Band structure, phase transition, phonon and elastic instabilities in calcium polonide under pressure: A first-principles study

Ab initio self-consistent calculations were performed to study the structure, electronic, elastic and lattice dynamical properties of calcium polonide under hydrostatic pressure. Enthalpy calculations show that the material undergoes a first-order phase transition from rocksalt (RS) to CsCl structure at 16.7 GPa with 5.0% volume collapse. A soft transverse acoustic phonon mode at the zone boundary X point is identified at 21.1 GPa for RS phase, signifying a pressure-induced dynamical instability. Moreover, the shear modulus C44 in RS phase softens with increasing pressure. Our results imply that the underlying physical origin of pressure-induced RS → CsCl phase transition might be in close relation with the coupling between unstable X-point phonon mode and shear modulus C44. The RS phase under pressure undergoes a direct → indirect bandgap transition. The anomalous behavior of band structure originates from the fact that the top valence and bottom conduction states at different reciprocal lattice points show the different dependence on hydrostatic pressure.