Simultaneous softening of Cu3N phonon modes along the T2 line under pressure: A first-principles calculation

Cu3N band structures and phonon dispersion curves under pressure are calculated using first-principles density functional theory and density functional perturbation theory with the local density approximation by the plane-wave pseudopotential method. The band structures are similar to other full-potential calculations. The indirect band gap is about 0.13 eV and decreases with increasing pressure. Simultaneous softening of the M3 and R25 zone boundary phonon modes was found and the possible associated successive structural phase transitions were discussed. The mode Grüneisen parameters for optic modes were obtained and the frequency versus pressure relationship was well fitted to second-order polynomials. The quadric relationship between the soft-mode frequency and pressure was also well reproduced for the M3 and R25 soft modes. The large difference of the soft-mode-driven transition pressures for the first high-pressure phases of ReO3 and Cu3N were also discussed.