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.