Theoretical Study On The Structures And Properties Of Ternary Li-Au-X(X=P,I,F) Compounds At High Pressure

Pressure,one of basic thermodynamic variables,can effectively change the chemical attributes of elements and free energy of matters,thus inducing phase transition and the stabilization of compounds with unconventional stoichiometries.Besides,high-pressure compounds usually have unusual structures,bonding patterns,and properties,playing an important role in promoting fundamental research and material application.Compared that high pressure synthesis is demanding and costly for experimental equipment,theoretical calculations can estimate the synthesis conditions at a lower cost and speed up the experimental synthesis of compounds,exhibiting great advantage and guiding significance in high pressure science research.Gold（Au）is a very old but vibrant charming element.The strong relativistic effects cause the Au5d orbital to stretch and the Au6s orbital to shrink,which makes that Auhas considerable inertness under environmental conditions,but high chemical acitivity under certain conditions,leading to the formation of various unexpected compounds.For example,at high pressure,Aucan form Li-rich aurides with Li that has stong electropositivity,in which Auhas a large negative oxidation states up to-3 and acts as a 6p element.On the other hand,Au,reacting with F under pressure,can achieve up to+4 and+6 valences in AuF₄ and AuF₆,respectively,which become potential strong oxidants.In ternary Aucompounds,it has achieved the stabilization of Au-H bonding and superconducting transition of the compounds,originating from the synergistic charge transfer among different elements.These findings not only refresh our knowledge about Au,but also stimulate us to explore ternary Aucompounds under high pressure.In this work,with the aid of first-priciples swarm structure calculations,by introducing different p-block elements into Li-Ausystem,we explored the potential ternary Aucompounds stabilized by pressure and synergistic charge transfer,focusing on their stabilizing mechanism,structure,and electronic properties.A series of ternary Aucompounds with novel structures and bonding patterns,extended Aunegative oxidation states,and superconductivity are found.The main research contents and results are as follows.Li₅AuP₂ is identified to contain Auwith sp³ hybridization,which is a semiconductor and has a large second-order nonlinear optical response.Phosphorus（P）,with moderate electronegativity and the highest negative oxidation state of-3,can form abundant phosphides and diverse P motifs.When P is introduced into the reaction of Li and Auat high pressure,multiple Li-Au-P compounds are identified to be stable and have distinct electronic properties.In Li₅AuP₂,the charge transfer from Li to P and Aumakes Au6p orbitals activate and hybridize with the 6s one,generating Ausp³ hybridization.Meanwhile,Auis covalently bonded with sp³-hybridized P atoms,leading to the structural features with tetrahedral units（AuP₄,AuLi₄,Li P₄ and blende-like Li-P units）and noncentrosymmetry.The formation of Au-P covalent bonds and lone pairs of P atoms makes the constituent atoms satisfy the octet rule,rendering Li₅AuP₂ an indirect-gap semiconductor.Moreover,Li₅AuP₂ is estimated to have a large second harmonic generation susceptibility in the near-infrared region.Metallic Li-Au-P compounds are estimated to exhibit phonon-modulated superconductivity,which is associated with covalent Au-P network.Except Li₅AuP₂,other high-pressure Li-Au-P compounds are metals.The Li-richest composition is up to Li₁₀AuP,which is an electride.The increasing Li content makes the electron occupation in Au6p orbitals increase,reaching the highest negative oxidation state of more than-3 in Li₁₀AuP.For metallic Li-Au-P compounds containing distinct covalent Au-P motifs,electron-phonon coulpling calculations indicate that C2/m Li₄AuP₃ has the highest superconducting transition temperatures,whose superconductivity is dominated by quasi-planar AuP₃ covalent grids.By comparison of two inequivalent P atoms in Li₄AuP₃,the P atom that forms stronger covalent bonds with Aumakes a larger contribution to its superconductivity.The increase of pressure can induce the disappearance of phonon softening and the decrease of the density of states at the Fermi level,causing Li₄AuP₃ being non-superconductive.In contrast with semiconducting Li-Au-F compounds,Li-Au-I compounds are metallic.F is more conductive to modulate Auatomic charge than I,originating that F induces Auto coordinate with more Li atoms.Halogen can accept only one electron at ambient pressure.When I and F with different electronegativities are introduced into Li-Ausystem under high pressure,there are tendency to stablize Li-rich Li_nAuX（X=I and F,n≥6）.In these compounds,Auand I/F atoms are surrounded with Li atoms,impeding the bonding between Auand I/F atoms,leading to their hypercoordination and the high negative oxidation states of Auatoms.Among them,Li₁₀AuI and Li₁₀AuF are electrides,in which the negative oxidation states of Auatoms reach more than-4 and-5,respectively,extending the understanding of Au’s electron reserve ability.In contrast to metallic Li_nAuI,Li₆AuF and Li₁₀AuF are indirect-gap semiconductors,indicating that the introduction of F realize the nonmetalization of Li-Aucompounds.The physical origin of structural stability,interstitial electron and Aucharge transfer differences in ternary Aucompounds induced by the three p-block elements is explored.For Li-Au-X systems,with X from P,I to F,the stable compositions gradually reduce,which is mainly attributed to the decreasing number of electrons accepted by element X at high pressure.By comparison of the three electrides Li₁₀AuX（X=P,I,and F）,X can effectively modulate the coordination configuration of Auatoms and interstitial sites as well as their obtained electron.More interestingly,Li₁₀AuP and Li₁₀AuI can be viewed as electron-deficient electrides with respect to Li₁₀AuF.Furthermore,the energy level difference of P 3d,I 5d and F 3s with respect to Au6p orbitals at high pressure is the internal mechanism of charge regulation of different X elements.The increase of electron gained by Auatoms and interstitial sites also induce a transition of Li₁₀AuX from metal,weak metal to semiconductor.Our work enriches ternary Aucompounds with novel structures and bonding patterns,and achieves the coexistence of electride nature with Au’s high negative oxidation state,thus being of great significance to deepen the understanding of high-pressure electride and Au’s attribute.