Structure of fluid phosphorus at high temperature and pressure: An x-ray diffraction study

The structure of fluid phosphorus has been studied by in situ high pressure x-ray diffraction in order to characterize the first-order transition at high temperature and pressure between a molecular and a polymeric structural form. The structure factor of the molecular form can be reasonably well represented in terms of a simple model for the structure of a molecular fluid of uncorrelated tetrahedra. This allows us to estimate the density in a large thermodynamic range. Moreover, the coordination number of the polymeric form is deduced from the radial distribution function and comes out to be <4. This result can be interpreted as the survival of a Peierls distortion in the p-bonding mechanism in this form of phosphorus. This distortion is found to be basically insensitive to temperature up to 2200 °C.