Structure of high-temperature fluid selenium

A semiempirical tight-binding energy model is developed for selenium. It in cludes s and p electrons as well as an empirical description of the dispers ion forces that proves necessary at the liquid densities under study. The b and-structure parameters are obtained by fitting ab initio calculations. Th e simulated liquid structures are in very good agreement with the most rece nt x-ray scattering and extended x-ray absorption fine-structure experiment s in a broad temperature and density range. The Monte Carlo simulations per formed show that the complex liquid structures observed result from the bre aking and branching of the selenium chains. The total coordination number i s shown to result from the balance between one-, two-, and threefold coordi nated atoms. The role of these defects is discussed in relationship with th e electrical conductivity of the liquid, i.e., the semiconductor-metal and metal-nonmetal transitions observed at high pressures.