Theoretical Investigations On The Structures And Properties Of Novel Carbon And Boron Materials

Boron（B）is next to carbon（C）in the periodic table.Both B and C have extremely light atomic mass and can form complex and diverse structures with themselves or other elements,resulting in unique and excellent properties.The carbon-based materials represented by two-dimensional（2D）carbon isomers have shown excellent application potential in many fields with their unique properties,prompting us to continuously explore new high-quality carbon isomers with excellent properties.In addition,boron compounds have become another research hotpot due to their various structures and excellent properties.Among them,the discovery of the coexistence of nontrivial topological properties and superconductivity in Mg B₂,arousing the enthusiasm of scientists to study the coexistence of nontrivial topological properties and superconductivity of metal borides（MBs）.Searching new materials and exploring novel properties in materials is of great significance in establishing the correlation between structures and physical properties.Experimentation,high-throughput computing and machine learning can effectively help us accelerate the design and application of materials.Based on the above discussion,the specific research contents are as follows:（1）Biphenylene monolayer(C₄₆₈):a novel nonbenzenoid carbon allotrope with potential application as an anode material for high-performance sodium-ion batteriesAllotrope metal structures are composed of carbon as anode materials for metal-ion batteries is a hot research topic.In this work,the recent synthesized graphene allotrope,2D biphenylene,consisting of tetragonal,hexagonal,and octagonal carbon rings,was explored theoretically.Our first-principles calculations verified that 2D biphenylene not has the stability of dynamic,mechanics and thermodynamics but exhibits metallic feature.Its novel structure can provide multiple adsorption sites for Na ions,a fast charge–discharge rate（low Na migration barriers of<0.2 e V）and high theoretical capacity(1075.37 m Ahg^-1).These superior properties,combined with its carbon abundance and light mass,making the biphenylene monolayer a promising anode for sodium-ion batteries（SIBs）with high-performance.（2）Two-dimensional potassium borides with hidden kagome-like lattice:ideal topology,van Hove singularities and superconductivityThe coexistence of nontrivial topological properties and superconductivity which is in the material provides an opportunity to realize topological superconductivity and majorana zero modes.However,rare research has been done on ideal topological materials combined with superconductivity.In this work,by means of first-principles calculations,based on the globally minimum inverse sandwich configuration of K₂B₉^-cluster,we designed a K₂B₉monolayer structure with the inverse sandwich configuration,and determined that the structure is a 2D global minimum configuration with the same chemical formula.Then,based on the K₂B₉structure,we predicted three new 2D monolayers(KB₉,KB₁₈and B₉).We found that these four monolayer structures both with hidden kagome-like lattice,and both were phonon-mediated superconductors with transition temperatures（T_C）of 12.56 K,14.46 K,10.50 K,and 0.96K,respectively.Furthermore,K₂B₉and KB₁₈were topological nodal-line semimetals,while KB₉was a dirac semimetal.Their topology originated from the band inversion of p_x+p_yand p_zorbitals of boron atoms from the containing B₉layer.Interestingly,the nodal lines and dirac points coexist with van Hove singularities（v HSs）stemming from the hidden kagome-like lattice,which plays a key role in enhancing their T_C.Our research not only unveiled the orbital character of the B₉layer plays an essential role in kagome-like lattices of potassium borides,but also provided an opportunity to realize the coexistence of topological states,v HSs,and superconductivity.（3）Superconductivity of two-dimensional alkali metal and alkaline-earth metal borides with inverse sandwich motifInspired by the superconductivity of K₂B₉monolayer with an inverse sandwich configuration,we systematically investigated the stability and superconductivity of a series of 2D alkali（M=Li,Na,Rb,Cs）and alkaline-earth（M=Be,Mg,Ca,Sr,Ba）metal boride M₂B₉monolayers using first-principles calculations combined with Eliashberg functions.Through stability-screening calculations,we found that Li₂B₉is dynamically and thermally stable.Moreover,the Li₂B₉monolayer has the superconductivity,whose T_Cis as high as 26 K,much higher than the T_C（0.98-14.46 K）from a previous study of the K-B structures.This work,on the one hand enriches the 2D family of MBs,on the other hand,it provides a candidate material with inverse sandwich configuration and superconductivity.（4）Accuracy test of different machine learning force field featuresPredicting energies and forces using machine learning force field（MLFF）depends on accurate descriptions（features）of the chemical environment.Despite the numerous features proposed,there is a lack of controlled comparison among them for their universality and accuracy.In this work,we compared several commonly used feature types for their ability to describe physical systems.These different feature types include piecewise cosine functions,gaussian symmetric functions,multiple tensor potential（MTP）,spectral neighbor analysis potential（SNAP）,simplified smooth deep potential with chebyshev polynomials（dp1）and gaussian polynomials（dp2）,and atomic cluster expansion（ACE）.We evaluated the training root mean square error（RMSE）for the atomic group energy,total energy,and force using linear regression（LR）model.We applied these MLFF models to an amorphous sulfur system and carbon system,and the fitting results showed that MTP feature can yield the smallest RMSE results compared with other feature types for either sulfur system or carbon system in the disordered atomic configurations.Furthermore,for these two systems the MTP feature combined with LR model can reproduce similar quantities along its first-principles molecular dynamics（FPMD）trajectory.Our results are helpful in selecting the proper features for the MLFF development.Based on the above work,on the one hand,based on the structure of the biphenylene monolayer synthesized by experiments,we studied the properties of the biphenylene monolayer as anode material for SIBs,which enriched the properties of carbon allotrope-C₄₆₈,also provided a reference for the development of properties of carbon-based materials.On the other hand,five new 2D monolayer structures were designed and their properties were studied by means of high-throughput calculation,which not only enriched the structure library of MBs,but also provided ideas for obtaining novel structure development.Moreover,appropriate features have been selected for the MLFF development of sulfur and carbon systems,which will lay a foundation for our subsequent performance development of carbon and sulfur-based materials.