Phonon states of B12P2 crystals: Ab initio calculation and experiment

Using methods of density functional theory (DFT) and Raman spectroscopy (RS) we investigate the phonon structure of the B12P2 crystal. The theory is consistent with the experiment for all twelve Raman-active modes. We have calculated Born charges, oscillator strengths, longitudinal-transverse splitting, and dielectric functions of dipole modes. The results are used to interpret the known IR spectra of B12P2. Analysis of phonon dispersion shows that the crystal is dynamically stable. The energy gap separating acoustic and optical phonons is absent due to the overlap of the phonon states at the Brillouin zone boundary. Vibrations of phosphorus and boron atoms contribute to the low- and high-frequency regions of the phonon density of states, respectively.