Phosphorus under pressure: Ba-IV-type structure as a candidate for P-IV

Pressure-induced transformations in phosphorus are examined by means of ab initio density-functional calculations. Attempts are made to identify the structure of the P-IV high-pressure phase, and transformation paths between a self-hosting incommensurate Ba-IV-type structure and the body-centered cubic structure are discussed in this context. Total energy calculations for a series of commensurate approximations are used to optimize the Ba-IV-type structure with a body-centered tetragonal ``guest.'' This suggests that an optimized structure with a c-axis host-guest ratio cH∕cG~1.14 is energetically favorable as a candidate for the structure of P-IV. The cH∕cG found here differs significantly from the values determined experimentally for Ba-IV-type structures in Bi and Sb.