The stability of the tetragonal phase under high pressure

Abstract

The Madelung energies for the transformations from the diamond or zinc-blende structure to the tetragonal β-Sn-type structure and to the cubic NaCl-type structure are calculated. A high-pressure Cs IV phase of alkali metals is equivalent to our previously predicted β-Sn(II) structure. At higher pressure, β-Sn(II) phase tends to become stable compared with β-Sn(I), which corresponds to the well-known β-Sn-type structure as a high-pressure phase of the diamond-type structure. Numerical calculations to determine the relative stabilities among diamond or zinc-blende, β-Sn(I) and β-Sn(II) structures are performed for Si, GaSb, and among BCC, FCC, β-Sn(I) and β-Sn(II) structures for Rb and Cs using our perturbational treatment based on the pseudopotential formalism. Then, numerical results of transition pressure, relative volume, volume discontinuity and transition heat are discussed together with the observed data.