Search

We propose BiCrX3 as a new family for hosting skyrmions. The intrinsic large SOC and inversion asymmetry lead to large DMI. The isolated skyrmions and skyrmion lattices emerge, and are robust within wide magnetic field and temperature range.

Herein, first-principles calculations were carried out to investigate the mechanism of metal-ion, such as K, Li, Na, Mg and Al, adsorption and diffusion on the SnCo (0 0 1) surface. Our results demonstrated that K, Li, and Na are more readily adsorbed on the pristine SnCo (0 0 1) surface than Mg and Al. Based on the calculations of migration barriers for metal atom on SnCo (0 0 1) surface, we identified the most energetically favorable metal atom diffusion pathway. The influence of vacancies on the K and Li storage capabilition of the SnCo (0 0 1) surface was also investigated. Our calculations show that Sn vacancies (VSn) tend to improve Li adsorption, but have no effect on K adsorption. Unfortunately, the Co vacancy (VCo) tends to inhibit the adsorption of Li and K on the SnCo (0 0 1) surface.

Transition-metal aluminium nitrides widely used as protective tool coatings are a class of materials with a combination of high hardness, outstanding wear resistance as well as good chemical stability. In this work, through a well developed structure searching method, the ground-state phase of V0.5Al0.5N is verified and systematically studied on its mechanical and electronic properties by comparing with Ti0.5Al0.5N and W0.5Al0.5N via first-principles calculations. Our results show that the ground-state phase of V0.5Al0.5N adopts a hexagonal structure of P63/mmc symmetry. Mechanical property studies demonstrate the hexagonal phase has a surprisingly improved hardness of about 38 GPa and enhanced ideal strengths relative to its well-known metastable cubic B1 phase whose hardness is only about 20 GPa. This mechanical enhancement greatly expands the upper limit of the strength and hardness for this type of Al-containing ternary systems. Meanwhile, detailed analysis on strength and elastic anisotropy indicates it also exhibits much better mechanical isotropy. Underlying mechanism of the mechanical enhancement is explored by the electronic analysis in-depth. The position of the EF is tuned by the introduction of the Al element and this electronic tuning leads to a metallic-to-semiconductor transformation from B1 to the hexagonal phase and the strengthening of the bonds between the metal elements and the N atoms.

There is an urgent need to develop phosphors with high quantum efficiencies (QEs) since white light-emitting diodes (WLEDs) have emerged as a new generation of illumination materials. Utilizing energy transfer to improve the absorption of activators and adjust the emission colors of samples is one effective strategy. Here, color-tunable phosphors of the form CaAl4O7:Ce,Tb were synthesized aiming at efficient energy transfer from Ce3+ to Tb3+. Since Tb3+ can be suitably sensitized by Ce3+, the co-doped phosphors can be effectively excited by near-ultraviolet (NUV) light. The internal QE of CaAl4O7:0.04Ce,0.04Tb under 350 nm excitation is as high as 92.55%, and the external QE is 71.02%. A WLED fabricated from BAM:Eu2+, CaAl4O7:0.04Ce,0.04Tb, and CaAlSiN3:Eu2+ with a 365 nm LED chip exhibited a correlated color temperature (CCT) of 4706 K and a color rendering index (CRI, Ra) of 81.44. The energy transfer mechanism and thermal stability of the phosphor were also investigated. The results provide an effective approach for developing highly efficient green-emitting phosphors for NUV WLEDs.

A new tetragonal carbon allotrope (named T-C64) is predicted by swarm structural searches combined with first principles calculation. It contains 64 carbon atoms in a Tetragonal unit cell with I41/amd symmetry and exhibits distinct topologies including C28 cages. This new carbon phase has an sp2-sp3 network with calculated hardness of 68.2 GPa. In order to examine the stability of T-C64 under ambient pressure, we calculated the properties of elastic constant and phonon spectrum. In addition, by calculating the electronic properties of the crystal, it is concluded that T-C64 is an indirect band gap semiconductor with a band gap of 2.23 eV.

High-throughput first principles calculations for 109 carbon allotropes were performed. The elastic constants and phonon calculations suggest that these new structures are mechanically and dynamically stable at ambient pressure. Seven direct band gap semiconductor carbon allotropes were uncovered. The Vickers hardness of all seven structures exceeds 40 GPa, indicating that these allotropes are potential superhard materials.

A new type of ternary monocarbides M0.5Re0.5C (M = V, Nb, Ta) with outstanding mechanical properties were firstly reported by introducing the Re into their well-known hard parent compounds MC to tune the valence electron concentration (VEC). By an efficient structure searching method, a universal orthorhombic structure with space group of Cmc21 has been uncovered and demonstrated to be the ground-state phase of these three monocarbides, which can be sustained up to the ultrahigh pressure of 300 GPa for V0.5Re0.5C and Nb0.5Re0.5C. The thermodynamic and dynamical stabilities of the predicted phases were confirmed by the formation enthalpy and phonon spectra calculations. The structural, mechanical as well as electronic properties of M0.5Re0.5C in comparison with their parent compounds MC were thus fully investigated using first-principles density functional theory (DFT) calculations. Compared to their parent compounds MC, results on the mechanical calculations indicated that they not only possess larger Vickers hardness and ideal indentation shear strength values but also exhibit much smaller elastic and ideal strength anisotropy which are advantageous to their potential applications. Detailed analysis of electronic structures reveals that the origin of the improved mechanical properties of M0.5Re0.5C is mainly attributed to the enhanced hybridization of metals-d and C-2p orbitals by locating the fermi level at the valley of the pseudogap.

Refractory tungsten silicides have been identified as typical high-temperature structural materials with excellent mechanical and thermal properties in various industrial applications. Here, we show the first extensive crystal structures search of W–Si system over a wide range of stoichiometries at ambient condition, using first-principles swarm intelligence structure search methods. Besides the well-known W5Si3 and WSi2, three novel and unexpected stoichiometries of W2Si, WSi, and W3Si4 with tetragonal ground-state structures were firstly uncovered and suggested to be experimentally synthesizable according to the convex hull calculations. It is found that the high silicon content benefits the formation of the puckered double W–Si zigzag covalent chains in WmSin, resulting in the enhanced resistance to shear deformation related to the macroscopic hardness. Results on the ideal tensile and shear strengths calculations indicated that the failure modes in the studied WmSin are mainly dominated by the shear type along the (110) [ 1 1 ‾ 0 ] directions. Further analysis of atomic shear deformation demonstrates that the collapse of Si–W building blocks is responsible for the lattice stability and achievable mechanical strength of WmSin.

High-pressure structure transition of nontoxic all-inorganic MHP CsSnBr3 was fully explored up to 15 GPa using an advanced structure search technique CALYPSO combined with first-principles calculations. Besides the known orthorhombic Pnma ground state phase, two high-pressure semiconducting Cmcm and P21/m phases of CsSnBr3 were firstly uncovered above 2.37 and 6.8 GPa, respectively. Both phase transitions of the Pnma → Cmcm at 2.37 GPa and Cmcm → P21/m at 6.8 GPa were characterized as first order with a volume reduction of 4.7% and 10.8%. The occurrences of high-pressure Cmcm and P21/m phases follow the enhanced distortions of Sn–Br polyhedrons and increased coordination of Sn atoms from 6 to 8 at elevated pressures. Compared to the direct band gap of the ambient-pressure Pnma phase, the Cmcm and P21/m phases exhibit a larger indirect band gap of 2.347 and 3.143 eV, respectively, originating from the movement away from the Fermi level of conduction bands driven by the twisting of Sn–Br polyhedrons under pressure. The light absorption performances of two high-pressure phases in comparison with the Pnma phase were studied by the calculated optical absorption coefficients.

Using first-principles calculations, a new type of the full-Heusler alloy, namly, Ag2ScGe, with XA configuration, is proposed. The unit cell of the Ag2ScGe alloy consists of 16 atoms, and it is found that there may be two different phases: XA and L21. To determine the stable configuration of the Ag2ScGe alloy, we calculated the formation energy, energy curve, and phonon spectrum of the two configurations of the Ag2ScGe alloy and finally determined the new stable structure, i.e. the XA configuration of the Ag2ScGe alloy. Calculations of the mechanical properties and electronic properties show that this new structure exhibits the properties of a ductile and metallic material with elastic anisotropy. In addition, by calculating the optical properties of this new alloy, it is concluded that Ag2ScGe alloy is a good absorber in the visible light region.

Unbiased structure searching methods and first-principles density functional theory (DFT) calculations were employed to explore new stable crystal structures of helium azide under high pressure. Five new phases of HeN3 were discovered, namely, C2/m-I, P21/c, R-3c, R-3m and C2/m-II. The study of formation enthalpy of these structures reveals that the C2/m-II phase is the most energetically favorable structure with a pressure in the range of 40–98 GPa, while the R-3m phase is the most stable one with a pressure between 98–300 GPa. Electronic structure calculations from DFT demonstrate that all five newly predicted phases of HeN3 shows semiconducting characteristics. Among them, the R-3m phase under the pressure of 300 GPa has the largest band gap of 5.4 eV. Strong covalent bonding and sp3 hybridization among nitrogen atoms in the structures are revealed by a detailed analysis of electronic localization function distributions and Bader atoms-in-molecules method.

Transition metal nitrides have been widely used in many scientific and technical areas because of their unique physical and mechanical properties. We report two new nitrogen-rich transition metal nitrides, MoN6 and ReN6, by crystal structure searching technique. Under high pressure, MoN6 will undergo phase transition (from R-3m to Pm-3 phase) at 54 GPa, and ReN6 always keep the R-3m phase in the pressure range from 50 to 100 GPa. There are benzene-like six-membered “N6” rings with nitrogen single bonds in the R-3m phase structures, indicating that MoN6 and ReN6 are expected to be the high-energy-density materials.

Titanium pernitride (TiN2) with CuAl2-type structure (I4/mcm, Z = 4), the first reported member to the family of early transition metal pernitride was recently synthesized via direct chemical reactions between titanium mononitride and nitrogen molecules at 73 GPa and 2400 K. Using an unbiased structure searching method combined with first principles calculations, we here have fully investigated the energy landscape of MN2 (M = Ti, Zr, and Hf) at high pressure up to 100 GPa and suggested that the synthesized I4/mcm structure is a metastable form for TiN2 at ambient conditions. A novel tetragonal I4/mmm (Z = 4) structure composed of the MN6 octahedrons connected by peculiar double N N bonded dimers, was identified as the universal thermodynamic ground-state structure for three MN2. Under high pressure, the I4/mmm structure undergoes a first-order phase transition to the discovered I4/mcm structure with a large volume drop, accompanying the increase of coordination and oxidation number of metal M atoms in MN2. The configuration of M+2 [N2]−2 for MN2 in the I4/mmm structure was demonstrated by electronic structure and chemical bonding analyses. The mechanical properties including elastic parameters and elastic anisotropy behaviors of MN2 within I4/mmm structure were systematically studied. Results on the elastic anisotropy and ideal tensile strengths indicate that the (001) planes may be viewed as their cleavage planes.

Motivated by successful synthesis of the first early transition metal dinitride TiN2 under high pressure, the possible crystal structures of another family member vanadium dinitride (VN2) have been systematically explored up to 300 GPa by using a well-developed CALYPSO (Crystal structure Analysis by Particle Swarm Optimization) method for crystal structural prediction. Three theoretical phases with P63/mmc, Cmc21, and I4/mcm space group have been determined at different pressures. And their dynamical and mechanical stabilities have been confirmed by the phonon dispersion and elastic constants calculations. Among three predicted phase, the tetragonal I4/mcm-VN2 exhibits highly improved Young's, bulk and shear moduli relative to the traditional fcc-VN, indicating its potential mechanical applications. In addition, the elastic anisotropy of each phase was fully investigated by the dependence of Young's modulus on the crystal orientation. To further study the mechanical property, the ideal tensile, shear, and compressive strengths are calculated. Our results imply the tetragonal phase of VN2 is not an intrinsic superhard material. The electronic structures are analyzed to study the chemical bonding nature so as to confirm the driving force of the enhancement of the mechanical property.

Six new silicon phases with direct band gaps were found through silicon atomic substitution of carbon in the known carbon structures via high-throughput calculations. The six newly discovered Si phases are in the space groups of Im3[combining macron]m, C2/c, I4/mcm, I4/mmm, P21/m, and P4/mbm, respectively. Their crystal structures, stabilities, mechanical properties, elastic anisotropy, and electronic and optical properties were systematically studied using first-principles density functional theory calculations. All the new phases were proved to be thermodynamically and mechanically stable at ambient pressure. The direct band gap values in the range of 0.658-1.470 eV and the excellent optoelectronic properties of these six Si allotropes suggest that they are promising photovoltaic materials compared to diamond silicon.

Based on structure prediction method, the machine learning method is used instead of the density functional theory (DFT) method to predict the material properties, thereby accelerating the material search process. In this paper, we established a data set of carbon materials by high-throughput calculation with available carbon structures obtained from the Samara Carbon Allotrope Database. We then trained a machine learning (ML) model that specifically predicts the elastic modulus (bulk modulus, shear modulus, and the Young’s modulus) and confirmed that the accuracy is better than that of AFLOW-ML in predicting the elastic modulus of a carbon allotrope. We further combined our ML model with the CALYPSO code to search for new carbon structures with a high Young’s modulus. A new carbon allotrope not included in the Samara Carbon Allotrope Database, named Cmcm-C24, which exhibits a hardness greater than 80 GPa, was firstly revealed. The Cmcm-C24 phase was identified as a semiconductor with a direct bandgap. The structural stability, elastic modulus, and electronic properties of the new carbon allotrope were systematically studied, and the obtained results demonstrate the feasibility of ML methods accelerating the material search process.

Silicon is a preferred material in solar cells, and most of silicon allotropes have an indirect band gap. Therefore, it is important to find new direct band gap silicon. In the present work, a new direct band gap silicon allotrope of o-Si32 is discovered. The elastic constants, elastic anisotropy, phonon spectra, and electronic structure of o-Si32 are obtained using first-principles calculations. The results show that o-Si32 is mechanically and dynamically stable and is a direct semiconductor material with a band gap of 1.261 eV.

Searching for superhard metallic carbon materials has always been an enduring topic in scientific research. In this work, based on first-principles calculations combined with RG2 method, three new sp2–sp3 hybridized superhard metallic carbon phases were proposed, namely, Fddd-C96, Ccca-C32, and Ibam-C48. The structural, mechanical, and electronic properties of these phases are systematically studied. The results show that they are dynamically, thermally, and mechanically stable. The hardness of Fddd-C96, Ccca-C32, and Ibam-C48 are 77, 69, and 43 GPa, respectively. Electronic properties show that all three materials are metallic, the origin of metallicity mainly comes from sp2 hybridization. To compare the properties of carbon materials with high superhardness and low energy, a P parameter was defined. We find that only two carbon allotropes in the well-known Samara Carbon Allotrope Database have higher P parameters than Fddd-C96. The method of P parameter can quickly screen out materials that meet expectations from a large amount of data based on the characteristics of the materials. These three structures are expected to become candidate materials for high-pressure electronic devices.

A new metallic superhard carbon allotrope C5 with five carbon atoms per unit cell is theoretically predicted to be stable at ambient pressure through first-principles calculations combined with unbiased swarm structure searching techniques. This novel carbon allotrope consisting of a mixture of sp2 and sp3 carbon network exhibits excellent mechanical properties with claimed hardness of 58.5 GPa. Results from our calculated strain-stress relationship reveals that the failure mode of crystal lattice for C5 is dominated by shear type in the (010)[101] direction with shear stress magnitude of 74.3 GPa. The calculated electronic band structure of C5 suggests its metallic nature. Detailed analyses of band decomposed charge density show that the metallic nature is contributed by the sp2 carbon atoms, which form conducting pathways along the a- and b-axes.

First-principles calculations combined with particle swarm optimisation technique are employed to provide an in-depth understanding of the structures, stabilities, electronic properties, and hardne...

The effects of scandium (Sc) solute addition on the structural stability, mechanical strength, and electronic structure of magnesium-lithium (Mg-Li) ordered alloy were comprehensively studied by first-principles calculations. Two kinds of MgLi-Sc substitutional alloys (Mg-substituted Mg8−xLi8Scx and Li-substituted Mg8Li8−xScx) hold cubic lattice symmetry and become more stable with the increase in Sc content. The mechanical properties, including single crystal elastic constants, polycrystalline elastic moduli, and elastic anisotropy behaviors, have been systematically studied for two considered MgLi-Sc substitutional alloys. It was found that Mg8Li8−xScx exhibits larger elastic moduli than those of Mg8−xLi8Scx with the same Sc content; however, the derived elastic moduli of two substitutional alloys showed nonlinear variations as a function of Sc addition. Calculated stress–strain curves show an increasing of ideal tensile strengths and critical strains of the MgLi-Sc substitutional alloys along the and directions with Sc addition, indicating that the cubic MgLi-Sc substitutional alloy is mechanically harder and less brittle at higher Sc content. Furthermore, the developments of the electronic structures of two types of the studied alloys with increasing Sc addition were also investigated by the density of states and electronic localization function calculations.

An investigation of the mechanical, electronic, and optical properties of the recently reported material K4 phosphorus was made in this work. K4 phosphorus has been proved to be mechanically and dynamically stable up to 7 GPa under hydrostatic pressure. We compared the elastic anisotropy, average acoustic velocity, and Debye temperature of K4 phosphorus at 0 and 7 GPa. The ideal tensile properties at large strains of K4 phosphorus were also examined, with the results showing that it would cleave under the tensile strength of 8.5 GPa with the strain of 0.3. In addition, the effect of tensile strain and pressure on optical properties and band gap were studied.

Tungsten borides with excellent mechanical properties have been recognized as a class of ultrahard compounds in various industrial applications. Here, motivated by the recent experimental work, a quantitative comparison analysis on the structure, mechanical strength, and electronic structure of the tantalum-strengthened superhard W 0.5 Ta 0.5 B monoboride and its parent material WB has been studied by first-principles calculations. Excellent agreements of the calculated lattice parameters and simulated X-ray diffraction between present results and experimental data have confirmed the crystal structure of the synthesized W 0.5 Ta 0.5 B. Compared to the WB, the calculated stress-strain curves show an enhanced shear strength and improved ductility of W 0.5 Ta 0.5 B on (100) and (010) crystal planes, originating from the reduction of antibonding states between the W- e g states which enables strenuous sliding of metal bilayer in W 0.5 Ta 0.5 B. Furthermore, the lattice instability of W 0.5 Ta 0.5 B under large shear strain with an intriguing sequential bond-breaking mode that is derived from the first breaking of zigzag B chains and the subsequent collapsing of WB 7 and TaB 7 polyhedrons by simultaneously breaking of B–W and B–Ta bonds. These findings shed strong light on the strengthening mechanism of W 0.5 Ta 0.5 B and the design for novel ultra-incompressible and superhard solids in transition metal monoborides.

A novel orthorhombic oI16 structure, formed by strong covalent sp2-sp3 B C and B O bonds, was identified as to be the thermodynamic ground-state phase for B2CO compound by using an unbiased structure searching method. The energy of this new oI16 phase is considerably lower than all those of previously proposed candidates, and it is dynamically stable at atmosphere pressure through the phonon calculations. The evidences of the indirect semiconducting and chemical bonding nature are manifested by the electronic structure calculations and AIM analyses. The pronounced elastic anisotropy of oI16 structure has been revealed by the calculated distributions of elastic moduli along different crystal orientations. The calculated stress-strain relations of oI16 structure disclose its common hard nature but with an extended ductility.

A new superhard carbon orthorhombic allotrope oC20 is proposed, which exhibits distinct topologies including C4, C3 and two types of C6 carbon rings. The calculated elastic constants and phonon spectra reveal that oC20 is mechanically and dynamically stable at ambient pressure. The calculated electronic band structure of oC20 shows that it is an indirect band gap semiconductor with a band gap of 4.46 eV. The Vickers hardness of oC20 is 75 GPa. The calculated tensile and shear strength indicate that the weakest tensile strength is 64 GPa and the weakest shear strength is 48 GPa, which means oC20 is a potential superhard material.

Based on the first-principles calculations, the strain effects on the optical properties of direct band gap silicon crystal D135-Si were systemically investigated. The stress-strain relations and phonon spectra show that the realistic peak tensile strengths in three principle symmetry crystallographic directions [100], [010] and [001] are 4.5, 9.7 and 12.1 GPa, respectively. By imposing the pressure and strain on structure, we studied the direct-indirect band gap transitions and obtained the ranges of the direct band gap, namely 0–4 GPa on pressure, −0.07–0, −0.04–0.08, and −0.01–0.03 on strain along a-, b- and c-direction, respectively. The imaginary part of the dielectric function was calculated to analyze the optical absorption property, which shows the strong adsorption coefficients in the visible range of the sunlight. The effect of pressure on the optical absorption property of structure is the smallest, and the strain effects are gradually enhanced along c-, b- and a-direction, respectively.

The pressure-induced phase transitions of MoAlB have been fully studied to provide insights into the typical MAB phases under high pressure. A novel high-pressure monoclinic structure (C2/m, Z = 4) was firstly identified for the MoAlB above 234 GPa, using the first principles crystal structure prediction approach. First-order structure transition character of MoAlB from the ambient-pressure orthorhombic Cmcm phase to this high-pressure monoclinic C2/m phase was demonstrated by a volume drop of 1.28%. Inspections of crystal structures showed that the occurrence of C2/m phase follows the slip in the metallic Al-Al layers and the fracture of linear zigzag B-B chains in the Cmcm phase under compression, confirming the earlier reports. Our argument has been also supported by the softening of elastic constant under high pressure. Phonon curves calculations suggest the high-pressure C2/m phase can be quenchable to ambient pressure. The electronic and mechanical properties of this novel C2/m phase have thus been systematically investigated.