First Principle Study Of The Defects In Ti₄AC₃?A=Ga,Si,Ge?

MAX phases exhibit a unique combination of the merits of metals and ceramics.Compared with other MAX phases,M₄AX₃ recevied less attention on their structures and properties for the difficulty to synthesize.In order to better understand M₄AX₃,we systematically investigated the effects of C vacancies and C/N solid solutions on the structures,phase stabilities,elastic properties and electronic properties in three typical MAX phases,Ti₄GaC₃,Ti₄SiC₃ and Ti₄GeC₃ by using the Vienna Ab initio Simulation Package?VASP?based on first-principles study.Lattice parameter a and bond lengths decrease as more carbon vacancies introduced in optimized Ti₄AC₃.When more C atoms are replaced by N atoms,lattice constant a decrease,c increase in Ti₄GaC₃.Lattice constant a decrease,the highest value of c was in Ti₄SiC_1.5N_1.5,and then decrease.Lattice constants and all bond lengths decrease in Ti₄GeC₃.The introduction of C vacancies leads to bondings rebuild and atoms are more closely spaced.The diameter differences between C and N atoms and repulsion between Ti and N lead to lattice parameters change in Ti₄A(C_3-xN_x)?x=0-3?.The stability of Ti₄AC_3-x decrease as more C vacancies introduced.Ti₄AC₃ with carbon vacancies in 2a are more stable than in 4f sites.The stability of its solid sloutions decreases as more C atoms substituted with N atoms,which results from the repulsion in Ti₄A(C_3-xN_x).Hardness and stiffness decrease as more C vacancies introduced in Ti₄AC₃.Ti₄GaC_1.5N_1.5has the lowest hardness and stiffness.Bulk modulus increase,hardness decrease and Ti₄SiC_1.5N_1.5 has the most stiffness.Elastic moduli changes when more N are introduced.Doping N in Ti₄AC₃ would enhance interatomic bondings.The mechanical properties under different solid solubility are different.The study of the density of states?DOS?at the Fermi level demonstrated that,the DOS increases with C vacancies introduced in Ti₄AC₃,which causes electronic conductivity increase.With C replaced by N in Ti₄A(C_3-xN_x)solid solutions,the Ti₄GaC_1.5N_1.5 has the highest DOS.The DOS increases when more N are introduced in Ti₄SiC₃ and Ti₄GeC₃,electronic conductivity increases.The increases in DOS caused by the introduction of C vacancies and N doping in Ti₄AC₃ are attributed to the enhanced Ti-Ti hybridization.