Theoretical Studies On Prediction Of Structure And Physical Properties Of Cobalt Arsenide Under High Pressure

As a clean and effective means to adjust various physical and chemical properties of materials,pressure has been applied by researchers to find materials with various excellent properties.Arsenide,as an important member of topological superconductors,has attracted extensive attention because of its unique structural characteristics and excellent performance.Herein,based on the first-principles calculation method,combined with the crystal structure searching technology,the cobalt-arsenic(Co-As)compounds were systematically studied in the pressure range up to 100 GPa.The phase transition sequences of Co-As binary compounds with different chemical ratios are obtained,and the structural phase diagram of Co-As binary compounds is proposed for the first time.The structures at ambient pressure are consistent with the experiment structures.At high pressure,three new high-pressure phases are predicted:Co2As-Pnma,CoAs2-Pnnm and CoAs3-C2/m structure.In order to determine the stability of these three structures,we verified them from two aspects of dynamics and thermodynamics:(1)The phonon dispersion relation is calculated,and the results show that the phonons of the three structures have no imaginary frequency phenomenon in Brillouin zone,which indicates that the structures have dynamic stability;(2)According to the calculation results of enthalpy of formation,it is shown that the three structures are thermodynamically stable in the studied pressure range.In order to clarify the electronic properties and bonding behavior of the new structure under high pressure,we first calculated the electronic band and density of states(DOS)of the three high pressure phases.The electronic band results show that the conduction bands of the three phases all pass through fermi surface,which indicates that the three new high-pressure phases all have metal characteristics.The results of electron density of states show that in the structure of Co2As-Pnma,the fermi surface is mainly contributed by the d orbital of Co atom.However,in the CoAs2-Pnnm and CoAs3-C2/m structures,in the range of-2-2 eV,there is a strong hybridization between the d orbital of Co atom and the p orbital of As atom,and the vicinity of fermi surface is also contributed by both orbitals.In addition,it is worth noting that the pressure-induced electronic topological transition is found in the CoAs3-C2/m structure,which is not found in other structures.The calculated electronic local function results show that there are metal bonds and ionic bonds in all three new high-pressure phases,and there are polar covalent interactions between Co atoms and As atoms in CoAs2-Pnnm and CoAs3-C2/m structures.Through the calculation of elastic properties,the results show that all three phases have good ductility,and the Co2As-Pnma structure has high compression resistance and strong rigidity.We systematically studied the Co-As binary compound,put forward its structural phase diagram,described the electrical properties and bonding behavior of the new phase under high pressure in detail,and showed the excellent properties of Co-As compound under high pressure,so it has important research significance.