AB-INITIO CALCULATION OF PHONON-SPECTRA FOR GRAPHITE, BN, AND BC2N SHEETS

Phonon spectra for graphitic sheets of carbon, BN, and BC2N are comput ed and studied. The dynamical matrices are constructed from force cons tants obtained by performing supercell Hellmann-Feynman force calculat ions within the framework of the local-density approximation. The calc ulated phonon dispersions of the BC2N sheet are found to be similar to a superposition of those of graphite and BN sheets. Geometrical relat ions between the calculated phonon modes of the BC2N sheet and those o f graphite and BN sheets are discussed.