Phonon instabilities leading to structural phase transition in YN and YP: A first-principles study

The dynamical instabilities of YN and YP are investigated using ab initio density functional theory to understand the mechanism of pressure-induced phase transitions from the rocksalt (B1) to the CsCl(B2) structure. We predict a transition from B1 to B2 phase at 130 GPa for YN and 55 GPa for YP. To support these results we have additionally developed ab initio lattice dynamics calculations. Analysis of the phonon calculations suggests that the pressure induced instabilities of the transverse acoustic modes at W and X points of the Brillouin zone for YN and YP respectively are responsible of the phase transition of these compounds. Pressure induced dynamical instabilities has been predicted for the range between 70 and 90 GPa for YP and between 180 and 190 GPa for YN.