Lattice dynamical properties of black phosphorus under pressure studied by inelastic neutron scattering.

Acoustic phonon dispersion relations of black phosphorus in its orthorhombic A11 phase have been measured along three principal directions mainly at 1 bar and 15.4 kbar. An anomalous softening has been observed on the [100]LA branch whose vibrational patterns (eigenvectors) have been analyzed based on a force-constant- model fit by Kaneta et al. A large change in bonding angles associated with this phonon mode is implied to cause the softening through an electron-phonon interaction.