Theoretical Study On The Structures And Properties Of Fluorine-rich Transition Metal Fluorides At High Pressures

The preparation of fluorine-rich transition metal fluorides is one of the most challenging and attractive topics in the field of chemistry and condensed matter physics,mainly due to their unique physical and chemical properties.On the one hand,the extreme electronegativity of fluorine endows the central metal atom with a high oxidation state,enabling a deeper understanding of the reaction behavior of the element.On the other hand,fluorine-rich transition metal fluorides usually exhibit intrinsic and large oxidation potential and can be used as strong oxidating and fluorinating agents,promoting the synthesis of new materials.Up to now,the highest fluorine stoichiometry is 7.The only example is ReF7.Pressure,as a basic thermodynamic variable,has become an effective method to obtain novel materials.In general,pressure has unique advantages in stabilizing unconventional compositions,inducing structural phase transitions,acquiring new materials,broadening chemical reaction types,and tuning the electronic structure of materials.This is mainly attributed to the fact that pressure can effectively overcome the reaction barrier,rearrange the atomic orbital energy level,shorten the interatomic distance,and tune the chemical properties of elements,etc.Compared with the high pressure experiment,first-principles structural prediction method can find stable compounds under high pressure with lower cost and shorter time,which plays an important role in the research and development of new materials.Therefore,based on the first-principles structure search method,this thesis focuses on exploring the transition metal fluorides under high pressure for the novel compositions,oxidation states,as well as physical and chemical properties.The main contents and results are as follows:Gold?Au?is a well-known fascinating element exhibiting unusual physical and chemical properties.The strong relativistic effect of Au induces the expansion of 5d orbital,which gives rise to a higher reactivity of the Au 5d electrons.On the other hand,theoretical calculation predicts that AuF6 molecule has extremely high oxidizing power.Once AuF6 can be successfully synthesized,it will become the strongest oxidant thus far.Our study found that Au can be fluorinated to form AuF6 molecular crystal under the pressure of 5 GPa,in which Au shows the oxidation state of+6,breaking through the highest oxidation state that Au has achieved.Further compression can stabilize another new composition of gold fluoride,AuF4,filling the intermediate+4 oxidation state of Au.Both AuF4 and AuF6 molecules show metallicity.The calculation also shows that AuF6 has extremely high electron affinity and is expected to be a strong oxidant.In addition,the congeners of Au,silver?Ag?and copper?Cu?,can also stabilize their more F-rich compositions,AgF4 and CuF4,under higher pressure.Compared with Ag and Cu,the possibility of Au to achieve its tetrafluoride or even hexafluoride under lower pressure than Ag and Cu is mainly attributed to its strong relativistic effect.Iridium?Ir?is able to engage all its 9 valence electrons in bonding in its oxide,but the most fluorine-rich stoichiometry of Ir at ambient pressure is only IrF6.This thesis predicted that Ir can stabilize its octafluoride,IrF8 molecular crystal,above 39GPa.This is the first example of stable neutral transition metal octafluoride.IrF8molecular crystal undergoes phase transitions and the spatial symmetry of the basic building block?IrF8 molecule?in IrF8 phases gradually increases with pressure?e.g.,dodecahedron?square antiprism?quasi-cube?.Ir?5d⁷6s²?loses eight valence electrons bonding with fluorine,leaving only one valence electron to form an open-shell configuration,so that all the three IrF8 phases show metallic characters.More interestingly,the predicted electron affinities of the predicted IrF8 phases are larger than IrF6 and comparable to PtF6,and the high-pressure phase?R-3?even exceeds PtF6.Considering the diverse d electron configuration of transition metal elements and inspired by the above results,this thesis explores the possibility of the 5^th and 6^th row elements containing more than six valence electrons(the 4^th row elements are excluded due to their small atomic radii)to form more F-rich compounds.Three new F-rich compounds?TcF7,OsF8 and CdF3?were found.Specifically,osmium?Os?can form stable octafluoride under lower pressure?4 GPa?than Ir,in which Os also shows+8 oxidation state.Unlike IrF8,OsF8 is semiconducting.The original unoccupied Os6p levels are brought down in energy in OsF8 by the combined effects of pressure and a strong ligand field.The valence expansion of Os is found to strengthen the Os-F bonds while allowing for ligand-to-metal F 2p?Os 6p charge transfer,which decreases the overall bond polarity.The necessity of occupying M-F antibonding levels in octafluorides where the metal valence electron counts exceed 8 explains a lower stability of IrF8,and the instability of PtF8 and several other compounds.The results in this thesis show that pressure can effectively regulate the formation of F-rich transition metal fluorides and extend the cognition of oxidation state and fluorination limit of transition metals.