XF₃ (X=Be, Mg, Ca, Sr, Ba), Ti₂CoTl_1-xPb_{x And TiHflrZ (Z = Al, Ga, In) First-principles Studies}

Heusler alloys have simple structures,multiple constituent elements and a variety of excellent physical properties,such as ferromagnetic(FM)property,half metallic(HM)property,shape memory and thermoelectric properties.It was discovered by German chemist F.Heusler in 1903 and named after it.The HM properties of the Heusler alloys are mainly reflected in that they have band gap in one spin channel,while the electrons pass through the Fermi level(EF)in the other spin channel.Therefore,the spin polarization of electrons at EF can be as high as 100%.The HM property also exist in metallic oxides,perovskites,double perovskites,dilute magnetic semiconductors and zinc-blende and wurtzite structural compounds.Because of their high Curie temperature and large magnetic moment,Heusler alloys stand out among the above materials and have a broad application prospect in the field of spintronics and magnetoelectronics.In this paper,the first-principles calculations method were used to study the structural,electronic,magnetic and mechanical properties of the d0 binary Heusler alloys XF3(X=Be,Mg,Ca,Sr,Ba),full Heusler alloys Ti2CoTl1-xPbx(x=0.00,0.25,0.50,0.75,1.00)and quaternary Heusler alloys TiHflrZ(Z=Al,Ga,In).The conclusions are as follows:(1)The first-principles calculations within generalized gradient approximation(GGA)method are employed to study the structural,electronic,magnetic and mechanical properties of the d0 binary Heusler alloys XF3(X=Be,Mg,Ca,Sr,Ba).They have integer magnetic moments per formula unit(f.u.)1 ?B/f.u.,and the most stable state is FM state.They are thermodynamic stability because the formation energies are negative and the cohesion energies are positive.The elastic constants Cij satisfy the mechanical stability conditions.Their HM properties are robust under the effect of hydrostatic strain and tetragonal strain.(2)We investigate the structural,magnetic,electronic and mechanical properties of the full Heusler alloys Ti2CoTl1-xPbx(x=0.00,0.25,0.50,0.75,1.00)by using first-principles calculations.The Ti2CoTl1-xPbx have the HM characters within the lattice constant regions of 5.799?6.707,5.749?6.821,5.749?6.982,5.789?7.115 and 5.976?6.943 (?) for x=0.00,0.25,0.50,0.75 and 1.00,respectively.The negative formation energy,positive cohesion energy and higher than room temperature Curie temperature Tc indicate that the Ti2CoTl1-xPbx are thermodynamic stable and can be used in the field of spintronics and magnetoelectronics.The total magnetic moment per f.u.?t of the Ti2CoTl1-xPbx satisfies the Slater-Pauling rule ?t=Zt-18,where Zt represents the total number of valence electrons per f.u..With increasing the Pb atom concentration x,the band structures in both spin-up and spin-down channels move towards low energy region but the spin-down indirect band gap Eg? increases.In addition,the elastic constants and elastic modulus indicate that the Ti2CoTl1-xPbx have mechanical stability,as well as ductility and strong resistance to fracture and plastic deformation.(3)We investigated the structural,electronic,magnetic and mechanical properties of quaternary Heusler alloys TiHfIrZ(Z=Al,Ga,In)based on first-principles calculations within GGA method.The most stable configurations of the TiHfIrZ(Z=Al,Ga,In)are all Y-type(?)in FM phase.The large absolute values of both negative formation energy and positive cohesion energy verify the TiHfIrZ(Z=Al,Ga,In)are thermodynamically stable.The higher than room-temperature Curie temperatures Tc,as large as 2 total magnetic moments per f.u.and the HM characters indicate the TiHfIrZ(Z=Al,Ga,In)can be used in the field of magnetoelectronics and spintronics.The TiHfIrZ(Z=Al,Ga,In)have mechanical stabilities,ductility and high hardness.The lattice constant regions of maintaining the HM characters are 5.722?7.062 (?),5.786?7.008 (?),5.839?7.034 (?) for TiHfIrZ(Z=Al,Ga,In),respectively.