principles computations

Based on a combination of the CALYPSO method for crystal structure prediction and first-principles calculations, we explore the crystal structures of VH2 under the pressure range of 0-300 GPa. The cubic Fm-3m phase with regular VH8 cubes is predicted to transform into orthorhombic Pnma structure with fascinating distorted VH9 tetrakaidecahedrons at 47.36 GPa. Both the Fm-3m phase at 0 GPa and the Pnma phase at 100 GPa are mechanically and dynamically stable, as verified with the calculations of elastic constants and phonon dispersions, respectively. Moreover, the calculated electronic band structure and density of states indicate both stable phases are metallic. Remarkably, the analyses of the Poisson's ratio, electron localization function (ELF) and Bader charge substantiate that both stable phases are ionic crystals on account of effective charges transferring from V atom to H. On the basis of the microscopic hardness model, the Fm-3m and Pnma crystals of VH2 are potentially incompressible and hard materials with the hardness values of 17.83 and 17.68 GPa, respectively.

The Microstructure and Electronic Properties of Yttrium Oxide Doped With Cerium: A Theoretical Insight

Trivalent Cerium (Ce³⁺) doped Yttrium Oxide (Y₂O₃) host crystal has drawn considerable interest due to its popular optical 5d-4f transition. The outstanding optical properties of Y₂O₃:Ce system have been demonstrated by previous studies but the microstructures still remain unclear. The lacks of Y₂O₃:Ce microstructures could constitute a problem to further exploit its potential applications. In this sense, we have comprehensively investigated the structural evolutions of Y₂O₃:Ce crystals based on the CALYPSO structure search method in conjunction with density functional theory calculations. Our result uncovers a new rhombohedral phase of Y₂O₃:Ce with R-3 group symmetry. In the host crystal, the Y³⁺ ion at central site can be naturally replaced by the doped Ce³⁺, resulting in a perfect cage-like configuration. We find an interesting phase transition that the crystallographic symmetry of Y₂O₃ changes from cubic to rhombohedral when the impurity Ce³⁺ is doped into the host crystal. With the nominal concentration of Ce³⁺ at 3.125%, many metastable structures are also identified due to the different occupying points in the host crystal. The X-ray diffraction patterns of Y₂O₃:Ce are simulated and the theoretical result is comparable to experimental data, thus demonstrating the validity of the lowest energy structure. The result of phonon dispersions shows that the ground state structure is dynamically stable. The analysis of electronic properties indicate that the Y₂O₃:Ce possesses a band gap of 4.20 eV which suggests that the incorporation of impurity Ce³⁺ ion into Y₂O₃ host crystal leads to an insulator to semiconductor transition. Meanwhile, the strong covalent bonds of O atoms in the crystal, which may greatly contribute to the stability of ground state structure, are evidenced by electron localization function. These obtained results elucidate the structural and bonding characters of Y₂O₃:Ce and could also provide useful insights for understanding the experimental phenomena.

Effects of boron defects on mechanical strengths of TiB2 at high temperature: ab initio molecular dynamics studies

We report the determination of diffusion paths and potential barriers of boron point defects in TiB2 calculated using the climbing image nudged elastic band method at T = 0 K, and ab initio molecular dynamics studies on the structural stabilities, diffusion behavior of boron point defects and mechanical strengths of TiB2 at elevated temperatures. In contrast to the previous conjecture that TiB2 with boron vacancies are thermodynamically unstable based on the calculations at T = 0 K that boron vacancies have positive formation energies and shift electronic Fermi energies from the pseudogap valleys to the bonding states, our results show that boron vacancies in TiB2 are very robust and they have negligible effects on the structural stabilities and mechanical strengths of TiB2 at least up to 2000 K within the vacancy concentration we studied (<2.5 at%). On the other hand, our results reveal that the boron interstitials can diffuse easily in TiB2 at a moderately high temperature (1000 K) or under large shear and tensile deformations, which give rise to significant deteriorations (more than 50% reduction) in the mechanical strength of TiB2 at a high temperature (2000 K) with a boron interstitial density below 2.5 at%. Under all the shear and tensile deformations we applied, the boron interstitials in TiB2 eventually diffuse into the boron layers, causing deformations of these boron layers, which weakens their interactions with metal layers nearby and consequently reduces the mechanical strengths of the materials as temperature and boron interstitial density increase. The present findings expand our understandings on the material strength of TiB2 at high temperatures with boron point defects, and offer new insights for its applications as a high-strength ultra-high temperature ceramic.

Mechanical properties of tantalum carbide from high-pressure/high-temperature synthesis and first-principles calculations

The mechanical strength of ceramic material TaC can be described well with atomistic simulations if realistic deformation models are considered.

Structural evolution and electronic properties of medium-sized boron clusters doped with scandium

Doping of boron-based materials with transition metal atoms allows one to tune or modify the properties and structure of the materials. In this work, an extensive search for the global minima on potential energy surfaces of ScB _n and ScB[Formula: see text] clusters has been performed using the CALYPSO method. The structural evolution of scandium doped boron clusters of this range is found to proceed in three steps; namely, the formation of half-sandwich type structures is followed by the formation of drum-like structures with the Sc atom located at the center and terminates with the cage-like structures. It is also found that highly symmetrical geometric structures are more common for the smaller size range of [Formula: see text]. The neutral ScB₁₃ cluster is identified as magic on the basis of an analysis of relative stabilities in the ScB _n series. Our analysis of chemical bonding has shown that the stability of this cluster is mainly due to the formation of several delocalized [Formula: see text]-bonding molecular orbitals composed of Sc 3d and B 2s atomic orbitals. These bonds appear to be responsible for the enhanced stability of ScB₁₃ with respect to other Sc-doped boron clusters.

Systematic Theoretical Study on Structural, Stability, Electronic, and Spectral Properties of Si2 Mg n Q (Q = 0, ±1; n = 1-11) Clusters of Silicon-Magnesium Sensor Material

By using CALYPSO searching method and Density Functional Theory (DFT) method at the B3LYP/6-311G (d) level of cluster method, a systematic study of the structures, stabilities, electronic and spectral properties of Si₂MgnQ (n = 1–11; Q = 0, ±1) clusters of silicon-magnesium sensor material, is performed. According to the calculations, it was found that when n > 4, most stable isomers in Si₂MgnQ (n = 1–11; Q = 0, ±1) clusters of silicon-magnesium sensor material are three-dimensional structures. Interestingly, although large size Si₂MgnQ clusters show cage-like structures, silicon atoms are not in the center of the cage, but tend to the edge. The Si₂Mg1,5,6,8-1 and Si₂Mg13,4,7,9,10+1 clusters obviously differ to their corresponding neutral structures, which are in good agreement with the calculated values of VIP, AIP, VEA, and AEA. |VIP-VEA| values reveal that the hardness of Si₂Mg_n clusters decreases with the increase of magnesium atoms. The relative stabilities of neutral and charged Si₂MgnQ (n = 1–11; Q = 0, ±1) clusters of silicon-magnesium sensor material is analyzed by calculating the average binding energy, fragmentation energy, second-order energy difference and HOMO-LUMO gaps. The results reveal that the Si₂Mg30, Si₂Mg3-1, and Si₂Mg3+1clusters have stronger stabilities than others. NCP and NEC analysis results show that the charges in Si₂MgnQ (n = 1–11; Q = 0, ±1) clusters of silicon-magnesium sensor material transfer from Mg atoms to Si atoms except for Si₂Mg1+1, and strong sp hybridizations are presented in Si atoms of Si₂MgnQ clusters. Finally, the infrared (IR) and Raman spectra of all ground state of Si₂MgnQ (n = 1–11; Q = 0, ±1) clusters of silicon magnesium sensor material are also discussed.

Smooth Flow in Diamond: Atomistic Ductility and Electronic Conductivity

Diamond is the quintessential superhard material widely known for its stiff and brittle nature and large electronic band gap. In stark contrast to these established benchmarks, our first-principles studies unveil surprising intrinsic structural ductility and electronic conductivity in diamond under coexisting large shear and compressive strains. These complex loading conditions impede brittle fracture modes and promote atomistic ductility, triggering rare smooth plastic flow in the normally rigid diamond crystal. This extraordinary structural change induces a concomitant band gap closure, enabling smooth charge flow in deformation created conducting channels. These startling soft-and-conducting modes reveal unprecedented fundamental characteristics of diamond, with profound implications for elucidating and predicting diamond's anomalous behaviors at extreme conditions.

Prediction of the Reactivity of Argon with Xenon under High Pressures

Pressure significantly modifies the microscopic interactions in the condense phase, leading to new patterns of bonding and unconventional chemistry. Using unbiased structure searching techniques combined with first-principles calculations, we demonstrate the reaction of argon with xenon at a pressure as low as 1.1 GPa, producing a novel van der Waals compound XeAr₂. This compound is a wide-gap insulator and crystallizes in a MgCu₂-type Laves phase structure. The calculations of phonon spectra and formation enthalpy indicate that XeAr₂ would be stable without any phase transition or decomposition at least up to 500 GPa.

Detwinning Mechanism for Nanotwinned Cubic Boron Nitride with Unprecedented Strength: A First-Principles Study

Synthesized nanotwinned cubic boron nitride (nt-cBN) and nanotwinned diamond (nt-diamond) exhibit extremely high hardness and excellent stability, in which nanotwinned structure plays a crucial role. Here we reveal by first-principles calculations a strengthening mechanism of detwinning, which is induced by partial slip on a glide-set plane. We found that continuous partial slip in the nanotwinned structure under large shear strain can effectively delay the structural graphitization and promote the phase transition from twin structure to cubic structure, which helps to increase the maximum strain range and peak stress. Moreover, ab initio molecular dynamics simulation reveals a stabilization mechanism for nanotwin. These results can help us to understand the unprecedented strength and stability arising from the twin boundaries.

Exploration of high-pressure structural transition and electronic properties of BaFe2S3

The iron-based compound BaFe₂S₃ has received considerable attention in recent years due to its unique electronic properties with important applications. But, there has been relatively little attention devoted to the structure evolution of BaFe₂S₃ under high pressure. Here, we report a detailed theoretical study of the structural, electronic and sound velocity properties of BaFe₂S₃ under high pressure by CALYPSO structure search method in combination with the first-principles calculations. We have uncovered three novel structures of BaFe₂S₃ under high pressure with space group P222₁, Imm2, and C2/m symmetries. Theoretical calculations reveal that the structures of BaFe₂S₃ under high pressure satisfy the phase transition sequence of Cmcm  →  P222₁  →  Imm2  →  C2/m, and the corresponding transition pressures are 31.6, 47.4 and 57.0 GPa, respectively. Our results enrich the high pressure phases of BaFe₂S₃ and will stimulate future experiments to synthesize these new phases.

New Theoretical Insights into the Crystal-Field Splitting and Transition Mechanism for Nd3+-Doped Y3Al5O12

There has been considerable research interest paid to rare-earth transition-metal-doped Y₃Al₅O₁₂, which has great potential for application as a laser crystal of new-type laser devices because of its unique optoelectronic and photophysical properties. Here, we present new research conducted on the structural evolution and crystal-field characteristics of a rare-earth Nd-doped Y₃Al₅O₁₂ laser crystal by using the CALYPSO structure search method and our newly developed WEPMD method. A novel cage-like structure with a Nd³⁺ concentration of 4.16% is uncovered, which belongs to the standardized C₂₂₂ space group. Our results indicate that the impurity Nd³⁺ ions are likely to substitute the Y³⁺ at the central site of the host Y₃Al₅O₁₂ crystal lattice. The laser emission ⁴F_3/2 → ⁴I_11/2 occurring at 1077 nm is in accord with that of the experimental data. By introducing the proper correlation crystal field, three transitions, ⁴G_5/2 → ⁴I_9/2, ⁴F_7/2 → ⁴I_9/2, and ⁴S_3/2 → ⁴I_9/2, are predicted to be good candidates for laser action. These findings can provide powerful guidelines for further experiments of rare-earth-metal-doped laser crystals.

Computational discovery and characterization of new B2O phases

We present computational discoveries of new structural phases of the B2O compound exhibiting novel bonding networks and electronic states at ambient and elevated pressures. Our advanced crystal structure searches in conjunction with density functional theory calculations have identified an orthorhombic phase of B2O that is energetically stable at ambient pressure and contains an intriguing bonding network of icosahedral B12 clusters bridged by oxygen atoms. As pressure increases above 1.9 GPa, a structural transformation takes the orthorhombic B2O into a pseudo-layered trigonal phase. We have performed extensive studies to investigate the evolution of chemical bonds and electronic states associated with the B12 icosahedral unit in the orthorhombic phase and the covalent B-O bonds in the trigonal phase. We have also examined the nature of the charge carriers and their coupling to the lattice vibrations in the newly identified B2O crystals. Interestingly, our results indicate that both B2O phases become superconducting at low temperatures, with transition temperatures of 6.4 K and 5.9 K, respectively, in the ambient and high-pressure phase. The present findings establish new B2O phases and characterize their structural and electronic properties, which offer insights and guidance for exploration toward further fundamental understanding and potential synthesis and application.

Probing the structures and electronic properties of anionic and neutral BiAun−1,0 (n = 2–20) clusters: a pyramid-like BiAu13 cluster

The geometric structures and electronic properties of bismuth-doped gold clusters, BiAu_n^−1,0 (n = 2–20), are studied via a combination of the Crystal structure AnaLYsis by Particle Swarm Optimization structure prediction software and the density functional theory approach.

NbB12−: a new member of half-sandwich type doped boron clusters with high stability

The global minimum structure of a NbB₁₂⁻ cluster of half-sandwich type.

Magnetism, stability and electronic properties of a novel one-dimensional infinite monatomic copper wire: a density functional study

In the present work, the structural, magnetic and electronic properties of a novel one-dimensional infinite monatomic copper wire (1D-IMCW) have been investigated using first-principles computational calculation.

Rare Helium-Bearing Compound FeO_{2}He Stabilized at Deep-Earth Conditions

There is compelling geochemical evidence for primordial helium trapped in Earth's lower mantle, but the origin and nature of the helium source remain elusive due to scarce knowledge on viable helium-bearing compounds that are extremely rare. Here we explore materials physics underlying this prominent challenge. Our structure searches in conjunction with first-principles energetic and thermodynamic calculations uncover a remarkable helium-bearing compound FeO_{2}He at high pressure-temperature conditions relevant to the core-mantle boundary. Calculated sound velocities consistent with seismic data validate FeO_{2}He as a feasible constituent in ultralow velocity zones at the lowermost mantle. These mutually corroborating findings establish the first and hitherto only helium-bearing compound viable at pertinent geophysical conditions, thus providing vital physics mechanisms and materials insights for elucidating the enigmatic helium reservoir in deep Earth.

Higher Strength and Ductility than Diamond: Nanotwinned Diamond/Cubic Boron Nitride Multilayer

Recently, the nanotwinned structure has attracted considerable attention because of unprecedented improvement in its mechanical properties, thermal stability, and other properties. Here, we introduce the nanotwinned structure between two superhard materials [diamond and cubic boron nitride (cBN)] and obtain a nanotwinned diamond/cBN multilayered material with ultrahigh strength and unprecedented ductility. Under continuous shear deformation, the stress and total energy in the material develop in a zigzag way because of atomic reconfiguration. Further research shows that atomic reconfiguration occurs preferentially in the cBN region, followed by that in the diamond region by partial slip, and finally occurs at the interface through alternate "exchange" of the positions of C and B atoms. This multilevel stress release model can account for the significant increase in the strain range and peak stress of nanotwinned materials. These results could provide available information for the design of superhard materials with multilevel resistance to plastic deformation.

The Structure and Properties of Magnesium-Phosphorus Compounds Under Pressure

Inspired by the emergence of compounds with novel structures and unique properties (i.e., superconductivity and hardness) under high pressure, we systematically explored a binary Mg-P system under pressure, combining first-principles calculations with structure prediction. Several stoichiometries (Mg₃ P, Mg₂ P, MgP, MgP₂ , and MgP₃ ) were predicted to be stable under pressure. Especially, the P-P bonding patterns are different in the P-rich compounds and the Mg-rich compounds: in the former, the P-P bonding patterns form P₂ , P₃ , quadrilateral units, P-P⋅⋅⋅P chains or disordered "graphene-like" sublattice, while in the latter, the P-P bonding patterns eventually evolve isolated P ions. The analysis of integrated-crystal orbital Hamilton populations reveals that the P-P interactions are mainly responsible for the structural stability. The P-rich compounds with stoichiometries of MgP, MgP₂ and MgP₃ exhibit superconductive behaviors, and these phases show T_c in the range of 4.3-20 K. Our study provides useful information for understanding the Mg-P binary compounds at high pressure.

A novel superhard tungsten nitride predicted by machine-learning accelerated crystal structure search

Transition metal nitrides have been suggested to have both high hardness and good thermal stability with large potential application value, but so far stable superhard transition metal nitrides have not been synthesized. Here, with our newly developed machine-learning accelerated crystal structure searching method, we designed a superhard tungsten nitride, h-WN₆, which can be synthesized at pressure around 65 GPa and quenchable to ambient pressure. This h-WN₆ is constructed with single-bonded armchair-like N₆ rings and presents ionic-like features, which can be formulated as W^2.4+N₆^2.4-. It has a band gap of 1.6 eV at 0 GPa and exhibits an abnormal gap broadening behavior under pressure. Excitingly, this h-WN₆ is found to be the hardest among transition metal nitrides known so far (Vickers hardness around 57 GPa) and also has a very high melting temperature (around 1,900 K). Additionally, the good gravimetric (3.1 kJ/g) and volumetric (28.0 kJ/cm³) energy densities make this nitrogen-rich compound a potential high-energy-density material. These predictions support the designing rules and may stimulate future experiments to synthesize superhard and high-energy-density material.

High-Pressure Evolution of Crystal Bonding Structures and Properties of FeOOH

Recent conflicting reports on the high-pressure structural evolution of iron oxide-hydroxide (FeOOH) offer starkly contrasting scenarios for the hydrogen and oxygen cycles in Earth's interior. Here we explore the crystal structures of FeOOH using an advanced search algorithm combined with first-principles calculations. Our results indicate a phase transition around 70 GPa from the known ε-FeOOH to a new pyrite-type FeOOH (P-FeOOH) phase, and the two phases remain nearly degenerate in an unusually large pressure range. These findings clarify and explain the experimentally observed structural evolution and extensive phase coexistence. Moreover, our structure search identifies a previously unknown monoclinic (M-FeOOH) phase that is energetically close to P-FeOOH at pressures near the core-mantle boundary. We further reveal that the high-pressure FeOOH phases exhibit remarkably distinct sound-velocity profiles, providing key material properties essential to interpreting seismic data and elucidating FeOOH's influence on geophysical and geochemical processes in deep Earth.

Unexpected stable phases of tungsten borides

Superhard materials are generally synthesized under high pressure, which makes them expensive. We discovered new hard and superhard tungsten borides at low pressure and even at zero pressure with interesting properties.

Theoretical research on novel orthorhombic tungsten dinitride from first principles calculations

Tungsten nitrides have been intensely studied for technological applications owing to their unique mechanical, chemical, and thermal properties. Combining first-principles calculations with an unbiased structural searching method (CALYPSO), we uncovered a novel orthorhombic structure with a space group Cmc2₁ as the thermodynamically most stable phase for tungsten dinitride (WN₂) between 46–113 GPa. The computed elastic constants and phonons reveal that the Cmc2₁-WN₂ structure is dynamically stable at atmospheric pressure. Moreover, hardness calculations indicate that this structure is likely to become a hard material. Our current results may stimulate further experimental work on synthesizing these technologically important materials and improve the understanding of the pressure-induced phase transitions of other transition-metal light-element compounds.

We uncovered a novel WN₂ structure (Cmc2₁, 46–113 GPa) which is dynamically stable and ultra-incompressible at atmospheric pressure.

Allotropes of tellurium from first-principles crystal structure prediction calculations under pressure

We investigated the allotropes of tellurium under hydrostatic pressure based on density functional theory calculations and crystal structure prediction methodology. Our calculated enthalpy-pressure and energy-volume curves unveil the transition sequence from the trigonal semiconducting phase, represented by the space group P3₁21 in the range of 0–6 GPa, to the body centered cubic structure, space group Im3̄m, stable at 28 GPa. In between, the calculations suggest a monoclinic structure, represented by the space group C2/m and stable at 6 GPa, and the β-Po type structure, space group R3̄m, stable at 10 GPa. The face-centered structure is found at pressure as high as 200 GPa. As the pressure is increased, the transition from the semiconducting phase to metallic phases is observed.

Through first-principles simulations, we suggest the phase stability of the allotropic transition sequence of tellurium from the trigonal structure up to the cubic structure.