Theoretical Study On The Structures And Electronic Properties Of Vanadium Fluoride And Tungsten Fluoride At High Pressure

Pressure is one of the most important parameters that we use to study the structure and properties of matter.It can effectively shorten the distance between atoms,lead to the overlap of adjacent electron orbitals,change the electron spin,modulate the interaction between electronic structures and atoms,induce new phenomena and effects that cannot occur under conventional conditions,and produce high-pressure phases with new structures and properties.High-pressure studies can deepen the understanding of the structure,properties and change laws of materials,and are important for the design of new functional materials and the understanding of planetary structures.With the rapid development of computer technology,theoretical calculations have become an important part of accelerated scientific research.Theoretically,simulating the physical and chemical properties of materials can effectively shorten the experimental cycle and accelerate the research and development of new materials.It has become an important tool for scientists to design new materials and has greatly contributed to the progress of scientific research and industry.Vanadium（V）and tungsten（W）have very active chemical properties due to their partial occupation of d orbitals,and they readily react with the electronegative fluorine（F）to form a variety of compounds.In addition,some of the V-F compounds and W-F compounds both exhibit complex structural phase transitions at high pressures.Further studies on the phase stability under high pressure can help to gain insight into the chemical reaction properties,structure and performance of transition metal fluorides.In this paper,binary V-F and W-F systems at high pressures are systematically investigated based on first-principles calculations and the CALYPSO structure search method,as follows.1.using CALYPSO software,the high-pressure phases of V-F were systematically searched at 0-100 GPa,and new components of VF and VF₂ with dimeric structures were found.Two new high-pressure phases of VF₅ with space group symmetry C222₁ and P-1are proposed,and rare decahedral structural units appear in these two new phases.A new high-pressure phase transition sequence of VF₅ is determined:the Pnma phase VF₅ at atmospheric pressure transforms into the C222₁ phase at 32 GPa and finally into the P-1phase at 53.71 GPa.Calculations of the electronic structure revealed that the predicted stable VF₅ high-pressure phases are all semiconductors,in which all the valence electrons of the transition metal vanadium atoms are used for bonding,combining with the F atoms through ionic bonds.2.The high-pressure phases of W-F were searched systematically at 0-300 GPa,and two new high-pressure phases of WF₆ with space group symmetry of P2₁/m and P-1 were found,and decahedral structural units appeared in these two new phases.The high-pressure phase transition sequence of WF₆ was determined:the Pnma phase WF₆ at atmospheric pressure transitions to P2₁/m at 20.29 GPa and finally to P-1 phase at 155.8GPa.In addition,the P-1 phase WF₄ and P-1 phase WF₅,which are stable at high pressure,are also found.The electronic structure of the W-F compound was then investigated and it was found that the predicted stable WF₆ high-pressure phases are all semiconductors,in which the bonds between tungsten and fluorine atoms are ionic,with the tungsten atom providing all the valence electrons.