The intermediate high-pressure phase of silicon

Recent X-ray diffraction experiments (M. Hanfland, U. Schwarz, K. Syassen, K. Takemura, Phys. Rev. Lett., 82 (1999) 1197) show that the intermediate h igh-pressure phase of Si-VI is isostructural with Cs-V. The structure is ba se-centered orthorhombic, space group Cmca, with 8 atoms in the primitive c ell (16 in the orthorhombic unit cell, oC16). We present here ab initio cal culations that include relaxations of Si in the primitive hexagonal (ph), t he orthorhombic (Cmca) as well as the hexagonal close packed (hcp) structur e. The force calculations confirm the atomic positions obtained by analyses of the experimental data, the calculated P-V relations for the three phase s are very close to the observed isotherms, and the calculated ph --> oC16 --> hcp transition pressures, 33 +/- 2 and 41 +/- 5 GPa, agree well with ex periments. A comparative calculation for Cs-V is briefly discussed. (C) 199 9 Elsevier Science Ltd. All rights reserved.