Local structure of liquid Rb-Se mixtures near the miscibility gap

In the liquid Rb-Se mixture there exists a liquid-liquid phase separation (miscibility gap) in the Se-rich concentration region. In order to study how the local structure is associated with the occurrence of the miscibility gap, EXAFS and neutron diffraction measurements for liquid Se, Rb10Se90, Rb15Se85 and Rb20Se80 mixtures have been carried out. The results reveal that the interaction between Rb and Se atoms is very weak and the liquid mixtures are composed of short Se chain-molecules of which chain ends are terminated by Rb atoms. We have carried out detailed structural modeling studies using reverse Monte Carlo simulation. It is found that the chain-like or ring-like Se segments are separated by voids. The Rb atoms adsorbed to Se chain ends segregate in the void and the shortened Se chain-molecules link up with each other when one approaches the miscibility gap.