A comparative study of the structural, electronic, vibrational, dielectric and elastic properties of the two phases CaAgBi through first-principles calculations

Abstract

• The 186 and 194 phases of CaAgBi have been systematically comparative study via first-principles calculations. • The structural, electronic, vibration and elastic properties of hexagonal CaAgBi vary with the degree buckling of the Ag-Bi atomic layers. • The 194 phase of CaAgBi is found to be mechanical stable and dynamical unstable, while the ground-state of 186 phase could remain mechanical and dynamical stable within small buckling of the Ag-Bi atomic layers. • The electronic properties of two phases of CaAgBi are similar due to their similar structures, but the elastic properties of theirs are quite different, the 194 phase is shown greater anisotropic. Recently, CaAgBi was predicted to be a Dirac semimetal, which has been verified by its electrical resistivity and magneto-transport properties. However, its mechanical related properties have not been investigated yet. Here, we have studied the electronic, vibrational, dielectric and mechanical properties of CaAgBi materials (space group of 186 and 194 phases, which of the intrinsic difference is whether buckling between the Ag and the Bi atomic layers) by first-principles calculations. The results reveal that the both phases are metallic due to the Fermi level across valence bands, which are mainly dominated by the d orbitals of Ca atoms and the p orbitals of Bi atoms. In addition, the 186 phase shows more covalent quality in its electron density difference of the (1 1 0) plane. From the phonon dispersion, we found the 186 phase with small buckling is dynamically stable due to its absence imaginary frequency, while the 194 phase without buckling is unstable. The degree of buckling Ag-Bi layer, which can stabilize structure, had also been tested. The huge deviation between the Born effective charges and the corresponding nominal ionic charges indicates the 186 phase has a strong covalent nature, which is consistent with the above conclusion by electron density difference. Moreover, we also found the two phases of CaAgBi are mechanical stable, ductile and ionic-covalent, the bulk and shear modulus of which have rotational symmetry around z axis. Furthermore, the 186 phase is harder, stiffer, more covalent and better thermoelectric properties, while the 194 phase is more ductile, showing great anisotropy in directional modulus.