Ab initioMolecular Dynamics Simulations of Structural Transformations in Silicon

Abstract

Constant-pressure ab initio molecular dynamics simulations were carried out to study structural phase transitions of silicon at high pressure region. In a compression process, a structural transformation from the diamond to the simple hexagonal structure was observed. In a decompression process, the fcc structure was transformed to the simple hexagonal one accompanying with slight deformation. In spite of our small system size and crude electronic state calculations, we could realize several structural transformations. In this report, structural transformations of crystalline Si are investigated by ab initio molecular dynamics (MD) simulations under constant-pressure and constant- temperature condition. To carry out these simulations, the Parrinello-Rahman (PR) method 1) and Nose-Hoover thermostats 2) are combined with the conventional Car- Parrinello method. 3) The combination of the CP and PR method is often referred as the constant-pressure CP method. 4) Crystalline Si takes the diamond structure at room temperature and atmospheric pressure. However, at higher pressure, many other structures are observed in exper- iment. At 11GPa, the diamond structure is transformed to the β-tin structure and at 16GPa, β-tin to the simple hexagonal (sh) structure. At higher than 40GPa, Si forms the hcp structure and at higher than 80GPa, the fcc structure is observed. We have tried to simulate these transition sequences. In our simulations, struc- tural transformations were realized and several types of transformed structures were obtained depending upon transition pressures and initial conditions. In this report, we present simulation results in which the sh structure was obtained. Electronic state calculations were implemented within the local density approximation (LDA). 5) Electronic wave functions are expanded in a plane wave basis at a single point (Γ point) in the Brillouin zone. The electron-ion interaction was described by a pseu- dopotential 6) with a separable form. 7) Our simulation cell contained 64 atoms and temperature of atoms was kept to 300 K throughout MD runs by a thermostat. The number of atoms is slightly larger than that in the previous study. 4) We started our simulation accompanying a compression process with the dia- mond structure. Cutoff energy Ecut of a plane wave basis was 20 Ry and external pressure was set to 0 GPa. After equilibration in the diamond structure at 0 GPa, pressure was raised to 14 GPa instantaneously, but no structural change was ob- served. After ∼ 1ps from this compression, pressure was again raised up to 26 GPa. A drastic structural change was observed during MD run at this pressure. We found that the transformed structure is almost the sh structure, but it contains a defect. Radial distribution function g(r) for this structure shows clearly that the sh-like