The Phase Transition And Mechanical Properties Of Transition-Metal Compounds

Superhard materials are widely used in many fields such as marine ships,aerospace,and blade molds with excellent properties such as high hardness,high strength,and corrosion resistance.However,as humans continue to explore the deep sea and other fields,traditional superhard materials such as diamond,cubic boron nitride,etc.due to their low heat resistance and poor chemical stability have greatly limited their application in engineering.Therefore,it is urgent to explore new multifunctional superhard materials.The transition-metals compounds,using transition metals to incorporate light elements,may become novel superhard materials with metallic,superconducting,and other versatility.We calculated based on the first-principles and VASP to target transition-metal boride TMB_3+x?TM=W,Mo?,TMB₁₂?TM=Sc,Y,Zr,Hf?and transition-metal oxide VO₂.It systematically studies the structural stability,mechanical behavior and phase transition properties.And explain its microscopic mechanism.Provide relevant theoretical basis for the research and synthesis of transition-metal compounds.?1?For TMB_3+x?TM=W,Mo?,We used the structure of TMB₃-hP16 to create1×2×3 and 2×2×1 supercells,adjust the occupied components of metal and boron for the positions of Wyckoff 2b and 2c,and construct a variety of TMB_3+x structure formation systems.Through first-principles calculations,We found that with boron gradually occupying the 2b position of the metal layer,its structural stability and mechanical properties are gradually enhanced at x?2,and at x=2,the mechanical properties are best.More importantly,we found that MoB₅,when its boron occupied the2b position 50%,became a new thermodynamic ground state phase in the Mo-B system.At the same time,we also analyzed its superhard mechanism.The boron portion occupies the 2b position in the metal layer,so that the boron between the boron layer and the newly occupied metal layer form a B-B covalent bond,increasing the incompressibility in the c direction,thereby enhancing its hardness.Thus,for the actual synthesis of TMB_3+x compounds,a reasonable ratio of metal and boron to the original material provides a theoretical basis for achieving optimal mechanical properties.?2?Transition dodecaborides(TMB₁₂)is currently raising great expectations for multifunctional materials,but their refined structures have never been fully resolved,which severely limits the understanding of structure-property relationships.Here,we report that the tI26 structure is thermodynamic ground states of TMB₁₂?TM=Sc,Y,Zr,and Hf?whereas the widely perceived cF52 structure is,in fact,metastable phases of ScB₁₂ and YB₁₂ and high-temperature phases of ZrB₁₂ and HfB₁₂.Among them,ZrB₁₂and HfB₁₂ undergo phase transition under the induction of about 350 K and 450 K,respectively.In both crystal structures,a prominent feature is that the boron atoms are linked into a rigid three-dimensional?3D?network with the metal atoms in large cuboctahedral cages.It is the uniqueness of these configurations that achieves the unusual functionalities,i.e.,the coexistence of high hardness,low density,and good electrical conductivity,and is expected to be applied to machining tools and lightweight protective coatings.Furthermore,we elucidate the electronic origins of their relative stability and mechanical properties.These findings resolve the longstanding structural puzzle of this family of TMB₁₂ and provide crucial insights into the underlying nature of their remarkable properties.?3?Metal-insulator transition underlies many remarkable and technologically important phenomena in VO₂.Even though its monoclinic structure had before been the reserve of the insulating state,Several experiments recently observed an unexpected monoclinic metallic state.Here,we use a modified approach combining first-principles calculations with orbital-biased potentials to reproduce the correct stability ordering and electronic structure of different phases of VO₂.We identify a ferromagnetic monoclinic metal that is likely to be the experimentally observed mysterious metastable state.Furthermore,our calculations show that an isostructural insulator-metal electronic transition is followed by the lattice distortion from the monoclinic structure to the rutile structure.These results not only explain the experimental observations of the monoclinic metallic state and the decoupled structural and electronic transitions of VO₂,but also provide a understanding for the metal-insulator transition in other strongly correlated d electron systems.