Structures And Properties Of Non-hydrostatic Compressive Molecular Hydrogen And Twninned Boron Carbide

High pressure can significantly reduce the atomic distance and change the electronic orbital distribution,enriching the crystal structures and regulating the physical and chemical properties.The traditional high-pressure modulations usually exert isotropic hydrostatic pressure influence on the crystal.Nonhydrostatic high pressure,broadening the dimension of high-pressure research and increasing the stress parameters from one to six dimensions,breaks the limit of lattice symmetry under hydrostatic pressure and expands the structural deformation modes or phase transition paths.Previous research shows that the superhard insulating diamond can be transformed into metal and even superconductivity under nonhydrostatic pressure,which breaks the traditional understanding of the intrinsic properties of diamond.In addition,the introduction of defects will lead to the local strain of the crystal,triggering the internal stress field of the crystal and resulting in the equivalent high-pressure effects similar to the external pressure.The introduction of dense twin structures will enhance structural thermal stability of diamond and promote its Vickers hardness from 100 GPa to 200 GPa.Based on two new high-pressure tuning strategies–nonhydrostatic pressure and pressure-induced dense twinning method,we have selected two typical light-element covalent materials and conducted systematically studies on structural and physical properties of dense molecular hydrogen under nonhydrostatic pressure and superhard twin crystal boron carbide,considering the abundant structural and bonding prototypes of nonmetallic elements and compounds.1.Since Wigner and Huntington’s pioneering work 87 years ago,pressure-induced metallization of hydrogen has attracted immense interest and impelled advances of theoretical and computational methods,ultrahigh-pressure devices,and related measurement and characterization techniques.Later predictions by Ashcroft that metallic hydrogen may be a high-temperature superconductor further invigorated the study of this fascinating material and its enigmatic properties.Theoretical studies of metallization of solid hydrogen have focused on its formation under hydrostatic pressure.However,the ultrahigh pressures required introduce anisotropic stresses in the vicinity of the sample,and the effect of such anisotropic stresses on the compressed material has remained largely unexplored.It is therefore instructive to establish the influence of anisotropic stresses on the evolution of structural and electronic properties of molecular hydrogen under nonhydrostatic pressures with the goal of reaching very high T_c superconductivity in this unique material under accessible conditions in the laboratory.This paper reports a detailed study of metallization and superconductivity of solid molecular hydrogen under anisotropic stresses,i.e.,at non-hydrostatic pressures.Our first-principles stressstrain calculations establish that despite being a soft and plastic crystal at low pressures（and temperatures）solid hydrogen at megabar pressures can sustain considerable uniaxial compressive and shear stresses,and such anisotropic stresses can have major impact on physical properties.The resulting symmetry reduction of the crystal lattice and ensuing changes in electronic structure accelerate bandgap closure leading to metallization and enhanced electron-phonon coupling to produce superconductivity.In particular,we find that shear deformations are highly effective in inducing metallic and superconducting states at much reduced hydrostatic pressures.These phenomena are driven by robust underpinning mechanisms and are therefore expected to remain intact when the possible corrections by thermal and quantum effects are further considered.The findings highlight a hitherto unexplored mechanism for creating potential very high Tc superconductivity in dense hydrogen at accessible static compression conditions.The results also offer explanations for the enigmatic differences in reported experimental results over the past 30 years.Our results also have broad implications for exploring similar phenomena in hydrogen-rich compounds and molecular crystals.2.Superhard material is an important component of high-pressure generator and high temperature and high pressure is an important means to synthesize superhard material.High pressure induced densed twinning is an effective strategy to improve the mechanical properties and structural stability of superhard materials and to design superhard materials with excellent comprehensive properties.The boron-rich solids are a prominent class of hard materials that possess excellent mechanical properties,but their crystal structures contain complex cage structure units causing more complexity and computational cost in design of these structures than simple atomic unit crystals.Here,we develop a structure design method for twin structures with complex structural units and report on an intriguing crystal twinning induced structural stabilization mechanism that optimizes the relation of local bonding symmetry and global stacking symmetry in a broad class of icosahedral boronrich solids,which host complex multi-atom cages and chains as the basic structural building units.Remarkably,a symmetry guided optimization procedure produces twinning structures that possess energies below that of the single crystal,which become progressively lower with rising defect（twinning）density.This behavior runs counter to the common wisdom that structural defects would raise the crystal energy.Such unusual behaviors stem from a twinning-mediated release of native strains inherent in the complex bonding network,and this phenomenon is widely present in a variety of icosahedral boron-rich solids,such as B₄C,B₆O,B₆N,and B₁₃CN,that are conducive to hosting multiple energetically close densely twinned crystal domains.We further identified a broader variety of twinned boron-rich crystals that exhibit energies only slightly above that of the single crystal.These findings showcase a robust twinninginduced crystal stabilization mechanism among a prominent class of complex covalent bonding crystals,creating a new paradigm that enriches fundamental structurestability relation for defects in crystalline solids.