Ab initio pseudopotential studies of equilibrium lattice structures and phonon modes of bulk BC3

Different stacking arrangements of BC3 layered crystals are studied with the use of the ab initio pseudopotential density-functional method. The total energies, lattice constants, electron energy band structures and density of states, as well as phonon frequencies are calculated for the possible bulk BC3 structures obtained by full relaxations starting from different initial atomic configurations of ABAB (or ABCABC)... layer stacking. Two stable BC3 structures, one semiconductor and the other metal, are obtained, which have lower total energies comparing with those of the structures proposed previously. Our calculations show that except for these two BC3 structures, all the structures we studied, including the BC3 structures proposed previously, have imaginary phonon frequencies corresponding to the relative, parallel motion of the adjacent BC3 layers, indicating the instability of the layer stacking in these structures.