A first-principles study of the tetragonal and hexagonal R2Al (R = Cr, Zr, Nb, Hf, Ta) phases

The crystal structures, elastic moduli, electronic structure, and phonon dispersion of the tetragonal R2Al (R = Cr, Zr, Nb, Hf, Ta) intermetallic compounds are investigated by using the first-principles method. The space group number is 139 for tetragonal Cr2Al, 136 for tetragonal Nb2Al and Ta2Al, and the space group numbers are 140 and 194 for tetragonal and hexagonal Zr2Al and Hf2Al, respectively. The results of elastic constants and phonon dispersion indicate that the present intermetallic compounds are thermodynamically stable. The stability of hexagonal Zr2Al and Hf2Al is analyzed via the electronic density of state, compared to the tetragonal Zr2Al and Hf2Al compounds. For the R2Al intermetallic compounds, the less ductility and strong anisotropy are predicted. The more negative formation enthalpy and thermodynamic stability of R2Al (R = Nb, Zr, Hf) shed light on the Nb2Al, Zr2Al, Hf2Al phases found experimentally in refractory high entropy alloys.