First-principles Study Of Alkali Metal Borosilicide Compounds Under High Pressure

In recent years,researchers have witnessed tremendous advances of new phonon-mediated superconductors(from SH₃to La H₁₀to C-S-H system)and achieved high superconducting critical temperatures(T_c)close to room temperature,which have attracted wide attention.In numerous of superconductors,cage hydrides have attracted the attention of researchers because of their unique crystal structure(hydrogen atoms formed the host framework)and high superconducting critical temperature under high pressure,such as Ca H₆,Mg H₆,Sc H₆,YH₆,La H₁₀,etc.Among them,the La H₁₀compound(space group is Fm-3m)theoretically predicted that when the pressure reached about 200 GPa,the T_cvalue could reach 260 K,which was later confirmed by experiments.However,these rich hydrogen cages can only be stable under high pressure.Once the pressure is released,none of them are stable,and will be decomposed into classic stoichiometry hydrides and hydrogen molecules.Compared with cage like hydrides,some metal silicide clathrates[such as Ba₈Si₄₆,(Na,Ba)_xSi₄₆,Ba₈Al₆Si₄₀,etc.]are also superconductors.They can be synthesized and exist stably at atmospheric pressure,but their superconducting transition temperature is low.With the development of computer science and physical chemistry theory,theoretical simulation provides a good way for crystal structure design with good physical properties.In conclusion,this paper selected alkali-metal elements(Na,K,Rb,Cs),B and Si as the research objects,and systematically explored the crystal structure and physical properties of alkali-metal borosilicate compounds under high pressure by substitution and crystal structure prediction method.The innovative results are as follows:1.We chose VII-type cage with sodalite-like structure as the prototype and B-Si consist of"host"cage which was formed by replacing part of Si atoms with B atoms,and different alkali-metal were doped as"guest"to form AB₃Si₃(A=Na,K,Rb,Cs)cage.It was found that KB₃Si₃and Rb B₃Si₃are stable within the pressure range of 6.7-36.8 GPa and 7.3-34.9 GPa,respectively;while the enthalpies of Na B₃Si₃and Cs B₃Si₃are positive within the pressure range of 0-100 GPa,indicating that their thermodynamic properties are unstable,which means that they can't be synthesized experimentally.In order to further study their dynamic stability,phonon calculations were carried out of theses phases at atmospheric pressure and 10 GPa,respectively.The results show that only Rb B₃Si₃and Cs B₃Si₃were stable at ambient pressure and high pressure,while Na B₃Si₃and KB₃Si₃have partial imaginary frequency modes throughout the Brillouin Zone,which indicates that their dynamic properties were unstable.In order to get a better understanding of the phase stability,a DFT-chemical pressure(CP)analysis was performed.It was concluded that the sizes of Rb and Cs atoms are more suitable for B-Si cage,while the sizes of Na and K atoms are too small to match the cage,so only Rb B₃Si₃and Cs B₃Si₃are stable.It can better explain the structure of sodalite with different alkali-metal and B-Si composition.Furthermore,through the calculation of electron-phonon coupling and electron local function of Rb B₃Si₃,the results show that the T_cvalue of Rb B₃Si₃at ambient pressure is 14 K.This is mainly due to the fact that vibration between B-Si is beneficial to the electron-phonon coupling,and the strong sp³covalent bond between B-Si is also found.Finally,the elastic modulus,tensile and shear strength along different directions of AB₃Si₃(A=Na,K,Rb,Cs)and their mechanical properties under atmospheric pressure were calculated.The Vickers hardness values of these structures were 7.5-8.5 GPa,11.6-15.5 GPa,11.8-16.1 GPa and 11.2-14.8 GPa,respectively.This work provides an important theoretical support for the study of alkali-metal borosilicate with cage structure.2.CALYPSO crystal structure prediction method was applied to systematically explore ternary alkali-metal borosilicate compounds Cs B₃Si₃,Rb_xB₃Si₃(x=1,2,3),KB₃Si₃and Na B₃Si₃at 0-100 GPa.From the perspective of energy,we have found14 new structures and an analogous sodalite-like clathrate which have been studied by substitution have been reproduced.The calculation results of enthalpy of formation show that the thermodynamic properties of the new structures are stable within a certain pressure range,further analysis of lattice dynamics stability show that all the structures of Rb₂B₃Si₃compounds are stable at the corresponding pressure range,except P1 phase throughout the Brillouin Zone.The calculated results of electronic band structure and density of states show that only three structures of KB₃Si₃compound(space group:R-3m,R-3,P2₁2₁2₁)and Cm structure of Cs B₃Si₃compound are indirect band gap semiconductors,and other compounds are all metals.