Theoretical Design On Structures Of Unusual Stoichiometric Compounds At High Pressure

As well known,materials under high pressure are compressed to a denser packing mode,accompanying with shortening interatomic distance and changing crystal structure and electronic properties,which are prone to obtaining materials with novel properties and unusual stoichiometry.Hence,studying the pressure-induced change for materials comes to the topic in material,chemistry,and physics field,which is also considered as the promising candidate for obtaining major scientific breakthroughs.Theoretically,this paper systematically studied the chemical reactions of halogen elements iodine and fluorine,alkaline earth metal barium and fluorine,and transition metal osmium and oxygen by using structure prediction software CALYPSO combined with first principles.The obtained innovative results are as follows:1.Considering hypercoordinate iodine reagents are the promising candidates of metal-free environmentally benign catalysts.Thus,the preparation of novel hypercoordinate iodine compounds has been a longstanding objective.Up to now,the maximum coordinate number of iodine（I）is seven in electrically neutral iodine heptafluoride（IF₇）and eight in anionic octafluoride（IF₈^-）.Here,we bring forward pressure as a controllable method for realizing new hypercoordinate I via its reaction with F.First-principles swarm structure calculations disclose a neutral octafluoride compound（IF₈）consisting of eight-coordinated I.Interestingly,IF₈ adopts quasi-cube molecular configuration with R-3 symmetry,which is in sharp contrast to the square antiprismatic structure in IF₈^-.Its underlying mechanism can be attributed to more electrons occupying d_z² orbital of the central I atom with respect to IF₈^-.The unusual metallicity of IF₈ originates from the electron-deficient covalent bonds.Moreover,we reliably built the high-pressure phase diagram of I-F binary compounds.Iodide fluorides undergo complex structural phase transitions under high pressure,simultaneously accompanying semiconductor to metal transition.Our work provides a useful strategy for achieving hypercoodinate iodine compounds.2.The oxidation state of an element influences its chemical behavior of reactivity and bonding.Finding unusual oxidation state of elements is a theme of eternal pursuit.As labeled by an alkali-earth metal,barium（Ba）invariably exhibits an oxidation state of+2 by a loss of two 6s valence electrons while its inner 5p closed shell is known to remain intact.Here,we show through the reaction with fluorine（F）at high pressure that Ba exhibits hitherto unexpected high oxidation state greater than+2 in three pressure-stabilized F-rich compounds BaF₃,BaF₄ and BaF₅,where Ba takes on the role of a 5p element by opening up its inert 5p shell.Interestingly enough,these pressure stabilized Ba fluorides share common structural features of Ba-centered polyhedrons but exhibit a diverse variety of electronic properties showing semiconducting,metallic,and even magnetic behaviors.Our work modifies the traditional knowledge on the chemistry of alkali-earth Ba element established at ambient pressure,and highlights the major role of pressure played in tuning the oxidation state of elements.3.Osmium is the densest and stiffest metal among transition metals,having a very large bulk modulus comparable to that of diamond.Therefore,the structure of osmium under high pressure makes a small change with maintaining the P6₃/mmc phase.The special character is beneficial for obtaining functional materials relating light elements,such as peroxides.Here,we studied the chemical reactions between osmium and oxygen,and built the high-pressure phase diagram,and obtained two osmium peroxide OsO₃ and OsO₄,which has broadened the traditional applications of osmium at normal conditions.