Creeplike Stress Response In New Light-Element Strong Covalent Solids

Materials that exhibit superior mechanical strength and toughness hold great importance in science,technology,and industrial applications.Diamond and cubic boron nitride(c-BN)are well-known traditional superhard materials with outstanding abilities to resist structural deformations.They have made great contributions to the development of scientific research and technological innovation,and have greatly broadened the scope of application in related fields.On search for new materials over the past few decades,a lot of previous studies have focused on the exploration of light elements(B,C,N)and their compounds.These elements are capable to form strong covalent bonds with short bond length,low ionicity and high bond density,which is also a necessary condition for superhard materials.In addition,these materials provide a low-cost alternative to traditional superhard materials like diamond and c-BN that require high-temperature and high-pressure synthesis conditions.Therefore,light elements and their compounds become the primary candidates for new superhard materials.Among them,carbon and boron have attracted much attention because of their unique bonding ability.The former exhibits extremely versatile bonding abilities that produce a rich variety of crystalline forms with novel properties in sp³-,sp²-,and sp-hybridized bonding states,which stimulated the discoveries of a series of carbon allotropes.The latter with the coexistence of two-center and three-center bonding configurations plays a central role in compensating for the electron deficiency of the B₁₂ clusters in boron and boron-rich compounds,leading to striking nonmetallic characteristics and structural diversity.The most notable characteristics of these two types of materials are their diverse structures and excellent mechanical properties.The discoveries of these two materials play a vital role in enriching the types of materials and promoting the progress of materials science.In the past few decades,extensive theoretical and experimental explorations have made vigorous efforts to design and synthesize advanced materials that showcase improved and more versatile mechanical characteristics.However,a wide variety of materials including strong covalent solids transition-metal carbide,nitride,and boride are highly anisotropic in their mechanical properties with directionally dependent stress responses that induce undesirable structural changes or even failure,which may complicate or impede their performances.The occurrence of graphitization under large tensile and shear loading conditions is associated with a bond-breaking process that is synonymous with these superhard covalent materials.Under increasing strains,the stress responses usually rise steeply and then drop abruptly past the peak stress that determines the limiting mechanical strength,which also defines the corresponding brittle structural failure modes without any appreciable plastic deformation in this class of materials.Materials serving under extreme loading conditions are required to be both strong and tough to withstand high stresses at large strains without developing catastrophic failures.And materials that exhibit mechanical characters with the ability of undergoing substantial plastic deformation before fracturing are highly desired and in great demand.In view of the development status of superhard materials and existing problems in the field,we choose the two strong covalent light elements,boron(B)and carbon(C),as the research object,and put forward the following viewpoints:(i)We proposed that the reduced symmetry of the bonding arrangements containing multiple inequivalent atomic sites and bond types in materials will lead to the isotropic characteristics and the more gradual and gentle decrease of stresses past the peak values without the precipitous drop that normally occurs in strong covalent crystals like diamond and c-BN that contain more directional bonding configurations.Such distinct mechanisms extend deformation range past peak stress and enhance ductility,thereby toughening such superstrong covalent crystal.(ii)Boron and boron-rich compounds possess rich structural features and outstanding mechanical properties due to their fascinating and intricate bonding structures containing icosahedral B₁₂ cages as structural units.However,the complex charge distribution in the B₁₂ structural unit is an important factor affecting the mechanical properties of this type of material.We use element replacement to realize charge compensating to ensure the boron-rich compounds is in the"electronically accurate"state,so as to realize the improvement of material mechanical response from"brittle fracture"to"toughness enhancement".The structure-property relation is one of the cornerstones in condensed-matter and materials physics,and it is fundamentally important in exploring,understanding and predicting material properties and in designing and tailoring new material systems.To explore the mechanical responses at equilibrium and extreme conditions,and to design a series of superstrong covalent crystals that possess favorable bonding structures that balance strength and ductility,making such materials more durable and versatile for wide-ranging applications.In this paper,we select two typical materials--carbon allotropes with low bond symmetry and"electronically accurate"ternary boron-rich compounds,and perform mechanical calculations at equilibrium and extreme conditions.The quest for such materials requires an in-depth understanding of structure-strength relations under diverse structural deformation modes to determine atomistic mechanisms that can guide further development and potential application.Based on this,this thesis mainly conducts the following two parts of research:?.We have chosen a recently predicted complex orthogonal carbon structure[C.Y.He et al.,Phys.Rev.Lett.121,175701(2018)]as a prototype to showcase the distinct mechanical properties associated with intriguing structural features in the crystal.The Pbam-32 carbon possesses unique bonding configurations containing alternating fivefold,sixfold,and sevenfold carbon rings,forming a three-dimensional sp³network that exhibits a reduced directional bonding environment compared to diamond and c-BN,which meets the requirements of our expectation.We perform systematic computational studies of Pbam-32 carbon crystal using first-principles calculations to examine the elastic moduli near equilibrium and to determine the stress-strain relations under a variety of tensile and shear deformations,obtaining stress responses and associated structural deformation modes at large strains past the peak stress until the occurrence of bonding structural failure or transformation,which provides guidance for theoretical design and experimental synthesis of new materials with excellent mechanical properties.The current results show two important points:(i)The elastic moduli of Pbam-32 carbon are even slightly higher than the results for diamond obtained from the same computational procedure,despite that its density is lower than that of diamond by about 5%.(ii)The intriguing deformation modes with distinct sequential bond weakening and breaking mechanisms at large strains impede or even suppress the graphitization process commonly seen in traditional superstrong solids like diamond and c-BN.Such distinct mechanisms extend deformation range past peak stress and enhance ductility,thereby toughening this superstrong carbon crystal.The present findings advance the fundamental knowledge of structural and mechanical properties of the Pbam-32 carbon allotrope,and these results have broad implications for elucidating the stress-strain relations and the atomistic toughening mechanisms that offer guidance for rational design and optimization of an important class of prominent superstrong covalent crystals.?.In the present work,we report findings on boron-rich compounds of B₁₃CN and B₁₃C₂,which are isostructural but differ in electron fillings,with the former being electron precise and the latter electron deficient.We have performed systematic first-principles studies to examine structural and stress responses over a large range of strains of these two compounds under a variety of loading conditions.Surprisingly,we find that the subtle differences in bonding states of these two isostructural compounds lead to significantly divergent and contrasting stress responses and mechanical properties.(i)B₁₃CN exhibits a creeplike deformation pattern with a gentle descent trend throughout the entire tensile deformation range.(ii)B₁₃CN possesses markedly higher ideal strengths than B₁₃C₂ under most strain conditions.A close analysis of the stress responses and associated structural deformation modes show that the enhanced icosahedral stability by the extra electron provided by the N atom leads to the more pronounced mechanical stability of B₁₃CN,while the icosahedral premature distortion and collapse due to its electron-deficient nature in the bonding network leads to the earlier mechanical instability of B₁₃C₂.And the hardness of B₁₃CN calculated using the hardness model formula is 41.0 GPa,indicating that it is a potential superhard material.Such knowledge offer insights into atomistic processes associated with the contrasting behaviors in B₁₃CN and B₁₃C₂atomistic processes associated with the contrasting behaviors in B₁₃CN and B₁₃C₂over diverse strain conditions.These findings may also help elucidate fundamental mechanisms in a broad class of boron-rich compounds possessing B₁₂ icosahedra as structural units and prove useful for rational design of additional boron-rich compounds with outstanding properties.