New carbon-nitrogen-oxygen compounds as high energy density materials

Molecular crystals are complex systems exhibiting various crystal structures,and accurately modeling the crystal structures is essential for understanding their physical behaviors under high pressure.Here,we perform an extensive structure search of ternary carbon-nitrogen-oxygen(CNO)compound under high pressure with the CALYPSO method and first principles calculations,and successfully identify three polymeric CNO compounds with Pbam,C2/m and I4m2symmetries under 100 GPa.More interestingly,these structures are also dynamically stable at ambient pressure,and are potential high energy density materials(HEDMs).The energy densities of Pbam,C2/m and I4m2 phases of CNO are about2.30 kJ/g,1.37 kJ/g and 2.70 kJ/g,respectively,with the decompositions of graphitic carbon and molecular carbon dioxide andα-N(molecular N_(2))at ambient pressure.The present results provide in-depth insights into the structural evolution and physical properties of CNO compounds under high pressures,which offer crucial insights for designs and syntheses of novel HEDMs.

Structure prediction, electronic, and mechanical properties of alkali metal MB_(12)(=Be, Mg, Ca, Sr) from first principles

Using the particle swarm optimization algorithm on structural search methods, we focus our crystal structures search on boron-rich alkali metal compounds of MB_(12)(M = Be, Mg, Ca, Sr) with simulation cell sizes of 1–2 formula units(f.u.)at 0 GPa. The structure, electronic, and mechanical properties of MB_(12) are obtained from the density functional theory using the plane-wave pseudopotential method within the generalized gradient approximations. The formation enthalpies of MB_(12) regarding to solid metal M and solid alpha-boron suggested the predicted structures can be synthesized except for BeB12. The calculated band structures show MB_(12)(M = Be, Mg, Ca, Sr) are all indirect semiconductors. All the calculated elastic constants of MB_(12) satisfy the the mechanical stable conditions. The mechanical parameters(i.e., bulk modulus,shear modulus, and Young’s modulus) are derived using the Voigt–Reuss–Hill method. The G/B ratios indicated that the MB_(12) should exhibit brittle behavior. In addition, the hardness, Debye temperature, universal anisotropic index, and the percentage of anisotropy in compression and shear are also discussed in detail. We hope our results can inspire further experimental study on these boron-rich alkali-metal compounds.

Two dimensional GeO_(2)/MoSi_(2)N_(4)van der Waals heterostructures with robust type-Ⅱ band alignment

Constructing two-dimensional(2D)van der Waals heterostructures(vdWHs)can expand the electronic and optoelectronic applications of 2D semiconductors.However,the work on the 2D vdWHs with robust band alignment is still scarce.Here,we employ a global structure search approach to construct the vdWHs with monolayer MoSi_(2)N_(4)and widebandgap GeO_(2).The studies show that the GeO_(2)/MoSi_(2)N_(4)vdWHs have the characteristics of direct structures with the band gap of 0.946 eV and typeII band alignment with GeO_(2)and MoSi_(2)N_(4)layers as the conduction band minimum(CBM)and valence band maximum(VBM),respectively.Also,the direct-to-indirect band gap transition can be achieved by applying biaxial strain.In particular,the 2D GeO_(2)/MoSi_(2)N_(4)vdWHs show a robust type-II band alignment under the effects of biaxial strain,interlayer distance and external electric field.The results provide a route to realize the robust type-II band alignment vdWHs,which is helpful for the implementation of optoelectronic nanodevices with stable characteristics.

Novel two-dimensional PdSe phase:A puckered material with excellent electronic and optical properties

By combining structural search and first-principles calculations,we predict a new stable twodimensional PdSe monolayer,and systematically investigate its structural,electronic and optical properties.The calculated formation enthalpy,phonon spectra and molecular dynamic simulations confirm that PdSe monolayer possesses excellent thermodynamic and dynamic stability.PdSe monolayer is a semiconductor with an indirect band gap of∼1.10 eV.The carrier transport of PdSe monolayer is dominated by hole and exhibits remarkable anisotropy due to the intrinsic structure anisotropy.The optical properties also show obvious anisotropic characteristic with considerable absorption coefficient and broad absorption from the visible to ultraviolet regions.Benefiting from these excellent physical properties,PdSe monolayer is expected to be a promising candidate as electronic and optoelectronic devices.

Prediction of stable BC_(3)N_(2)monolayer from first-principles calculations:Stoichiometry,crystal structure,electronic and adsorption properties

In this paper,a novel BC_(3)N_(2)monolayer has been found with a graphene-like structure using the developed particle swarm optimization algorithm in combination with ab initio calculations.The predicted structure meets the thermodynamical,dynamical,and mechanical stability requirements.Interestingly,the BC_(3)N_(2)plane shows a metallic character.Importantly,BC_(3)N_(2)has an in-plane stiffness comparable to that of graphene.We have also investigated the adsorption characteristics of CO_(2)on pristine monolayer and Mo functionalized monolayer using density functional theory.Subsequently,electronic structures of the interacting systems(CO_(2)molecule and substrates)have been preliminarily explored.The results show that Mo/BC_(3)N_(2)has a stronger adsorption capacity towards CO_(2)comparing with the pristine one,which can provide a reference for the further study of the CO_(2)reduction mechanism on the transition metal-functionalized surface as well as the new catalyst’s design.

Pushing the limit of thermal conductivity of MAX borides and MABs

The emergence of MAX borides as well as MAB phases attracted great attention because of the renewable developments of ternary ceramics and offering great opportunities in potential applications.However,the number of borides remains limited,and further fundamental descriptions and detailed investigations on various properties are still lacking.In this report,we employ an integrated computational scheme that combines density functional theory with the evolutional algorithm to search for the favorable structures of P-and S-glued ternary borides terminated by Nb metal.We discover that the structures of 212-type,as e.g.Nb_(2)PB_(2)and Nb_(2)SB_(2),belong to the P6m2 space group,while those of 211-type,as e.g.Nb_(2)PB and Nb_(2)SB,prefer to crystallize in the P6_(3)/mmc space group,and the corresponding carbides Nb_(2)PC and Nb_(2)SC are also considered for the sake of completeness and comparative analsys.The predicted Nb_(2)PB_(2),Nb_(2)PB,Nb_(2)SB,Nb_(2)PC and Nb_(2)SC are energetically stable,as revealed by the negative formation energies and by the proposed reaction paths with respect to the most competing phases,as well as dynamically stable,as suggested by the non-imaginary phonon spectra.The thermal conductivities of the six materials show unusual behaviors,particularly for the acoustic and optical contributions,and are accompanied by a strong anisotropy.Most importantly,Nb_(2)PB_(2) is found to be an excellent thermal conductor with a total thermal conductivity of~65 W/(m K),while Nb_(2)SC is found to be an ultra-low thermal conductor,with a total thermal conductivity of~5 W/(m K).These values are clearly outside the currently reported range of thermal conductivities,which makes Nb_(2)PB_(2)and Nb_(2)SC extremely interesting for fundamental research as well as prospective applications with the aid of artificial tunings on the almost independent MB block and the A layer.The discovery of these novel materials is expected to contribute substantially to the rapid development of ternary ceramics and to accelerate attempts in the applicability of MAX phases for heat conduction.

Enhancing crystal structure prediction by decomposition and evolution schemes based on graph theory

Crystal structure prediction algorithms have become powerful tools for materials discovery in recent years, however, they are usually limited to relatively small systems. The main challenge is that the number of local minima grows exponentially with the system size. In this work, we proposed two crossover-mutation schemes based on graph theory to accelerate the evolutionary structure searching by automatic decomposition methods. These schemes can detect molecules or clusters inside periodic networks using quotient graphs for crystals, and the decomposition can dramatically reduce the searching space. Sufficient examples for test, including the high-pressure phases of methane, ammonia, MgAl2O4 and boron, show that these new evolution schemes can significantly improve the success rate and searching efficiency compared with the standard method in both isolated and extended systems.

Two-dimensional Pt_(2)P_(3)monolayer:A promising bifunctional electrocatalyst with different active sites for hydrogen evolution and CO_(2)reduction

Green hydrogen production and CO_(2) fixation have been identified as the fundamental techniques for sustainable economy.The open challenge is to develop high performance catalysts for hydrogen evolution reaction(HER)and CO_(2) electroreduction(CO_(2)ER)to valuable chemicals.Under such context,this work reported computational efforts to design promising electrocatalyst for HER and CO_(2)ER based on the swarm-intelligence algorithm.Among the family of transition-metal phosphides(TMPs),Pt_(2)P_(3) monolayer has been identified as excellent bifunctional catalysts due to high stability,excellent conductivity and superior catalytic performance.Different from typical d-block catalysts,p-band center presented by P atoms within Pt_(2)P_(3) monolayer plays the essential role for its reactivity towards HER and CO_(2)ER,underlining the key value of p-electrons in advanced catalyst design and thus providing a promising strategy to further develop novel catalysts made of p-block elements for various energy applications.