First-principles Study Of Novel Ca-X(X=Si,Ge) Intermetallic Compounds

With the development of science and technology,elements such as Ga,As,Cd,In,Te and Pb have excellent performance and are widely used in electronic devices.However,most of these elements are toxic,pollute the environment,and the reserves on the earth are very limited.The alkaline earth metal compound is composed of environmentally friendly elements such as Ca,Mg,Si,etc.,and is rich in resources,which solves the above problems well and becomes a hot spot of current research.In this paper,based on the density functional theory,we conduct a first-principles study on the novel Ca-X（X=Si,Ge）intermetallic compounds,predict the possible stable crystal structure,and explore the electronic structure and potential applications.We first studied the Ca₂Ge compounds of the Ca-Ge system and found four possible Ca₂Ge phases through structural search.By calculating the ground state static enthalpy and phonon dispersion curve,we confirmed that the cubic phase Fm-3m Ca₂Ge is stable in a certain pressure range.We studied the electronic structure of cubic phase Fm-3m Ca₂Ge.The results show that it is a direct bandgap semiconductor.The calculated band gap of GGA-PBE is 0.582 eV,and the band gap obtained by HSE06 is 1.133 eV.Using Boltz Tra P in combination with theVASP algorithm,we studied the thermoelectric properties.The calculation results show that the ZT value of the intrinsic cubic phase Fm-3m Ca₂Ge reaches its maximum value of 0.076 at 1000 K.When the chemical potential is μm=-0.301 eV,the corresponding maximum ZT value is 0.547 at1000 K,indicating that the thermoelectric performance can be optimized at a high temperature by appropriate p-type doping.Through the structural search,low-lying criteria and stability analysis,potential new electronic compounds appearing in the Ca-Si system were explored.We first predicted that Ca₃Si,a space group with P6₃/mmc,I4/mcm,and C2/c structures,is a potential electronic compound.The ground state static enthalpy and phonon dispersion curves show that the hexagonal phase P6₃/mmc phase is the most stable structure under the large positive pressure range of 13.5～104GPa.Based on the band structure,partial charge density,and the electron localization function around the Fermi level,the three predicted Ca₃Si structures have anion electrons confined within the one-dimensional channel.Among them,the P6₃/mmc Ca₃Si is a one-dimensional electron compound,and the I4/mcm and C2/c Ca₃Si is a quasi-one-dimensional electron compound.Their chemical formulas can all be expressed as[Ca₃Si]²⁺:2e^-,which have potential applications in the fields of electronics and catalysis.The hexagonal phase Ca₂Si has a distinct layered structure.Inspired by the two-dimensional material,we stripped the hexagonal phase Ca₂Si to obtain a two-dimensional Ca₂Si crystal structure.The band structure shows that two-dimensional Ca₂Si is an indirect bandgap semiconductor,and the band gap values calculated by GGA-PBE and HSE06 are 0.38 eV and0.72 eV,respectively.The carrier mobility calculations show that the electron carrier mobility in the x and y directions is much larger than their hole carrier mobility,showing anisotropic characteristics.This high carrier mobility indicates that the two-dimensional Ca₂Si single layer has a good application prospect in the fie ld of high-efficiency solar cells.