Structure of high-temperature fluid selenium

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