Since the discovery of the Seebeck effect and the Peltier effect in the 19th century,thermoelectric materials have entered people's field of vision,attracting the exploration of countless researchers.So far,many thermoelectric materials have been discovered,which have their own advantages and can be applied to different occasions.However,all thermoelectric materials have problem of thermoelectric conversion efficiency.It is very important for researchers to improve the figure of merit.The environmentally friendly silicide thermoelectric material SrSi2 is limited in application due to the low figure of merit at room temperature.For thermoelectric material SrSi2,many literatures have pointed out that doping can improve its thermoelectric properties.Based on these,we investigated the structural,electronic and optical properties of Sr1-xMxSi2?M=Ca and Ba??x=0,0.25,0.5,0.75 and 1?by the first-principles calculation in the framework of density-function theory.First,we calculated the lattice constant,bond length,Mulliken population,energy band,density of states and some optical constants?including dielectric function,absorption coefficient,reflectivity,and loss function?of pure SrSi2 and Sr1-xCaxSi2?x=0.25,0.5,0.75 and 1?.We found the lattice constant of Sr1-xCaxSi2 decreases and the crystal structure are compressed evidently with the increasing Ca.It is reasonable that doped Ca atoms cause a positive chemical pressure in the SrSi2 system.There was a structural transition for Sr0.5Ca0.5Si2 from cubic to tetragonal when the doping concentration is 0.5.And the total energy of the tetragonal phase is lower than that of the cubic phase,which suggesting the tetragonal structure is more stable.In addition,the calculation of the band structure shows that SrSi2 is a semiconductor with an indirect band gap.The band gap of Sr1-xCaxSi2 decreases with the increasing doping concentration.The band gap disappears when the doping concentration is over 0.5.That is to say,substitution with Ca atoms causes SrSi2 to be converted from a semiconductor to a metal.Resistivity is related to the energy gap value,so the resistivity is reduced with the decreased band gap,and the figure of merit is correspondingly improved.In addition,we calculated the static dielectric constant of pure SrSi2 and Sr1-xCaxSi2.Our results show that the static dielectric constant has a large increase after doping with Ca atoms,which indicates that Sr1-xCaxSi2 is a good dielectric material.Secondly,we calculated the lattice constant,bond length,Mulliken population,energy band,density of states and optical constants of Ba atoms doped SrSi2.The lattice constant of Sr1-xBaxSi2 increases with the increasing doping concentration,a negative chemical pressure is generated in the system and crystal structure has obvious expansion.This result is completely opposite to the case of doping Ca atoms,because the radius of the Ca atoms is smaller than the Sr atoms and the radius of the Ba atoms is larger than the Sr atoms.Similar to the Sr1-xCaxSi2,Sr0.5Ba0.5Si2 has a transition from cubic to tetragonal structure when the doping concentration is 0.5,and the total energy of the tetragonal phase is lower.We calculated the energy band of Sr1-xBaxSi2 and found that the energy gap of Sr1-xBaxSi2 is larger than that of pure SrSi2,which means that the doping of Ba atoms enhances the semiconductor properties of SrSi2,and this result is accompanied by the reduction in the high temperature electrical resistivity.Therefore,the thermoelectric performance will be increased.Doping Ba atoms also causes a great change in the optical properties of SrSi2.The static dielectric constant increases first and then decreases with the increase of Ba concentration.Doping can change the electronic band structure in the vicinity of the Fermi level of the material,which has the opportunity to improve the thermoelectric properties of the material.Therefore,studying the structural and electronic properties of the thermoelectric material can provide theoretical guidance for finding thermoelectric materials with excellent performance. |