First-principles investigations on structure stability, elastic properties, anisotropy and Debye temperature of tetragonal LiFeAs and NaFeAs under pressure

First-principles methods combined with homogeneous deformation methods are used to investigate the second- and third- order elastic constants of LiFeAs and NaFeAs with tetragonal structure. The effective second-order elastic constants are obtained from the second- and third- order elastic constants for LiFeAs and NaFeAs. Pugh's modulus ratio, Poisson's ratio, Vickers hardness, elastic anisotropy and Debye temperature under high pressure are further investigated. The ductility increases and the hardness decreases with pressure, and the ductility of NaFeAs is better than that of LiFeAs and the hardness of NaFeAs is always lower than that of LiFeAs. For LiFeAs and NaFeAs, the shear anisotropy factor A{100} along the {100} plane decreases quickly and the shear anisotropy factor A{001} along the {001} plane increases with increasing pressure.