Genetic algorithm prediction of crystal structure of metastable Si-IX phase

We performed genetic algorithm search for the atomic structure of the long time unsolved Si-IX phase. We found two new structures with space groups of P42/m and P-4, respectively, which have lattice parameters in excellent agreement with the experimental data. The phonon calculations showed that the P42/m structure exhibits a soft phonon mode, while the P-4 structure is dynamically stable. Our calculation also showed that the P-4 structure is a meta-stable structure in a pressure range from 0 to 40 GPa. The Si-IX phase could be a mixed phase consisting of the P42/m and the P-4 structures.