As a first step, a search for the lowest energy structures was undertaken to distinguish between stable and unstable phases. A conjugate-gradient algorithm was called to relax the atoms onto ground state structure. For this purpose, Blöchl tetrahedron method was used with corrections brought by Methfessel-Paxton . Reciprocal space Brillouin-zone (BZ) integrals were approximated with a special k-point sampling . The calculational protocol consists of successive self-consistent cycles with increasing k-mesh (k x ,k y ,k z ) until the forces on atoms were below 0.02 eV/Å, and the stress components less than 0.003 eV/Å 3 .